JPH1183788A - Acidity measuring apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable automatic execution from the washing of a working electrode to the measurement of acidity by a method wherein the potential of the working electrode for a comparison electrode is swept to perform an electrolytic washing of the working electrode and then, a control section is provided to measure acidity. SOLUTION: A working voltage control circuit 24 for applying a voltage E1 to a working electrode W and a D/A converter 22 for outputting a voltage E2 to a counter pole voltage control circuit 21 are connected to a control section 20 made up of a microcomputer or the like. A comparison electrode R is connected to an input terminal of the counter pole voltage control circuit 21 and a counter pole C to an output terminal thereof. The control section 20 cause a change in the voltage applied to the working electrode W so that the potential of the working electrode W for the comparison electrode R is swept in a specified range to perform an electrolytic washing of the working electrode W. Then, the acidity is measured. This enables automation from the washing of the working electrode W to the measurement of acidity to eliminate the need for the work of grinding and replacing the working electrode W, thereby getting rid of scattering in measured values associated with changes in the surface condition of the working electrode W.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an acidity for measuring the acidity of free fatty acids contained in edible oils, malic acid or tartaric acid contained in fruit drinks, acids contained in alcoholic drinks, or coffee acid etc. in coffee. It relates to a measuring device.

[0002]

2. Description of the Related Art In recent years, foods have been required to have a quality above a certain level from the viewpoint of health and safety. Above all, acids contained in foods have a great effect on the quality of foods. Alkaline foods are often considered better due to the health boom, and foods with a low acidity tend to be exclusively preferred these days.

[0003] As described above, the acidity of various foods greatly affects the consumption of foods, and the degree of the influence and the method of measurement differ depending on the foods. Therefore,
As typical examples of such foods, we explain what kinds of acids and how they were measured conventionally in fruit drinks such as edible oils and juices, alcoholic drinks such as whiskey, liquor and wine, and coffee. I do.

First, the acid contained in the edible oil will be described. Japan's dietary habits are changing rapidly, but it appears that there is a large trend of instantization first and a diversification trend represented by handmade tastes in the first place. . Especially, the instant orientation reflects the times, and many processed foods are on the rise. Above all, the increase in fried foods is remarkable. This is because fried foods are preferred in taste and are relatively resistant to spoilage.

However, when this fried food is exposed to an environment affected by temperature and light for a long time, fats and oils are automatically oxidized by oxygen in the air, causing a perishable smell and other deterioration in quality. . For these reasons, there has been general interest in the deterioration and deterioration of edible fats and oils and processed fats and oils, such as the establishment of a regional food certification system for fried foods, or the regulation of oil confectionery, and Legal regulations are also being considered for deterioration of fats and oils in the guidelines for lunches and prepared dishes.

There are several analytical methods for measuring the degree of damage of fats and oils, particularly the degree of deterioration of heated fats and oils, by measuring an acid value, a peroxide value, a viscosity and an iodine value. Here, as described above, considering that temperature, humidity, and light have a large effect on food deterioration, and that the acidity changes greatly in the early stages of deterioration, the acid value of the acidity that directly measures the acidity is considered. Measurements are appropriate for determining thermal degradation, and are commonly used.

Next, the acid of drinking water will be described. Fruit drinks, such as juice, are juices obtained by extracting raw fruit through a juicer.Many fruit drinks use concentrated juice or frozen juice as a raw material rather than using fresh fruit juice as it is. There are many. For example, in the case of orange juice, after removing diseased fruits and immature fruits of mandarin oranges, the epidermis is washed, squeezed to extract pulp and juice, and furthermore, the pericarp, germ membrane and the like are removed from the juice. At this point, the sugar content and the acidity are adjusted to conform to the Japanese Agriculture and Forestry Standards, and the acidity is measured at that time. When orange juice is made from concentrated juice or frozen juice, the acidity is measured when water is added to the concentrated juice or frozen juice to make orange juice.

Next, an alcoholic beverage will be described. Distilled liquor that increases the yield of ethanol by repeating distillation, such as whiskey and shochu, over and over, or brewed liquor obtained by fermenting and filtering the raw material, typified by liquor and wine, and other fruit liquors There are various types of alcoholic beverages such as sparkling liquor and beer and the like, and their production processes are also different. However, in the production of any alcoholic beverage, acidity is measured in the process to ensure product quality.

