RU2206089C1 - Method of determining weight proportion of functional groups of polyuronides - Google Patents

Abstract

FIELD: analytical methods. SUBSTANCE: invention relates to standardizing of pectins and alginates used as heavy metal detoxicants. Aqueous solution of polysaccharides is titrated potentiometrically or conductometrically with 0.1 M sodium hydroxide solution (determining free carboxyl groups). To titrate polysaccharide solution, 0.1 M sodium hydroxide solution is poured and resulting solution is allowed to stand during 24 h at room temperature until hydrolysis of ester groups is completed, after which solution is titrated with 0.1 M hydrochloric acid solution. More 0.1 M hydrochloric acid solution is then poured and resulting solution is heated for 10 min on boiling water bath, cooled, and filtered. Polyuronic acid precipitate is washed on filter with water until negative acid test of wash water is attained, after which precipitate is quantitatively transferred into beaker with the aid of 0.1 M sodium hydroxide solution and dissolved during 30 min with periodic stirring. Finely, solution is titrated with 0.1 M hydrochloric acid solution. Reference experiment is simultaneously performed. EFFECT: increased accuracy and information content. 11 tbl, 4 ex

Description

 The invention relates to the pharmaceutical and food industries and for the standardization of pectins and alginates used as detoxifiers of heavy metals.

Polyuronides (pectins, alginic acid) having functional groups: —COOH, —COOCH ₃ , —OH, —OCOCH ₃ , have sorption properties and can act as ion exchangers. These properties of polysaccharides are due to the presence of uronic acids in them: polygalacturonic in pectin, polyguluronic and polymannuronic in alginic acid. To a large extent, the ion-exchange properties of polyuronides are associated with the content of the functional groups listed above: the higher the content of free carboxyl and hydroxyl groups, the higher the sorption properties of polysaccharides. Conversely, the high quantitative content of carboxyl groups esterified with methanol and acetic acid hydroxyl groups determines the pronounced gel-forming properties of the polymers.

 Methods are described for determining the content of free carboxyl groups of polyuronides by titration with sodium hydroxide solutions in the presence of a phenolphthalein indicator (at the equivalence point, a transition from a colorless color to a slightly pink solution is observed). The content of esterified carboxyl groups is determined after saponification of polysaccharides with an excess of sodium hydroxide solution for 2 hours at room temperature, followed by the addition of an equal volume of hydrochloric acid to the reaction mixture and titration of the excess acid with alkali (phenolphthalein is an indicator). The content of acetyl groups is determined after saponification of polyuronides with a 0.6% sodium hydroxide solution by distillation of the resulting acetic acid and titration with alkali (indicator-phenolphthalein). (Shelukhina N.P., Abaeva R.Sh., Aymukhamedova GB.B. Pectin and the parameters for its preparation. - Frunze: Ilim, 1987. - P.87-88, 91-92).

 The disadvantages of the described method are: a) the complexity of determining the content of acetyl groups associated with the distillation of the resulting acetic acid, b) the inability to quantify acetic acid, because after saponification with alkali, sodium acetate is predominantly formed, which will not be titrated with alkali in the future; c) determination of the content of free and esterified carboxyl groups and the content of acetyl groups is carried out in various samples of the preparation, which increases the error of determination, d) the equivalence point in the determination of all functional groups is established visually using phenolphthalein, which also increases the determination error due to a fuzzy color transition ; e) the volumes of titrants — sodium hydroxide solution — are overestimated due to the fact that water saturated with carbonates and bicarbonates (pH 5-6.5) is also partially titrated with alkali.

