Indicator impurity corrections for accurate spectrophotometric pH measurements

Spectrophotometric measurements of seawater pH with an indicator dye offer a relatively robust and calibration-free method of measuring pH with excellent short-term precision (repeatability of ~0.0004 in pH). These distinct advantages have made spectrophotometric pH particularly desirable for use in calibrating autonomous pH sensors deployed in observational networks for studying the ocean’s CO₂ system. However, colored impurities in the indicator dye, which have been found at appreciable levels both in commercially available indicators and in some batches of imperfectly purified indicators, can significantly compromise the comparability and accuracy of spectrophotometric pH measurements. We present a spectral-based method for correcting for the effect of impurities by comparing the full absorbance spectrum of a sample indicator dye solution to the spectrum expected for a solution of purified indicator, whose absorption coefficient spectra had been carefully characterized. We use this method to assess the purity of several different samples of impure and purified m-cresol purple and to evaluate the pH-dependence of the impurity contribution to a dye solution’s absorbance spectrum. The full spectrum method for indicator impurity corrections was compared to two alternate methods in the literature to evaluate the likely uncertainty in indicator impurity corrections. Our method can be used with the full spectral data of a reference material indicator dye to improve the comparability of spectrophotometric pH measurements.