Kinetic salt effects on the inversion of sucrose

Abstract

The rate of inversion of sucrose by strong acids has been measured at 24·7° C by a method which eliminated the mutarotation lag and the end-point uncertainty. The rate was proved to be strictly first order for at least 54 hr. and the first-order rate constant was determined with an internal consistency better than 1%. In 0·1 M hydrochloric acid the first-order rate constantk1is linear inS, the number of grams of sucrose in a litre, according to the formulak1x 104/min.-1= 7·13 + 0·97 x 10-2S. For sucrose at 30 g./l. hydrolysed by single strong acids at molarities up to 0·2, the first order rate constantk1is related to the molaritycof strong acids by the formulak1min.-1= 6·95 x 10-3cx 10Bje, whereBjis a constant determined by the anion as follows: Cl¯, 0·28; Br¯, 0·35; ClO¯4, 0·38; NO¯3, 0·30. In similar experiments with mixed strong acids or a strong acid with the addition of a neutral uni-univalent salt, the experimental results can be expressed by the formulak1min.-1= 6.95 x 10¯3CHx 10ΣBjcj, whereCHdenotes the molarity of strong acids andcjthe molarity of each anion. EachBjhas the same value as in the formula for single acids. This formula is in accordance with Brönsted’s principle of specific interaction. In experiments on solutions containing bi-univalent and tri-univalent salts there is a further small negative salt effect of the multivalent cations increasing with their valency. This effect indicates that the principle of specific interaction becomes detectably inaccurate at these higher ionic strengths. Our results are compared with those of other workers. On the whole there is good agreement within the experimental accuracy of the various data. Certain discrepancies in absolute values of the rates are probably attributable to uncertainties of 0·1° C or less in the temperature.