JPH0474959A - Measuring method of acidity of neutral sample - Google Patents

Abstract

PURPOSE:To attain a measuring method of acidity of a neutral sample which enables attainment of a highly precise result in a short time and can be used also for automatic measurement in an in-line manner, by a method wherein a working curve is prepared beforehand, a pH value obtained in regard to an analysis sample is applied thereto and the acidity is read out. CONSTITUTION:After a sample to be measured is taken in a sampling valve 3 from a transfer piping 1 by a pump 2, the sampling valve 3 is inverted, ion- exchange water in a tank 5 is taken partially in the sampling valve 3 by a volume-regulating pump 4, and the sample in the sampling valve 3 is washed away into a measuring tank 6 by this ion-exchange water. A standard solution of sodium hydroxide in a tank 7 is taken partially by a volume-regulating pump 8 and injected into the measuring tank 6 and pH is measured by a pH sensor 10 with the solution agitated by an agitator 9. A measuring signal thus obtained is processed successively in a signal processing device 11, and a value at the time when the pH is maximum is read and converted into acidity by a working curve stored in the signal processing device 11.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for measuring acidity of a neutral sample. Specifically, the present invention provides a method for measuring the acidity of samples that have a buffering capacity and a pH around neutrality, such as raw materials, semi-finished products, or finished products used in the production of foods, feeds, pharmaceuticals, etc. made from natural raw materials or organic materials. It is related to. More specifically, this invention covers amino acid-related substances (including peptides and proteins), nucleic acid-related substances, carboxylic acids (oxyacids, etc.)
This invention relates to a method for measuring the acidity of neutral samples containing other substances with pH*-adjusting ability (including specimens).

(Conventional technology and its issues) The acidity of raw materials, semi-finished products, or finished products (which can also be expressed as the amount of alkali-consuming substances j) often has a large effect on the quality of the product and the condition of a culm. In particular, for foods, feeds, pharmaceuticals, etc. that have a pH near neutrality, minute differences in acidity have a large impact on product quality and process conditions.
The acidity of semi-finished products or finished products must be measured accurately and quickly.

Conventional methods for measuring acidity include, for example, weighing out a certain amount of a sample (or weighing out a certain amount and dissolving it in water), performing neutralization titration with an alkaline standard solution, and directly reading the consumption amount of the alkaline standard solution. is common. In this case, whether or not the neutralization equivalent point (end point) has been reached can be determined by adding an indicator and visually determining the change in color due to the change in pH, by measuring with a colorimeter, or by measuring the pH with a pH electrode. It is used. However, one of the problems with these conventional methods is that the analysis takes a long time, and even in manual analysis as well as measurement using automated analytical equipment, the entire analysis process of one sample (sample weighing, titration, (calculation) takes at least several minutes. Another problem is that it is difficult to accurately determine the neutralization equivalent point and requires a high degree of skill in titration. For example, when phenolphthalein, which is often used in official methods, is used as an indicator, the color change range is DH8
, 0 to 9.8, and when the sample contains a substance with pHI adsorption capacity, the pH change due to alkali addition is slow, so the wide range of this discoloration can be a hindrance to judgment. Become. Also, when measuring the neutralization equivalent point using a pH electrode, good results can be obtained if the sample concentration is relatively dilute and does not contain a substance that has a buffering effect against the addition of alkali. , samples containing natural products and organic compounds that are the subject of analysis in this invention, i.e., pH
[For samples containing substances that contain substances, the change in DH due to alkali addition is slow, making it difficult to obtain good results (highly accurate analytical values), and the accuracy is particularly low for neutral samples. .

By the way, regarding the acidity measurement of an "acidic sample", JP-A-63-16263 discloses a method of measuring the pH after adding a certain amount of alkaline solution and measuring the strength of the acid from the pH value. . This is a pH value that is measured when a certain amount of alkaline solution is added to a standard sample with a desired acidity (target acidity), and the same amount of alkaline solution is also added to the sample to be analyzed. Then, the pH value shown by the sample is measured and compared with the pH value shown by a standard sample having the target acidity to determine whether or not the analysis sample has the target acidity. In addition, as a simple method, the pH after adding an equivalent amount of alkali or acid to the sample is
A method of measuring acidity or alkalinity by measuring is disclosed in JP-A-1-262462. This method utilizes the rapid change in l)H near the equivalence point, and is applicable only to strong acids and strong alkalis that do not contain substances containing pHM binding capacity.

