CITRIC ACID–SODIUM CITRATE–WATER SOLUTIONS ACID-BASE AND ELECTROCHEMICAL BEHAVIOR

Abstract

The protolytic equilibria in the system citric acid (H3Cit)–sodium citrate (Na3Cit)–water at a total content of citrates forms (citric acid, dihydrocitrate (H2Cit-), hydrocitrate (HCit2-) and citrate (Cit3-) anions) 1.0 mol/l have been studied by pH- and conductometric methods in the temperature range 293 ÷ 313 K. The first concentration ranges at which pH – lg(νH3Cit/νNa3Cit) and κ – CNa+/Ccit functions are linear correspond to the H3Cit/H2Cit- buffer system; the second ones – to H3Cit/H2Cit- and H2Cit-/HCit2-; the third ones to – HCit2-/Cit3- buffer system. The investigated solutions ion-molecular composition and ionic strength (m, mol/l) have been calculated using the mathematical model taking into account the law of mass action, material balance for citrates and electrical neutrality principle. The ionic strength concentration dependences have a complex character due to the ion-molecular composition multicomponent nature for the studied solutions. The ionic strength values are directly proportional to the ratio [Na+]/CCit in the concentration intervals 0 ≤ [Na+]/CCit ≤ 1.0 and 2.0 ≤ [Na+]/CCit ≤ 3.0. In the concentration range 1.0 ≤ [Na+]/CCit ≤ 2.0, the values of m = 1.15 ± 0.27 mol/l) weakly depend on the [Na+]/CCit ratio and practically do not depend on temperature. Citric acid concentration and thermodynamic constants for the first, second, and third dissociation stages have been determined. The obtained data on the acid-base and electrochemical characteristics of the solutions HOC3H4(COOH)3 – HOC3H4(COONa)3 – H2O(СCit = 1.0 mol/l; СNa+ = 0 ÷ 1.0 mol/l) can be used in chemical analysis, microbiological and biochemical studies, and the acidity data of the solutions studied can simulate chemisorption of acidic (SO2) and/or basic (NH3) gases.