JP4356130B2 - Acidity measuring device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an acidity measuring device for measuring the acidity of free fatty acids contained in edible oils, malic acid, tartaric acid, citric acid, acids contained in alcoholic beverages, caffeic acid in coffee, and the like. .
[0002]
[Prior art]
In recent years, food products have been required to have a certain level of quality in terms of health and safety. Above all, the acid contained in the food greatly affects the quality of the food. In addition, it is often considered that alkaline foods are better because of the health boom, and recently, foods with low acidity tend to be exclusively preferred.
[0003]
As described above, the acidity of various foods has a great influence on the consumption of food, but the degree of influence and the measuring method differ depending on the food. Therefore, as representative examples of such foods, (1) cooking oil, (2) fruit drinks such as fruit and juice, (3) alcoholic drinks such as whiskey, liquor, wine, and (4) coffee, I will explain what was measured in the past.
[0004]
First, the acid contained in edible oil will be described. Japan's dietary habits are changing rapidly. Looking at the trend, it seems that there is a major trend of instantization first, and secondly the trend of diversification represented by handmade tastes. . In particular, instant orientation is a reflection of 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 also preferred for taste and are relatively resistant to corruption.
[0005]
However, when this fried food is also exposed to an environment affected by temperature and light for a long time, fats and oils are auto-oxidized by oxygen in the air, resulting in a foul odor and other quality deterioration. For these reasons, there has been a general interest in the deterioration and deterioration of edible oils and fats and processed products, such as the establishment of a local food certification system for frying, or the regulation of oil confectionery, etc. Laws and regulations regarding oil and fat deterioration are also being examined in the guidelines for lunch boxes and prepared dishes.
[0006]
By the way, there are several analysis methods for measuring the acid value, the peroxide value, the viscosity, the iodine value, and the like as methods for knowing the degree of damage of such oils and fats, particularly the degree of deterioration of heated oils and fats. Here, considering the fact that, as described above, temperature, humidity, and light have a great influence on the deterioration of food, and the change in acidity is large at the early stage of deterioration, the acidity value for directly measuring the acidity among these Measurement is appropriate for determining thermal degradation (degradation due to temperature or light), and is usually used in many cases.
[0007]
Next, the acid of a fruit and fruit drink is demonstrated. For fruits, especially mandarin oranges, acid and sugar are indicators of deliciousness. In other words, the acidity of mandarin oranges is also a measure of the harvest time, so the acidity is always measured before harvesting. Also, the acidity is measured at the mandarin orange shipping factory, which affects the shipping price.
[0008]
Fruit drinks such as juice are juices obtained by squeezing raw fruit into a juicer, but many fruit drinks are made from concentrated or frozen fruit juice rather than using fresh fruit juice as it is. There are many. For example, in the case of orange juice, after removing diseases and immature fruits of mandarin oranges, the epidermis is washed and squeezed to remove the flesh and fruit juice, and the fruit skin, gills, etc. are removed from the fruit juice. At this point, the sugar content and acidity are prepared so as to conform to the Japanese Agricultural Standards, and the acidity is measured at that time. Moreover, when making orange juice from concentrated fruit juice or frozen fruit juice, acidity is measured when water is added to concentrated fruit juice or frozen fruit juice to produce orange juice.
[0009]
Subsequently, the alcoholic beverage will be described. Distilled liquor that increases the yield of ethanol by repeating distillation such as whiskey and shochu many times, or brewed liquor obtained by fermenting and filtering the material itself typified by liquor and wine, and other fruit liquors There are various types of alcoholic beverages, such as sparkling liquor such as beer, etc., and the manufacturing process varies. However, in the production of any alcoholic beverage, acidity is measured in the process to ensure product quality.
[0010]
Finally, the coffee acid will be described. As described below, there are many kinds of substances that give sourness that affects the taste of coffee, but the acid content is important as an index for evaluating the sourness of coffee. Representative examples of the acid contained in the coffee include chlorogenic acids. Its content also varies during the roasting process of coffee beans. In addition, there are many compounds involved in the sourness of coffee, such as caffeic acid, quinic acid, and citric acid. And although the content of each acid is very small, it is thought that a delicate balance and the total amount are decisive factors of sourness.
[0011]
As described above, in various foods, the acidity is measured in the production process, and there are various measuring methods.
[0012]
As an example of the conventional acid measurement method, there are methods defined by the standard oil and fat analysis method, Japanese Agricultural Standards, JIS, Japanese Pharmacopoeia oil and fat test method, hygiene test method food and drink test method, water supply test method, etc. In any case, the basis of measurement is a neutralization titration method using phenolphthalein as an indicator. Therefore, in order to explain this neutralization titration method, the water supply test method and the neutralization titration method defined in the reference oil analysis method will be described below.
[0013]
The acidity in the water test method is defined as the number of mg when acid is converted into calcium carbonate contained in 1 liter of a sample. Specifically, 100 mL (milliliter) of test water is taken, about 0.2 mL of phenolphthalein indicator is added thereto, and a 0.02 mol / L sodium hydroxide solution is further added. Then, tightly plug and lightly rock, and if the red color disappears, the titration is continued until the slight red color remains without disappearing, and the mLa of sodium hydroxide is obtained as the end point of neutralization. The acidity at that time is given by the following equation.