The determination of the content of functional groups of polysaccharides by conductometric titration of a 0.1-0.2% pectin solution with a 0.1 mol / L sodium hydroxide solution is described; during titration, the resistance value of the solutions (R, Ohm) is determined and the conductivity of the solutions (I / R, Ohm ^-1 ) is calculated, then the equivalence point, which corresponds to the neutralization of free carboxyl groups, is determined graphically by I / R = f (V _NaOH ). Then, 10 ml of a 0.1 mol / L sodium hydroxide solution (excess alkali) is added to the titrated polyuronide solution to saponify the esterified carboxyl and hydroxyl groups (saponification is carried out for 30-40 minutes at room temperature), after which an aliquot of the saponified polysaccharide solution is titrated with solution 0 , 1 mol / l hydrochloric acid. In the coordinate system: I / RV _NaOH , two equivalence points are graphically determined: the first point characterizes the neutralization of sodium hydroxide, which did not react during the saponification, and the curve between the first and second equivalence points corresponds to the titration of all carboxyl groups (before and after saponification of the polyuronide, as well as sodium acetate). Another aliquot of polysaccharide solution consecutively saponified under static conditions was treated with the cation exchanger in H ⁺ form (to remove the sodium ions) and an anion exchanger in the OH ^- -form (to remove the chloride and acetate ions) then the solution was filtered and the filtrate was again titrated conductometric 0 , 1 mol / l sodium hydroxide solution. Graphically determine the equivalence point, which corresponds to the neutralization of all carboxyl groups. (Golubev V.N., Shelukhina N.P. Pectin: chemistry, technology, application. - M., 1995. - P.105-110).

 Schematically, the definition of functional groups in pectin by the specified method can be represented in the form of equations shown in scheme 1 (see the end of the description).

The disadvantages of this method are: 1) the inability to quantify pectic acid, because a) after treatment with anion exchange resin, pectic acid as a poorly soluble compound partially dissociates an ion and a hydrogen ion (K _dis • 10 ^-4 ) into pectate, while the pectate ion is exchanged for OH ^- anion exchange groups, precipitating on the anion exchange resin; b) elution of pectic acid from cation exchange resin and anion exchange resin with water cannot be quantitative, because pectic acid adsorption on resins is possible; c) after treatment with ion exchangers, the formed pectic acid (precipitate) is filtered and remains on the filter, which is not taken into account in the described method, since the determination of pectic acid, according to the method, is carried out not in the insoluble part, but in the filtrate; 2) the complexity and duration of the process associated with the need for regeneration of cation exchangers and anion exchangers; 3) since Since water has a slightly acid reaction due to the formation of carbonates and bicarbonates (pH 5-6.5), when titrating polyuronides with alkali, part of the latter is spent on titration of water, therefore, the alkali consumption in the analysis is somewhat overestimated.

 A known method for determining the content of functional groups of polysaccharides is as follows: a pectin solution is titrated with a 0.1 mol / L sodium hydroxide solution in the presence of phenolphthalein (determination of free carboxyl groups), then an additional 10 ml of alkali is added and left at room temperature for 30 minutes. The excess alkali is titrated with a 0.1 mol / L hydrochloric acid solution in the presence of phenolphthalein (determination of all functional groups) and another 50 ml of a 0.1 mol / L hydrochloric acid solution is added, placed in a boiling water bath for 10 minutes, cooled, adjusted to a certain volume, filtered. The measured amount of filtrate is titrated with 0.1 mol / L sodium hydroxide solution in the presence of phenolphthalein (determination of all carboxyl groups of pectin or pectic acid). Similarly carry out a control experiment. (Arasimovich V.V., Baltaga S.V., Ponomareva N.P. Methods of analysis of pectin substances, hemicelluloses and pectolytic enzymes in fruits. - Chisinau: Editorial and Publishing Department of the Academy of Sciences of the Moldavian SSR, 1970. - P. 38). This method of determining the mass fraction of functional groups of polyuronides is closest to the proposed one and is selected as a prototype.

 The disadvantages of this method are: a) the impossibility of quantitative determination of pectic acid, because pectic acid as a poorly soluble compound precipitates (after treatment with hydrochloric acid and heating), which is filtered, therefore, the determination of pectic acid must be carried out in the precipitate, and not in the filtrate; b) the impossibility of quantitative determination of all ester groups, because saponification with 10 ml of alkali and for 30 minutes is not enough, c) the establishment of equivalence points by visual means using phenolphthalein due to a fuzzy color transition of the solutions contributes to a high measurement error.

 The purpose of the invention is the quantitative determination of various functional groups of polyuronic acids, improving the accuracy of the analysis.

