Spectrophotometric pH measurements of freshwater

The use of cresol red (CR) indicator for determination of freshwater pH is evaluated. Ionic strength effects and indicator pH perturbation are discussed and quantified using theoretical and empirical approaches. Spectrophotometric and potentiometric methods are directly compared by repeated analyses of a low ionic strength pH buffer. The mean and standard deviation of the two methods were 7.618 ± 0.008 (spectrophotometric) and 7.484 ± 0.040 (potentiometric) (N = 18) with systematic errors of 0.003 and 0.137 pH units relative to the true pH (7.621). Field data from an alkaline river (pH ∼ 7.8-8.8) show that measurement reproducibility is better than 0.01 pH units, making it possible to resolve very small spatial and temporal changes in riverine pH. Uncertainty in the indicator apparent dissociation constant limits the accuracy of the pH measurement to ∼0.05 pH units. An alternative method for estimating the dissociation constant, based on calculation of pH from two other carbonate parameters, is proposed.