JP2001141688A - Electrode for measuring acidity and acidity-measuring apparatus using the same and its measurement method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrode for measuring an acidity whereby an acid contained in a sample to be measured can be directly measured without diluting the sample, an acidity-measuring apparatus which is compact and easy to operate for highly accurately and directly measuring an organic acid or the like in the sample, and a method for measuring an acidity which generates few errors when calculating the acidity because of the absence of a solvent for measurement and the absence of dilution of the sample and can output stable measured results by controlling a voltage and a potential impressed at the measurement. SOLUTION: The electrode for measuring an acidity includes a working electrode, a counter electrode and a reference electrode formed on a base. The working electrode and the reference electrode are formed of an inactive conductive material not generating an oxidation-reduction reaction, and the counter electrode is formed of a conductive material. A proton attack layer with an immobilized proton attack substance is formed on the working electrode and is exposed to a surface.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an acidity measuring electrode capable of directly measuring an organic acid or the like contained in food or the like without diluting it in a solvent, an acidity measuring device for measuring acidity using the electrode, and an acidity measuring device. It relates to the measuring method.

[0002]

2. Description of the Related Art In various foods, the acidity of each food is measured in the production process. There are various methods for measuring the acidity. An example of a conventional acid measurement method is as follows: standard oil and fat analysis method, Japanese Agricultural Standards, JI
S, Japanese Pharmacopoeia Oil and Fat Test, Hygiene Test, Food and Beverage Test,
There are methods specified in the tap water test method and the like, and the measurement is based on a neutralization titration method using phenolphthalein as an indicator. Then, in order to explain this neutralization titration method, the neutralization titration method specified in the clean water test method and the standard fat and oil analysis method will be described below.

[0003] The acidity in the clean water test method is defined as the number of mg when acid is converted to calcium carbonate contained in one liter of a sample. Specifically, test water 100mL
And add about 0.2 mL of the phenolphthalein indicator to the test water, add a 0.02 mol / L sodium hydroxide solution, stopper tightly and shake gently. The end point of neutralization is defined as the time when the titration is continued until the remaining sodium hydroxide remains, and the mL number a of the sodium hydroxide is determined. Acidity θ at that time (calculated as calcium carbonate mg /
L) is calculated according to (Equation 1).

[0004]

(Equation 1)

Next, a neutralization titration method specified in the standard fat and oil analysis method will be described. The definition of acid value in the reference fat analysis method is as follows:
This refers to the number of mg of potassium hydroxide required to neutralize free fatty acids contained in 1 g of a sample. In the case of a liquid sample, the sample is taken from its estimated acid value (for example, if the acid value is 1 or less, collect 20 g,
If the acid value is more than 1 and 4 or less, 10 g is collected, and if the acid value is more than 4 and 15 or less, 2.5 g is collected) and properly measured in an Erlenmeyer flask. To this is added 100 mL of neutral solvent and shaken thoroughly until the sample is completely dissolved. However, the neutral solvent referred to here is ethyl ether, ethanol 1: 1.
About 0.3 mL of a phenolphthalein indicator was added to 100 mL of the mixed solvent, and the mixture was neutralized with a 1/10 normal (N) potassium hydroxide-ethanol solution immediately before use.

In the case of a solid sample, it is heated and melted on a water bath and then dissolved by adding a solvent. This is defined as 1/10 (N)
Titrate with a potassium hydroxide-ethanol standard solution, and determine the end point of neutralization when the color change of the indicator lasts for 30 seconds. Then, the mg number of potassium hydroxide at this time is calculated.

[0007] However, in addition to the neutralization titration method described above, there is a method of measuring acidity that is completely different from that described above by electrochemically, ie, voltammetry. The method of electrochemically measuring the acidity is disclosed in Japanese Patent Application Laid-Open No. 5-264503, in which a measuring electrolyte solution in which a free fatty acid and a naphthoquinone derivative coexist is measured by voltammetry by a potential regulation method. The magnitude of the current value of the pre-reduction wave for reducing the naphthoquinone derivative is proportional to the concentration of the free fatty acid for all fatty acids from lower fatty acids such as formic acid to higher fatty acids such as oleic acid and linoleic acid,
The fact that the value obtained by superimposing the current values of each fatty acid corresponds to the total concentration of fatty acids is used. That is, the acid concentration is measured by measuring the magnitude of the current value of the pre-reduction wave in the naphthoquinone derivative. The data measured by this method is shown by the solid line in FIG. FIG. 4 is a current-potential relationship diagram for measuring acidity by voltammetry of a measurement electrolyte solution in which a conventional naphthoquinone derivative coexists. In FIG.
The horizontal axis shows the potential of the working electrode with respect to the reference electrode when silver-silver chloride was used as the reference electrode and glassy carbon of φ3 was used as the working electrode, and the vertical axis shows the current value flowing at this time. However,
The current value changes depending on conditions such as the surface area of the working electrode and the concentration of acid. On the other hand, the voltage value on the horizontal axis slightly varies depending on the acid concentration, but the influence of the conditions such as the surface area is negligible. A in FIG. 4 is a pre-peak showing a pre-reduction wave proportional to the acid concentration, and B is a main peak of the naphthoquinone derivative.

[0008] This electrochemical measurement method enables measurement with a relatively simple operation. However, as a measurement electrolyte solution, a support electrolyte solution and an organic mixed solvent in which a naphthoquinone derivative is dissolved are mixed with a free fatty acid-containing solution. Since the measurement sample is mixed and produced in a very small amount, there is a problem that variation in weighing of the liquid greatly affects measurement accuracy.

[0009]

As described above,
In the conventional acidity measuring device using the neutralization titration method, since the measurer judges the color change due to the phenolphthalein indicator and determines the end point of the titration, the end point varies depending on the measurer, and the acidity is measured by the measurer. Could change.

Among them, in the case of the neutralization titration method for fatty acids in the standard fat and oil analysis method, a mixed solution of ether and ethanol is used as a neutral solvent, and the boiling point of ether is 34.6 ° C.
It is difficult to handle because it is easy to catch fire. In addition, when the color of the sample itself is dark, for example, when the material itself is colored, such as oil, juice, wine, etc. fried in large quantities, the change in the color of phenolphthalein near the end point of titration is accurately determined. There was a problem that the end point could be read incorrectly and the measured value could not be grasped. And the amount of the sample is
0 g and 100 mL of the neutral solvent are required, so that a large amount of sample is required for one measurement.