Finally, the acid of coffee will be described.
As described below, there are many types of substances that impart sourness that affects the taste of coffee, and the acid content is important as an index for evaluating the sourness of coffee. Representative of acids contained in coffee include chlorogenic acids. Its content also varies during the roasting of coffee beans. There are many other compounds that contribute to the sourness of coffee, such as caffeic acid, quinic acid, and even citric acid. And, while the content of each acid is very small, it is considered that the delicate balance and the total amount are decisive factors for the acidity.

As described above, in various kinds of foods, the acidity of each food is measured in the production process, and there are various measuring methods.

As an example of the conventional acid measurement method, a method defined by a standard method for analyzing fats and oils, Japanese Agricultural Standards, JIS, a method for testing oils and fats in the Japanese Pharmacopoeia, a method for testing food and drink, a method for testing food and water, and the like are used. However, the measurement is based on a neutralization titration method using phenolphthalein as an indicator. Therefore,
In order to explain the neutralization titration method, the neutralization titration method specified in the water supply test method and the standard fat and oil analysis method will be described below.

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 phenolphthalein indicator
mL, and then add a 0.02 mol / L sodium hydroxide solution. Then, the bottle is sealed and shaken lightly, and when the red color has disappeared, the time when titration is continued until the slight red color remains without disappearing is determined as the end point of neutralization, and the mL number a of sodium hydroxide is obtained. The acidity at this time is given by acidity (mg / L in terms of calcium carbonate) = a × 10.

Next, the neutralization titration method specified in the standard fat and oil analysis method will be described. The definition of acidity in the reference fat analysis method is
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 acidity (for example, if the acidity is 1 or less, collect 20 g,
If the acidity is more than 1 and 4 or less, collect 10 g, and if the acidity is more than 4 and 15 or less, collect 2.5 g), and weigh it correctly 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.

Incidentally, regarding the measurement of fatty acids, there is a method of measuring the acidity by voltammetry instead of such a neutralization titration method.

This 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 of the naphthoquinone derivative is proportional to the concentration of free fatty acids 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 superimposed value 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 of the naphthoquinone derivative.

FIG. 11 shows data measured by this method. Here, FIG. 11 is a graph showing a current-potential relationship in the measurement of acidity by voltammetry of a measurement electrolyte solution in which a conventional naphthoquinone derivative coexists.

In FIG. 11, the horizontal axis represents the potential of the working electrode with respect to the reference electrode when silver-silver chloride is used as the reference electrode and glassy carbon of φ3 is used as the working electrode (hereinafter referred to as “electrode potential”). The axes indicate the values of the current flowing through the counter electrode at this time. However, the current value changes depending on conditions such as the surface area of the working electrode, roughness, and acid concentration. On the other hand, the voltage value on the horizontal axis is negligible, though there is some variation depending on the acid concentration.

In FIG. 11, A is a pre-peak showing a pre-reduction wave proportional to the acid concentration, and D is a main peak of the naphthoquinone derivative.

When the acidity of a fatty acid is measured using the technique disclosed in Japanese Patent Application Laid-Open No. 5-264503, voltammetric control using a potentiostat or the like is generally required.

Here, the operation principle of the potentiostat will be described with reference to FIG. FIG. 12 is a schematic circuit diagram of a potentiostat.

In FIG. 12, a sweep voltage setting power supply 31 is connected to a plus terminal which is a non-inverting input terminal of the operational amplifier 28.
However, a comparison electrode R is connected to a minus terminal which is an inverting input terminal. The counter electrode C is connected to the output terminal. A resistor 29 is provided between the output terminal of the operational amplifier 28 and the counter electrode C, and a voltage amplification circuit 30 is connected to both ends of the resistor 29. The working electrode W is set to the ground potential.

When the electrode potential is swept by such a potentiostat and the current I ₁ flowing through the counter electrode C at that time is to be measured, first, the sweep voltage setting power supply 31 is set to a predetermined voltage V ₁ . Then, a voltage is applied to the counter electrode C by the feedback circuit including the operational amplifier 28 so that the voltage V ₁ = the comparison electrode voltage V ₂ . Then, the current I ₁ flowing through the counter electrode is converted into a voltage V ₃ by the resistor 29, further amplified by the voltage amplifying circuit 30 and output as the voltage V ₄ . The voltage V ₄ and recorded on a plotter or the like, is intended to complete the relative graph of electrode potential and the counter electrode current I _1.

In this case, it is necessary to grasp the relationship between the counter electrode current I ₁ and the output voltage V _4.
Therefore, a power supply circuit for generating a dual power supply as a power supply for the entire system is also required.

