Apparent dissociation constant of phenol red as determined by spectrophotometry and by visual colorimetry.

The apparent pK’ of phenol red was determined in buffered NaCI solutions of known compositionat 25#{176} and 370, by both spectrophotometric and visual colonmetric measurement of the concentration ratios of the alkaline to acid form of the indicator. A small but statistically significant difference was obtained by the two procedures. The pK’ values obtained in 0.15M NaCI spectrophotometrically (7.67 at 25#{176} and 7.57 at 37#{176}) were approximately 0.03 unit lower than those obtained colorimetrically. Measurements made as a function of NaCI concentration at 25#{176} were in accord with the Debye-H#{252}ckel th ory. Small variation was observed between solutions buffered with phosphate or tris at the higher NaCI concentrations. The results indicate that in addition to ionic strength and temperature, variations in method of measurementand in the sourceof the preparation are important factors in the determination and use of pK’ values for phenol red. Attention is called to the pertinence of these results to the measurement of blood pH by colorimetric and spectrophotometricmethods.