The electrochemical acidity measuring apparatus disclosed in Japanese Patent Application Laid-Open No. 5-264503 requires a coexisting electrolyte containing an organic solvent such as quinone and alcohol and a supporting electrolyte such as sodium chloride as a measuring reagent. Further, since the coexisting electrolyte solution and the measurement sample must be mixed at a predetermined volume ratio, the mixing variation affects the measurement accuracy, so that there is a problem that the device is inconvenient in use.

Therefore, the acidity measuring device by the electrochemical method as well as the measurement by the neutralization titration method still cannot reliably measure the acidity. In addition, when the acidity is measured by an electrochemical method, the potential of the electrode is not stabilized depending on the state of the electrode before the measurement, so that the measured current value is not stabilized and the measurement accuracy is affected. As described above, even though the technology is excellent in principle, it has a problem that it is difficult to make it a practical measuring device.

However, among the acidity measuring devices by the electrochemical method, a device using a biosensor typified by a blood glucose level sensor directly drops a sample to be measured without adverse effects of solvent dilution or electrode condition before measurement. Although the above coexisting electrolyte is unnecessary and simple, as described in JP-A-58-99746, the enzyme adsorbed on the electrode is used for the measurement sample and the catalyst. To transfer the electrons emitted during the reaction directly to the electrode by the medium compound, dissolve the medium compound such as ferrocene in an organic solvent, attach it to the electrode surface, and then attach a phosphate buffer solution of the enzyme. is necessary. For this reason, the substance being measured at the electrode is not directly contained in the sample,
The substances generated after the enzymatic reaction are measured. Therefore, there is a possibility that an error may occur when converting to a measured value.

Therefore, the present invention solves such a conventional problem, and it is not necessary to dilute a measurement sample, and it is possible to measure a substance directly contained in the sample. An object of the present invention is to provide an acidity measurement electrode that can be operated and can measure acidity with high accuracy, and an acidity measurement device that measures acidity using the electrode.

Another object of the present invention is to provide an acidity measuring method capable of measuring acidity with a high degree of accuracy because there is no solvent for measurement and no dilution.

[0016]

According to the present invention, there is provided an electrode for measuring acidity according to the present invention, wherein a working electrode and a reference electrode are made of an inert conductive material which does not cause a redox reaction, and a counter electrode is provided. A proton attack layer formed of a conductive material and having a preproton attack substance immobilized thereon is exposed on the surface of the working electrode.

With this configuration, it is possible to provide an electrode for measuring the acidity, which can directly measure the contained acid without having to dilute the sample to be measured.

According to another aspect of the present invention, there is provided an acidity measuring apparatus comprising: a power supply for applying a voltage between the working electrode and the counter electrode when a sample containing an acid is dropped on the acidity measuring electrode; A control unit that controls a potential between the working electrode and the reference electrode to a predetermined potential, a detection unit that detects a current flowing between the working electrode and the counter electrode, and a current value detected by the detection unit. And an arithmetic unit for converting into an acidity.

With this configuration, it is possible to directly measure the acid in the sample to be measured with high accuracy, and to provide a compact and easy-to-operate acidity measuring apparatus.

Further, in order to solve the above-mentioned problems, the acidity measuring method of the present invention provides a method for measuring the acidity on a working electrode, a counter electrode, and a reference electrode, which are formed by exposing a proton attack layer having a preproton attack substance immobilized on the surface. After the acid-containing sample is dropped, a voltage is applied between the working electrode and the counter electrode, and when the potential between the working electrode and the reference electrode is maintained at a predetermined potential, the voltage between the working electrode and the counter electrode is reduced. The acidity is calculated by measuring the value of the current flowing through the device.

According to this configuration, it is possible to provide an acidity measuring method capable of measuring an acidity with high accuracy because the solvent is not diluted without the measurement solvent.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention described in claim 1 is an electrode for measuring acidity provided with a working electrode, a counter electrode and a reference electrode formed on a base, wherein the working electrode and the reference electrode are provided. The counter electrode is made of a conductive material, and a proton attack layer in which a pre-proton attack substance is immobilized is exposed on the surface of the working electrode while being made of an inert conductive material that does not cause a redox reaction. Since it is an electrode for measuring acidity characterized by being formed, a sample to be measured is dropped directly onto the electrode for measuring acidity without making a measuring solvent mixed with a proton attack substance and immersing the electrode for measuring acidity. Alone can directly measure hydrogen dissociated from acids and undissociated hydrogen.

The proton attack substance is not only a substance having an action of binding to a proton (H +) dissociated from an acid, but also a substance having an action of attacking and extracting a non-dissociated proton contained in an acid. That is. But,
Proton attack substances do not have such an effect when placed at a spontaneous potential. That is, a substance at this natural potential is called a preproton attack substance.
When a voltage on the negative side of the natural potential is applied to this preproton attack substance, the preproton attack substance becomes an anionized active state (radical) and becomes a proton attack substance. Therefore, the proton attack layer is a layer in which a stable preproton attack substance is fixed when the electrode is not used.However, when the sample to be measured is dropped and anionized to form a proton attack substance, the acid in the sample to be measured is applied with an applied voltage. Since it is polarized to be δ +, the proton of the acid can be extracted.

According to a second aspect of the present invention, the working electrode, the counter electrode, and the reference electrode are each formed as a thin film, and the electrode for measuring the acidity according to the first aspect, Each electrode is flattened and thin, and the acidity measurement electrode can be made compact.

According to a third aspect of the present invention, since the base is a substrate, the entirety of the electrode for measuring the acidity is compactly formed. can do.

According to a fourth aspect of the present invention, in the electrode for measuring acidity according to any one of the first to third aspects, the preproton attack substance is a quinone compound or an azo compound. Since the quinone compound and the azo compound are chemically stable substances, the electrodes can be stored for a long time. Furthermore, even if a quinone compound or an azo compound is given the energy of decomposition by light, the activation energy required for the decomposition becomes higher than the light energy by immobilization, so that the light stability is improved and photodecomposition may occur. And stable measurement can be performed.

Here, as the quinone compound, p-benzoquinone, o-benzoquinone, diphenoquinone, naphthoquinone, anthraquinone, benzeneazohydroquinone and the like are used. Examples of the azo compound include azobenzene, azophenol, benzeneazomethane, benzeneazoethane, azonaphthalene, azotoluene, azobenzoic acid, azoaniline, azoanisolemethylazobenzene, 1-benzeneazonaphthalene, benzeneazonaphthol, oxyazobenzene, , 4-dioxyazobenzene and the like are used.