In the conventional measurement, the working electrode W
When using for a long time, even if a solution with the same acidity is measured,
Measurement results could be different. This is because the working electrode W deteriorates and the current waveform changes. here,
Deterioration of the working electrode W means that deposits are formed on the surface of the electrode because the electrode is immersed in a solvent for a long time or an electrode voltage is applied. As means for regenerating the deterioration of the working electrode W, polishing with sandpaper or replacement of the electrode itself is performed.

[0026]

As described above,
The conventional acidity measuring device measures the acidity using a neutralization titration method, and determines the color change due to the phenolphthalein indicator and uses this as the end point of the titration. In some cases, the acidity was not determined objectively.

According to the neutralization titration method for fatty acids in the standard fats and oils 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., and it is easy to catch fire. Is difficult. Moreover, for example, when the color of the sample is dark, such as oil fried in large quantities, or when the material itself is colored, such as juice or wine, the color of phenolphthalein near the end point of titration I can't grasp the change accurately,
There was a problem in that the reading of the end point was incorrect and the measured values varied. Furthermore, the amount of the sample is several tens g, and the neutral solvent is 100 mL.
There is also a problem that a large number of samples are required for one measurement and the increase in the number of measurements becomes a burden.

In the technique of measuring using a potentiostat disclosed in Japanese Patent Application Laid-Open No. 5-264503, the handling of the potentiostat is not easy and specialized knowledge is required. take time. Further, also in the circuit configuration, the voltage of the W terminal is 0
Since it is fixed at V, it is necessary to use both positive and negative power supplies as the power supply of the whole system so that the voltage of the C terminal can be applied to plus and minus, and it has an analog voltage output circuit etc. which has a wide range output function. There was a problem that had to be.

Further, the polishing operation performed for the surface deterioration of the working electrode is a difficult operation that requires specialized knowledge and skill. This is because the roughness of the electrode surface is one of the factors that greatly influence the measurement result, and therefore, it must be adjusted to the same level every time the surface roughness is measured. Here, a current waveform obtained by voltammetry using the working electrode in the initial stage of polishing is shown as a waveform 1 in FIG. FIG. 13 is a graph showing a voltammetric current waveform when the working electrode is polished. In FIG. 13, the waveform after repeating the measurement with the same electrode is the waveform 2, but the pre-peak can hardly be distinguished from this waveform. The waveform 3 in FIG. 13 was measured after re-polishing, and it is apparent that the waveform 3 is different from the waveform 1 after the previous polishing.

There is a similar problem with the replacement of the working electrode. Since errors in the manufacturing stage extend not only to the surface roughness but also to the surface area, there is a possibility of further erroneous measurement by measuring with a different electrode. Was.

Accordingly, an object of the present invention is to provide an acidity measuring apparatus capable of automatically performing from the cleaning of the working electrode to the measurement of the acidity.

[0032]

In order to solve this problem, an acidity measuring apparatus according to the present invention comprises a working electrode and a counter electrode immersed in a coexisting electrolyte in which an electrolyte and an acid-containing solution to be measured are mixed. And, by changing the voltage applied to the working electrode and the reference electrode portion, the potential of the working electrode with respect to the reference electrode of the comparison electrode portion is swept within a predetermined range to perform electrolytic cleaning of the working electrode. And a control unit for performing the control.

According to this, the potential sweep width between the working electrode and the reference electrode is amplified, and the working electrode can be swept with a potential sweep width suitable for electrolytic cleaning and acidity measurement. From washing to acidity measurement can be performed automatically. Therefore, the work of polishing and replacing the working electrode becomes unnecessary, and a long-term, high-accuracy acidity measurement can be performed.

[0034]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention according to claim 1 of the present invention is characterized in that a working electrode, a counter electrode and a reference electrode section immersed in a coexisting electrolyte in which an electrolyte and an acid-containing measurement liquid are mixed, A control unit that changes the voltage applied to the working electrode, sweeps the potential of the working electrode with respect to the comparison electrode of the comparison electrode unit within a predetermined range, performs electrolytic cleaning of the working electrode, and then measures the acidity. The acidity measuring device has an effect that it is possible to automatically perform from the cleaning of the working electrode to the measurement of the acidity.

The invention according to claim 2 of the present invention provides
In the invention according to claim 1, the control unit is an acidity measuring device for performing the electrolytic cleaning of the working electrode by sweeping the potential of the working electrode with respect to the reference electrode between -1 V and +2 V. Since sweeping can be performed at a potential at which electrolytic polymerization is not performed, deterioration of the working electrode is suppressed, so that high-accuracy acidity measurement can be easily performed.

