A buffer solution for colorimetric comparison has been developed by T. C. McIlvaine. This system requires only two stock solutions: 0.2 M disodium phosphate and 0.1 M citric acid, combined in such volumes to make 20 cc of the mixture. The disodium phosphate was recrystallized three times and standardized by titration against HCl. The citric acid was also recrystallized before use and standardized by titration against NaOH. The pH values of the mixtures were determined electrometrically using a calomel electrode and a K potentiometer. The pH values were calculated using Clark's extension of Sorensen's values for the normal calomel electrode. A graph was constructed showing the pH values of various mixtures of phosphate and citric acid solutions, with the total volume always being 20 cc. By interpolation, the proper volumes of the two solutions that would give a solution with any desired pH were determined. The values in Table I were obtained this way and checked by preparing the solutions and measuring the pH values. The variation between observed and calculated pH values was 0.01 or less. Figure 1 shows the titration curve for the phosphate-citric acid buffer system, along with titration curves for well-known standard buffer solutions. The shape of the phosphate-citric acid curve indicates that the mixtures are well suited for colorimetric determinations of pH.A buffer solution for colorimetric comparison has been developed by T. C. McIlvaine. This system requires only two stock solutions: 0.2 M disodium phosphate and 0.1 M citric acid, combined in such volumes to make 20 cc of the mixture. The disodium phosphate was recrystallized three times and standardized by titration against HCl. The citric acid was also recrystallized before use and standardized by titration against NaOH. The pH values of the mixtures were determined electrometrically using a calomel electrode and a K potentiometer. The pH values were calculated using Clark's extension of Sorensen's values for the normal calomel electrode. A graph was constructed showing the pH values of various mixtures of phosphate and citric acid solutions, with the total volume always being 20 cc. By interpolation, the proper volumes of the two solutions that would give a solution with any desired pH were determined. The values in Table I were obtained this way and checked by preparing the solutions and measuring the pH values. The variation between observed and calculated pH values was 0.01 or less. Figure 1 shows the titration curve for the phosphate-citric acid buffer system, along with titration curves for well-known standard buffer solutions. The shape of the phosphate-citric acid curve indicates that the mixtures are well suited for colorimetric determinations of pH.