The invention according to claim 5 is characterized in that a part of the proton attack layer is exposed to the surface and the periphery is molded with an insulating material. Since it is a measurement electrode, the diffusion of the substance to be measured (acid) into the electrode is stabilized, so that the acidity can be measured stably and the area of the electrode exposed on the surface can be easily adjusted.

The invention described in claim 6 is the first invention.
5.The electrode for measuring acidity according to any one of 5, wherein a power supply for applying a voltage between the working electrode and the counter electrode when a sample containing acid is dropped on the electrode for measuring acidity, the working electrode and the working electrode A control unit that sweeps a potential between reference electrodes, a detection unit that detects a current flowing between the working electrode and the counter electrode, and a calculation unit that converts a current value detected by the detection unit into an acidity. The acidity measurement device is characterized by using an electrode formed by directly exposing the proton attack layer on the surface, so the chronoamperometry measurement is performed directly without diluting the sample to be measured in a solvent. In addition, the measurement time can be shortened, and the acidity can be easily measured simply by measuring the reduction current of the acid contained in the sample to be measured.

The invention described in claim 7 is the first invention.
5.The electrode for measuring acidity according to any one of 5, wherein a power supply for applying a voltage between the working electrode and the counter electrode when a sample containing acid is dropped on the electrode for measuring acidity, the working electrode and the working electrode A control unit that sweeps a potential between reference electrodes, a detection unit that detects a current flowing between the working electrode and the counter electrode, and a calculation unit that converts a current value detected by the detection unit into an acidity. Because the acidity measurement device is characterized by using an electrode formed by directly exposing the proton attack layer on the surface, direct voltammetry measurement without diluting the sample to be measured in a solvent Can be. Further, even if the characteristics of the apparatus vary, highly accurate measurement can be performed, and the acidity can be easily measured only by measuring the reduction current of the acid contained in the sample to be measured.

According to an eighth aspect of the present invention, in the acidity measuring apparatus according to the seventh aspect, the sweep speed of the control unit is 10 to 100 mV / s. Since the reaction between the acid and the proton attack substance on the electrode surface is stabilized, the measurement can be performed with higher accuracy.

Here, as the sweep speed becomes slower than 10 mV / s, there is a tendency that the reaction on the electrode surface becomes excessive and it becomes difficult to obtain a stable potential.
As the speed becomes higher, the speed at which the potential is swept exceeds the reaction speed of the electrode, so that it tends to be difficult to obtain a stable current waveform.

According to a ninth aspect of the present invention, in the acidity measuring device according to any one of the sixth to eighth aspects, the sample to be measured is a juice. Acid can be suitably measured.

[0034] The invention described in claim 10 is the acidity measuring apparatus according to any one of claims 6 to 8, wherein the sample to be measured is an alcoholic beverage. Organic acids can be suitably measured.

[0035] In the eleventh aspect of the present invention, the sample to be measured is a food oil or an engine oil. The free fatty acids contained in the engine oil can be suitably measured.

When measuring the edible oil or the engine oil, the edible oil or the engine oil is dissolved in an organic solvent in advance, and then a sample solution mixed with a supporting electrolyte such as lithium perchlorate or lithium chloride is prepared. It is preferable to measure free fatty acids in the sample solution.

According to a twelfth aspect of the present invention, in the acidity measuring device according to any one of the sixth to eighth aspects, the sample to be measured is serum. Can be measured.

Here, when measuring serum, it is preferable to measure fatty acids obtained by decomposing and releasing lipid components in serum by enzymes.

The invention described in claim 13 of the present invention is as follows:
A working electrode formed by exposing a proton attack layer on which a preproton attack substance is immobilized, and a counter electrode;
After dropping the acid-containing sample to be measured on the reference electrode, a voltage is applied between the working electrode and the counter electrode, and when the potential between the working electrode and the reference electrode is held at a predetermined potential, the working electrode The acidity measurement method is characterized in that the acidity is calculated by measuring the value of the current flowing between the counter electrode and the electrode, so that the working electrode formed by exposing the proton attack layer is used. Chronoamperometry can be directly performed without dilution, and the measurement time can be shortened.

The invention described in claim 14 of the present invention is as follows:
A working electrode formed by exposing a proton attack layer on which a preproton attack substance is immobilized, and a counter electrode;
After dropping the acid-containing sample to be measured on the reference electrode, a voltage is applied between the working electrode and the counter electrode to sweep a potential between the working electrode and the reference electrode. Since the acidity measurement method is characterized in that the acidity is calculated by measuring the value of current flowing through the working electrode formed by exposing the proton attack layer, the sample to be measured is not diluted in a solvent. Can be directly measured by voltammetry.

The invention described in claim 15 of the present invention provides:
15. The preproton attack substance is a quinone compound or an azo compound.
The acidity measurement method described in (1) above is a stable substance among the preproton attack substances, and can perform highly accurate measurement with little change even after long-term storage.

The invention described in claim 16 of the present invention provides:
The pre-proton attack substance which is initially partially hydrolyzed by dropping the sample to be measured is returned to the pre-proton attack substance with the potential between the working electrode and the reference electrode as an oxidation potential, and then the working electrode and the reference electrode Since the potential between the two is maintained at a predetermined reduction potential, the acidity measurement method according to claim 13 or 15, wherein the preproton attack substance hydrolyzed by dropping the sample to be measured is converted to an oxidation potential. Since the acidity is measured by applying a reduction potential after returning, the sensitivity is increased and the accuracy of the chronoamperometry measurement is improved.

The invention described in claim 17 of the present invention provides:
The pre-proton attack substance which is initially partially hydrolyzed by dropping the sample to be measured is returned to the pre-proton attack substance with the potential between the working electrode and the reference electrode as an oxidation potential, and then the working electrode and the reference electrode 15. The method according to claim 14, wherein the potential is swept by a reduction potential.
Since the acidity measurement method described in 5, the preproton attack substance hydrolyzed by dropping the sample to be measured is returned to its original state by the oxidation potential, and then the reduction potential is applied to measure the acidity. The accuracy is improved.

An embodiment of the present invention will be described below with reference to the drawings.