According to a third aspect of the present invention, there is provided the acidity measuring device according to the first or second aspect, wherein the control unit performs an electrolytic cleaning operation of the working electrode every time the control unit performs the acidity measuring operation. Since the deposits on the surface of the working electrode can be removed immediately before the measurement of the acidity, it has the effect of obtaining a stable reproducible acidity measurement result.

According to a fourth aspect of the present invention, in the first or the second aspect of the present invention, the control section operates at a rate of once in a plurality of acidity measuring operations or at a rate of once in a predetermined period. An acidity measuring device for performing an electrolytic cleaning operation of an electrode. Since the surface of a working electrode is constantly activated, it has an effect that a stable reproducible acidity measurement result can be obtained.

Hereinafter, an embodiment of the present invention will be described with reference to FIG.
This will be described with reference to FIG. In these drawings, the same members are denoted by the same reference numerals, and duplicate description is omitted.

FIG. 1 is an external perspective view showing an acidity measuring apparatus according to an embodiment of the present invention, FIG. 2 is an external perspective view showing the acidity measuring apparatus shown in FIG. 1 with an upper lid opened, and FIG. FIG. 4 is a cross-sectional view showing a measuring container in the acidity measuring device, FIG. 4 is a graph showing a current-potential relationship of acidity measurement by voltammetry of a co-existing electrolyte mixed with a benzoquinone derivative, and FIG. FIG. 6 is a graph showing an example of an electrode potential sweep waveform at the time of electrolytic cleaning of the working electrode, FIG. 7 is a graph showing an example of an electrode potential sweep waveform at the time of measuring the acidity of the working electrode, and FIG. FIG. 9 is a block diagram showing a control system of the acidity measuring apparatus of FIG. 1, FIG. 9 is a graph showing voltage waveforms of the working electrode and the reference electrode during electrolytic cleaning of the working electrode, and FIG. Is a graph showing the voltage waveform.

In FIG. 1, the main unit 7 of the acidity measuring device A
Is provided with an upper lid 1 that covers an internal space in which the measurement container 8 (FIG. 2) is set. An opening button 2 for releasing the lock of the upper lid 1 is provided adjacent to the upper lid 1. The main body 7 is provided with an LCD 3 as display means for displaying the measured acidity, and further includes a switching button 4 for switching an area according to the magnitude of the acidity, and a start / stop button 5 for starting the measurement.
A power button 6 for turning on and off the power of the apparatus is arranged. Then, when the release button 2 shown in FIG. 1 is pressed, the upper lid 1 is opened as shown in FIG. 2, and the set measurement container 8 is exposed. Note that the measurement container 8 is detachable.

As shown in FIG. 3, the counter electrode 11, the working electrode 9, and the comparative electrode 1
2 are installed. In addition, a co-existing electrolyte 13 in which a quinone derivative, an organic solvent, an electrolyte, and a liquid to be measured are mixed is provided in a solution accommodating section 14 to which a container cover 15 is attached.
Is housed. As the quinone derivative, an orthobenzoquinone derivative or a parabenzoquinone derivative is desirable. According to these quinone derivatives, the pre-peak value of the voltammetry current waveform appears shifted from the dissolved oxygen reduction current waveform, and the influence of the dissolved oxygen reduction can be cut off. Then, by attaching the container cover 15 to the solution accommodating portion 14, one end of the counter electrode 11, the working electrode 9, and the comparison electrode portion 12 is protruded to the outside, and the other end is immersed in the coexisting electrolyte 13. Here, as a material of the working electrode 9 whose side surface is coated with the furan resin 10, glassy carbon called carbon or glassy carbon or plastic foam called PFC is used.
Carbon sintered at between 2000C and 2000C is suitable. The material of the counter electrode 11 is preferably platinum, graphite, or gold which does not corrode even in the coexisting electrolyte 13 and is chemically stable, but may be stainless steel, aluminum, a platinum-containing alloy or the like which does not corrode. Although not shown, a connector for connecting the counter electrode 11, the working electrode 9, and the comparison electrode section 12 to a circuit side described later is provided on the main body section 7.

As shown in FIG. 3, the comparison electrode section 12 includes a container 16 made of, for example, glass, a comparison electrode 17 protruding from the container 16, a buffer solution 18 contained in the container 16, And a liquid junction 19 provided on the bottom surface.

Here, the material of the comparison electrode 17 is silver-
Silver chloride is preferred, but saturated calomel, saturated calomel chloride, silver-silver ion, mercury-saturated mercury sulfate, and copper-saturated copper sulfate may also be used. The display of, for example, silver-silver chloride indicates that the surface of the comparative electrode 17 made of silver is covered with silver chloride.