[0014]
Acidity (calcium carbonate equivalent mg / L) = a × 10
Next, the neutralization titration method defined in the standard fat analysis method will be described.
[0015]
The definition of acidity in the standard fat analysis method 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 subjected to its estimated acidity (for example, 20 g is collected when the acidity is 1 or less, 10 g is collected when the acidity exceeds 1 and 4 or less, and 2.5 g is collected when the acidity exceeds 4 and 15 or less). Collect and measure correctly in an Erlenmeyer flask. Add 100 mL of neutral solvent and shake well until the sample is completely dissolved. However, the neutral solvent referred to here is about 0.1 mL of phenolphthalein indicator added to 100 mL of a mixed solvent of ethyl ether and ethanol 1: 1, and a 1/10 N (N) potassium hydroxide-ethanol solution just before use. Neutralized.
[0016]
In the case of a solid sample, it is heated and melted on a water bath, and then dissolved by adding a solvent. This was titrated with 1/10 normal (N) potassium hydroxide-ethanol standard solution, and the end point of neutralization was determined when the color change of the indicator continued for 30 seconds. And the mg number of potassium hydroxide at this time is calculated.
[0017]
By the way, as for the measurement of fatty acids, there is a method of measuring acidity by voltammetry without using such neutralization titration method.
[0018]
This is disclosed in, for example, Japanese Patent Application Laid-Open No. 5-264503, and a measurement electrolyte solution in which a free fatty acid and a naphthoquinone derivative coexist is measured by voltammetry according to a potential regulation method. In this measurement, the current value of the reduction wave of the naphthoquinone derivative is proportional to the free fatty acid concentration for all fatty acids from lower fatty acids such as formic acid to higher fatty acids such as oleic acid and linoleic acid. The value obtained by superimposing the current values of each fatty acid corresponds to the total concentration of fatty acids. That is, the acid concentration is measured by measuring the magnitude of the current value of the reduction pre-wave of the naphthoquinone derivative.
[0019]
Data measured by this method is shown in FIG. Here, FIG. 11 is a graph showing a conventional current-potential relationship of acidity measurement by voltammetry of a measurement electrolyte solution in which a naphthoquinone derivative coexists.
[0020]
In FIG. 11, the horizontal axis indicates the potential of the working electrode (electrode potential) relative to the potential of the comparative electrode when silver-silver chloride is used for the comparative electrode and glassy carbon having a diameter of 3 mm is used for the working electrode. The current value flowing through the working electrode (= current value flowing through the counter electrode) is shown. However, the current value varies depending on conditions such as the surface area size and roughness of the working electrode, and the acid concentration. On the other hand, the value of the electrode potential on the horizontal axis is negligible, although it varies slightly depending on the acid concentration.
[0021]
In FIG. 11, the point A is a pre-peak showing a reduction pre-wave proportional to the acid concentration, and the point D is the main peak of the naphthoquinone derivative.
[0022]
By the way, when measuring the acidity of a fatty acid using the technique disclosed in Japanese Patent Application Laid-Open No. 5-264503, voltammetric control using a potentiostat or the like is generally required.
[0023]
Here, the operating principle of the potentiostat will be described with reference to FIG. FIG. 12 is a circuit diagram showing a potentiostat. In FIG. 12, 27 is an operational amplifier, 28 is a resistor, 29 is a voltage amplifier circuit, and 30 is a sweep voltage setting power source.
[0024]
In FIG. 12, the sweep voltage setting power source 30 is connected to the plus terminal which is the non-inverting input terminal of the operational amplifier 27, and the comparison electrode R is connected to the minus terminal which is the inverting input terminal. A counter electrode C is connected to the output terminal through a resistor 28. Further, a voltage amplification circuit 29 is connected to both ends of the resistor 28. The working electrode W is at a ground potential.
[0025]
When the electrode potential is swept by such a potentiostat and the current I1 flowing through the counter electrode C which is the same as the current flowing through the working electrode at that time is to be measured, the sweep voltage setting power supply 30 is first set to a predetermined voltage V1. Then, a voltage is applied to the counter electrode C by the feedback circuit composed of the operational amplifier 27 so that the voltage V1 = the comparison electrode voltage V2. The current I1 flowing to the counter electrode is converted into a voltage V3 by the resistor 28, further amplified by the voltage amplifier circuit 29, and output as the voltage V4. This voltage V4 is recorded on a plotter or the like, the current I1 is calculated backward from the voltage V4, and a relative graph of the electrode potential and the counter electrode current (working electrode current) I1 is completed.
[0026]
In this case, it is necessary to grasp the relationship between the counter electrode current I1 and the output voltage V4, and since the working electrode W is fixed at 0 V, a power supply circuit that generates both power supplies as a power supply for the entire system is also required. It becomes.