 This goal is achieved in that 50 ml of water is added to a weighed portion of polysaccharides of about 0.1 g (exact weighed portion) and mixed on a magnetic stirrer with heating until a solution is obtained. The resulting solution is titrated potentiometrically on an I-135 ionomer (indicator electrode is glass, reference electrode is silver chloride) or conductometric on a conductivity meter KP (platinum electrodes) with 0.1 mol / L sodium hydroxide solution. To the titrated polysaccharide solution, 50 ml of a 0.1 mol / L sodium hydroxide solution are added and the solution is left for 24 hours at room temperature for complete saponification of the ester groups. Then, the entire solution is potentiometrically or conductometrically titrated with 0.1 mol / L hydrochloric acid solution. In this case, the first equivalence point characterizes the neutralization of sodium hydroxide, which did not enter the saponification reaction, and the portion of the curve between the first and second equivalence points corresponds to the titration of all carboxyl groups (before and after saponification of polyuronides, as well as sodium acetate). Then, to precipitate pectic or alginic acids, another 50 ml of a 0.1 mol / L hydrochloric acid solution is poured, a beaker with the solution is placed in a boiling water bath for 10 minutes, cooled, filtered through a thick paper filter (blue ribbon), the precipitate is washed with filter with water until a negative reaction of the washings to acetic and hydrochloric acids according to the universal indicator. Then the precipitate (pectic or alginic acid) from the filter is quantitatively transferred into a beaker using 50 ml of a 0.1 mol / L sodium hydroxide solution and left for 30 min with periodic stirring to completely dissolve, after which potassium or alginate solution is titrated potentiometrically or conductometrically. sodium 0.1 mol / L hydrochloric acid solution: the first equivalence point corresponds to neutralizing the excess sodium hydroxide that has not reacted, and the portion of the curve between the first and second equivalence points There is a titration of the carboxyl groups of the polyuronide. In parallel, a control experiment is carried out (without polyuronide).

При использовании кондуктометрического титрования точку эквивалентности определяют графически в системе координат I/R-V_{титранта}, где I/R - электропроводность растворов (Ом^-1).When applying potentiometric titration to reliably determine the equivalence point, a dependency graph is constructed in the coordinate system

When using conductometric titration, the equivalence point is determined graphically in the coordinate system I / RV of the _titrant , where I / R is the conductivity of the solutions (Ohm ^-1 ).

 As a result of the studies, the following optimal conditions were determined for determining the mass fraction of the functional groups of polyuronides (tables 1-4).

 From table 1 it follows that potentiometric and conductometric titration determine the good reproducibility of the results obtained at all stages of determining the functional groups. Visual fixation of the equivalence point gives different results in the analysis of various functional groups, possibly due to a “blurry” transition of the color of the solutions at the equivalence point, incomplete saponification of polysaccharides, and also due to the lack of a control experiment involving titration of water with pH <7. With visual fixation, it is impossible to determine two equivalence points characteristic of titration of an excess of alkali that has not reacted and that has reacted alkali. When determining the carboxyl groups of polyuronic acids (3rd titration) by the prototype method, only one equivalence point corresponding to titration of water with sodium hydroxide solution is also determined; there is no second point of equivalence due to the loss of polyuronic acids as a result of filtration (acids remain in the form of precipitates on the filter).

 From table 2 it follows that the optimal volume of alkali for complete saponification of esterified groups is 50.0 ml. With smaller volumes of alkali, the pH of the solutions is ≤7, i.e. alkali is not enough for almost complete saponification.

 From table 3 it is seen that the most optimal duration of saponification is 24 hours, with a longer reaction time, the pH of the solutions of polyuronides does not change.

 As follows from table 4, within 30 min precipitation of pectic and alginic acids dissolve in a solution of sodium hydroxide, forming true solutions.

 The statistically processed results of the determination of the content of functional groups are shown in table 5 (with n = 6, P = 0.95).

 As follows from table 5, the error in the definitions of the proposed method is on average 2.3 times less than the error of the analysis by the method adopted for the prototype. In this case, the potentiometric and conductometric methods of determination give comparable results, while the prototype method, the content of free -COOH and the sum of all carboxyl groups is different, apparently due to over titration of the analyzed solutions and insufficiently saponification of polysaccharides. When analyzing the carboxyl groups of polyuronic acids, the prototype method allows to obtain underestimated results in connection with the loss of polyuronic acids during filtration.