The spontaneous potential generated between the comparison electrode 17 and the working electrode 9 differs depending on the material of the comparison electrode 17. This is because the electrode potential with respect to the standard hydrogen electrode is different. For example, the difference between the case where silver-silver chloride is used for the comparison electrode 17 and the case where silver-silver ions are used is slightly less than 600 mV. In the present embodiment, silver-silver chloride is used for the comparison electrode 17, and the natural potential of the working electrode 9 with respect to the comparison electrode 17 is +300 to +500 mV.

Suitable materials for the buffer solution 18 include chlorine compounds such as silver chloride, sodium chloride, and lithium chloride, acetonitrile, copper sulfate, and other solutions that exhibit a buffering action in the oxidation-reduction reaction of the comparative electrode 17.

The liquid junction 19 is located between the buffer solution 18 and the co-existing electrolyte 13 and has the function of not allowing these solutions to pass but allowing electrons or ions to pass therethrough. It is made of porous Vycor glass or the like.

Next, the coexisting electrolyte 13 accommodated in the solution accommodating section 14 will be described. In the present embodiment, lithium perchlorate is used as the electrolyte. The coexisting electrolyte 13 of the present embodiment has a solvent of 65% ethanol.
Is mixed with 35% isooctane to make 10 mL, and 10 mM of orthobenzoquinone and 50 mM of lithium perchlorate are melted. The measurement is performed by mixing the liquid to be measured with this solvent. Ethanol can easily melt the electrolyte,
At the same time, it has the effect of cleaning the electrode surface. Further, isooctane can be melted even if it has been thermally degraded oil, and has good compatibility with ethanol.

Here, in the embodiment of the present invention, an orthobenzoquinone derivative or a parabenzoquinone derivative is mixed in the coexisting electrolyte 13 as described above.

Therefore, measurement of the acidity by voltammetry of the electrolyte in which such a benzoquinone derivative coexists will now be described.

In FIG. 4, the horizontal axis represents the potential of the working electrode and the passage of time with respect to the reference electrode when silver-silver chloride was used as the reference electrode and plastic foam carbon of φ2 was used as the working electrode, and the vertical axis represents the counter electrode at this time. Is the value of the current flowing through. Here, the current value varies depending on conditions such as the size of the surface area of the working electrode and the concentration of the acid, but the voltage value on the horizontal axis is negligible though slightly fluctuated depending on the concentration of the acid. The solid line shows the voltammetric current waveform of the benzoquinone derivative, and the broken line shows the reduction waveform of dissolved oxygen.

As shown in FIG. 4, the voltammetric current waveform of the benzoquinone derivative having a side chain on the benzene ring is as follows.
It appears with a large shift from the region where the dissolved oxygen reduction waveform appears. According to FIG. 4, the pre-peak waveform is 0 m
It exists from the vicinity of V to the positive potential side, and is shifted by about 400 mV from the negative potential side where the reduction waveform of the dissolved oxygen exists. And even at the position of this peak, there is almost no effect of the reduction of dissolved oxygen.

As described above, since the pre-peak value can be measured in a region free from the influence of dissolved oxygen, it is possible to accurately measure the acidity without being affected by the dissolved oxygen even if the dissolved oxygen is not removed in advance. Can be.

FIG. 5 shows the relationship between the acidity and the reduction current of the acidity measuring device A according to the present embodiment.

In FIG. 5, the horizontal axis represents the acidity of the liquid to be measured, and the vertical axis represents the prepeak current value shown in FIG. As shown in the figure, there is a proportional relationship between the current value giving the pre-peak value and the acidity of the acid mixed into the liquid to be measured.

An operation method of the acidity measuring apparatus A of the present embodiment will be described. First, 0.5 g of deteriorated oil, which is a liquid to be measured, was mixed with 10 mL of the above-described solvent and stirred.
This is stored in the solution storage unit 14. Then, the counter electrode 11,
The container cover 15 having the working electrode 9 and the comparison electrode unit 12 is attached to the solution container 14. When the preparation of the measurement container 8 is completed in this way, the measurement container 8 is set in the acidity measuring device A, the upper lid 1 is closed, and the measurement is enabled.

Then, the power button 6 and the start / stop button 5 are pressed to start the measurement. Then, LCD
3, the remaining time of the measurement is displayed, for example, in seconds,
When it becomes "0", the acidity is displayed, and a series of acidity measuring operations is completed.