[0027]
Moreover, in the measurement by the conventional potentiostat, even if the solution of the same acidity is measured according to the state of the working electrode W before the measurement, the measurement result may be different. This is a phenomenon that occurs because the current waveform varies depending on the initial state of the working electrode W, resulting in different pre-peak current values. For example, when the working electrode W is dried and measured immediately after being immersed in the measurement electrolyte, or when measured immediately after measuring a high acidity, the measurement result tends to be high, and the working electrode after washing When measured with, there was a problem that it tends to be lower. This is considered to be due to the difference in physical and electrical permeability with respect to the working electrode active surface of the liquid to be measured. That is, when measuring the correct acidity, it is necessary to perform storage management of the working electrode sufficiently and to stabilize the electrode surface even after the working electrode W is immersed in the measurement electrolytic solution.
[0028]
[Problems to be solved by the invention]
As described above, in the conventional acidity measuring apparatus, the acidity is measured using the neutralization titration method, and the color change due to the phenolphthalein indicator is judged and this is used as the end point of the titration. Therefore, the determination of the end point varies and the acidity is not objectively determined.
[0029]
And according to the neutralization titration method of 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 easily flammable at 34.6 ° C., which makes it difficult to handle. In addition, 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 titration end point There was a problem that the change could not be accurately grasped, and the measured value varied due to a wrong reading of the end point. Furthermore, since the amount of the sample is several tens of grams and 100 mL of the neutral solvent is required, and a large amount of sample is required for one measurement, there is a problem that an increase in the number of measurements becomes a burden.
[0030]
Moreover, in the technique using a potentiostat disclosed in Japanese Patent Application Laid-Open No. 5-264503 as a technique relating to a conventional acidity measuring apparatus, handling of the potentiostat is not easy, and specialized knowledge is also required. There were problems such as taking a long time to measure.
[0031]
Furthermore, it is very difficult to strictly control the surface state of the working electrode before measurement, and it takes human intuition and time for preparation before measurement even after being immersed in the measurement electrolyte. Here, the current waveform due to the difference in the dry state of the working electrode is shown in waveform S1 and waveform S2 in FIG. FIG. 13 is a graph showing two voltammetric current waveforms with different working electrode storage states. A waveform S1 in FIG. 13 is a waveform when the working electrode is dried at 105 ° C. and 10% temperature and humidity and then returned to room temperature, and the waveform S2 is 25 ° C. and 60% temperature and humidity. It is an electric current waveform when it measures at normal temperature using the preserve | saved working electrode. Thus, even when the same acidity is measured at the same temperature, the current waveform is different, and a measurement result with low accuracy is obtained.
[0032]
And even when trying to stabilize the working electrode surface by immersing it in the measurement electrolyte, if it is soaked for a long time, if the acidity of the fruit is measured, other fibers such as fruit fibers will be adsorbed on the electrode surface. Measurement errors will occur.
[0033]
In view of the above problems, the acidity measuring apparatus is required to automatically perform from the stabilization of the working electrode to the acidity measurement in order to perform the measurement with high accuracy.
[0034]
Therefore, an object of the present invention is to provide an acidity measuring apparatus capable of automatically performing from the stabilization of the working electrode to the acidity measurement.
[0035]
[Means for Solving the Problems]
  In order to solve this problem, the acidity measuring apparatus of the present invention includes a working electrode, a counter electrode, a reference electrode, and a potential of the reference electrode immersed in a coexisting electrolyte obtained by mixing an electrolyte and an acid-containing solution to be measured. A control unit that sweeps the potential of the working electrode with respect to the electric field, and the control unit can electrically stabilize the working electrode.Infiltrate the working electrode active surface with a new measuring solventThe first sweep is performed at the first sweep speed in a potential range where physical separation does not occur, and the second sweep is performed in one direction within the predetermined potential range in place of the second sweep speed. It has a configuration for measuring acidity.
[0036]
Thereby, the acidity measuring apparatus which can perform automatically from stabilization of a working electrode to acidity measurement is obtained.
[0037]
DETAILED DESCRIPTION OF THE INVENTION
  According to the first aspect of the present invention, there are provided a working electrode, a counter electrode, a comparative electrode, and a working electrode with respect to the potential of the comparative electrode, which are immersed in a coexisting electrolytic solution in which an electrolytic solution and an acid-containing liquid to be measured are mixed. An acidity measuring device having a control unit that sweeps the potential of the electrode, wherein the control unit can electrically stabilize the working electrode.Infiltrate the working electrode active surface with a new measuring solventThe first sweep is performed at the first sweep speed in a potential range where physical separation does not occur, and the second sweep is performed in one direction within the predetermined potential range in place of the second sweep speed. The acidity measuring device is characterized in that it measures acidity, and can perform two types of sweeping. For example, the working electrode is electrically stabilized by the first sweep, and the acidity is measured by the second sweep. Since the working electrode can be automatically measured from the stabilization of the working electrode to the acidity measurement, the working of the working electrode can be stabilized by strictly managing the working electrode and relying on human intuition. And has the effect that it becomes unnecessary. The surface of the working electrode is always stabilized immediately before the acidity measurement, and it has an effect that a stable acidity measurement result having good reproducibility can be obtained even when measuring irregularly or continuously measuring acids having greatly different values.
