JPH1183800A - Method and apparatus for measuring acidity - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method and an apparatus for measuring an acidity whereby the acidity of a sample to be measured can be measured highly accurately. SOLUTION: This measuring apparatus includes the coexisting electrolytic solution of the mixture of an electrolyte and a sample to be measured which contains acid, a measurement container 8 equipped with an acting electrode 10 soaked in the coexisting electrolytic solution, an opposite electrode 9 and a comparison electrode part 11, a controller 16 which carries out a voltammetric measurement to the coexisting electrolytic solution and measures a peak current value of a reduction prewave of a current running in the coexisting electrolytic solution, a memory part 27 which measures a plurality of standard reagents of known acidities by voltammetry and stores data, an operation process part 28 which calculates a proportional constant of a relationship between the acidity of the standard reagent and the peak current value from the stored data, and an acidity calculation part 26 which calculates an acidity of the sample to be measured from the peak current value of the reduction prewave of the current running in the coexisting electrolytic solution and the proportional constant.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention measures the acidity of free fatty acids contained in edible oils, citric acid, malic acid, tartaric acid contained in fruit drinks, acids contained in alcoholic drinks, and chlorogenic acid in coffee. The present invention relates to a method and an apparatus for measuring acidity.

[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. Recently, foods with low acidity tend to be exclusively enjoyed.

[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, temperature, humidity, and light have a large effect on food deterioration. Considering that the acidity changes greatly in the early stages of deterioration, the measurement of the acid value that directly measures the acidity deteriorates. Suitable for making a judgment,
Usually, this is often 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.

By the way, as for the measurement of the oxidation of fats and oils, there is a method of measuring fatty acids by voltammetry instead of the 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. 9 shows data measured by this method.
Here, FIG. 9 is a graph showing the relationship between the current measured for the acidity measurement by voltammetry and the conventional electrolytic solution in which a naphthoquinone derivative coexists.
It is a graph which shows the relationship of an electric potential.

In FIG. 9, the horizontal axis represents the potential of the working electrode with respect to the comparison electrode when silver-silver chloride was used as the comparison electrode and glassy carbon of φ3 was used as the working electrode, and the vertical axis represents the value of the current flowing through the counter electrode at this time. Shown respectively. However, the current value varies depending on conditions such as the surface area of the working electrode and the 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. 9, A is a pre-peak indicating a pre-reduction wave proportional to the acid concentration, and C is a main peak of the naphthoquinone derivative. The acidity can be quantified by utilizing the fact that the pre-peak current value is proportional to the acid concentration.

Incidentally, in order to measure the acidity of fatty acids by the method disclosed in Japanese Patent Application Laid-Open No. 5-264503, nitrogen gas or the like must be supplied to the coexisting electrolyte to remove dissolved oxygen in the coexisting electrolyte. No. The data indicated by the solid line in FIG. 9 is obtained without dissolved oxygen. If the dissolved oxygen is not removed, a current for reducing the dissolved oxygen flows, and it becomes difficult to determine the magnitude of the current value of the pre-reduction wave. The broken line shown in FIG. 9 shows the reduction waveform when oxygen is not removed, but the pre-peak is hardly discriminated. Therefore, the reason will be described.

FIG. 10 is a graph showing a pre-peak waveform when measuring the acidity by voltammetry of a conventional electrolytic solution in which a naphthoquinone derivative coexists. FIG. FIG. 1 is a graph showing the main peak waveform of FIG.
2 is a graph showing a dissolved oxygen reduction waveform at the time of measuring the acidity by voltammetry of a measurement electrolyte solution in which a conventional naphthoquinone derivative coexists.

When voltammetry is performed after removing oxygen,
It is considered that the pre-peak waveform clearly appears as a synthesis of the pre-peak waveform and the main peak waveform, but when deoxidization is not performed, the pre-peak waveform, the main peak waveform, and the dissolved oxygen reduction waveform are synthesized, and the pre-peak becomes unclear. . As described above, it is difficult to measure with the conventional method unless dissolved oxygen is removed.If dissolved oxygen is to be removed, a gas cylinder must be installed and supplied to the acidity measuring device, and the device becomes large. This has practically prevented the practical use of this measuring method.