 The proposed method for determining the mass fraction of functional groups of polyuronides is illustrated by the following examples of specific performance.

затем строят график зависимости в системе координат

и определяют объем титранта в точке эквивалентности (таблица 6, фиг.1).EXAMPLE 1. To 0.1 g (accurately weighed) of beet pectin is poured 50 ml of purified water and stirred on a magnetic stirrer with heating until the pectin dissolves. The resulting solution was titrated with 0.086 mol / L sodium hydroxide solution potentiometrically on an I-135 ionomer. A glass electrode is used as an indicator electrode, and silver chloride is used as a reference electrode. According to the results - pH and titrant volume (V _T , ml) - determined by the differential method (Ponomarev VD Analytical chemistry. - M .: VSH. 1982. - Part 2. - S.219-228) ratio

then build a dependency graph in the coordinate system

and determine the titrant volume at the equivalence point (table 6, figure 1).

 As can be seen from figure 1, the titrant volume of 0.086 mol / l sodium hydroxide solution at the equivalence point is 3.4 ml.

 After titration, the electrodes are washed with a small amount of water (~ 5 ml), the wash water is poured into a glass with a titrated pectin solution. Then, 50 ml of a 0.086 mol / L sodium hydroxide solution is added to this solution and the solution is left at room temperature for 24 hours to completely saponify the ester groups. Further, the entire solution is potentiometrically titrated with 0.1 mol / L hydrochloric acid solution. The titration results are shown in table 7 and figure 2.

 As can be seen from figure 2, the titrant volume - 0.1 mol / l hydrochloric acid solution - at the first equivalence point is 35.4 ml, at the second equivalence point - 44.3 ml.

 After titration, the electrodes are washed with a small amount of water (~ 5 ml), the wash water is poured into a glass with a titrated pectin solution. Then, 50 ml of a 0.1 mol / L hydrochloric acid solution is poured into this solution, the beaker with the solution is placed in a boiling water bath for 10 minutes, cooled, filtered through a dense paper filter ("blue ribbon"). The precipitate (pectic acid) on the filter is washed with water until the washings react negatively to acetic and hydrochloric acids according to the universal indicator. Then the filter cake was quantitatively transferred into a beaker using 50 ml of a 0.086 mol / L sodium hydroxide solution and left for 30 minutes with periodic stirring to completely dissolve, after which potassium pentate solution was titrated with a 0.1 mol / L hydrochloric acid solution. The results of potentiometric titration of sodium pectate are presented in table 8 and figure 3.

 As can be seen from figure 3, the titrant volume - 0.1 mol / l hydrochloric acid solution - at the first equivalence point is 30.7 ml, at the second equivalence point - 37.05 ml. In parallel with the titration of the analyzed pectin solutions, control experiments are carried out (without the addition of pectin). In this case, the titrant volumes in the control experiments are 0.2 ml; therefore, to accurately determine the titrant volumes at the equivalence points in the experimental solutions, the titrant volume in the control experiments is subtracted. Thus, during the first titration of pectin, the volume of 0.086 mol / L sodium hydroxide solution is 3.4-0.2 = 3.2 ml; at the second titration, 35.2 ml and 44.1 ml of a 0.1 mol / L hydrochloric acid solution; in the third titration, 30.5 ml and 36.85 ml of a 0.1 mol / L hydrochloric acid solution.

где С_Т - концентрация титранта - гидроксида натрия, моль/л.The mass fraction of free carboxyl groups (ω ₁ ,%) of pectin is determined by the formula:

where C _T is the concentration of titrant - sodium hydroxide, mol / L.

V _T - titrant volume, ml,
E _COOH - equivalent carboxyl group (45 g / mol),
a - sample of pectin, g.