[0038]
  Claim 2The invention according to claim 1 is an invention subordinate to the invention described in claim 1, wherein the control unit determines that the potential between the comparison electrode and the working electrode is between the first sweep and the second sweep. 2. The acidity measuring apparatus according to claim 1, wherein a predetermined voltage is applied between the working electrode and the counter electrode for a predetermined time so as to be a natural potential.Since the ion state near the working electrode surface that has become unbalanced by the first sweep approaches the equilibrium as much as possible at the start of the second sweep, reproducible and highly accurate acidity measurement can be performed. Has the effect of being able to.
[0039]
  Claim 3The invention according to claim 1 is an invention subordinate to the invention described in claim 1, and in the first sweep, the potential of the working electrode with respect to the potential of the reference electrode is changed from NV (natural potential) to NV-1V, The acidity measuring device according to claim 1, wherein sweeping is performed at a predetermined speed from NV-1V to NV + 1V, and further from NV + 1V to NV.Although the electrical stabilization of the working electrode surface is performed, until the physical peeling of the working electrode surfaceNot performedSince it is swept by the potential, the working electrode is prevented from being deteriorated, and the acidity measurement can be easily performed with high accuracy.
[0042]
Hereinafter, embodiments of the present invention will be described with reference to FIGS.
[0043]
(Embodiment 1)
1 is a perspective view showing an acidity measuring apparatus according to Embodiment 1 of the present invention, and FIG. 2 is a perspective view showing a state in which a slide-type drawer lid on which a measurement container is placed is opened in the acidity measuring apparatus of FIG. It is.
[0044]
1 and 2, 1 is a drawer lid, 2 is an LCD, 3 is a start / stop button, 4 is a power button, 5 is a main body, and 6 is a measurement container.
[0045]
About the acidity measuring apparatus comprised in this way, the arrangement | positioning, a function, operation | movement, etc. are demonstrated using FIGS. 3 is a cross-sectional view showing a measuring container in the acidity measuring apparatus of FIG. 1, FIG. 4 is a graph showing the current-potential relationship of acidity measurement by voltammetry of a coexisting electrolyte mixed with a benzoquinone derivative, and FIG. 5 is acidity measurement. FIG. 6 is a graph showing an example of an electrode potential sweep waveform when the working electrode is electrically stabilized, and FIG. 7 is a graph showing an electrode potential sweep waveform when measuring the acidity of the working electrode. FIG. 8 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 at the time of electrical stabilization of the working electrode, and FIG. 10 is acidity. It is a graph which shows the voltage waveform of the working electrode at the time of a measurement, and a comparison electrode.
[0046]
In FIG. 3, 7 is a working electrode, 8 is a counter electrode, 9 is a reference electrode, 10 is a coexisting electrolyte, 11 is a solution storage section, and 12 is a container cover.
[0047]
In FIG. 8, 21 is a control unit, 22 is a comparison electrode voltage control circuit, 23 is a current detection circuit, 24 is a working electrode voltage control circuit, 25 is an operation unit, and 26 is a display unit.
[0048]
In FIG. 1, a slide-type drawer lid 1 having an internal space for setting a measurement container 6 (FIG. 2) is attached to the main body 5 of the acidity measuring apparatus. The main body 5 is provided with an LCD 2 which is a display means for displaying the measured acidity, and further includes a start / stop button 3 for starting the measurement and a power button 4 for turning on / off the power of the apparatus. Has been. When the drawer lid 1 shown in FIG. 1 is pulled out, the measurement container 6 is exposed as shown in FIG. The measurement container 6 is detachable.
[0049]
As shown in FIG. 3, a counter electrode 8, a working electrode 7, and a comparison electrode 9 are attached to the container cover 12 of the measurement container 6. A coexisting electrolytic solution 10 in which a quinone derivative, an organic solvent, an electrolyte, and a liquid to be measured are mixed is stored in the solution storage unit 11 to which the container cover 12 is attached. As the quinone derivative, an orthobenzoquinone derivative or a parabenzoquinone derivative is desirable. According to these quinone derivatives, the pre-peak value of the voltammetric current waveform appears to be shifted from the reduction current waveform of dissolved oxygen, which can cut off the effect of reduction of dissolved oxygen and the light stability that is a weakness of quinones. Can also be realized. Then, by attaching the container cover 12 to the solution storage unit 11, one end of the counter electrode 8, the working electrode 7, and the comparison electrode 9 is projected outside, and the other end is immersed in the coexisting electrolyte 10. Here, as a material of the working electrode 7 whose side surface is coated with an epoxy resin, glassy carbon called carbon or glassy carbon, or carbon obtained by sintering a plastic foam called PFC at 1000 ° C. to 2000 ° C. is suitable. . The counter electrode 8 is preferably platinum, graphite, or gold that does not corrode in the coexisting electrolyte 10 and is chemically stable, but may be stainless steel, aluminum, a platinum-containing alloy, or the like that does not corrode. Carbons used for 7 may also be used. In the present embodiment, PFC is used as the material for the working electrode 7 and the counter electrode 8. Although not shown, the main body 7 is provided with a connector for connecting the counter electrode 8, the working electrode 7, the comparison electrode 9 and a circuit side described later.
