Features of the results of conductometric studies of apple juices

. By the conductometric method, the dependence of the specific electrical conductivity (χ) of five apple juice samples on the volume fraction (φ) of apple juice in a solution with distilled water. The dependence χ in the range φ from 0 to 1 acquires a power-law character (the correlation coefficient is close to 1), since a significant part of the mineral composition of apple juices consists of various organic acids. The technique fully complies with the principles of "green" chemistry, since the "waste" is apple juice diluted with distilled water.


Introduction
Currently, the consumption of juices of domestic production is growing rapidly. The increase in the number of producers and the variety of juices, along with the weakening of the national quality control system, has led to a decrease in the quality and safety of juice products. Therefore, the falsification of natural juices and juice-containing beverages is a serious problem. It is no coincidence that a lot of documents are devoted to the identification of juice products, among which GOST 34460-2018 should be highlighted [1].
In this document, the identification of juice products is carried out in accordance with the approved list of physico-chemical indicators, the characteristics of which are determined by various analytical methods. Such indicators include: mass fraction of soluble solids in juices; mass fraction of sediment; volume fraction of pulp; mass fraction of carbon dioxide; mass fraction of preservatives; mass fraction of dyes; mass fraction of sweeteners [1,2]. There is no conductometry among the proposed analytical methods.
One of the most common forms of adulteration of natural juices is diluting them with water. To identify and detect such falsification of juice products, methods are recommended, each of which has its advantages and disadvantages. The most environmentally friendly method for assessing changes in the mineral composition of aqueous solutions is the conductometric method [3,4], the use of which has not found wide application in the chemical analysis of juice products.
The aim of the work is to conduct conductometric studies of the mineral composition of various samples of direct-pressed apple juice and to establish quantitative characteristics of the dependence of the specific electrical conductivity (χ) of direct-pressed apple juice on the degree of its dilution with distilled water.
The scientific novelty of the work is that the nonlinear dependences of apple juice on the degree of their dilution with distilled water are established.
Practical value -the methodology for identifying apple juice when it is diluted by the conductometric method has been expanded. According to literature data, carbohydrates predominate in the composition of apple juice, the content of which can be more than 100 g in 1 liter of juice [6]. At the same time, potassium ions predominate among the cations -the content of which is about 1 g / l. Based on the above, it can be concluded that the mineral composition of apple juice is represented by a wide range of organic compounds with a small part of inorganic substances. All this can significantly affect the dependence of juices when they are diluted with distilled water. Therefore, two variants of the study of the dependence of the investigated solution on the volume fraction of juice were chosen. Option 1. The dependence of the model solution on the volume fraction of the studied juice in the range φ from 0 to 50% was investigated. 40 ml of distilled water was measured. Next, 40 ml of the studied juice was added to this volume from a burette (pipette) in portions of 1.0 or 5.0 ml and the χ of the resulting solution was measured after 1 minute of intensive mixing of the solution using a magnetic stirrer.

Methodology and objects of research
Option 2. The dependence of the model solution on the volume fraction of the studied juice in the range φ from 100 to 50% was investigated. 40 ml aliquots of the studied juice were measured. 40 ml of distilled water from a burette (pipette) was added to this aliquot in portions of 1.0 or 5.0 ml and further studies are carried out similarly to option 1.
The measurements were carried out using a conductometer-an Anion 4100 series laboratory liquid analyzer model A4155. The operability of the measuring channels was checked according to the operating manual [7] during the execution of the current definitions of the comparison and analysis of the measurement results of distilled water and standard KCl solutions.
Standard solutions of KCl (GOST 4234-77, h.h.) were prepared from fixanal. Distilled water was used according to GOST 6709-72 [8] with χ 5·mcm/cm. Table 1 shows the results of measuring χ of the studied apple juice (sample No. 1) when it is diluted with distilled water.The titration sequence of the studied juice with distilled water was carried out according to option 2. As follows from the obtained statistically processed experimental data, the average value of χ for sample No. 1 decreases from 2254 mcm/cm to 1461 mcm/cm when the study is diluted with distilled water twice. It should be noted that the standard deviation (Sr) at n = 5 does not exceed 1%. This indicates a rather small error of the conductometric method, which is consistent with the literature data.  Table 2. Everyone knows that because of the fermentation process of various juices, including apple, wine drinks are formed. Earlier conductometric studies have shown that the dependence of χ on the volume fraction of a wine drink when it is diluted with distilled water has a linear character with a value of R 2 of at least 0.99.