In other words, when this method is applied to a neutral sample containing a substance with pHM absorption capacity, the pH changes only very slowly even when alkaline solution is added, and strong acids are neutralized with strong alkalis. In addition, in such a neutral sample, the pHM-containing substance contained in the sample reacts slowly with the added alkali, so the pH value remains constant. It also has the characteristic that the value of l)H after adding a certain amount of alkali is unstable over time.

In this way, when measuring the acidity of a neutral sample using conventional measurement methods, there are problems such as the long time required for analysis and the difficulty in obtaining good results due to slow pH changes due to the addition of alkali. was there.

This invention was made in view of the above circumstances, and provides a method for measuring acidity of neutral samples that can obtain highly accurate results in a short time and can also be used for in-line automatic measurement. Our aim is to provide the following.

(Means for Solving the Problems) This invention solves the above problems by: (1) adding a certain amount of a standard sample of the same type as the analytical sample whose acidity is to be measured and whose acidity is known, or an aqueous solution thereof; A certain amount of alkaline standard solution is added, and after a certain period of time, the pH average value within a certain period of time or the pHfi high value within a certain period of time after addition of the alkaline standard solution is measured, and the above known acidity and the above pH value are each used as coordinate axes. Create a calibration curve, add an alkaline standard solution to the analytical sample under the same conditions as above, and
H value is measured, and the acidity of the analytical sample is determined from the calibration curve obtained for the quasi-sample and the pH value obtained by measuring the analytical sample. Provide a measurement method.

In addition, this invention provides that a certain amount of alkaline standard solution added to a certain amount of sample or its aqueous solution is 70% of the amount of alkaline standard solution that corresponds to the inflection point of the titration curve when added to a certain amount of sample A preferred embodiment is a range of 130% to 130%.

Hereinafter, the principle and effects of this invention will be explained in detail.

In measuring the acidity of a neutral sample, this invention provides an analysis in which the acidity should be measured without adding an alkaline standard solution to the neutral sample or its aqueous solution up to the neutralization equivalent point and directly reading the consumption amount. A certain amount of alkaline standard solution is added to a certain amount of a neutral standard sample of the same type as the sample with known acidity, and the pH average value within a certain period of time after addition of the alkaline standard solution or within a certain period of time after addition of the alkaline standard solution is calculated. pH at
Measure the I & high values and create a calibration curve in advance with known acidity and pH values as the coordinate axes.For the analysis sample, add an alkaline standard solution and adjust the pH under exactly the same conditions as for the standard sample. The acidity is read by performing measurement and applying the H value obtained for the analytical sample to the above calibration curve.

In the following, explanation will be given using milk (raw milk) as an example of a neutral sample.

When an alkaline standard solution is added to milk, the pH cannot be stabilized after the addition of the alkaline standard solution near the equivalence point. In other words, if you add an alkaline standard solution to or near the equivalence point and observe the change in pH value over time, you will notice that the pH value rises immediately after addition, and then immediately begins to fall.This drop in pH value is quite significant. Figure 1 shows this situation, with pH on the vertical axis and elapsed time (minutes) on the horizontal axis.From the results of an experiment conducted with air shut off, this pH value is It was confirmed that the change in pH was not due to the influence of carbon dioxide gas in the air, so it is presumed that the change was caused by a substance containing pHM contained in the milk itself. This is an important issue in determining the neutralization equivalent point, and this is also the reason why titration requires skill in conventional methods.