[0050]
Here, although the silver-silver chloride electrode is desirable as the configuration of the comparative electrode 9, it may be saturated calomel, saturated calomel chloride, silver-silver ion, mercury-saturated mercury sulfate, or copper-saturated copper sulfate. Further, the same carbon as the working electrode 7 may be used. In addition, for example, a display such as silver-silver chloride indicates that the surface of the reference electrode 9 made of silver is coated with silver chloride.
[0051]
The natural potential generated between the comparison electrode 9 and the working electrode 7 varies depending on the material and configuration of the comparison electrode 9. This is because the electrode potentials with respect to the standard hydrogen electrode are different. For example, when the silver-silver chloride electrode is used as the comparison electrode 9 and when the carbon electrode is used, the difference is slightly less than 300 mV. In the present embodiment, the same material as that of the working electrode 7 is used for the comparison electrode 9, and the natural potential of the working electrode 7 with respect to the comparison electrode 9 is 0 mV.
[0052]
Next, the coexisting electrolyte solution 10 stored in the solution storage unit 11 will be described. In the present embodiment, sodium chloride is used as the electrolyte. The coexisting electrolyte solution 10 of the present embodiment is a mixture of ethanol 60%, isopropyl alcohol 20% and distilled water 20% to make 5 mL, and melted orthobenzoquinone 20 mM (millimolar) and sodium chloride 150 mM. Measurement is performed by mixing the solution to be measured in a solvent. Ethanol can easily melt the electrolyte, and at the same time has the effect of cleaning the electrode surface. Distilled water can be melted even if it is a concentrated fruit juice, and isopropyl alcohol serves to increase the solubility of ethanol and distilled water.
[0053]
Here, in the embodiment of the present invention, as described above, the coexisting electrolyte 10 is mixed with an orthobenzoquinone derivative or a parabenzoquinone derivative.
[0054]
Next, the acidity measurement by voltammetry of the electrolyte solution in which such a benzoquinone derivative coexists will be described.
[0055]
In FIG. 4, the horizontal axis represents the potential of the working electrode 7 with respect to the comparative electrode 9 and the passage of time when plastic foamed carbon having a diameter of 2 mm is used for the working electrode and the comparative electrode, and the vertical axis represents the current flowing to the counter electrode 8 at this time. Value. Here, the current value varies depending on conditions such as the surface area of the working electrode 7 and the acid concentration, but the voltage value on the horizontal axis is negligible although there is some variation depending on the acid concentration. The solid line represents the voltammetric current waveform of the benzoquinone derivative, and the broken line represents the reduced waveform of dissolved oxygen.
[0056]
As shown in FIG. 4, the voltammetric current waveform of the benzoquinone derivative having a side chain on the benzene ring appears greatly shifted from the region where the reduced waveform of dissolved oxygen appears. According to FIG. 4, the pre-peak waveform exists from around 350 mV to the positive potential side, and is shifted by a width of about 400 mV from the negative potential side where the reduced waveform of dissolved oxygen exists. Even at the position of this peak, there is almost no influence of the reduction of dissolved oxygen.
[0057]
Thus, since the pre-peak value can be measured in a region where there is no influence of dissolved oxygen, it is possible to accurately measure the acidity without being affected by dissolved oxygen without removing the dissolved oxygen in advance, and thus without variation.
[0058]
FIG. 5 shows the relationship between the acidity of the acidity measuring apparatus according to this embodiment and the reduction current. In FIG. 5, the horizontal axis represents the acidity of the liquid to be measured, and the vertical axis represents the pre-peak current value shown in FIG. As shown in the figure, the current value giving the pre-peak value and the acidity of the acid mixed in the liquid to be measured are in a proportional relationship.
[0059]
An operation method of the acidity measuring apparatus according to this embodiment will be described. First, 0.1 ml of orange juice, which is a liquid to be measured, is mixed and stirred with respect to 5 mL of the solvent described above, and this is stored in the solution storage unit 11. Next, a container cover 12 having a counter electrode 8, a working electrode 7, and a comparison electrode 9 is attached to the solution storage unit 11. When the preparation of the measurement container 6 is completed in this way, the measurement container 6 is set on the drawer lid 1 and the drawer lid 1 is closed to make the measurement possible.
[0060]
Then, the power button 4 and the start / stop button 3 are pressed to start measurement. Then, the remaining measurement time is displayed on the LCD 2 in units of seconds, for example. When the value becomes “0”, the acidity is displayed, and the series of acidity measurement operations is completed.
[0061]
Next, operation | movement of the acidity measuring apparatus by this Embodiment is demonstrated.