[0023]

As described above,
In the conventional acidity measuring apparatus, since the neutralization titration method is used, the measurer judges the color change by the phenolphthalein indicator and determines the end point of the titration. For this reason, the end point may vary depending on the measurer, and the acidity may vary depending on the measurer.

In addition, for example, when the color of the sample is deep, such as oil fried in a large amount, or when the material itself is colored, such as juice or wine, phenolphthalein near the end point of titration is used. There was a problem that the change in color could not be accurately grasped, and the end point was incorrectly read and the measured value fluctuated.

In the technique disclosed in Japanese Patent Application Laid-Open No. 5-264503, when a naphthoquinone derivative is used without deoxygenation in order to make the apparatus compact, as shown by the broken line in FIG. An electric current is generated due to the reduction of oxygen, and overlaps with the electric current value of the acid originally intended to be measured.
There has been a problem that measured values vary depending on the amount of dissolved oxygen, and reliability is lacking. Therefore, if a gas cylinder is installed to perform oxygen removal, it becomes large and difficult to handle, and practically difficult to put into practical use.

Further, when measured by the technique disclosed in JP-A-5-264503, the measurement solution adheres to each of the working electrode, the counter electrode, and the reference electrode. Adhere to the surface of the electrode, and the measurement results vary. Also, when considered as a measuring device, the measured value changes when the measuring device is different due to the variation in the surface area of the working electrode. Therefore, although this is an excellent technique in principle, there is a problem in that the measured value varies and lacks reliability when used as a measuring device.

Therefore, the present invention solves such a problem in the related art, and can perform measurement without deoxygenation, and is compact, easy to operate, and can measure acidity with high accuracy. An object of the present invention is to provide an acidity measuring device.

Another object of the present invention is to provide a method for measuring the acidity of a sample to be measured with high accuracy.

[0029]

In order to solve this problem, an acidity measuring method according to the present invention provides a quinone derivative-containing electrolytic solution and an acid-containing electrolytic solution in which a voltammetric current waveform appears shifted from a dissolved oxygen reduction current waveform. Prepare a coexisting electrolyte mixed with the sample to be measured, measure the coexisting electrolyte by voltammetry, measure the peak current value of the reduction pre-wave of the current flowing through the coexisting electrolyte, Prepare a standard reagent of the above, measure by the voltammetry of the standard reagent, calculate the proportional constant in the relationship between the acidity and the peak current value, and calculate the peak current value and the proportional constant of the reduction pre-wave of the current flowing through the co-existing electrolyte. Is used to calculate the acidity of the sample to be measured.

Further, the acidity measuring apparatus of the present invention is characterized in that a voltammetric current waveform shifts from a dissolved oxygen reduction current waveform, and the quinone derivative-containing electrolytic solution and the acid-containing sample to be measured are mixed. A working electrode immersed in the co-existing electrolyte, a measurement container having a counter electrode and a reference electrode, and voltammetric measurement of the co-existing electrolyte, to determine the peak current value of the reduction pre-wave of the current flowing through the co-existing electrolyte A control unit for measuring, a plurality of standard reagents whose acidity is known are measured by voltammetry, a storage unit for storing each data, and a relation between the acidity and the peak current value in the standard reagent from the stored data. An arithmetic processing unit for calculating a proportional constant, and an acidity for calculating the acidity of the sample to be measured from the peak current value of the reduction pre-wave of the current flowing through the coexisting electrolyte and the proportional constant It is obtained by a detecting section.

As a result, the measurement can be performed without oxygen removal, the operation is compact and easy, and the acidity of the sample to be measured can be measured with high accuracy.