Therefore, we focused on the instability of this pH value, that is, the change over time in the pH value after addition of the alkaline standard solution, and as a result of detailed investigation of the fluctuation, we found that a certain range of amounts of alkaline standard were added to the neutral sample. It has been found that when a liquid is added, the change in pH value over time is reproducible. Utilizing this reproducibility, the pH value when an alkaline standard solution is added to the same kind of neutral sample (acidity known) under certain conditions is measured under certain conditions, and calibration is performed using acidity and pH value as coordinate axes. If you create a line, you can add an alkaline standard solution under exactly the same conditions to a neutral sample with unknown acidity, and then calculate the p
Measure the H value and compare the obtained pH value with the above known acidity and p
By applying the H value calibration curve, it is possible to read the acidity of a neutral sample with unknown acidity in a short time and with high accuracy.

Next, test examples regarding the reproducibility of such acidity and pH values are shown below.

Test Example 1 A fixed amount of 1/10 N sodium hydroxide standard solution was added to 50+ r of raw milk, and the pH value of the sample was adjusted to 3 immediately after the addition.
The measurement was continued for 0 seconds, the highest value was taken, and the relationship between the amount (ml) of 1/10 N sodium hydroxide standard solution added and the pH value (maximum value) was investigated.

Measurements were performed 10 times on one type of sample, and the results are shown in FIG. The vertical axis is pH (high M value), and the horizontal axis is the amount (ml) of 1/10 N sodium oxide standard solution added.

As is clear from this Figure 2, the pH (fi
The curve connecting the high values) becomes an addition amount dependent curve with a slight S-shaped curvature.

Such a characteristic curve will not be seen if there is no substance with pt function in the sample, and it is because milk contains substances with pt function such as proteins, fats, sugars, etc. It is believed that there is.

Test Example 2 Next, it was investigated what kind of correlation existed between a plurality of samples of the same type at the inflection point of the above-mentioned S-shaped curve.

9 samples of milk (each sample was prepared using a separate official method [Ministry of Health and Welfare No. 52, December 27, 1951).

"Acidity is determined by the method described in the Ministerial Ordinance on Ingredient Standards for Milk and Dairy Products.The pH of each raw milk is 6.73.
6.72.6.72.6.70.6, 73.6.71.
6, 72, 6.72, and 6.75, with almost no difference), the same measurements as in Test Example 1 were performed.

The resulting measured values were plotted on coordinates with pH on the vertical axis and sodium hydroxide content on the horizontal axis, and an attempt was made to approximate the resulting nine curves to a linear function using the least squares method. However, in both cases, good results were obtained using the cubic equation.

Next, when we calculated the inflection point coordinates of each cubic equation, we found that while the I)H coordinate was almost a constant value, the alkali content coordinate was changing, and as shown in Figure 3, the alkali content coordinate value and A high correlation was observed between acidity and acidity. This means that the amount of alkali required to reach the inflection point is proportional to the acidity.In other words, since the pH coordinate value of the inflection point is constant, it is proportional to the increase in acidity. This means that the titration curve moves in parallel to the right along the alkali content axis.

Figure 4 schematically illustrates this, where the vertical axis is pH, the horizontal axis is the amount of alkali added, and the three curves are samples with different acidities, namely A1 is the sample with the lowest acidity, A1 is the sample with the lowest acidity, and A1 is the sample with the lowest acidity. , A2 is a sample with the highest acidity, and A2 is a sample with an intermediate acidity. Pt, P2, P
> is sample A.

This is the amount of alkali corresponding to the inflection point of A 2 and A s.

Therefore, the pH when a certain amount of alkali is added, which corresponds to the inflection point of a sample with average acidity, will show a change proportional to the horizontal translation of the titration curve of each sample. The pH obtained under such conditions has a good correlation with acidity. FIG. 5 schematically shows this, with the vertical axis representing pH and the horizontal axis representing the amount of alkali added.

A + A 2, A is the same as in FIG.

Test Example 3 Next, in order to examine the scope of application of this correlation, nine types of raw milk (50011) were used, and each acidity (according to the above official method) and each addition amount of 1/10N sodium hydroxide standard solution (0 , 2ml, 4ml, 6ml, 8a] 1.10m
p per 1.12m1.14m1.16m1.18m1)
The correlation with H value was investigated. p! As in Test Examples 1 and 2, the maximum value was determined as 1 value. The results are shown in FIG.