[0062]
The operation of the acidity measuring apparatus according to the present embodiment is roughly divided into two operations. One is an electrical stabilization operation of the working electrode 7, and the other is an actual acidity measurement operation. These operations are performed while the above-described LCD 2 displays the remaining measurement time. The electric stabilization operation of the working electrode 7 is performed, and then the acidity measurement operation is performed.
[0063]
Here, these two operations will be described with reference to FIGS. First, the electrical stabilization operation of the working electrode 7 will be described.
[0064]
The electrical stabilization of the working electrode 7 is performed by sweeping the potential of the working electrode 7 with respect to the potential of the comparison electrode 9 in a range of natural potential ± 1V. An example of the potential sweep is shown in FIG. In FIG. 6, the horizontal axis represents elapsed time (seconds), and the vertical axis represents the potential of the working electrode 7 relative to the potential of the reference electrode 9 (that is, the “electrode potential” described above) (mV).
[0065]
As shown in FIG. 6, since the electrode potential when no voltage is applied, that is, the natural potential is almost 0 mV, it is attached to the residual acid or the electrode surface measured last time by sweeping with a potential width of ± 1 V around 0 mV. The washed liquid can be electrically removed, and a new measurement solvent can penetrate into the working electrode active surface. In this embodiment, since the natural potential is about 0 mV, the sweep range of the electrode potential is ± 1V. If the sweep range of the electrode potential deviates significantly from ± 1 V, a large current change due to physical peeling of the electrode surface or electropolymerization on the electrode surface occurs, making measurement impossible. Therefore, the sweep range of the electrode potential is optimally a natural potential ± 1 V which is sufficient for electrical stabilization and does not cause physical peeling or electrolytic polymerization.
[0066]
In addition, since the surface of the working electrode 7 is automatically stabilized in this way, it is not necessary to perform strict management of the working electrode 7 or stabilization work that relies on human intuition.
[0067]
Next, the acidity measurement operation will be described. In FIG. 7, up to 20 seconds is a natural potential application operation performed between the stabilization operation and the acidity measurement operation described above, and after 20 seconds is an acidity measurement operation. In this acidity measurement operation, as shown in FIG. 7, the electrode potential is swept in the range of 0 mV to −800 mV (predetermined potential range) at a speed of 3 to 300 mV / s (predetermined speed), and the current flowing to the counter electrode 8 at that time The acidity is calculated from the waveform. Here, the range of 0 mV to −800 mV is a region where there is almost no influence of the above-described dissolved oxygen, and a region where the pre-peak value of the current can be accurately measured. When a predetermined potential difference is swept at a sweep speed of 3 to 200 mV / s, a stable voltammetric current waveform as shown in FIG. 4 can be obtained.
[0068]
At this time, if the sweep speed is 200 mV / s or more, the potential sweeping speed is faster than the electrode reaction speed, so that a stable current waveform cannot be obtained. Conversely, if the sweep rate is less than 3 mV / s, the reaction on the electrode surface occurs excessively and a stable potential cannot be obtained. Therefore, the potential sweep rate must be 3 to 200 mV / s.
[0069]
By performing such sweeping, the peak of the acid reduction current appears at a potential of around 400 mV. This is a pre-peak, and this potential shifts to the negative side as the acid concentration increases. However, even if there is a shift, if it is set in the range of 0 mV to -800 mV, it can be measured at any concentration of acidity.
[0070]
Here, the relationship between the stabilization operation of the surface of the working electrode 7 and the acidity measurement operation will be described. As described above, in the present embodiment, the procedure of stabilizing the surface of the working electrode 7 immediately before the acidity measurement is performed. In this method, after the surface of the working electrode 7 is stabilized, the measurement is started without giving time for the deposit to be generated. However, the working electrode 7 can be stabilized as appropriate in consideration of the measurement frequency and the measurement interval. That is, for example, when acids that differ greatly at short intervals are frequently measured, it is possible to stabilize once by measuring the acidity three times, or to perform once per hour by time control. In addition, stabilization can always be performed before the acidity measurement, and in addition, an appropriate number of stabilization can be performed in accordance with the required measurement accuracy.
[0071]
Next, the control system of the acidity measuring apparatus according to this embodiment will be described.
[0072]
As shown in FIG. 8, the control unit 21 composed of a microcomputer or the like is connected to the operation unit 25 composed of the start / stop button 3 and the power button 4 shown in FIG. 1, and the display unit 26 composed of the LCD 2 and LEDs. Has been. The control unit 21 is connected to a working electrode voltage control circuit 24 and a comparison electrode voltage control circuit 22 that apply a voltage to each electrode according to a predetermined signal output from the control unit 21. A working electrode W is connected to an input terminal of the working electrode voltage control circuit 24, and a working electrode W is connected to an output terminal via a current detection circuit 23. The comparison electrode R is connected to the input terminal of the comparison electrode voltage control circuit 22 and the counter electrode C is connected to the output terminal. When a signal having a voltage E1 is sent from the control unit 21 to the working electrode voltage control circuit 24, the working electrode voltage control circuit 24 outputs a voltage so that the voltage of the working electrode W becomes E1. And when the signal which becomes the voltage E2 is sent to the comparison electrode voltage control circuit 22 from the control part 21, the comparison electrode voltage control circuit 22 will apply to the counter electrode C so that the voltage of the comparison electrode R may become E2. The output terminal of the current detection circuit 23 is connected to the control unit 21, and a signal corresponding to the current flowing through the working electrode W is sent to the control unit 21. The control unit 21 calculates the acidity based on the information from the current detection circuit 23.
