High Precision Multiwavelength pH Determinations in Seawater Using Cresol Red

Observations of sulfonephthalein absorbance ratios provide the basis for very precise measurements of seawater pH. Our analyses, using cresol red, indicate that meaningful pH comparisons at sea can be obtained at the level of ±0.001 pH units or better. Comparison of replicate seawater samples obtained from different Niskin bottles exhibited, in most instances, agreement within 0.0005 pH units. The procedures involved in multiwavelength pH measurements are quite simple relative to the required methodology in single wavelength observations