(Embodiment 1) FIG. 1 is a perspective view of an acidity measuring apparatus according to Embodiment 1 of the present invention, and FIG.
FIG. 1 is a perspective view of an electrode for measuring acidity in the present embodiment. In the figure, reference numeral 1 denotes an acidity measuring device according to the present embodiment;
Is a main body of the acidity measuring device 1, 3 is an LCD as a display means for displaying the measured acidity disposed on the upper surface of the main body 2, and 4 is a starter formed on the upper surface of the main body 2 for starting the measurement.
A stop button, 5 a power button formed on the upper surface of the main body 2 for turning on and off the power of the acidity measuring device 1, 6 a button for retrieving the acidity measurement data formed on the upper surface of the main body 2,
Reference numeral 7 denotes an acidity measuring electrode for measuring the acidity of the sample to be measured, 8 denotes a main body side connector for connecting the main body 2 to the acidity measuring electrode 7, and 9 denotes an electrode side connector connected to the main body side connector 8. Reference numeral 10 denotes a support portion composed of a plate and a pole for supporting the acidity measurement electrode 7, and reference numeral 11 denotes glassy carbon called gold, carbon or glassy carbon, or PF.
1000 ° C to 20 ° C plastic foam called C
A surface made of an inert conductive material such as carbon sintered at 00 ° C. (However, even if the inert conductive material is slightly less stable, it is better not to use platinum.) Working electrode with layer 19 formed, 1
Reference numeral 2 denotes a reference electrode made of an inert conductive material such as glassy carbon called gold, carbon or glassy carbon, or carbon obtained by sintering a plastic foam called PFC at 1000 ° C. to 2000 ° C. Reference numeral 13 denotes platinum, graphite, gold, If necessary, a counter electrode made of a conductive material that is chemically stable without corrosion even in a measuring solution such as stainless steel, aluminum and its alloy, and 14 is a working electrode 11 and a reference electrode 1
2 is a resin of an insulating material for fixing 2, 15 is a base on which the three electrodes are arranged, 16 is a cable for electrically connecting the three electrodes to the main body 2, 17 is a frame-shaped measurement target A guard 18 for preventing the sample from flowing out is a cover for covering the working electrode 11, the reference electrode 12 and the counter electrode 13 other than the exposed portions.

Here, the electrode 7 for measuring the acidity is shown in FIG.
In addition to the configuration in which the working electrode 11 and the reference electrode 12 are provided by molding with a resin 14 inside the box-shaped counter electrode 13 as shown in FIG. A structure having a cover 17 for preventing the sample from dripping, or a method in which a thin electrode is formed by screen printing, sputtering, CVD, or the like of a carbon paste or the like on a substrate 15 as shown in FIG. Alternatively, the configuration may be such that portions other than the exposed portion are covered with the cover 18.

The following describes the proton attack layer which is a feature of the present invention. Proton attack layer 1
There are the following four methods for forming 9.

FIG. 3A is a model diagram showing a method of physically adsorbing the proton attack layer, FIG. 3B is a model diagram showing the chemical surface bonding of the proton attack layer, and FIG. 3C is a model diagram showing the physical surface of the proton attack layer. Model diagram showing fixing method, (d)
FIG. 3 is a model diagram showing a method for chemically fixing a proton attack layer on a surface. In the figure, 19 is a proton attack layer, 20 is a preproton typified by a quinone compound, an azo compound, etc., which is exposed and fixed to the surface to form the proton attack layer 19 and has a deprotonation effect when radicalized. An attack substance, 21 is an adhesive mainly composed of an epoxy resin, a urethane resin or the like, and 22 is a polymer film made of a porous polymer. The preproton attack substance 20 is anionized when the sample to be measured is dropped, becomes active, and becomes a proton attack substance. This proton attack substance has an action of not only binding to a proton (H +) dissociated from an acid but also attacking and extracting a non-dissociated proton contained in an acid. Does not exhibit such an effect. The substance at this natural potential is a preproton attack substance.

The physical adsorption method shown in FIG.
Preproton attack substance 2 in organic solvent such as ethanol
0, in this case, a method in which a coexisting solution in which 5 to 30 mM of a quinone compound is dissolved is uniformly spread on the surface of the working electrode 11 to 30 μm or less and applied, and then the organic solvent is dried and fixed.

In the chemical surface bonding method shown in FIG. 3B, the preproton attack substance 20 is hydrolyzed or mercapto-formed with sulfur, and then acts on a co-existing electrolytic solution dissolved in an organic solvent containing a supporting electrolyte. The electrode 11, the reference electrode 12, and the counter electrode 13 are immersed and fixed by applying a voltage between the working electrode and the counter electrode so that an oxidation voltage of 0.5 to 2.0 V is applied between the working electrode 11 and the reference electrode 12. Or 0.5 to 2.0 between the working electrode 11 and the reference electrode 12
There is a method in which a voltage is applied and fixed between the working electrode and the counter electrode so that an oxidation voltage of V and a reduction voltage of -0.5 to -1.0 V are alternately applied.

To give a specific example, mercaptohydroquinone 10m in 200mL of a standard phosphate buffer of pH 6.86.
The working electrode 11, the reference electrode 12, and the counter electrode 13 are immersed in a solution in which M is dissolved, and the potential of the working electrode 11 is set to ± 1 V with respect to the reference electrode 12 at a sweep rate of 100 mV / s.
After sweeping 00 times, the polymer of mercaptoquinone is immobilized on the surface of the working electrode 11 and the proton attack layer 19
Is formed. Alternatively, even when the potential of the working electrode 11 is set to 1 V with respect to the reference electrode 12, the amount of immobilization is small, but quinone can be immobilized.

The working electrode 11, the reference electrode 12, and the counter electrode 13 were immersed in the co-existing electrolyte without using sulfur,
Even when an oxidation voltage of 2.0 V is applied, the proton attack layer 1
9 are possible. However, when using this method,
The fixing power is inferior to the case of mercapto conversion.

In the physical surface fixing method shown in FIG. 3C, the pre-proton attack substance 2 is applied to the insulating adhesive 21.
This is a method in which 0 particulate matter is mixed and uniformly spread on the surface of the working electrode 11 to 30 μm or less. As a method of applying to the electrode surface, a method of applying a coating solution having a certain viscosity by spin coating, screen printing, or the like is preferable.