[0073]
According to such a control system, when the power button 4 (FIG. 1) of the operation unit 25 is pressed, a signal is sent to the control unit 21 and the display unit 26 is driven. Next, when the start / stop button 3 (FIG. 1) of the operation unit 25 is pressed, a signal of a predetermined voltage set value is sent from the control unit 21 to the working voltage control circuit 24 and the comparison voltage control circuit 22. In response, the working voltage control circuit 24 applies the voltage E1 to the working electrode W while monitoring the voltage. At the same time, the comparison electrode voltage control circuit 22 applies the voltage to the counter electrode C so that the voltage value of the comparison electrode R becomes the voltage E2. A predetermined voltage is applied. At this time, the electrode potential E3 has a relationship of E3 = E1-E2. And the control part 21 outputs the signal of a voltage value in consideration of this relationship. For example, when it is desired to set the electrode potential E3 to 500 mV, the control unit 21 outputs signals for E1 = 700 mV and E1 = 200 mV.
[0074]
Here, FIG. 9 and FIG. 10 show voltage changes of the working electrode W and the comparison electrode R during the electrode potential sweep in the present embodiment.
[0075]
FIG. 9 is a waveform diagram showing the voltage waveform of each electrode when sweeping the electrode potential at the time of electrical stabilization of the working electrode W shown in FIG. 6, and FIG. 10 is an electrode potential at the time of acidity measurement shown in FIG. It is a wave form diagram which shows the voltage waveform of each electrode when performing this sweep. 9 and 10, the solid line indicates the voltage waveform SW of the working electrode W, and the broken line indicates the voltage waveform SR of the comparison electrode R.
[0076]
In FIG. 9, first, the voltage of the working electrode W is fixed to 2000 mV (predetermined voltage). At the same time, the voltage of the reference electrode R is set to 1000 mV, and 2 seconds are allowed to elapse. Next, the reference electrode R is swept from 1000 mV to 3000 mV at 500 mV / second (predetermined speed). The above is the method of sweeping the electrode potential from + 1V to -1V. Similarly, the reference electrode R is then swept from 3000 mV to 1000 mV, and further swept from 1000 mV to 2000 mV. Thereby, a waveform as shown in FIG. 6 can be swept.
[0077]
In the acidity measurement shown in FIG. 10, the voltage of the reference electrode R is increased from 2000 mV to 2800 mV to 100 mV / 20 seconds after a lapse of 20 seconds (predetermined time) with the voltage of the working electrode W and the reference electrode R fixed at 2000 mV. Sweep at a rate of seconds. As a result, the electrode potential can be swept as shown in FIG.
[0078]
Here, a current detection method when voltammetry is performed in the acidity measurement operation will be described.
[0079]
In FIG. 8, a current Ia flowing through the working electrode W is converted into a voltage Ea by the current detection circuit 23. The converted voltage Ea is input to the control unit 21 as a signal corresponding to the current Ia. And the control part 21 reads the time-dependent change of the electric current Ia from the voltage Ea, and determines the pre-peak value shown in FIG.
[0080]
Next, a method for calculating acidity will be described.
[0081]
As shown in FIG. 5, the pre-peak value Ip and the acidity θ of the acid mixed in the liquid to be measured are in a proportional relationship. That is, there is a relationship of acidity θ = current value Ip × constant K + constant B. Therefore, if the proportional constants K and B obtained by measuring two or more standard reagents whose acidity is known in advance are stored in the control unit 21, the control unit 21 can measure any acidity. Thus, the measured current value Ip is converted into acidity θ.
[0082]
As described above, according to the present embodiment, the potential of the working electrode 7 is swept with respect to the potential of the reference electrode 9 twice by changing the sweep voltage and the sweep speed. Two types of sweeps can be performed. For example, the working electrode 7 can be electrically stabilized by the first sweep and the acidity can be measured by the second sweep. 7 can be automatically performed from the stabilization to the acidity measurement, so that the strict management regarding the preservation of the working electrode and the stabilization work of the working electrode surface depending on human intuition can be eliminated. In addition, by applying a voltage to the working electrode-counter electrode so that the comparison electrode 9 and the working electrode 7 are at 0 V between the first sweep and the second sweep, an unbalance is caused by the first sweep. Since the ion state in the vicinity of the surface of the working electrode can be brought as close as possible to the equilibrium at the time of the second sweep start, reproducible and highly accurate acidity measurement can be performed.