[0032]

BEST MODE FOR CARRYING OUT THE INVENTION The invention according to claim 1 of the present invention is characterized in that a quinone derivative-containing electrolytic solution in which a voltammetric current waveform shifts from a dissolved oxygen reduction current waveform and a sample containing an acid are mixed. Prepared coexisting electrolyte solution,
Measure the co-existing electrolyte by voltammetry, measure the peak current value of the reduction pre-wave of the current flowing through the co-existing electrolyte, prepare a plurality of standard reagents with known acidity, and measure by voltammetry of the standard reagent. An acidity measurement method that calculates a proportional constant in the relationship between the acidity and the peak current value, and calculates the acidity of the sample to be measured by the peak current value and the proportional constant of the reduction prewave of the current flowing through the coexisting electrolyte, The effect of reducing the dissolved oxygen can be eliminated and the acidity of the sample to be measured can be measured with high accuracy.

[0033] The invention described in claim 2 of the present invention provides:
A voltammetric current waveform is shifted from a dissolved oxygen reduction current waveform, and a quinone derivative-containing electrolytic solution and an acid-containing sample to be measured are mixed together, and a working electrode and a counter electrode immersed in the coexisting electrolytic solution are mixed. And a measurement container having a comparison electrode part, and a control unit that performs voltammetry measurement of the co-existing electrolyte and measures a peak current value of a reduction pre-wave of a current flowing through the co-existing electrolyte, and a plurality of units whose acidity is known. A storage unit that measures the standard reagent by voltammetry and stores each data, an arithmetic processing unit that calculates a proportional constant in the relationship between the acidity and the peak current value in the standard reagent from the stored data, and a coexisting electrolyte. An acidity calculation unit for calculating the acidity of the sample to be measured based on the peak current value of the pre-reduction wave of the current flowing through and the proportional constant, Reducing effect can be cut off, and it is possible to measure the acidity of the sample to be measured with high accuracy, also,
This has the effect that the structure is simple and the device is compact.

Hereinafter, an embodiment of the present invention will be described with reference to FIG.
9 to FIG. 13, FIG. 13 and FIG. In all of the drawings for describing the embodiments, the same members are denoted by the same reference numerals, and redundant description is omitted.

FIG. 1 is an external perspective view showing an acidity measuring apparatus according to one 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 apparatus shown in FIG. 1, and FIG. FIG. 4 is a perspective view showing a measuring container in the acidity measuring device, FIG. 4 is an explanatory diagram showing a comparison electrode part in the acidity measuring device in FIG. 1 in detail, and FIG. 5 is a block diagram showing a control system of the acidity measuring device in the present embodiment. 6 is a graph showing the output voltage from the operational amplifier of the control system of the acidity measuring device according to the present embodiment, FIG. 7 is a graph showing the output voltage from the integrating circuit of the control system of the acidity measuring device according to the present embodiment, and FIG. FIG. 9 is a graph showing the relationship between the acidity and the reduction current of the acidity measuring device according to the embodiment of the present invention. FIG. 9 is a graph showing the current-potential of the acidity measurement by voltammetry of a sample to be measured in which a naphthoquinone derivative coexists. Graph showing the engagement, Figure 13 is a current of acidity measurement by voltammetry measurements electrolyte coexisting ortho benzoquinone derivative - a graph showing the relationship between potential,
FIG. 14 is a graph showing a current-potential relationship in acidity measurement by voltammetry of a measurement electrolyte solution in which a parabenzoquinone derivative coexists.

In FIG. 1, a main body 1 of the acidity measuring device A is shown.
5 is provided with an upper lid 1 that covers an internal space in which the measurement container 8 (FIGS. 2 and 3) 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 15 is provided with an LCD 3 which is a display means for displaying the measured acidity. Further, a mode switching button 7 for switching between a measurement mode and a mode for calibrating the electrodes is provided. , A start / stop button 5 for starting measurement, and a power button 6 for turning on / off the power of the apparatus. Then, when the release button 2 shown in FIG.
The upper lid 1 is slid as shown in FIG. 4 to expose the set measuring container 8. Note that the measurement container 8 is detachable.