Next, the operation of the acidity measuring apparatus A of the present embodiment will be described. The operation of the acidity measuring device A of the present embodiment is roughly divided into two operations. One is an electrolytic cleaning operation of the working electrode 9 and the other is an actual acidity measuring operation. These operations are performed while the above-described LCD 3 displays the remaining time of the measurement. The electrolytic cleaning operation of the working electrode 9 is performed, and then the acidity measurement operation is performed.

Here, these two operations will be described with reference to FIGS. First, the electrolytic cleaning operation of the working electrode 9 will be described.

The electrolytic cleaning of the working electrode 9 is performed by
Is swept in the range of -1V to 2V. Examples of the potential sweep are shown in Examples 1 and 2 in FIG.
This is shown in Example 3. In FIG. 6, the horizontal axis represents elapsed time (seconds), and the vertical axis represents the potential of the working electrode 9 with respect to the comparison electrode 17 (hereinafter, referred to as “electrode potential”) (mV).

As shown in FIG. 6, since the electrode potential when no voltage is applied, that is, the natural potential is +300 to +500 mV, by sweeping the potential width of ± 1.5 V around 500 mV, the previous acidity measurement can be performed. Foreign matter attached to the electrode surface can be electrically removed by immersion in the solvent. In the case of the present embodiment, the natural potential is between +300 mV and +300 mV.
Since it is 500 mV, the sweep range of the electrode potential is -1 V to
+ 2V. And, the sweep range of the electrode potential is -1V to
If the voltage greatly deviates from +2 V, electrolytic polymerization occurs on the electrode surface, and measurement becomes impossible. Therefore, the sweep range of the electrode potential is optimally -1 V to +2 V, which is sufficient for electrolytic cleaning and does not cause electrolytic polymerization.

Since the working electrode 9 is automatically cleaned in this way, the work for polishing and replacing the working electrode 9 is not required.

Next, the operation of measuring the acidity will be described. As shown in FIG. 7, the acidity measuring operation is performed by setting the electrode potential to +5.
The sweep is performed at a speed of 3 to 10 mV / s in the range of 00 mV to -300 mV, and the acidity is calculated from the current waveform flowing to the counter electrode 11 at that time. Here, +500 mV--3
The range of 00 mV is a region where there is almost no influence of the dissolved oxygen described above and in which the pre-peak value of the current can be accurately measured. And a sweep speed of 3 to 10 mV
When a predetermined potential difference is swept at / s, a stable voltammetric current waveform as shown in FIG. 4 can be obtained.

At this time, if the sweep speed is 10 mV / s or more, a stable current waveform cannot be obtained because the potential sweep speed is faster than the electrode reaction speed. Conversely, if the sweep speed is less than 3 mV / s, excessive reaction occurs on the electrode surface, and a stable potential cannot be obtained. Therefore, it is necessary to make the potential sweep speed 3 to 10 mV / s.

By performing such a sweep, the peak of the acid reduction current appears at a potential near 0 mV. This is the pre-peak, and this potential shifts to the negative side as the acid concentration increases. But even if there is a shift +50
If it is set in the range of 0 mV to -300 mV, the acidity can be measured at any concentration.

Here, the relationship between the operation of cleaning the working electrode 9 and the operation of measuring the acidity will be described. As described above, in the present embodiment, the procedure is such that the working electrode 9 is electrolytically cleaned immediately before the acidity measurement is performed. This is intended to start the measurement after the working electrode 9 has been washed without giving time for the formation of the deposit. However, cleaning of the working electrode 9 can be appropriately performed in consideration of the measurement frequency and the measurement interval. That is, for example, in the case where measurement is frequently performed at short intervals, it is possible to perform cleaning once for three times or to perform the cleaning once an hour by time control. In addition, the measurement can be performed in accordance with the required measurement accuracy.

Next, a control system of the acidity measuring apparatus A of the present embodiment will be described. As shown in FIG. 8, a control unit 20 including a microcomputer and the like includes an operation unit 25 including a start / stop button 5 and a power button 6 illustrated in FIG. 1 and a display unit 2 including an LCD 3 and an LED.
6 are connected. The control unit 20 includes the control unit 2
0 to the working electrode W by a predetermined signal output from
Action voltage control circuit 24 for applying ₁ and control unit 2
Counter voltage control circuit 2 according to a predetermined signal output from
D / A converter 22 for outputting a voltage value E ₂ is connected to one. A reference electrode R is further connected to the input terminal of the counter electrode voltage control circuit 21, and a counter electrode C is connected to the output terminal. A resistor 27 is provided between the counter electrode voltage control circuit 21 and the counter electrode C. A voltage amplifying circuit 23 that measures a voltage value between the resistors 27 and amplifies the voltage value is connected to both ends of the resistor 27. I have. And the voltage amplifying circuit 2
3 is connected to the control unit 20 and the control unit 2
At 0, the acidity is calculated based on the voltage value from the voltage amplification circuit 23.

