Buffer pH and pKa values as affected by added glycerol and sucrose

Many properties of foods and beverages depend upon being formulated to the proper pH. Should this pH shift unexpectedly, the stability and quality of the product could be affected. Although a pH-lowering effect of sucrose added to phosphate buffer has been observed, an explanation for the phenomenon has not been provided. The purpose of this study was to determine why the addition of non-ionic polyols changed buffer pH. Titration curves of citrate and phosphate buffers, with and without added sucrose or glycerol were constructed. Apparent pKa values were determined from titration curves and by measuring the pH values of equimolar buffer solutions. Sucrose appeared to lower the apparent pKa of phosphate buffer, but not citrate buffer. Glycerol appeared to have no effect on the apparent pKa value of phosphate buffer while causing a slight increase in the values for citrate buffer. These results parallel the effects of the polyols on buffer pH; the largest change was a pH decrease from adding sucrose to phosphate buffer. A pKa change appears responsible for this pH-lowering effect, possibly due to differences in solute–water interactions and changes to the hydration sphere around the buffer anions. Potential pH changes from food additives should be recognized during product development in order to optimize product quality and shelf life.