The Quinhydrone Electrode for pH Determinations in Milk and Dairy Products

Abstract

Summary Preliminary observations showed that the time interval between the addition of quinhydrone to milk and the potential reading had a highly significant influence upon the potential obtained. In spite of using a constant time interval, however, a difference between the pH obtained with a quinhydrone electrode and a glass electrode was found. The differences (pH glass electrode-pH quinhydrone electrode) averaged 0.08, with a standard deviation of 0.05 for the 60 whole milk samples included in the determinations. Increasing the temperature in the milk-quinhydrone mixture caused increasing differences in the pH values determined by the glass electrode and the quinhydrone electrode in similar milk samples. Increasing amounts of serum solids also increased the differences. When the fat content or the pH of the milk was increased, a decreasing difference between the value measured by the pH glass electrode and pH quinhydrone electrode was observed. Increasing temperature, serum solids, and pH in milk samples were shown to increase the pink color development in the milk-quinhydrone mixture as determined with a Hunter Color and Color Difference Meter, while addition of formaldehyde to the milk prevented the color formation. The results indicated that to obtain the best agreement between the two electrodes with milk samples a temperature of about 19° C. ought to be used, both in the calomel half-cell and in the milk-quinhydrone mixture. The differences observed between the pH of milk obtained with the glass electrode and the quinhydrone electrode probably can be explained by a reaction between quinone and proteins.