As shown in FIG. 3, a counter electrode 9 and a working electrode are provided on a container cover of a measuring container 8 which contains an electrolytic solution comprising a quinone derivative, an organic solvent and an electrolyte and a coexisting electrolytic solution in which a sample to be measured is mixed. 10 and a comparison electrode unit 11 are 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 voltammetry current waveform appears shifted from the dissolved oxygen reduction current waveform, and the influence of the dissolved oxygen reduction can be cut off. One end of each of the counter electrode 9, the working electrode 10, and the comparison electrode section 11 is protruded to the outside, and the other end is mounted on the measurement container 8 in a state of being immersed in the coexisting electrolyte.

The material of the counter electrode 9 is preferably platinum, graphite, or gold which does not corrode even in the co-existing electrolyte and is chemically stable, but may be stainless steel, aluminum, an alloy thereof or the like which does not corrode. As a material of the working electrode 10, glassy carbon called carbon or glassy carbon or plastic foam called PFC is used.
Carbon sintered at between 2000C and 2000C is suitable.

Next, the comparison electrode section 11 will be described.
As shown in FIG. 4, the comparison electrode unit 11 is
The reference electrode 12 includes a reference electrode 12 protruding into the glass container 11a, an internal liquid 13 contained in the glass container 11a, and a liquid junction 14 forming an end face of the glass container 11a.

Here, the material of the comparison electrode 12 is silver-
Although silver chloride is desirable, saturated calomel, saturated calomel chloride, silver-silver ion, mercury-saturated mercury sulfate and the like may be used.
Note that, for example, a display such as silver-silver chloride indicates that the surface of the comparative electrode 12 made of silver is covered with silver chloride. As a material of the internal liquid 13, a chloride compound such as silver chloride, potassium chloride, sodium chloride, and lithium chloride, acetonitrile, copper sulfate, and other solutions having a buffering action in the oxidation-reduction reaction of the comparative electrode 12 are suitable. In addition, liquid junction 1
Numeral 4 is located between the internal solution 13 and the co-existing electrolyte, and has the function of not passing these solutions but passing electrons or ions, and is made of porous ceramics or porous Vycor glass. ing.

Next, the coexisting electrolyte contained in the measuring container 8 will be described. In the present embodiment, lithium perchlorate is used as the electrolyte. The coexisting electrolyte solution is a solvent containing 65% ethanol and 35% isooctane.
Is mixed to make 10 mL, and 3 mmol of orthobenzoquinone and 50 mmol of lithium perchlorate are dissolved, and the liquid to be measured is mixed with this solvent. Ethanol can easily dissolve the electrolyte, and at the same time 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. However, the thermally degraded oil will not melt unless the isooctane content is 35% or more, so it is necessary to mix isooctane at least 35%. When the degree of thermal deterioration of the oil increases, it is necessary to increase the isooctane content correspondingly. Thus, isooctane is 35
% Or more, the degraded oil can be melted in ethanol, which is a protic organic solvent, and the acidity can be measured without conventional stirring and centrifugation.

The present invention is characterized in that the voltammetric current waveform contains a quinone derivative which appears to shift from the dissolved oxygen reduction current waveform. As this quinone derivative, an orthobenzoquinone derivative or a parabenzoquinone derivative is suitable.

In FIG. 13, the abscissa represents the potential of the working electrode with respect to the reference electrode when silver-silver chloride was used as the reference electrode and plastic-formed carbon of φ2 was used as the working electrode, and the ordinate represents the current flowing through the counter electrode. Is the current value. However, the current value varies depending on conditions such as the surface area of the working electrode and the acid concentration. On the other hand, the voltage value on the horizontal axis is negligible though slightly fluctuating depending on the acid concentration. The solid line shows the voltammetric current waveform of the orthobenzoquinone derivative, and the broken line shows the reduction waveform of dissolved oxygen.

As shown in FIG. 13, the voltammetric current waveform of the orthobenzoquinone derivative having a side chain on the benzene ring appears significantly shifted from the region where the dissolved oxygen reduction waveform appears. According to FIG. 13, the pre-peak waveform exists from around 0 mV to the positive potential side, and is shifted by about 400 mV from the negative potential side where the dissolved oxygen reduction waveform 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.

Here, the solid line is the voltammetric current waveform of the parabenzoquinone derivative, and the dashed line is the reduction waveform of dissolved oxygen.