,
Содержание суммы карбоксильных групп пектина и уксусной кислоты (ω₂, %, 2-е титрование) определяют по формуле:

где С_Т ^', С_Т ^'' - концентрации титранта - 0,1 моль/л раствора соляной кислоты;
V_Т ^', V_Т ^'' - объемы титранта соответственно в первой и второй точках эквивалентности, мл

Содержание всех карбоксильных групп пектина (ω₃, %; 3-е титрование) определяют аналогично ω₂:

Содержание ацетилированных групп (ω₄, %) определяют по формуле:
ω₄ = ω₂-ω₃ = 43,94-31,35 = 12,59%
Количество ацетилированных групп (п) равно количеству гидроксильных групп:

тогда содержание гидроксильных групп (ω₅, %) составляет: ω₅=п•М(ОН)= 0,280•17=4,76%
Содержание этерифицированных карбоксильных групп (ω₆, %):
ω₆ = ω₃-ω₁ = 31,35-13,59 = 17,76%
Количество этерифицированных карбоксильных групп равно количеству метанола:

п(СООН)=п(СН₃ОН)=0,395 моль,
тогда массовая доля метанола (ω₇, %) составляет:
ω₇=0,395•32=12,64%
Степень этерификации карбоксильных групп (ω₈, % определяют по формуле:

ПРИМЕР 2. К 0,02 г (точная навеска) альгиновой кислоты приливают 50 мл воды очищенной и перемешивают на магнитной мешалке с нагревом до растворения кислоты. Определение функциональных групп альгиновой кислоты проводят потенциометрически аналогично примеру 1.

,
The content of the sum of the carboxyl groups of pectin and acetic acid (ω ₂ ,%, 2nd titration) is determined by the formula:

where C _T ^' , C _T ^'' - titrant concentration - 0.1 mol / l hydrochloric acid solution;
V _T ^' , V _T ^'' - titrant volumes, respectively, at the first and second points of equivalence, ml

The content of all carboxyl groups of pectin (ω ₃ ,%; 3rd titration) is determined similarly to ω ₂ :

The content of acetylated groups (ω ₄ ,%) is determined by the formula:
ω ₄ = ω ₂ -ω ₃ = 43.94-31.35 = 12.59%
The number of acetylated groups (p) is equal to the number of hydroxyl groups:

then the content of hydroxyl groups (ω ₅ ,%) is: ω ₅ = n • M (OH) = 0.280 • 17 = 4.76%
The content of esterified carboxyl groups (ω ₆ ,%):
ω ₆ = ω ₃ -ω ₁ = 31.35-13.59 = 17.76%
The amount of esterified carboxyl groups is equal to the amount of methanol:

p (COOH) = p (CH ₃ OH) = 0.395 mol,
then the mass fraction of methanol (ω ₇ ,%) is:
ω ₇ = 0.395 • 32 = 12.64%
The degree of esterification of carboxyl groups (ω ₈ ,% is determined by the formula:

EXAMPLE 2. To 0.02 g (accurately weighed) of alginic acid is added 50 ml of purified water and stirred on a magnetic stirrer with heating until the acid dissolves. The determination of functional groups of alginic acid is carried out potentiometrically as in example 1.

 Taking into account control experiments during the first titration of alginic acid, the volume of 0.086 mol / L sodium hydroxide solution was 0.60 ml; in the second titration, 41.10 ml and 43.80 ml of a 0.1 mol / L hydrochloric acid solution; in the third titration, 36.20 ml and 38.45 ml of a 0.1 mol / L hydrochloric acid solution.

ω₄=71,60-59,66=11,94%,

ω₅=0,265•17=4,51%,
ω₆=59,66-13,68=45,98%,

ω₇=1,022•32=32,70%,

ПРИМЕР 3. К навеске свекловичного пектина около 0,1 г (точная навеска) приливают 50 мл воды очищенной и перемешивают на магнитной мешалке с нагревом до растворения пектина. Полученный раствор титруют 0,086 моль/л раствором гидроксида натрия кондуктометрически на кондуктометре КП. По полученным результатам - величинам сопротивления раствора (R, Ом) и вычисленным по ним значениям электропроводности растворов (I/R, Ом^-1) и объему титранта (V_Т, мл) графически определяют объем титранта в точке эквивалентности. Результаты кондуктометрического титрования свекловичного пектина приведены в таблице 9 и на фиг.4.Analogously to example 1, the content of functional groups of alginic acid is calculated:

ω ₄ = 71.60-59.66 = 11.94%,

ω ₅ = 0.265 • 17 = 4.51%,
ω ₆ = 59.66-13.68 = 45.98%,

ω ₇ = 1,022 • 32 = 32.70%,

EXAMPLE 3. To a sample of beet pectin, about 0.1 g (exact sample) is poured with 50 ml of purified water and stirred on a magnetic stirrer with heating until the pectin dissolves. The resulting solution was titrated with 0.086 mol / L sodium hydroxide solution conductometry on a conductivity meter KP. According to the results obtained - the values of the resistance of the solution (R, Ohm) and the values of the electrical conductivity of the solutions (I / R, Ohm ^-1 ) and the titrant volume (V _T , ml) calculated on them, the titrant volume is graphically determined at the equivalence point. The results of conductometric titration of beet pectin are shown in table 9 and figure 4.

 As can be seen from figure 4, the titrant volume of 0.086 mol / l sodium hydroxide solution at the equivalence point is 3.35 ml.

 After titration, the electrodes are washed with about 5 ml of water, the wash water is poured into a conductometer cell with a titrated pectin solution. Then, 50 ml of a 0.086 mol / L sodium hydroxide solution is added to this solution and the solution is left at room temperature for 24 hours to completely saponify the ester groups. Further, the whole solution is conductometric titrated with 0.1 mol / L hydrochloric acid solution. The titration results are shown in table 10 and figure 5.

 As can be seen from figure 5, the titrant volume - 0.1 mol / l hydrochloric acid solution - at the first equivalence point is 35.45 ml, at the second equivalence point - 44.35 ml.

 After titration, the electrodes are washed with about 5 ml of water, the wash water is poured into a glass with a titrated pectin solution. Then, 50 ml of a 0.1 mol / L hydrochloric acid solution is added to this solution, the beaker with the solution is placed in a boiling water bath for 10 minutes, cooled, filtered through a blue ribbon paper filter. The precipitate (pectic acid) on the filter is washed with water until the washings react negatively to acetic and hydrochloric acids according to the universal indicator. Then, the filter cake was quantitatively transferred into a beaker using 50 ml of a 0.086 mol / L sodium hydroxide solution and left for 30 minutes with periodic stirring until completely dissolved, after which the sodium pectate solution was conductometric titrated with a 0.1 mol / L hydrochloric acid solution. The results of conductometric titration of sodium pectate are shown in table 11 and Fig.6.

 As can be seen from Fig.6, the titrant volume - 0.1 mol / l hydrochloric acid solution - in the first point of equivalence was 30.70 ml, in the second point - 37.05 ml.

 Taking into account control experiments during the first titration of pectin, the volume of 0.086 mol / L sodium hydroxide solution amounted to 3.15 ml; with a second titration of 35.25 ml and 44.15 ml of a 0.1 mol / L hydrochloric acid solution, with a third titration of 30.50 ml and 36.85 ml of a 0.1 mol / L hydrochloric acid solution.

 The mass fraction of the functional groups of pectin is calculated by the formulas given in example 1.

ω₄=43,96-31,37=12,59%,

ω₅=0,280•17=4,76%,
ω₆=31,37-13,38=17,99%,

ω₇=0,400•32=12,80%,

ПРИМЕР 4. К навеске альгиновой кислоты около 0,02 г (точная навеска) приливают 50 мл вода очищенной и перемешивают на магнитной мешалке с нагревом до растворения кислоты. Определение функциональных групп альгиновой кислоты проводят кондуктометрически аналогично примеру 3.

ω ₄ = 43.96-31.37 = 12.59%,

ω ₅ = 0.280 • 17 = 4.76%,
ω ₆ = 31.37-13.38 = 17.99%,

ω ₇ = 0.400 • 32 = 12.80%,

EXAMPLE 4. To a weighed sample of alginic acid, about 0.02 g (exact weighed sample) was added 50 ml of purified water and stirred on a magnetic stirrer with heating until the acid was dissolved. The determination of functional groups of alginic acid is carried out conductometrically analogously to example 3.