The chemical surface fixing method shown in FIG. 3 (d) uses a porous polymer, Nafyon (manufactured by DuPont),
After dissolving polyhydroxyethyl methacrylate or the like in an organic solvent such as ethanol by 1 to 3% by weight, a solution prepared by dissolving a preproton attack substance 20, here a quinone compound, in an organic solvent such as ethanol in a concentration of 10 to 30 mM is added to the solution.
This is a method in which 0% is mixed and uniformly spread on the surface of the working electrode 11 to 30 μm or less. As a method of applying to the electrode surface, when the application area is about 100 square mm, 0.06 mL of the application liquid is used using a micro syringe.
A method of dropping and performing spin coating is preferable. When the surface of the working electrode is polished to near the mirror surface, the coating liquid may be dropped and dried after being spread naturally and uniformly. At this time, if the amount of the applied liquid and the surface roughness of the working electrode are controlled, a stable working electrode 11 can be manufactured.

The proton attack layer 19 is formed by the above-described fixing method, and a part of the proton attack layer 19 is exposed and molded with the insulating resin 14 as shown in FIGS. 2 (a) and 2 (b). Or cover 1 as shown in FIG.
Exposing and covering a part of the proton attack layer 19 in step 8 facilitates control of the electrode area, stabilizes diffusion of the sample to be measured to the electrode, and improves measurement accuracy.

By the way, the working electrode 11 of the first embodiment
Is a method in which a pre-proton attack substance such as a quinone compound or an azo compound is immobilized in an exposed state and a proton attack layer 1
The formation of No. 9 stabilizes the preproton attack substance, which is liable to cause photolysis, and enables highly accurate and stable measurement. In other words, even if the preproton attack substance is given the energy of decomposition by light, photoactivation hardly occurs because the activation potential required for decomposition becomes higher than light energy due to immobilization, and stable measurement can be performed. Because it becomes possible.

The operation of the acidity measuring apparatus using the acidity measuring electrode according to the first embodiment described above will be described below.

First, an acidity measuring method of the acidity measuring apparatus of the present invention will be described. FIG. 4 is a current-potential relationship diagram for measuring acidity by voltammetry of a measurement electrolyte solution in which a conventional naphthoquinone derivative coexists. In the figure, the horizontal axis represents the potential of the working electrode 11 with respect to the reference electrode 12, and the vertical axis represents the value of the reduction current flowing through the counter electrode 13. The portion indicated by A is a peak due to the concentration of the acid, and the portion indicated by B is a peak due to the concentration of quinone. However, the reduction current value varies depending on conditions such as the size of the surface area of the working electrode 11, the concentration of the acid, and the sweep speed of the potential. The potential at which the peak on the horizontal axis appears varies slightly depending on the acid concentration, but the effect of this condition is negligible. FIG. 5 is a graph showing the relationship between the reduction current value and the acidity. In the figure, the horizontal axis represents the reduction current value of the counter electrode 13, and the vertical axis represents the acidity of the sample to be measured. FIG. 5 shows that the acidity of the sample to be measured can be calculated by measuring the reduction current value because the reduction current value and the acidity of the sample to be measured maintain a constant relationship.

Next, a control circuit of the acidity measuring device according to the present embodiment will be described.

FIG. 6 is a control circuit diagram of the acidity measuring device according to the first embodiment of the present invention. It should be noted that FIGS.
The same components as those described in (c) are denoted by the same reference numerals and description thereof is omitted. In the figure, 4 'is a start / stop switch which is turned on / off by a start / stop button 4 for starting measurement, 5' is a power switch which turns on / off the power of the acidity measuring device 1 by a power button 5, and 23 is a microcomputer A control unit of an internal circuit for voltammetry, which is composed of an oscillator 24 for transmitting a generated signal to the control unit 23; a frequency dividing circuit 25 for internally generating a clock based on a signal generated by the oscillator 24 , 26 are timer means for counting the clock generated by the frequency dividing circuit 25 and counting the time, 27 is a D / A converter for converting a digital signal sent from the control unit 23 into an analog signal, and 28 is sent from a D / A converter 27. The analog signal
An operational amplifier for outputting to the C integrating circuit, 29 is an operational amplifier 2
Counter electrode 1 using the imaginary short output from 8
Monitoring circuit for controlling the voltage applied to 3, 30
Is a resistor, 31 is a differential amplifier that converts a current flowing through the counter electrode 13 into a voltage, 32 is an A / D converter that converts an analog signal into a digital signal and re-inputs it to the control unit 23,
Reference numeral 3 denotes a calculation unit configured by a microcomputer to calculate the acidity of the sample to be measured.

First, when the power button 5 is pressed, the LCD 3 becomes operable. Next, when the measurement is started by pressing the start / stop button 4, the control unit 23 generates a clock inside the control unit 23 by the frequency dividing circuit 25 based on the signal generated by the oscillator 24, counts the clock, and sets the timer 2.
6 starts timing in units of one second. The control unit 23 sends a digital signal of a predetermined voltage to the D / A converter 27 in synchronization with the counting of the timer 26. The digital signal is converted into an analog signal by a D / A converter 27 and output to an operational amplifier 28.

FIG. 7 is an output diagram from an operational amplifier of the acidity measuring device according to the first embodiment of the present invention, and FIG. 8 is an output diagram from an integrating circuit of the acidity measuring device according to the first embodiment of the present invention. In the figure, the horizontal axis represents the time measured by the timer 26, and the vertical axis represents the voltage of the digital signal sent from the control unit 23. From FIG. 7, the time is 1, 2, 3,.
The voltage is 5, 10, 15,.
It turns out that it increases by mV and 5mV. The signal output from the operational amplifier 28 is integrated by passing through an RC integration circuit, converted into an analog signal shown in FIG. 6, and then input to a monitoring circuit 29.

In the monitoring circuit 29, the operational amplifier 2
8 using the imaginary short circuit, and the counter electrode 13 on the output side.
Is controlled in accordance with the analog signal so that the voltage R of the reference electrode 12 on the minus input side has the same value as the analog signal. As a result, the potential difference between the reference electrode 12 and the working electrode 11 becomes a predetermined value +800 to -1000 m.
V range. On the other hand, the current flowing through the counter electrode 13 is converted into a voltage by the voltage across the resistor 30 passing through the differential amplifier 31 and is converted from an analog signal to a digital signal via the A / D converter 32, and further controlled. Input to the unit 23.

