STABILITY OF THE BUFFER SYSTEM OF LEMON JUICE

Abstract

The buffer system of lemon juice is similar to that of citric acid-citrate solns. of equivalent concs. In lemon juice samples and pure citric acid solns., the increase in pH with the dilution ratios is influenced by the concs. of total acid and salt. The smallest change in pH on dilution occurred in systems where the ratio of free acid to salt was unity. As lemon juice contains a large ratio of free acid to salt, the changes in pH are relatively large with increase or decrease in juice volume. The increase in pH with dilution is approx. the same for lemon juice and pure citric acid solns. of equivalent cones. The efficient buffer capacity of lemon juice extends over an approx. range of pH = pKa2 [plus or minus] 1.8. This agrees with the maximum range of the buffer index curve. The buffer index value of lemon juice is at a max. at pH 4.4. This value is in agreement with the pKa2 of a pure citric acid soln. The 2 minor inflections on the curve indicate pKa1 and pKa3 values of lemon juice. In concs. as high as 10 m.e. per ml. of lemon juice, KC1 and NaCl produced no significant change in pH. With the same concs. of K2SO4 and KNO3, increases in pH of 0.18 and 0.19, respectively, were recorded. In concs. as high as 50% by wt., sucrose did not change the pH values of various lemon juices and citric acid solutions which had different ratios of free acid to salt. That the buffer capacity of lemon juice is not affected by heat is demonstrated by the fact that boiled and normal juices have identical titration curves.