As shown in FIG. 14, the pre-peak of the voltammetric current waveform of the parabenzoquinone derivative appears slightly shifted to the region where the dissolved oxygen reduction waveform appears. According to FIG. 14, the pre-peak waveform is approximately 200 points from the negative potential side where the dissolved oxygen reduction waveform exists.
It is shifted by a width of mV. At the position of this peak, the effect of the reduction of dissolved oxygen is slightly observed, but the measurement of the acid is hardly affected.

As described above, since the pre-peak value can be measured in a region where the influence of dissolved oxygen is small, even if the dissolved oxygen is not removed in advance, the effect of dissolved oxygen is negligible. Can be measured.

Now, the operation of the apparatus for operating the acidity measuring apparatus according to the present embodiment will be described.

First, 0.5 g of the deteriorated oil, which is the sample to be measured, is mixed with 10 mL of the above-mentioned solvent and stirred, and the mixture is placed in the measuring container 8. Next, the measurement cover to which the counter electrode 9, the working electrode 10, and the comparison electrode unit 11 are attached is attached to the measurement container 8, and this is set in the acidity measuring device A, and the upper lid 1 is closed to be in a measurable state.

When the measurement is started by pressing the power button 6 and the start / stop button 5, a controller (control unit) 16 described later changes the potential of the working electrode 10 to the potential of the comparison electrode 12 which is a silver-silver chloride electrode. + 500m for
V to -300 mV (preferably +200 mV to -200
mV), a voltage is applied between the working electrode 10 and the counter electrode 9 so as to sweep at a sweep speed of 3 to 10 mV / s. The range of +500 mV to -300 mV is a region where there is almost no effect of dissolved oxygen, and in which the pre-peak value can be accurately measured. When a predetermined potential difference is swept at a sweep speed of 3 to 20 mV / s (preferably 3 to 10 mV), a stable voltammogram as shown in FIG. 9 can be obtained as described later.

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. However, even if there is a shift + 500mV ~
If it is set in the range of -300 mV, the acidity of any concentration can be measured.

Next, the control section of the acidity measuring apparatus A of the present embodiment will be described. As shown in FIG.
A start / stop switch 5 'operated by the stop button 5, a power switch 6' operated when the power button 6 is pressed, and a mode switch button 7 for switching between the measurement mode and the mode for calibrating the electrode are operated. A mode changeover switch 7 'is connected to the controller 16, which is composed of a microcomputer or the like. The controller 16 exchanges predetermined data to calculate the acidity of the liquid to be measured, and the LCD 3 which displays the calculated acidity. Is connected. Further, an oscillator 17 that oscillates at a predetermined cycle, a frequency dividing circuit 18 that generates a clock from a signal of the oscillator 17, and a timer (time measuring means) 19 that measures time with the clock of the frequency dividing circuit 18 are connected. Further, a storage unit 27 for storing measurement data and an arithmetic processing unit 28 for calculating a predetermined value from the measurement data stored in the storage unit 27 are connected. From the output side of the controller 16 toward the input side, the D / A converter 20, the operational amplifier 21, the monitoring circuit 2
2, resistor 23, differential amplifier 24, A / D converter 2
5 are sequentially electrically connected.

According to such a control unit, when the power button 6 in FIG. 1 is pressed, the LCD 3 becomes operable. Next, when the start / stop button 5 is pressed, the controller 16 generates a clock internally by the frequency dividing circuit 18 based on the signal generated by the oscillator 17, counts the clock, and the timer 19 starts counting time. The timer 19 measures time in units of one second.

The controller 16 is synchronized with the timer 19.
Sends a digital signal (pulse) of a predetermined voltage to the D / A converter 20. The D / A converter 20 converts the digital signal into an analog signal and outputs the analog signal to the operational amplifier 21.

As shown in FIG. 6, when time is plotted on the horizontal axis and voltage is plotted on the vertical axis, when sweeping at a voltage of 5 mV / s, the time is measured as 1 second, 2 seconds, 3 seconds,. Every time the voltage changes, the voltage changes stepwise by 5 mV, 10 mV, 15 mV,... Then, such a signal output from the operational amplifier 21 is integrated by passing through an RC integration circuit, becomes an analog signal shown in FIG. 7, and is input to the monitoring circuit 22.