The control unit 23 detects a current value that gives a pre-peak shown in FIG. 4A by comparing a voltage swept at a predetermined sweep speed described later with the input current. The calculation unit 33 calculates the acidity of the sample to be measured based on the pre-peak value of the current, and displays the value on the LCD 3. In FIG. 4, the potential is a potential difference between the working electrode 11 and the reference electrode 12, and the reduction current is a current flowing through the counter electrode 13. FIG. 9 is a graph showing the relationship between the acidity and the reduction current value. In the figure, the horizontal axis represents the acidity of the sample to be measured, and the vertical axis represents the value of the reduction current flowing through the counter electrode 13. The current value giving the pre-peak value indicated by A in FIG. 4 and the acidity of the sample to be measured are in a proportional relationship as shown in FIG. Here, assuming that the acidity of the sample to be measured is θ, the reduction current value at the pre-peak time is I, and the proportionality constant is K1, the relationship between the acidity and the current value is expressed as (Equation 2).

[0065]

(Equation 2)

(Equation 2) shows that the acidity θ can be measured by measuring the reduction current value I at the pre-peak time.

The preproton attack substance (a quinone compound in this embodiment) fixed to the base 15 to form the proton attack layer 19 has a pH value when a sample containing an acid is dropped. A part is hydrolyzed according to. For example, when quinone is used as the preproton attack substance, hydroquinone and quinone that are hydrolyzed are mixed at a certain ratio according to the pH. Then, in order to return the hydrolyzed preproton attack substance to the preproton attack substance, the control unit 23 controls the working electrode 11 and the reference electrode 1.
After applying an oxidation potential between the two, the preproton attack substance is preferably radicalized and swept at a potential on the minus side of the natural potential to perform voltammetry.
As a result, the amount of the quinone compound or azo compound which is a preproton attack substance is increased, so that the reactivity of the proton attack substance can be increased and the sensitivity can be improved.

Next, the operation of the acidity measuring device according to the present embodiment during operation will be described. After the sample to be measured is dropped onto the acidity measurement electrode 7 so that the working electrode 11, the reference electrode 12, and the counter electrode 13 are sufficiently immersed, the acidity measurement electrode 7 is fixed to the support portion 10. When the measurement is started by pressing the power button 6 and the start / stop button 4, the control unit 23 sets the potential of the working electrode 11 to +800 to the potential of the reference electrode 12.
A voltage is applied between the working electrode 11 and the counter electrode 13 so as to sweep at a sweep speed of 10 to 200 mV within a range of -1000 mV. Thus, the sweep speed is 10 to 200
When a predetermined potential difference is swept at mV, a stable voltammogram as shown in FIG. 4 can be obtained. By performing such a sweep, the peak of the acid reduction current is 0 m
Appears near V. This potential, called pre-peak,
When the concentration of the acid increases, it shifts to the minus side. Even if there is a shift, the potential of the working electrode 11 is increased by +800 to -10.
If it is set in the range of 00 mV, almost any concentration of acid can be measured.

As described above, since the acidity measuring device and the acidity measuring electrode according to the present embodiment are constituted, they have the following operations.

(1) When the acidity is measured by voltammetry, since the proton attack layer of the working electrode, for example, mercaptoquinone is polymerized and fixed, a proton attack substance and a solvent are separately mixed into the sample to be measured. It is not necessary, and the acidity of the acid contained in the liquid to be measured can be directly measured.

(2) In order to measure the current value while sweeping the potential between the working electrode and the reference electrode within a predetermined range,
A stable voltammogram can be obtained, and measurement by voltammetry with high accuracy can be performed.

(3) Further, the acidity measuring electrode molded with a resin can easily adjust the exposed area of the proton attack layer of the working electrode, and the acidity obtained by forming each electrode in a thin film on the substrate can be obtained. The measuring electrode can have a very compact shape.

(Embodiment 2) The acidity measuring apparatus according to Embodiment 2 is obtained by using chronoamperometry as the voltage applying method and the current value detecting method of the acidity measuring apparatus described in Embodiment 1.

FIG. 10 is a control circuit diagram of the acidity measuring device according to the second embodiment of the present invention. In addition, the same thing as what was demonstrated in FIGS. 1-9 is attached | subjected the same code | symbol, and description is abbreviate | omitted.

The operation of the acidity measuring apparatus of the present embodiment configured as described above will be described below.

First, an acidity measuring method of the acidity measuring apparatus according to the present embodiment will be described. When a voltage is applied between the working electrode 11 and the counter electrode 13 so that the potential shown in FIG. 4A is applied as a fixed potential between the working electrode 11 and the reference electrode 12, A reduction current flows in between. FIG. 11 is a graph showing the relationship between the reduction current value and time. In the figure, the horizontal axis represents time after voltage application, and the vertical axis represents reduction current value. Since the relationship shown in FIG. 11 changes depending on the concentration of the acid, there is a certain relationship between the data for a certain period of time after applying the voltage and the sample to be measured. As described above, a measurement method using a voltage application method and a current value detection method for observing a change in the amount of current with respect to time when a certain voltage is continuously applied from a state where no voltage is applied is called chronoamperometry. FIG. 12 is a graph showing the relationship between the logarithm of the reduction current value and the acidity. In the figure, the horizontal axis represents the logarithm of the reduction current value, and the vertical axis represents the acidity of the sample to be measured. From FIG. 12, it can be seen that since the logarithm of the reduction current value and the acidity of the sample to be measured maintain a constant relationship, the acidity of the sample to be measured can be calculated by measuring the reduction current value by chronoamperometry.

Next, an acidity measuring method of the acidity measuring apparatus according to the present embodiment will be described. First, power button 5
By pressing, the LCD 3 becomes operable. Next, when the measurement is started by pressing the start / stop button 4, the control unit 23 generates a clock inside the control unit 23 by the frequency dividing circuit 25 based on the signal generated by the oscillator 24, counts the clock, and sets the timer 26. Starts timing in units of one second.
The control unit 23 sends a digital signal of a predetermined voltage to the D / A converter 27 in synchronization with the counting of the timer 26. The digital signal is converted into an analog signal by a D / A converter 27 and output to an operational amplifier 28.

FIG. 13 is an output diagram from the operational amplifier of the acidity measuring apparatus according to the present embodiment. In the figure, the horizontal axis represents the time measured by the timer 26, and the vertical axis represents the voltage of the digital signal sent from the control unit 23. FIG. 13 shows that the output from the operational amplifier 28 is such that a positive voltage is applied for a certain time and then a negative voltage is applied. At this time, the positive voltage oxidizes the reactive group of the compound immobilized on the surface of the working electrode 11, so that the working electrode 1
By keeping the surface of No. 1 constant, it is possible to suppress the dispersion of the measured values. The signal output from the operational amplifier 28 is input to the monitoring circuit 29.

