RU2384841C1 - Method of authenticating wine - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: coulometric titration of 0.5-1.5 cm³ of wine in 50 cm³ of sodium chloride is carried out with generated OH^ˉ ions. A titration curve is plotted from the titration results and pH at the initial titration point is determined and the number of equivalents of H⁺ ions is calculated. pH discontinuity (equivalence point) is determined and the number of equivalents of H⁺ ions is calculated. The numerical value of the ratio of calculated number of equivalents found and if the numerical value lies in the 0.5±0.1 range, the wine is natural. The number of equivalents of H⁺ ions at the initial titration point and at the equivalence point is calculated using the following formulae: C=10^-pH·V_sol/V_v and C'= I·τ·1000/F·V_v, where V_sol is volume of the analysed solution in cm³; V_v is the volume of the wine sample in cm³; I is current in A; τ is time passed from the beginning of electrolysis to the equivalence point in s; F is the Faraday constant.

EFFECT: invention provides reliability of the method owing to the reduced effect of impurities and atmospheric carbon dioxide on the process of determining the number of equivalents of H⁺ ions in wine, and simplification of the process of authenticating wine owing to non-use of complex mathematical calculations.

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Description

The invention relates to the food industry and can be used to assess the authenticity of wine.

A known method for determining the content of organic acids for the identification of grape wines by high performance liquid chromatography [Instrumental analysis methods for the identification of grape wines / Z.E.Senkina, V.N. Arbuzov, B.M. Aleshin // Wine-making and viticulture. - No. 1. - 2004.].

The disadvantages of this method are the use of expensive equipment and lengthy preparation of reagents.

A known method of analysis of organic acids to identify the authenticity of wines based on capillary electrophoresis [Markovsky MG Improving the technology and methods for assessing the quality of grape wines based on the analysis and regulation of their acid composition: 05.18.01: Dis .... cand. tech. Sciences / M.G. Markovsky; Kuban State Technical University. - Krasnodar, 2006 .-- 170 p.: Tab. - Bibliographer .: S.120-138 (180 titles.)].

This method requires expensive equipment, sample preparation of samples, the presence of typical electrophoregrams and is based on the determination of the content of individual acids in the composition of wine products.

Closest to the claimed method is a method for identifying the authenticity of wine products, including assessing the acid composition of wine based on pH metric titration [Patent No. 2246108 of 02/10/2005]. The essence of the method lies in the fact that according to the results of potentiometric titration with an alkali solution of a mixture of the wine under study with a certain amount of hydrochloric acid, the dependence of the formation function on the pH of the solution (acid profiles) is calculated, followed by approximation of the sigmoid function, the parameters of which are used to make decisions about the authenticity of the wine.

The disadvantages of this method include the distortion of the results due to the absorption of carbon dioxide in the air during the experiment, the preparation of standard titrant solutions, as well as the need for complex calculations.

The technical result is to ensure reliability by reducing the influence of impurities and carbon dioxide of air on the process of determining the number of equivalents of H ⁺ ions in wine and simplifying the process of identifying the authenticity of wine due to the lack of complex mathematical calculations.

The technical result is achieved by the fact that in the method for identifying the authenticity of wine 0.5-1.5 cm ^{3 of} wine in 50 cm ³ 1 mol / dm ^{3 of a} solution of sodium chloride is subjected to pH-metric titration with coulometry-generated OH ions ^- , from which a curve is constructed titration, the pH is determined at the starting point of the titration and calculate the number of equivalents of H ⁺ ions, a pH jump determined (equivalence point) and counting the number of equivalents of ions H ^+, finding a numerical value of the ratio of numbers of equivalents calculated numerically and y value in the range 0.5 ± 0.1, judged wine naturalness, the calculation of the number of equivalents of H ⁺ at the starting point of titration and at the equivalence point lead respectively by the formulas C = 10 ^-pH · V _p-pa / V _v and C '= I · τ · 1000 / F · V _v , where V _p.pa is the volume of the test solution, cm ³ ; V _v is the sample volume of wine, cm ³ ; I is the current strength, A; τ is the time elapsed from the beginning of electrolysis to the equivalence point, s; F is the Faraday number.

