Quantitative analysis of total acidity in aqueous lactic acid solutions by direct potentiometric titration

In aqueous solutions of concentrated lactic acid (> 30 wt%), oligomers of lactic acid can produce via the intermolecular esterification of lactic acid, and thus the amount of titratable acidity is often less than that of total acidity. In this paper, the hydrolysis of oligomers of lactic acid was investigated at different temperatures in detail. The experimental results showed that when the content of total acidity is lower than 30 wt% in aqueous lactic acid solutions, the oligomers of lactic acid can been completely hydrolyzed to give monomeric lactic acid and the monomeric lactic acid is stable at temperatures of 8-33 degrees C, thus the total acidity is equivalent to the titratable acidity and can be determined quantitatively by a direct potentiometric titration. The results for determining total acidities were in good agreement with those obtained from a back titration by a two tailed t-test. Compared with the back titration, the novel method's procedure is simpler and reducing sugars do not interfere with measurements. The relative standard deviation (RSD) was less than 0.7% and 1.1% for the direct potentiometric and back titrations, respectively, which indicated that the direct potentiometric titration has a higher precision.