 Taking into account control experiments during the first titration of alginic acid, the volume of 0.086 mol / L sodium hydroxide solution was 0.60 ml; in the second titration, 41.10 ml and 43.80 ml of a 0.1 mol / L hydrochloric acid solution; in the third titration, 36.15 ml and 38.45 0.1 mol / L hydrochloric acid solution.

 The mass fraction of functional groups of alginic acid is calculated by the formulas given in example 1.

ω₄=71,55-60,95=10,60%,

ω₅=0,236•17=4,01%,
ω₆=60,95-13,67=47,28%,

ω₇=1,051•32=33,63%,

Таким образом, предлагаемый способ определения массовой доли функциональных групп полиуронидов обеспечивает следующий положительный эффект:
1) количественное определение полиуроновых кислот: в предлагаемом способе после нагревания, осаждения и фильтрации полиуроновых кислот анализ этих кислот проводят не в фильтрате, как предусмотрено по способу, принятому за прототип, а в нерастворимой части путем переведения осадков с помощью гидроксида натрия в раствор и последующего титрования соляной кислотой пектатов или альгинатов натрия. Данные таблицы 5 свидетельствуют о том, что при определении полиуроновых кислот по способу-прототипу при третьем титровании в фильтрате практически нет этих кислот, в то время как при анализе предлагаемым способом (анализ осадков) наблюдается высокое содержание этих кислот;
2) практически полное определение всех функциональных групп полиуроновых кислот при втором титровании, т.к. обеспечивается полное омыление сложноэфирных групп за счет увеличения объема щелочи (до 50 мл) и продолжительности омыления (24 ч), о чем свидетельствуют данные таблицы 5;
3) повышение точности анализа, или понижение погрешности определений в среднем в 2,3 раза за счет использования физико-химических методов анализа (потенциометрия, кондуктометрия) для фиксации точек эквивалентности.

ω ₄ = 71.55-60.95 = 10.60%,

ω ₅ = 0.236 • 17 = 4.01%,
ω ₆ = 60.95-13.67 = 47.28%,

ω ₇ = 1,051 • 32 = 33.63%,

Thus, the proposed method for determining the mass fraction of functional groups of polyuronides provides the following positive effect:
1) quantitative determination of polyuronic acids: in the proposed method, after heating, precipitation and filtration of polyuronic acids, the analysis of these acids is carried out not in the filtrate, as provided for by the method adopted for the prototype, but in the insoluble part by transferring the precipitates with sodium hydroxide into solution and subsequent titration with hydrochloric acid of sodium pectates or alginates. The data in table 5 indicate that when determining polyuronic acids by the prototype method, there are practically no these acids in the filtrate during the third titration, while the analysis of the proposed method (precipitation analysis) shows a high content of these acids;
2) almost complete determination of all functional groups of polyuronic acids in the second titration, because full saponification of ester groups is ensured by increasing the alkali volume (up to 50 ml) and the duration of saponification (24 hours), as evidenced by the data in Table 5;
3) increasing the accuracy of the analysis, or lowering the error of the determination by an average of 2.3 times due to the use of physico-chemical methods of analysis (potentiometry, conductometry) to fix the points of equivalence.

Claims (1)

 A method for determining the mass fraction of functional groups of polyuronides, including dissolving the sample in water, titrating the resulting solution with a 0.1 molar solution of sodium hydroxide and fixing the volume of the titrant, saponification of ester groups when adding a 0.1 molar solution of sodium hydroxide to the titrated solution and subsequent exposure at room temperature titration of the alkali excess with a 0.1 molar hydrochloric acid solution and fixing the titrant volume, adding a 0.1 molar hydrochloric acid solution to the titrated solution Then, placing the resulting solution in a boiling water bath for 10 minutes, followed by cooling and filtering, calculating the content of functional groups based on the recorded volumes of titrant, characterized in that the solution is soaked during saponification for 24 hours, after filtration, the precipitate is washed until a negative reaction washing water for acetic and hydrochloric acids according to a universal indicator, followed by dissolution in a 0.1 molar solution of sodium hydroxide for 30 min with periodic stirring and titration the resulting solution with a 0.1 molar solution of hydrochloric acid when fixing the titrant volume, titration is carried out potentiometrically or conductometry with fixing the titrant volumes at equivalence points.

Images