Impurities in indicators used for spectrophotometric seawater pH measurements: Assessment and remedies

Spectrophotometric pH measurements have been widely used in oceanic CO2−system research. However, impurities in indicator dyes from different manufacturers may cause uncertainty in measured pH values. In this work we report initial findings surrounding the potential significance of indicator sources (manufacturers) on pH measurements obtained with the sulfonephthalein indicator m-cresol purple. HPLC analyses were used for comparative assessments of impurities in m-cresol purple obtained from a range of manufacturers. Results indicate that m-cresol purple from different manufacturers, and perhaps from different batches of the same manufacturer, have different types and quantities of light-absorbing impurities. Impurities can contribute to pH offsets as large as 0.01 pH units. The pH offset caused by impurities decreases with decreasing sample pH. Indicators can be purified using preparative HPLC. Until purified indicators and refined indicator calibrations become available, we suggest that investigators should reserve some portion of each indicator batch for comparison with the properties of purified indicators.