As is clear from this Figure 6, the acidity of the sample and its pH
The correlation with the value becomes good when the amount of alkali added is in the range of 6 ml to 12 ml.

Therefore, applying this result to the pH inflection point mentioned above, we get 6
The range of 01 to 12ml is the amount of 1/10 N sodium hydroxide standard solution added (9ml) that gives the estimated pH value of the inflection point.
This corresponds to 70 to 130% of

In other words, in this invention, the amount of alkaline standard solution to be added to a fixed amount of sample or its aqueous solution is the amount of alkaline standard solution that corresponds to the inflection point of the titration curve when the alkaline standard solution is added to a fixed amount of the sample. The amount should be in the range of 70 to 130% of the amount of

Next, an alkaline standard solution was added to milk in such a range, and a calibration curve was created with its acidity (according to the official method) and pH value as coordinate axes. In other words, 9 types of milk 1
1710N sodium hydroxide standard m200 in 0ml
11 was added, and the maximum pH value was then measured for 30 seconds. As a result, a calibration curve as shown in FIG. 7 was obtained.

In addition, in each of the above test examples, the maximum pH value within a certain period of time after addition of the alkaline standard solution was used as the measure;
After 15 seconds), the pH value was measured every second for 15 seconds (16 measurement points) and the average value was used to investigate the same correlation, and good results were obtained in this case as well.

As described above, in this invention, a calibration curve is created in advance for the same type of standard sample, and based on this, the acidity of the analysis sample is determined. It is possible to easily output the acidity by inputting and processing the information.Such data processing is useful not only when samples are collected in batches for measurement, but also during the manufacturing process. It is also extremely useful for continuously sampling and measuring samples to monitor sample quality.

The present invention will be specifically explained below by showing examples of a measuring device and measurement.

Example work (measuring device and measurement example) The measuring device whose configuration is schematically shown in Figure 8 was installed in the process of receiving raw milk, and the acidity of raw milk brought in by tank truck was continuously measured. .

That is, the milk carried from the tank truck to the silo tank is sampled from the transfer pipe (1) to the sampling valve (3) using the pump (2), and then the sampling valve (3) is reversed and the metering pump ( 4), 5 ml of ion-exchanged water from the tank (5) is dispensed into the sampling valve (3), and the ion-exchanged water washes the sample in the sampling valve (3) into the measurement tank (6). Next, 1720N sodium hydroxide standard solution 4 in tank (7)
00 μm is collected using a metering pump (8), injected into a measurement tank (6), stirred with a stirrer (9), and then added to a pH sensor (
10), the pH was measured. In this case, the pH sensor (1
The signal from 0) is sent to the signal processing device (11) (PC98
01UX (Nippon Electric), the value when the pH reached the maximum was read, and the value was stored in the signal processing device (11) and converted into acidity using a calibration curve. Finally, the measuring tank (6) is drained using the pump (12), and then the metering pump (
4), ion-exchanged water was injected into the measurement tank (6), the measurement tank (6) and the pH sensor (10) were washed, and the measurement of one sample was completed. These operations required approximately 2 minutes. In addition, before measurement, the tank (13) and (14)
Standard Mum was injected into the measurement tank (6) using pumps (15) and (16), respectively, and the pH sensor (10) was calibrated.

Note that the above work is performed on the "raw material", which includes pumps, tanks, sampling valves (3), measurement tanks (6), 1) H sensor (10), etc., except for the signal processing device (11). All measurements were performed automatically using a lactic acid level measuring device (manufactured by Toa Denpa Kogyo).

The acidity of the milk obtained as a result showed no statistically significant difference from the acidity measured separately manually.

Example 2 (Measuring device and measurement example) Using a measuring device whose configuration is schematically shown in FIG. 9, the acidity of raw milk was measured in the receiving process in the same manner as in Example 1.

That is, a certain amount of milk is transported from a tank truck to a silo tank through a transfer pipe (1) and a valve (17), and a metering pump (18) (SJ1211H, manufactured by Atto ■) is used to collect a certain amount of milk, and a mixing valve ( 19).