It was experimentally found that in the inventive method to simplify the mathematical calculations allows you to pre-set the pH meter. To do this, the pH meter is adjusted according to two conditionally buffer solutions: 1 - 1 mol / dm ³ sodium chloride solution and 2 - 0.1 mol / dm ³ hydrochloric acid solution in the presence of 0.9 mol / dm ³ sodium chloride solution. The pH of the first solution is taken equal to 7.00, and the pH of the second 1.00.

Under the conditions of this pH meter setting, it is easy to calculate the number of equivalents of ions H ⁺ , C, mol / dm ³ using the formula according to the measured values of the pH of a wine sample in a sodium chloride solution at the initial titration point

where V _p.pa is the volume of the test solution, cm ³ ;

V _ν is the sample volume of wine, cm ³ .

Determination of the number of equivalents of H ⁺ ions found by titration (at the equivalence point on the titration curve), i.e. titratable acidity of wine, is complicated by the difficulty of obtaining titrant - a solution of a base that does not contain carbonates formed during the absorption of carbon dioxide from air, which significantly distorts the results. To eliminate the effect of carbon dioxide, it is proposed to carry out a pH-metric titration in the presence of sodium chloride in a titration cell with generator electrodes. The titrant is OH ^- ions, which are generated coulometrically during the electrolysis of sodium chloride and react with H ⁺ ions in wine samples.

The generator electrodes are a silver anode and a platinum cathode. At the anode, the silver chloride precipitate is allocated, and at the cathode - ions OH ^- and hydrogen gas. Hydrogen gas prevents the absorption of carbon dioxide from the air by the solution, which makes the titration more accurate. The error is lower (0.02%) drip (the error associated with measuring the volume with a pipette), since titrant volume measurement is not required. Electrolysis is carried out until a sharp jump in pH appears on the titration curve. The dependence of the change in pH values on the time of electrolysis is recorded by an indicator system consisting of glass and silver chloride electrodes, also placed in a titration cell and connected to a pH meter that is connected to a computer.

The number of equivalents of H ⁺ ions at the equivalence point, C ', mol-equiv / dm ^{3 is} calculated by the formula

where I is the current strength, A;

τ is the time elapsed from the beginning of electrolysis to the equivalence point, s;

F is the Faraday number;

V _ν is the sample volume of wine, cm ³ .

The number of equivalents of H ⁺ ions at the equivalence point determines the content of titratable acids in wine, and the number of equivalents of H ⁺ ions at the initial point of titration determines the active acidity of wine. Organic acids (tartaric, malic, succinic, citric, lactic, acetic) play an important role in the formation of organoleptic properties of wine, namely, one of the most important indicators of wine taste - acidity. The most significant contribution to the titratable acidity of natural wines is made by tartaric and malic acids. The share of each of the remaining acids, incl. lemon, does not exceed 6%. In falsified wines, citric acid prevails.

As our studies showed, when a wine sample is added to a solution of sodium chloride in the proposed ratios, tartaric and malic acids are completely ionized in the first stage and practically do not dissociate in the second, and citric ionized only by 20-30%. During titration, acids are completely ionized. Therefore, the C / C 'ratio in natural wine should be around 0.5. This is confirmed by numerous experiments, according to the results of which it was found that the C / C 'ratio in natural wine is 0.5 ± 0.1, and in falsified wine, in which citric acid was added to the requirements of the standard, it is much smaller.