Here, the controller 16 controls the working electrode 10
A voltage is applied between the working electrode 10 and the counter electrode 9 while monitoring the potential difference between the reference electrode 12 and the reference electrode 12.
At this time, if the potential difference between the working electrode 10 and the reference electrode 12 is swept at about 20 mV / s or more, a stable current waveform cannot be obtained because the potential sweeping speed is faster than the electrode reacting speed. . Conversely, the sweep speed is set to 3 mV / s
If it is less than the above value, an excessive reaction occurs on the electrode surface, and a stable current waveform cannot be obtained. Therefore, it is necessary to make the potential sweep speed 3 to 20 mV / s.

In the monitoring circuit 22, the voltage of the counter electrode 9 on the output terminal side is controlled in accordance with an analog signal by using the imaginary short circuit of the operational amplifier constituting the monitoring circuit. Make it the same as the analog signal. As a result, the potential difference between the comparison electrode 12 and the working electrode 10 falls within a predetermined range of +500 mV to -300 mV. On the other hand, the current flowing through the counter electrode 9 is converted into a voltage by passing the voltage between both ends of the resistor 23 through the differential amplifier 24, and is converted from an analog signal to a digital signal via the A / D converter 25, and is sent to the controller 16. Is entered.

Here, as shown in FIG. 7, the controller 16 compares the input digital signal with the voltage swept at a predetermined sweep speed, thereby obtaining the pre-peak signal shown in FIG. 9A. Is detected. Then, the acidity of the liquid to be measured is calculated by the acidity calculator 26 based on the pre-peak value of the current, and the value is displayed on the LCD 3.

Here, a calibration method and an acidity calculation method of the acidity measuring apparatus according to the present embodiment will be described.

In FIG. 8, the acidity of the juice is calculated by converting the amount of NaOH necessary for neutralizing the acid contained in the juice into the amount of citric acid (in the case of mandarin orange) and expressing it as the number of grams in 100 g of juice. I have. The acidity of edible oil is expressed in mg of KOH required to neutralize free fatty acids contained in 1 g of a sample. 9, the voltage E on the horizontal axis is the potential difference between the working electrode 10 and the comparison electrode 12, and
The current I on the vertical axis is the current flowing through the counter electrode 9.

The present inventors measured several kinds of samples in which the concentration of the acid mixed into the liquid to be measured was changed by voltammetry, and determined the current value I giving the pre-peak value indicated by A in the waveform of FIG. It was found that when a calibration curve was created, a proportional relationship as shown in FIG. 8 was obtained.

FIG. 8 was prepared by measuring the acid contained in the fruit juice and the free fatty acid contained in the edible oil by voltammetry, and a very good proportional relationship was obtained. That is, the prepeak current value I and the acidity θ
= Kθ + C (K: proportional constant, C: constant) holds. Therefore, at least 2 for each type of electrolyte and liquid to be measured.
By measuring point data, a highly accurate calibration curve for acidity measurement can be drawn.

Therefore, in the acidity measuring apparatus according to the present embodiment, the mode switching button 7 shown in FIG. 1 is pressed to set a calibration mode for calibrating the electrodes. Next, a plurality of standard reagents whose acidity is known in advance, for example, acidity 1, 3 are measured by voltammetry, and the data is stored in the storage unit 27. Then, the arithmetic processing unit 28 calculates a proportional constant K in this data from the relationship between the acidity θ and the current value I at the pre-peak A, and stores it in the storage unit 27.

In this way, the acidity calculating section 26 calculates the acidity θ of the sample to be measured based on the measured pre-peak current value I of the sample to be measured and the proportional constant K stored in the storage section 27. Is done.

This makes it possible to measure the acidity of the sample to be measured with high accuracy.

[0067]

As described above, according to the present invention, the peak current value of the reduction pre-wave of the current flowing through the coexisting electrolyte and the proportional constant in the relationship between the acidity of the standard reagent and the peak current value are measured. Since the acidity of the sample is calculated,
The acidity is measured automatically with a simple operation. This allows
An effective effect that the acidity of the sample to be measured can be measured with high accuracy is obtained.