In the monitoring circuit 29, the operational amplifier 2
8 using the imaginary short circuit, and the counter electrode 13 on the output side.
Is controlled in accordance with the analog signal so that the voltage R of the reference electrode 12 on the minus input side has the same value as the analog signal. As a result, the potential difference between the reference electrode 12 and the working electrode 11 becomes a predetermined value +800 to -1000 m.
V range. On the other hand, the current flowing through the counter electrode 13 is converted into a voltage by passing the voltage between both ends of the resistor 30 through the differential amplifier 31, and is converted from an analog signal to a digital signal via the A / D converter 32. Input to the unit 23. The control unit 23 detects a current value at each point by comparing a predetermined voltage with the input current. Based on the current value in the fixed time, the calculation unit 33
To calculate the acidity of the sample to be measured, and display the value on the LCD 3.

Here, assuming that the acidity of the sample to be measured is θ, the reduction current value is I, and the proportionality constant is K 2, the relationship between the acidity and the current value is expressed as (Equation 3).

[0081]

(Equation 3)

(Equation 3) shows that the acidity θ can be measured by taking the logarithm of the measured value of the reduction current value I.

The pre-proton attack substance fixed on the base 15 to form the proton attack layer 19 is partially hydrolyzed in accordance with the pH when a sample containing an acid is dropped. . Therefore, in order to return the hydrolyzed preproton attack substance to the preproton attack substance, the controller 23 controls the working electrode 11 and the reference electrode 12.
It is preferable to reduce the preproton attack substance by radicalizing while maintaining the reduction potential on the negative side of the spontaneous potential after applying an oxidation potential between the two.

Next, the operation of the acidity measuring device according to the present embodiment during operation will be described. After the sample to be measured is dropped onto the acidity measurement electrode 7 so that the working electrode 11, the reference electrode 12, and the counter electrode 13 are sufficiently immersed, the acidity measurement electrode 7 is fixed to the support portion 10. When the measurement is started by pressing the power button 5 and the start / stop button 4, the control unit 23 sets the potential between the working electrode 11 and the reference electrode 12 to +300 to +
After applying a voltage between the working electrode 11 and the counter electrode 13 for 5 to 30 seconds so that the oxidation potential becomes 1000 mV, the potential between the working electrode 11 and the reference electrode 12 becomes 0 to −300.
The working electrode 11 and the counter electrode 13 are set to have a reduction potential of mV.
When a current flowing between the working electrode 11 and the counter electrode 13 when a voltage is applied between the electrodes is measured, a response diagram as shown in FIG. 11 is obtained. At this time, since a current flows from the moment when the reduction potential is applied, it is considered that variations in the time lag at the time of measurement may affect the measurement value. Therefore, the measurement of the reduction current value is performed for a predetermined time by applying the reduction potential. It is preferable to measure after the elapse of. To give a specific example,
When wine is used as the sample to be measured using the working electrode 11 having an area of about 3 mm 2, the time is preferably 1 to 10 seconds.

As described above, since the acidity measuring apparatus according to the present embodiment is constituted, the function described in the first embodiment is obtained, and the potential between the working electrode and the reference electrode is set within a predetermined range. Since the current value is measured while holding the current value, by measuring the current value after a predetermined time has elapsed, stable and accurate measurement can be performed.

[0086]

As described above, according to the electrode for measuring acidity of the present invention, the following advantageous effects can be obtained.

According to the first aspect of the present invention, the sample to be measured is simply dropped onto the acidity measuring electrode without preparing a measuring solvent mixed with a proton attack substance and dipping the acidity measuring electrode. Hydrogen dissociated from acids and undissociated hydrogen can be measured directly.

According to the second aspect of the present invention, each electrode can be planarized and thin, and the electrode for measuring the acidity can be made compact.

According to the third aspect of the present invention, the entirety of the electrode for measuring the acidity can be formed compactly by forming a substrate.

According to the fourth aspect of the present invention, the quinone compound and the azo compound are chemically stable substances, so that the electrodes can be stored for a long time. Furthermore, even if the quinone compound or the azo compound is given the energy of decomposition by light, the activation energy required for decomposition becomes higher than the light energy due to the immobilization, so that the light stability is improved and photodecomposition occurs. It is possible to perform stable measurement without any problem.

According to the fifth aspect of the present invention, since the diffusion of the acid into the electrode is stable, the acidity can be measured stably, and the area of the electrode exposed on the surface can be easily adjusted. Can be.

According to the sixth aspect of the present invention, since the electrode formed by directly exposing the proton attack layer on the surface is used, the sample to be measured is directly diluted without diluting with a solvent. Can perform metrological measurements,
The measurement time can be shortened, and the acidity can be easily measured simply by measuring the reduction current of the acid contained in the sample to be measured.

According to the seventh aspect of the present invention, since an electrode formed by directly exposing the proton attack layer on the surface is used, direct voltammetry measurement can be performed without diluting the sample to be measured with a solvent. can do. Further, even if the characteristics of the apparatus vary, highly accurate measurement can be performed, and the acidity can be easily measured only by measuring the reduction current of the acid contained in the sample to be measured.

According to the invention described in claim 8, the reaction between the acid contained in the sample to be measured and the proton attack substance on the electrode surface is stabilized, so that the measurement can be performed with higher accuracy.

According to the ninth aspect, the organic acid contained in the juice can be measured preferably.

According to the tenth aspect, an organic acid contained in an alcoholic beverage can be suitably measured.

According to the eleventh aspect, free fatty acids contained in edible oil can be suitably measured.

According to the twelfth aspect of the present invention, fatty acids in serum can be suitably measured.

According to the thirteenth aspect of the present invention, since the working electrode formed by exposing the proton attack layer is used, the sample to be measured is directly diluted with the solvent without diluting it with the solvent. Can take measurements,
Measurement time can be shortened.

According to the fourteenth aspect of the present invention, since the working electrode formed by exposing the proton attack layer is used, the voltammetric measurement is performed directly without diluting the sample to be measured with a solvent. be able to.

According to the fifteenth aspect of the present invention, since the preproton attack substance is stable, it is possible to perform highly accurate measurement with little change even after long-term storage.

According to the sixteenth aspect of the present invention, the acidity can be measured by returning the preproton attack substance hydrolyzed by dropping the sample to be measured by the oxidation potential and then applying the reduction potential to the acidity. The sensitivity is improved, and the accuracy of the chronoamperometry measurement is improved.