The conclusions made are experimentally confirmed for various samples of wine. The research results are given for natural samples of wine: table semi-sweet Isabella, table semi-sweet Zemfira, table dry Cabernet and falsified samples of wine: special Anapa and Fruit Fruit + citric acid. For each sample of wine by the present method conducted an experiment. A 50 cm ³ 1 mol / dm ³ sodium chloride solution was placed in the titration cell, and 1.00 cm ³ samples of the test wine sample were added. Then, a mixer, generator and measuring electrodes were immersed in the titration cell. Measuring electrodes (glass and silver chloride) were connected to a pH meter connected to a computer. The generator electrodes (silver anode and platinum cathode) were connected to a DC power source. The agitator, pH meter, computer were turned on, and the pH measurements were started in 1 second increments recorded by the computer. After stabilizing the pH meter readings, a direct current source was switched on, at which a current of 0.01 A was set, and pH metric titration was performed with coulometrically generated OH ions ^, with a titration curve being built until a jump appeared on the titration curve and an inflection occurred (equivalence point). The construction of a pH-metric titration curve was carried out on a computer using the MathCAD program or other similar programs for graphical construction and mathematical processing of data.

Figure 1, 2 presents the titration curves of samples of wine "Isabella", "Zemfira", "Cabernet", "Anapa" and "Fruit" + citric acid. Using the pH-metric titration curve corresponding to each wine sample, the pH value was found at the initial point of titration and the number of H ⁺ , C ion equivalents was determined by formula (1), and by the time elapsed from the beginning of titration (the moment the current was turned on) until the kink occurred jump in the titration curve (equivalence points), the number of equivalents of ions H ⁺ , C 'was found by the formula (2). The numerical value of the ratio of the calculated numbers of equivalents C / C 'was found. The obtained ratios were compared with the declared limit of 0.5 ± 0.1. According to the results of the comparison, it was found that the numerical values of C / C 'for Isabella, Zemfira and Cabernet wines are within the declared limit, which indicates the naturalness of these samples, and for Anapa and Fruit fruit samples + lemon acid, the numerical values of C / C 'are much lower than the declared limit, which indicates their falsification. The results of the experiment are presented in table 1.

In the studied samples of wine in accordance with the methodology of the prototype, a pH-metric titration was carried out with the subsequent calculation of the parameters of the sigmoid curve of the formation function. In the sample of Zemfira wine, the parameters of the sigmoid curve of the formation function were calculated, the values of which amounted to ₁ = 2.0; k ₂ = -1.6; to ₃ = 3.3. The relative error of determination at a probability level of 0.95 for these coefficients was ± 3.5, respectively; 3.9; 3.4%. The obtained values of the parameters for _{1-to 3} fall into the range corresponding to genuine wines. However, the parameter k ₃ , taking into account the error, is at the boundary of the allowable range. Thus, one parameter is doubtful, which affects the reliability of the conclusion about the naturalness of the wine. According to the claimed method for a sample of wine "Zemfira" the numerical value of the ratio of the numbers of equivalents of H ⁺ at the starting point of titration and at the point of equivalence is C / C '= 0.487. The relative error of the C / C 'ratio at a probability level of 0.95 is ± 1.8%. The obtained value of the C / C 'ratio when compared with the claimed range of 0.5 ± 0.1 indicates the naturalness of the wine. For the remaining wine samples studied (Isabella, Anapa, Cabernet, Fruit Fruit + citric acid), the relative error of determination at a probability level of 0.95 for the three parameters is ₁ , ₂ , ₃ , which are used for acceptance the decision on the naturalness of the investigated wine in the prototype was ± (3.3-4.1), respectively; (3.6-4.4); (3.1-4.0)%, and for the claimed C / C 'ratio, the relative error of determination at a probability level of 0.95 was ± (1.2-2.0)%.