According to such a control system, the operation unit 25
When the power button 6 (FIG. 1) is pressed, a signal is sent to the control unit 20.
Is sent to drive the display unit 26. Next, the operation unit 25
Start / stop button 5 (FIG. 1) is pressed
And a predetermined voltage from the control unit 20 to the working voltage control circuit 24.
A set value signal is sent and the working voltage control circuit receives this signal.
24 is the voltage E₁Is applied to the working electrode W. Further
And a predetermined comparison from the control unit 20 to the D / A converter 22.
A digital signal of the electrode voltage set value is sent, and this signal is D
The signal is converted to an analog signal by the
To the pressure control circuit 21_TwoIs output.
In the counter voltage control circuit 21, the D / A converter 22
Input voltage value E_TwoAnd the voltage value E of the comparison electrode R_ThreeCompare with
E_Two= E_ThreeIs applied to the counter electrode C. This
, The electrode potential E_FourIs E_Four= E₁-E _Two(Or E_Three)
Relationship. The control unit 20 considers this relationship.
And outputs a signal of a voltage value. For example, electrodes
Potential E_FourIs set to 500 mV, E₁= 1000
mV, E_Two= 500mV is output from the control unit 20.
Is forced.

As described above, since the voltage applied to the working electrode W is variable, the sweep range of the electrode potential E ₄ is expanded.

That is, if the voltage output range of the D / A converter 22 is 0 to 1 V and the voltage of the working electrode W is fixed, the sweep width of the electrode potential E ₄ is 1 V regardless of the voltage of the working electrode W. Becomes However, the working electrode W
Of the electrode potential E4 can be varied from 0 V to 2 V in several ways, and the sweep width of the electrode potential E ₄ is 3 to 1 V to 2 V.
V, and can be swept at a potential sufficient for electrolytic cleaning as described above. Thus, by varying the voltage E ₁ of the working electrode W, sweep range of the electrode potential E ₄ is increased.

Here, as a means for expanding the sweep range of the electrode potential E ₄ , a method of simply increasing the voltage output range of the D / A converter 22 can be considered. However, D / A
The converter 22 has a resolution of 1/1 with respect to the reference voltage.
Since setting is made so that 2000 "or the like, by raising the reference voltage, the amount of change in the output voltage when the digital data is 1-bit change in the input is increased. With this, smooth electrode potential E ₄ do sweep no longer be, it is impossible to obtain a stable voltammetric current waveform. Therefore, as a means to extend the sweep range of the electrode potential E _4, it is desirable to vary the voltage of the working electrode W as described above.

Here, FIGS. 9 and 10 show voltage changes of the working electrode W and the reference electrode R during the electrode potential sweep in this embodiment.

FIG. 9 shows the voltage waveform of each electrode during the electrolytic cleaning of the working electrode W shown in FIG. 6, and FIG. 10 shows the voltage waveform of each electrode when the electrode potential is swept during the measurement of the acidity shown in FIG. The voltage waveform of W and the broken line are the voltage waveforms of the comparison electrode R.

In FIG. 9, the voltage of the working electrode W was set to 0 V for 20 seconds after the start of the cleaning, and the comparison electrode R was set at 1000 m.
Sweep from V to 0 mV. Similarly, the voltage of the working electrode W is switched to 1 V and 2 V every 20 seconds, and the sweep of the voltage of the comparison electrode R from 1000 mV to 0 mV is repeated.
The above is the method of sweeping the electrode potential from -1V to 2V, whereby the waveform as shown in FIG. 6 can be swept.

In the acidity measurement shown in FIG. 10, the voltage of the working electrode W is fixed at 500 mV, and the voltage of the comparison electrode R is swept from 0 mV to 800 mV at a speed of 10 mV / s. As a result, the electrode potential can be swept as shown in FIG.

Here, a method of detecting current when voltammetry is performed in the acidity measuring operation will be described.

[0076] In FIG. 8, the current I _a flowing through the counter electrode C is converted into a voltage E _a by the resistor 27. Converted voltage E _a is input is amplified by the voltage amplifier circuit 23 to the control unit 20 as the voltage E _b. Then, the control unit 20 reads the time course of the current I _a from the voltage E _b, the pre-peak value shown in FIG. 4 is determined.