At the same time, the 1720N standard sodium hydroxide solution in the tank (20) is continuously injected into the mixing valve (19) by the pump (21), and the milk and 1720N sodium hydroxide standard solution are mixed together by the valve (19). They were mixed at a ratio of 25:9.

Next, this liquid mixture is further mixed uniformly with a mixing coil (22), and a pH sensor (24) (H3-6O3 equipped with 5G-80, manufactured by Toa Denpa Kogyo) is connected via a switching valve (23). 1) The H value was measured.

The time from mixing at the mixing valve (19) to measurement was set to exactly 10 seconds, and after continuous measurement for 30 seconds, the valve (17) was switched, the ion-exchanged water in the tank (25) was drawn, and the flow path was opened. One analysis was completed by washing. Note that during this cleaning, the pump (21) was stopped.

The measured values are processed by a signal processing device (26) (PC9801UX, manufactured by NEC Corporation) for all measured values (31 points) for 30 seconds, averaged, and calculated using a calibration curve stored in the signal processing device (26) in advance. Converted to acidity.

The measurement of one sample was completed in about 1 minute. In addition, before measurement, the standard buffer solutions in tanks (27) and (28) are sequentially flowed through the switching valve (29), and 1) the H sensor (
24) was calibrated.

There was no statistically significant difference between the acidity of the milk obtained as a result and the acidity measured separately manually.

Of course, this invention is not limited to the above examples, and it goes without saying that various embodiments are possible with respect to the details of its structure. It is also possible to accurately measure the acidity of neutral samples such as

(Effects of the Invention) As explained in detail above, the method for measuring the acidity of a neutral sample of the present invention allows the acidity of a nearly neutral sample containing a substance containing pHW capacity to be measured quickly and with high accuracy. It becomes possible. Moreover, the measurement can be easily automated.

[Brief explanation of the drawing]

FIG. 1 is a diagram illustrating the change in pH value over time when an alkaline standard solution is added to milk up to its equivalence point. FIG. 2 is a correlation diagram between the maximum pH value and the standard amount of sodium hydroxide added when the volume of the standard sodium hydroxide solution is added to milk. FIG. 3 is a correlation diagram between the acidity at the inflection point and the amount of alkaline standard solution added. Figure 4 shows a schematic diagram showing that the titration curve moves in parallel along the alkali content axis in proportion to the acidity of the sample. FIG. 2 is a schematic diagram showing the relationship between the amount of alkali and 11H when a certain amount of alkali is added. FIG. 6 shows the correlation coefficient between the acidity of raw milk and the I)H value when the volume of the standard sodium hydroxide solution is added to the milk. FIG. 7 is an example of a calibration curve for milk. FIG. 8 and FIG. 9 are schematic diagrams each illustrating an apparatus used in the measuring method of the present invention. 1... Transfer piping 2.12.21... Pump 3... Sampling valve 4.8.15.16.18... Metering pump 5.7.1
3.14.20.25.27.28...tank 6...measuring tank 9...stirrer 0.24...pH sensor 1.26...signal processing device 7.23.29...・Switching valve 9...Mixing valve 2...Mixing coil Fig. 1

Claims (2)

[Claims] (1) [1] A certain amount of a standard sample of the same type as the analytical sample whose acidity is to be measured and whose acidity is known, or an aqueous solution thereof,
A certain amount of alkaline standard solution is added, and after a certain period of time, the average pH value within a certain period of time or the maximum pH value within a certain period of time after addition of the alkaline standard solution is measured, and the above-mentioned known acidity and above-mentioned pH value are plotted on the coordinate axes. Create a calibration curve with
[2] For the analytical sample, add an alkaline standard solution under the same conditions as above and measure the pH value, [3] The calibration curve obtained for the standard sample and the pH value obtained by measuring for the analytical sample. A method for measuring acidity of a neutral sample, characterized in that the acidity of the analysis sample is determined from the pH value. (2) A certain amount of alkaline standard solution added to a certain amount of sample or its aqueous solution corresponds to the inflection point of the titration curve when added to a certain amount of sample.
Claim (1) characterized in that it is in the range of 0-130%.
) method for measuring acidity of neutral samples.