The increase in the reliability of the results by reducing the relative error is due to the fact that the prototype requires a fairly lengthy analysis. The exact amount of hydrochloric acid is added to the wine sample to break down the associates, and the sample is titrated point by point by adding exact concentration of sodium hydroxide, which is difficult due to the instability of the titer in time. The titration curve is built on separately obtained points, and then processed by the correlation formula to obtain the parameters of the sigmoid curve of the formation function, which will also depend on the presence of errors in measuring the titration parameters, measuring the volume and accuracy of the concentrations of the solutions of hydrochloric acid and sodium hydroxide used. In the inventive method, a wine sample is diluted 50 times with a solution of sodium chloride (necessary to create electrical conductivity). Strong dilution of a wine sample with electrolyte destroys the associates of titratable acids with other components of wine much better. 1 molar solution of sodium chloride slightly absorbs carbon dioxide from the air, in contrast to caustic sodium. The inventive method does not lead to dilution of the solution with a titrant, since the latter is generated directly in the cell, and the resulting titration curve seems almost continuous, due to the high discreteness of the recorded pH values.

Thus, the advantage of the proposed method compared to the prototype lies in the conditions of the experiment — coulometric generation of OH ^— for pH metric titration, which eliminates the difficult task of obtaining a base that does not contain carbonates, as well as detailed information about the extreme sections of the titration curve.

The proposed method does not require lengthy calculations due to the appropriate adjustment of the pH meter before starting measurements, which simplifies the identification of the authenticity of wines.

The reliability of the results is increased due to a significant dilution of the investigated wine sample, which eliminates the interfering effect of various impurities, the absence of operations to measure volumes and establish the concentration of reagents, and the analysis of the results does not require special correlation functions. The relative error at a probability level of 0.95 of the claimed C / C 'ratio, which is used to judge the naturalness of the wine, is 2.2-2.6 times lower than the parameters used in the prototype.

Case Studies

Reagents

To prepare solutions for adjusting the pH meter, sodium chloride was used according to GOST 4233-77 Reagents. Sodium chloride. Technical conditions; hydrochloric acid prepared from a standard titer (fixanal) - Hydrochloric acid, C (HCl) = 0.1 mol / dm ³ according to TU 2642-001-33813273-97.

Requirements for physico-chemical parameters of sodium chloride in accordance with GOST 4233-77 are presented in table 2.

table 2 Physico-chemical characteristics of sodium chloride according to GOST 4233-77 Name of indicator Norm for the brand
Clean (h) 1. Mass fraction of sodium chloride (NaCl) in the calcined preparation,%, not less than 99.8 2. Mass fraction of substances insoluble in water,%, no more 0.010 3. Mass fraction of losses during ignition,%, no more 1,0 4. Mass fraction of total nitrogen,%, no more 0.0010 5. Mass fraction of iodides (I),%, no more Not standardized 6. Mass fraction of bromides (Br),%, no more Not standardized 7. Mass fraction of sulfates (SO ₄ ),%, no more 0.010 8. Mass fraction of phosphates (PO ₄ ),%, no more Not standardized 9. Mass fraction of barium (Ba),%, no more 0.010 10. Mass fraction of iron (Fe),%, no more 0.0010 11. Mass fraction of magnesium (Mg),%, no more 0.0050 12. Mass fraction of arsenic (As),%, no more 0.00010 13. Mass fraction of heavy metals (Pb),%, no more 0.0010 14. Mass fraction of potassium (K),%, no more 0,020 15. Mass fraction of calcium (Ca),%, no more 0.010 16. The pH of the drug solution with a mass fraction of 5% Not standardized

Device

The titration cell is equipped with measuring electrodes (glass and silver chloride) for recording changes in the pH of the solution and generator electrodes (silver anode and platinum cathode) for coulometric generation of OH ^- ions. The measuring electrodes are connected to an Expert-001 brand pH meter-ionomer, which is connected to a computer. A DC power source B5-49 is connected to the generator electrodes.