Next, a method of calculating the acidity will be described.
As shown in FIG. 5, the pre-peak value I _p and the acidity θ of the acid mixed into the liquid to be measured are in a proportional relationship. That is, there is a relationship of acidity θ = current value I _p × constant K + constant B. Therefore, the proportional constants K and B obtained by measuring two or more standard reagents whose acidity is known in advance are determined by the control unit 20.
When an arbitrary acidity is measured, the current value _Ip measured by the control unit 20 is converted into the acidity θ.

According to the present embodiment, since the acidity measurement is automatically performed in this manner, an objective and highly accurate acidity measurement result can be easily obtained.

[0079]

As described above, according to the present invention, the potential sweep width between the working electrode and the reference electrode is amplified, and the working electrode is swept with a potential sweep width suitable for electrolytic cleaning and acidity measurement, respectively. Therefore, an effective effect of automatically performing from the cleaning of the working electrode to the measurement of the acidity can be obtained.

This eliminates the need for polishing and replacement work of the working electrode, thereby eliminating variations in measured values due to changes in the surface condition of the working electrode, and making it possible to perform long-term highly accurate acidity measurement. Effects can be obtained.

Also, since the acidity measurement is automatically performed,
An effective effect that an objective and highly accurate acidity measurement result can be easily obtained is obtained.

The potential of the working electrode with respect to the reference electrode is -1 V
By performing the electrolytic cleaning of the working electrode by sweeping between +2 V, the working electrode can be swept at a potential at which the electrolytic cleaning of the working electrode is performed but the electrolytic polymerization is not performed. An effective effect that measurement can be performed is obtained.

By performing the electrolytic cleaning operation of the working electrode every time the acidity measurement operation is performed, it is possible to remove the deposits on the working electrode surface immediately before the acidity measurement, so that a stable acidity measurement result with good reproducibility can be obtained. An effective effect of being able to obtain is obtained.

By performing the electrolytic cleaning operation of the working electrode once in a plurality of acidity measurement operations or once in a predetermined period, the surface of the working electrode is always activated, so that good reproducibility is obtained. An effective effect that a stable acidity measurement result can be obtained is obtained.

[Brief description of the drawings]

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

FIG. 2 is an external perspective view showing a state in which an upper lid is opened in the acidity measuring device of FIG.

FIG. 3 is a sectional view showing a measuring container in the acidity measuring device of FIG. 1;

FIG. 4 is a graph showing a current-potential relationship in acidity measurement by voltammetry of a co-existing electrolyte mixed with a benzoquinone derivative.

FIG. 5 is a graph showing the relationship between the acidity and the reduction current of the acidity measuring device.

FIG. 6 is a graph showing an example of an electrode potential sweep waveform during electrolytic cleaning of a working electrode.

FIG. 7 is a graph showing an example of an electrode potential sweep waveform at the time of measuring the acidity of a working electrode.

FIG. 8 is a block diagram showing a control system of the acidity measuring device of FIG. 1;

FIG. 9 is a graph showing voltage waveforms of a working electrode and a reference electrode during electrolytic cleaning of a working electrode.

FIG. 10 is a graph showing voltage waveforms of a working electrode and a reference electrode during acidity measurement.

FIG. 11 is a graph showing a current-potential relationship in the measurement of acidity by voltammetry of a measurement electrolyte solution in which a conventional naphthoquinone derivative coexists.

FIG. 12 is a schematic circuit diagram of a potentiostat.

FIG. 13 is a graph showing a voltammetric current waveform when a working electrode is polished.

[Explanation of symbols]

 Reference Signs List 9 working electrode 11 counter electrode 12 comparison electrode part 13 coexisting electrolyte 17 reference electrode 20 control part A acidity measuring device

Claims (4)

[Claims] 1. A working electrode, a counter electrode and a reference electrode section immersed in a coexisting electrolyte solution in which an electrolyte solution and an acid-containing measurement solution are mixed, and the voltage applied to the working electrode is changed to perform the comparison. An acidity measuring apparatus, comprising: a control unit for performing electrolytic cleaning of the working electrode by sweeping a potential of the working electrode with respect to a comparison electrode of an electrode unit within a predetermined range, and thereafter measuring an acidity. 2. The apparatus according to claim 1, wherein the control section sweeps the potential of the working electrode with respect to the comparison electrode between -1 V and +2 V to perform electrolytic cleaning of the working electrode. . 3. The acidity measuring device according to claim 1, wherein the control section performs an electrolytic cleaning operation of the working electrode every time the acidity measuring operation is performed. 4. The method according to claim 1, wherein the control section performs the electrolytic cleaning operation of the working electrode once in a plurality of acidity measurement operations or once in a predetermined period. Acidity measuring device.