According to the seventeenth aspect of the present invention, the pre-proton attack substance hydrolyzed by dropping the sample to be measured is returned to its original state by the oxidation potential, and then the reduction potential is applied to measure the acidity. The sensitivity is increased, and the accuracy of voltammetry measurement is improved.

[Brief description of the drawings]

FIG. 1 is a perspective view of an acidity measuring device according to a first embodiment of the present invention.

2A is a perspective view of an acidity measurement electrode according to Embodiment 1 of the present invention. FIGS. 2B and 2C are perspective views of an acidity measurement electrode which is another embodiment of FIG. 2A.

3A is a model diagram showing a method of physically adsorbing a proton attack layer. FIG. 3B is a model diagram showing a chemical surface bonding of a proton attack layer. FIG. 3C is a model diagram showing a method of physically fixing a proton attack layer. (D) Model diagram showing the method for chemically fixing the proton attack layer on the surface

FIG. 4 is a diagram showing a current-potential relationship for measuring acidity by voltammetry of a measurement electrolyte solution in which a conventional naphthoquinone derivative coexists.

FIG. 5 is a graph showing a relationship between a reduction current value and an acidity.

FIG. 6 is a control circuit diagram of the acidity measuring device according to the first embodiment of the present invention.

FIG. 7 is an output diagram from an operational amplifier of the acidity measuring device according to the first embodiment of the present invention.

FIG. 8 is an output diagram from an integration circuit of the acidity measurement device according to the first embodiment of the present invention.

FIG. 9 is a graph showing the relationship between acidity and reduction current value.

FIG. 10 is a control circuit diagram of the acidity measuring device according to the second embodiment of the present invention.

FIG. 11 is a graph showing a relationship between a reduction current value and time.

FIG. 12 is a graph showing the relationship between the logarithm of the reduction current value and the acidity.

FIG. 13 is an output diagram from an operational amplifier of the acidity measuring device according to the second embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 1 acidity measuring device 2 main body 3 LCD 4 start / stop button 4 'start / stop switch 5 power button 5' power switch 6 button 7 acidity measurement electrode 8 main body side connector 9 electrode side connector 10 support section 11 working electrode DESCRIPTION OF SYMBOLS 12 Reference electrode 13 Counter electrode 14 Resin 15 Base 16 Cable 17 Guard 18 Cover 19 Proton attack layer 20 Preproton attack substance 21 Adhesive 22 Polymer film 23 Control part 24 Oscillator 25 Divider circuit 26 Timer means 27 D / A converter 28 operational amplifier 29 monitoring circuit 30 resistor 31 differential amplifier 32 A / D converter 33 operation unit

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Takeshi Kusakabe 1006 Kadoma Kadoma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. 72) Inventor Kazuyoshi Mori 1006 Kazuma Kadoma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (17)

[Claims] 1. An acidity measurement electrode provided with a working electrode, a counter electrode, and a reference electrode formed on a base, wherein the working electrode and the reference electrode are made of an inert conductive material that does not cause an oxidation-reduction reaction. And the counter electrode is made of a conductive material, and a proton attack layer on which a preproton attack substance is immobilized is exposed and formed on the surface of the working electrode. electrode. 2. The apparatus according to claim 1, wherein said working electrode, said counter electrode, and said reference electrode are each formed in a thin film shape.
2. The electrode for measuring acidity according to item 1. 3. The electrode according to claim 1, wherein the base is a substrate. 4. The method according to claim 1, wherein said preproton attack substance is a quinone compound or an azo compound.
4. The electrode for measuring acidity according to any one of claims 1 to 3. 5. The acidity measuring electrode according to claim 1, wherein a part of said proton attack layer is exposed to the surface and the periphery thereof is molded with an insulating material. 6. A voltage is applied between the working electrode and the counter electrode when an acid-containing sample to be measured is dropped on the acidity measurement electrode according to any one of claims 1 to 5, and the acidity measurement electrode. A power supply, a control unit that controls a potential between the working electrode and the reference electrode to a predetermined potential, a detection unit that detects a current flowing between the working electrode and the counter electrode, and a current detected by the detection unit. An arithmetic unit for converting a value into an acidity. 7. A voltage is applied between the working electrode and the counter electrode when an acid-containing sample to be measured is dropped onto the acidity measuring electrode according to claim 1 and the acidity measuring electrode. A power supply, a control unit that sweeps a potential between the working electrode and the reference electrode, a detection unit that detects a current flowing between the working electrode and the counter electrode, and a current value detected by the detection unit to an acidity. An acidity measuring device, comprising: a calculating unit for converting. 8. A sweep speed of 10 to 100 by said control unit.
The acidity measuring device according to claim 7, wherein the acidity is mV / s. 9. The acidity measuring apparatus according to claim 6, wherein the sample to be measured is juice. 10. The acidity measuring apparatus according to claim 6, wherein the sample to be measured is an alcoholic beverage. 11. The acidity measuring apparatus according to claim 6, wherein the sample to be measured is food oil or engine oil. 12. The acidity measuring apparatus according to claim 6, wherein the sample to be measured is serum. 13. An acid-containing sample to be measured is dropped onto a working electrode formed by exposing a proton attack layer having a preproton attack substance immobilized on its surface, a counter electrode, and a reference electrode. A voltage is applied between the counter electrodes, and when a potential between the working electrode and the reference electrode is maintained at a predetermined potential, an acidity is calculated by measuring a current value flowing between the working electrode and the counter electrode. Acidity measurement method. 14. An acid-containing sample to be measured is dropped on a working electrode formed by exposing a proton attack layer having a preproton attack substance immobilized on its surface, and a counter electrode and a reference electrode. Applying a voltage between the counter electrode and sweeping a potential between the working electrode and the reference electrode, measuring an electric current value flowing between the working electrode and the counter electrode to calculate an acidity, Method. 15. The method according to claim 13, wherein the preproton attack substance is a quinone compound or an azo compound. 16. The preproton attack substance, which was initially dropped and drastically hydrolyzed by dropping the sample to be measured, is returned to the preproton attack substance with the potential between the working electrode and the reference electrode as an oxidation potential. 16. The method according to claim 13, wherein the potential between the electrode and the reference electrode is maintained at a predetermined reduction potential. 17. The method according to claim 17, wherein said pre-proton attack substance, which is partially hydrolyzed by dropping said sample to be measured, is returned to said pre-proton attack substance by setting a potential between said working electrode and said reference electrode to an oxidation potential. 16. The method according to claim 14, wherein a potential between the electrode and the reference electrode is swept with a reduction potential.