Instrument Preset

The Expert-001 pH meter ionomer was adjusted using two conditionally buffered solutions: 1 - 1 mol / dm ³ sodium chloride solution and 2 - 0.1 mol / dm ³ hydrochloric acid solution in the presence of 0.9 mol / dm ³ sodium chloride solution. The pH of the first solution was taken equal to 7.00, and the second - 1.00. 50 cm ^{3 of} 1 mol / dm ³ sodium chloride solution was placed in the cell and the pH was measured, the value of which should lie within 7.00 ± 0.01 units. Otherwise, we checked either the adjustment of the pH meter or a solution of 1 mol / dm ³ sodium chloride for carbon dioxide content. Carbon dioxide can be removed both by boiling the solution, and by purging the solution with nitrogen or air purified from carbon dioxide.

Performance

Example 1

The object of the study was a sample of dry wine "Cabernet". Figure 3 presents the constructed pH-metric titration curve.

A 50 cm ³ 1 mol / dm ³ sodium chloride solution was placed in the titration cell and 0.5 cm ^{3 of} Cabernet dry wine sample was added. Then, a mixer, generator and measuring electrodes were immersed in the titration cell. The measuring electrodes were connected to an Expert-001 brand pH meter-ionomer connected to a computer. The generator electrodes were connected to a DC power source B5-49. The mixer was turned on, the pH meter ionomer, the computer, and pH measurements were started in 1 second increments recorded by the computer. After stabilizing the pH readings, a direct current power supply was turned on, at which a current of 0.01 A was set, and a pH metric titration was performed with coulometrically generated OH ions ^, with a titration curve being built until a jump appeared on the titration curve and an inflection occurred (equivalence point). The construction of a pH-metric titration curve was carried out on a computer using the MathCAD program or other similar programs for graphical construction and mathematical processing of data.

From the titration curve, the pH value was determined at the initial titration point, which amounted to 3.48, and the number of equivalents of H ⁺ , C ions, mol / dm ^{3 was} calculated by the formula (1):

where V _p-pa is the volume of the test solution, cm ³ ;

V _ν is the sample volume of wine, cm ³ .

Using the titration curve, we determined the time elapsed from the beginning of titration (the moment the current was turned on) to the inflection at the jump of the titration curve (equivalence point), which amounted to 325 s. Calculated the number of equivalents of ions H ⁺ , C 'according to the formula (2):

where I is the current strength, A;

τ is the time elapsed from the beginning of electrolysis to the equivalence point, s;

F is the Faraday number;

F _ν is the sample volume of wine, cm ³ .

Found the numerical value of the ratio of the calculated numbers of equivalents C / C ^/ :

C / C ^/ = 0.03344 / 0.06736 = 0.496

The resulting ratio was compared with a limit of 0.5 ± 0.1. The numerical value C / C '= 0.496 is included in the range 0.5 ± 0.1, which indicates the naturalness of the sample of dry wine "Cabernet".

Example 2

The object of the study was a sample of dry wine "Cabernet". Figure 4 presents the constructed pH-metric titration curve.

A 50 cm ³ 1 mol / dm ³ sodium chloride solution was placed in the titration cell and 1.0 cm ^{3 of} Cabernet dry wine sample was added. Then, a mixer, generator and measuring electrodes were immersed in the titration cell. The measuring electrodes were connected to an Expert-001 brand pH meter-ionomer connected to a computer. The generator electrodes were connected to a DC power source B5-49. The mixer was turned on, the pH meter ionomer, the computer, and pH measurements were started in 1 second increments recorded by the computer. After stabilizing the pH readings, a direct current power source was turned on, at which a current of 0.01 A was set, and a pH metric titration was performed with coulometrically generated OH ions ^, with a titration curve being built until a jump in the titration curve and an inflection point (equivalence point) appeared. The construction of a pH-metric titration curve was carried out on a computer using the MathCAD program or other similar programs for graphical construction and mathematical processing of data.

Using the titration curve, the pH value at the initial titration point was determined, which amounted to 3.17, and the number of equivalents of H ⁺ , C ions, mol / dm ^{3 was} calculated by the formula (1):

where V _p-pa is the volume of the test solution, cm ³ ;

V _ν is the sample volume of wine, cm ³ .