Further, since there is no need to add a complicated mechanism, an effective effect that the structure is simple and the apparatus is compact can be obtained.

Further, by measuring and calibrating the standard reagent on a regular basis, there is obtained an effective effect that contamination of the electrode surface due to aging and variation in the measurement result due to a difference in the surface area of the working electrode can be suppressed. Can be

[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 perspective view showing a measuring container in the acidity measuring device of FIG. 1;

FIG. 4 is an explanatory view showing in detail a comparative electrode part in the acidity measuring device of FIG.

FIG. 5 is a block diagram showing a control system of the acidity measuring device according to the present embodiment.

FIG. 6 is a graph showing an output voltage from an operational amplifier of a control system of the acidity measuring device according to the present embodiment.

FIG. 7 is a graph showing an output voltage from an integration circuit of a control system of the acidity measuring device according to the present embodiment.

FIG. 8 is a graph showing the relationship between the acidity and the reduction current of the acidity measuring device according to the embodiment of the present invention.

FIG. 9 is a graph showing the current-potential relationship in the measurement of acidity by voltammetry of a measurement electrolyte solution in which a conventional naphthoquinone derivative coexists.

FIG. 10 is a graph showing a pre-peak waveform at the time of measuring the acidity by voltammetry of a measurement electrolyte solution in which a conventional naphthoquinone derivative coexists.

FIG. 11 is a graph showing the present peak waveform at the time of measuring the acidity by voltammetry of a measurement electrolyte solution in which a conventional naphthoquinone derivative coexists.

FIG. 12 is a graph showing a dissolved oxygen reduction waveform when measuring the acidity by voltammetry of a measurement electrolyte solution in which a conventional naphthoquinone derivative coexists.

FIG. 13 is a graph showing a current-potential relationship in acidity measurement by voltammetry of a measurement electrolyte solution in which an orthobenzoquinone derivative coexists.

FIG. 14 is a graph showing a current-potential relationship in acidity measurement by voltammetry of a measurement electrolyte solution in which a parabenzoquinone derivative coexists.

[Explanation of symbols]

 Reference Signs List 8 Measurement container 9 Counter electrode 10 Working electrode 11 Reference electrode unit 16 Controller (control unit) 26 Acidity calculation unit 27 Storage unit 28 Operation processing unit

Claims (2)

[Claims] 1. A co-existing electrolytic solution in which a quinone derivative-containing electrolytic solution in which a voltammetric current waveform shifts from a dissolved oxygen reducing current waveform and an acid-containing sample to be measured is prepared, and Measure by voltammetry,
Measure the peak current value of the pre-reduction wave of the current flowing through the coexisting electrolyte, prepare a plurality of standard reagents having a known acidity, perform measurement by voltammetry of the standard reagent, and determine the difference between the acidity and the peak current value. An acidity measurement method comprising: calculating a proportional constant in the relationship; and calculating an acidity of the sample to be measured from a peak current value of a reduction pre-wave of a current flowing through the coexisting electrolyte and the proportional constant. 2. A co-existing electrolytic solution in which a quinone derivative-containing electrolytic solution and an acid-containing sample to be measured whose voltammetric current waveform shifts from a dissolved oxygen reducing current waveform are mixed, and immersed in the co-existing electrolytic solution. A working container having a working electrode, a counter electrode and a reference electrode portion, and a voltammetric measurement of the coexisting electrolyte is performed,
A control unit that measures a peak current value of a reduction pre-wave of a current flowing through the coexisting electrolyte, a storage unit that measures a plurality of standard reagents having known acidities by voltammetry, and stores respective data, An arithmetic processing unit for calculating a proportional constant in the relationship between the acidity and the peak current value in the standard reagent from the data; and the peak current value of the pre-reduction wave of the current flowing through the coexisting electrolyte and the proportional constant, An acidity measurement device comprising: an acidity calculation unit that calculates the acidity of a measurement sample.