Results and their discussion
Using the data in Tables 1 and 2 for each of the five studied apple juices, a graph of the dependence of the studied solution on its volume fraction was constructed. An example of such a graph for sample No. 1 is shown in Figure 1. As can be seen from the data presented in the figure, the dependence of the investigated solution (sample No. 1) on its φ in solution is linear, while the correlation coefficient is close to 1. The tangent of the slope angle of this dependence is conventionally fashionable to call the identification coefficient [4,5].
For samples No. 1, No. 2, No. 3, No. 4, No. 5 similar dependences were constructed, which also have a linear character in the investigated range φ. Table 3 summarizes the results of statistical data processing of the dependence χ (mcm/cm) on the juice φ in the test solution from φ=1.0 to φ= 0.5. The Sr of the results obtained ranges from 0.3 to 1.2%, which confirms the literature data on a small error (ranging from 1-2%) of conductometric measurements. The division of the studied juices into two groups, which differ significantly in the value of K ind by more than 300 microns / cm, indicates the possible use of K ind to assess the mineral composition of juices, which, as a rule, is individual depending on the geomorphological and climatic conditions of the area where the apple harvest was formed. Figure 2 shows all five dependences of the studied solutions on the apple juice in solution. The obtained dependencies are linear in nature, which is confirmed by all five values of the correlation coefficient, which are very close to 1. It should be noted that all five obtained dependencies, although linear in nature, do not pass through the origin. This feature may be a consequence of the influence of the ionic force on the values of. Table 4 shows the results of measuring distilled water when apple juice is injected into it (sample No. 1). The titration sequence with distilled water with the juice under study was carried out according to option 1.  0.00 0.02 0.07 0.11 0.20 0.27 0. As follows from the experimental data obtained, after statistical processing, the average value of χ for sample No. 1 increases from 29 mcm/cm to 1454 mcm/cm when apple juice is added to distilled water.At the same time, it should be noted that Sr at n = 5 with the first two additions of juice fluctuates at the level of 3-4%, and with subsequent additions of apple juice does not exceed 1%. This indicates a rather small error of the conductometric method, which is consistent with the literature data.

Aliquot Volume fraction of the test solution (φ, %) and χ of the solution (mcm/cm)
For samples No. 2,No. 3,No. 4,No. 5 The results of similar studies from their injected apple juice solution (25 o C) are presented in Table 5. Table 5. Dependence of χ distilled water (analyzed solution) on φ injected apple juice solution No. 2 (25 о С).

Aliquot
Volume fraction of the test solution (φ, %) and χ of the solution (mcm/cm) 0.00 0.02 0.07 0.11 0.20 0.27 0.33 0.38 0.43 0. As follows from the data presented, for sample No. 2, the value of χ when diluting the test solution is doubled from 37 mcm/cm to 1254 mcm/ cm, for sample No. 3 -from 29 mcm/cm to 1546 mcm/cm, for sample No. 4 -from 4 mcm/cm to 1439 mcm/cm, and for sample No. 5 -from 3 mcm/cm to 1138 mcm/cm. Conditionally, according to the initial value of χ, the studied solutions can be divided into two groups. The first group (sample No. 2  Using the data in Tables 4 and 5 for each of the five studied apple juices, a graph of the dependence of the studied solution on its φ was constructed. An example of such a graph for sample No. 1 is shown in Figure 3. As can be seen from the data presented in the figure, the dependence of the investigated solution (sample No. 1) on its φ in solution is linear, while the correlation coefficient is close to 1.
For samples No. 2,No. 3,No. 4, No. 5 similar dependences were constructed, which also have a linear character in the investigated range φ. Table 6 summarizes the results of statistical processing of the data of the dependence χ (mcm/cm) on the juice φ in the test solution from 0 to 0.5. The Sr of the results obtained ranges from 0.2 to 0.9%, which confirms the literature data on a small error (ranging from 1-2%) of conductometric measurements. The division of the studied juices into two groups, which differ significantly in the value of K ind by more than 400 microns / cm, indicates the possible use of K ind to assess the mineral composition of juices, which, as a rule, is individual depending on the geomorphological and climatic conditions of the area where the apples were harvested. Figure 4 shows all five dependences of the studied solutions on the apple juice in solution. The obtained dependencies are linear in nature, which is confirmed by all five values of the correlation coefficient, which are very close to 1. It should be noted that all five obtained dependencies, although linear in nature, do not pass through the origin. This feature may be a consequence of the influence of the ionic force on the values of.
Thus, considering separately the dependence of the studied solutions of five juice samples on the φ of apple juice in solution in the range from 0 to 0.5 and from 0.5 to 1 shows that this dependence is very close to linear. At the same time, when combining the obtained values and constructing the dependence χ on φ in the range from 0 to 1, it gives grounds to talk about the nonlinearity of this dependence.This can be said on the basis of the correlation values, which fluctuate around 0.97 for all five samples of apple juice studied ( Figure 5). Statistical processing of the dependence χ on φ in the range from 0 to 1 shows that the correlation coefficient has the greatest values in the case of the power dependence χ of the studied solutions on φ. In this case, the correlation values are very close to 1 ( Figure 6).

Conclusions
Thus, based on the results obtained, the following can be noted.
Statistical processing of the obtained results shows that the dependence of the studied solutions of five juice samples on the φ of apple juice in solution in the range from 0 to 0.5 and from 0.5 to 1 is very close to linear (correlation coefficient of at least 0.99).
According to the value of K ind , the studied apple juice samples can be divided into two groups, which differ significantly in the value of K ind by more than 300 microns /cm with a change in φ from 0 to 0.5 and by more than 400 microns / cm with a change in φ from 1 to 0.5. This difference can be used to identify apple juices by the Kind value .
When combining the obtained values and constructing the dependence χ on φ in the range from 0 to 1, the linear nature of the dependence is significantly distorted, since the value of the correlation coefficient decreases to 0.97, and the dependence acquires a powerlaw character (the correlation coefficient is close to 1).
The nonlinear nature of the dependence of the studied solutions on the apple juice can be explained by the fact that a significant part of the mineral composition of apple juice consists of various organic acids, so it is no coincidence that the method of potentiometric titration with an alkali solution is used to estimate the amount of these acids.