Using the titration curve, we determined the time elapsed from the beginning of titration (the moment the current was turned on) to the inflection at the jump of the titration curve (equivalence point), which amounted to 675 s. The number of equivalents of H ⁺ , C 'ions was calculated by the formula (2):

where I is the current strength, A;

τ is the time elapsed from the beginning of electrolysis to the equivalence point, s;

F is the Faraday number;

V _ν is the sample volume of wine, cm ³ .

Found the numerical value of the ratio of the calculated numbers of equivalents C / C ':

C / C '= 0.03448 / 0.06995 = 0.493

The resulting ratio was compared with a limit of 0.5 ± 0.1. The numerical value С / С '= 0.493 is included in the range of 0.5 ± 0.1, which indicates the naturalness of the sample of dry wine “Cabernet”.

Example 3

The object of the study was a sample of dry wine "Cabernet". Figure 5 presents the constructed pH-metric titration curve.

A 50 cm ³ 1 mol / dm ³ sodium chloride solution was placed in the titration cell, and 1.5 cm ^{3 of} Cabernet dry wine was added. Then, a mixer, generator and measuring electrodes were immersed in the titration cell. The measuring electrodes were connected to an Expert-001 brand pH meter-ionomer connected to a computer. The generator electrodes were connected to a DC power source B5-49. The mixer was turned on, the pH meter ionomer, the computer, and pH measurements were started in 1 second increments recorded by the computer. After stabilizing the pH readings, a direct current power source was turned on, at which a current of 0.01 A was set, and a pH metric titration was performed with coulometrically generated OH ions ^, with a titration curve being built until a jump in the titration curve and an inflection point (equivalence point) appeared. The construction of a pH-metric titration curve was carried out on a computer using the MathCAD program or other similar programs for graphical construction and mathematical processing of data.

From the titration curve, the pH value was determined at the initial titration point, which was 3.01, and the number of equivalents of H ⁺ , C ions, mol / dm ^{3 was} calculated by the formula (1):

where V _p.pa is the volume of the test solution, cm ³ ;

V _ν is the sample volume of wine, cm ³ .

Using the titration curve, we determined the time elapsed from the beginning of titration (the moment the current was turned on) to the inflection at the jump of the titration curve (equivalence point), which amounted to 1000 s. The number of equivalents of H ⁺ , C 'ions was calculated by the formula (2):

where I is the current strength, A;

τ is the time elapsed from the beginning of electrolysis to the equivalence point, s;

F is the Faraday number;

V _ν is the sample volume of wine, cm ³ .

Found the numerical value of the ratio of the calculated numbers of equivalents C / C ':

C / C '= 0.03355 / 0.06908 = 0.486

The resulting ratio was compared with a limit of 0.5 ± 0.1. The numerical value C / C '= 0.486 falls within the range of 0.5 ± 0.1, which indicates the naturalness of the sample of dry wine "Cabernet".

Claims (1)

A method for identifying the authenticity of wine, characterized in that 0.5-1.5 cm ^{3 of} wine in 50 cm ³ 1 mol / dm ³ of a sodium chloride solution is subjected to pH-metric titration with coulometry-generated OH ions ^- , from which a titration curve is constructed, determine the pH at the starting point of the titration and calculate the number of equivalents of H ⁺ ions, determine the pH jump (equivalence point) and calculate the number of equivalents of H ⁺ ions, find the numerical value of the ratio of the calculated numbers of equivalents and the numerical value, finding which is within 0.5 ± 0.1, the naturalness of the wine is judged, while the calculation of the number of H ⁺ equivalents at the initial titration point and at the equivalence point is carried out, respectively, by the formulas: C = 10 ^-pH · V _p-pa / V _v and C '= I · τ · 1000 / F · V _v , where V _p-pa is the volume of the test solution, cm ³ ; V _v is the sample volume of wine, cm ³ ; I is the current strength, A; τ is the time elapsed from the beginning of electrolysis to the equivalence point, s; F is the Faraday number.

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