[0083]
【The invention's effect】
  As described above, according to the acidity measuring apparatus according to claim 1 of the present invention, the working electrode, the counter electrode, and the reference electrode immersed in the coexisting electrolytic solution in which the electrolytic solution and the acid-containing measured solution are mixed, By having a control unit that sweeps the potential of the working electrode with respect to the potential of the comparison electrode by changing the sweep voltage and the sweep speed twice for the first time and the second time, two types of sweeps can be performed. Since the working electrode can be electrically stabilized by the first sweep and the acidity can be measured by the second sweep, the working electrode can be automatically stabilized from the stabilization to the acidity measurement. Therefore, it is possible to eliminate the need for strict management for working electrode storage and stabilization of the working electrode surface that relies on human intuition, and it is possible to obtain an advantageous effect that highly accurate acidity measurement can be performed over a long period of time. It is done.The surface of the working electrode is always stabilized immediately before the acidity measurement, and it has an effect that a stable acidity measurement result having good reproducibility can be obtained even when measuring irregularly or continuously measuring acids having greatly different values.
[0084]
According to the acidity measuring apparatus of claim 2, the control unit is configured so that the potential between the reference electrode and the working electrode becomes a natural potential between the first sweep and the second sweep. By applying a predetermined voltage between the counter electrode and the counter electrode for a predetermined time, the ion state in the vicinity of the working electrode surface that has become unbalanced by the first sweep can be made as close as possible to the equilibrium at the start of the second sweep. Therefore, an advantageous effect that highly accurate acidity measurement with reproducibility can be performed is obtained.
[0085]
According to the acidity measuring apparatus according to claim 3, in the acidity measuring apparatus according to claim 1, the control unit sets the potential of the working electrode with respect to the potential of the reference electrode to NV (natural potential) in the first sweep. To NV-1V, then from NV-1V to NV + 1V, and further from NV + 1V to NV at a predetermined speed, the working electrode surface is electrically stabilized but the working electrode surface is physically separated. Sweeping with no potential, the deterioration of the working electrode can be suppressed, and an advantageous effect that the acidity measurement with high accuracy can be easily performed is obtained.
[Brief description of the drawings]
FIG. 1 is a perspective view showing an acidity measuring apparatus according to Embodiment 1 of the present invention.
2 is a perspective view showing a state in which a slide-type drawer lid on which a measurement container is placed is opened in the acidity measuring apparatus of FIG.
3 is a cross-sectional view showing a measurement container in the acidity measurement apparatus of FIG.
FIG. 4 is a graph showing the current-potential relationship of voltammetric acidity measurement of a coexisting electrolyte mixed with a benzoquinone derivative.
FIG. 5 is a graph showing the relationship between the acidity of the acidity measuring device and the reduction current.
FIG. 6 is a graph showing an example of an electrode potential sweep waveform when the working electrode is electrically stabilized.
FIG. 7 is a graph showing an example of an electrode potential sweep waveform when measuring the acidity of the working electrode
8 is a block diagram showing a control system of the acidity measuring apparatus of FIG.
FIG. 9 is a graph showing voltage waveforms of the working electrode and the reference electrode when the working electrode is electrically stabilized.
FIG. 10 is a graph showing voltage waveforms of the working electrode and the reference electrode during acidity measurement.
FIG. 11 is a graph showing a conventional current-potential relationship of voltammetric acidity measurement of a measurement electrolyte coexisting with a naphthoquinone derivative.
FIG. 12 is a circuit diagram showing a potentiostat.
FIG. 13 is a graph showing two voltammetric current waveforms with different working electrode storage states;
[Explanation of symbols]
1 Drawer lid
2 LCD
3 Start / Stop button
4 Power button
5 Body
6 Measuring container
7 Working electrode
8 Counter electrode
9 Reference electrode
10 Coexisting electrolyte
11 Solution storage
12 Container cover
21 Control unit
22 Reference electrode voltage control circuit
23 Current detection circuit
24 Working electrode voltage control circuit
25 Operation unit
26 Display section

Claims (3)

A working electrode immersed in a coexisting electrolytic solution in which an electrolytic solution and an acid-containing measured solution are mixed, a counter electrode, a reference electrode, and a control unit that sweeps the potential of the working electrode with respect to the potential of the reference electrode An acidity measuring device,
The control unit is capable of electrically stabilizing the working electrode, and has a first sweep speed within a potential range in which a new measurement solvent penetrates the working electrode active surface and does not cause physical separation of the working electrode surface itself. The acidity measurement apparatus is characterized in that the first time sweep is performed and the second sweep speed is changed to the second sweep speed and the second sweep is performed in one direction within a predetermined potential range to perform acidity measurement. The control unit is arranged between the working electrode and the counter electrode so that the potential of the comparison electrode and the working electrode becomes a natural potential between the first sweep and the second sweep. The acidity measuring apparatus according to claim 1, wherein a predetermined voltage is applied for a predetermined time. In the first sweep, the control unit sets the potential of the working electrode with respect to the potential of the comparison electrode from NV (natural potential) to NV-1V, then from NV-1V to NV + 1V, and further from NV + 1V to NV. 2. The acidity measuring apparatus according to claim 1, wherein the acidity measuring apparatus sweeps at a predetermined speed.

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