Seawater pH measurements: an at-sea comparison of spectrophotometric and potentiometric methods

Abstract

Spectrophotometric procedures utilizing sulfonephthalein indicators provide rapid, quantitative measurements of seawater pH. In a series of shipboard comparison, the average difference between our spectrophotometric and potentiometric pH measurements was 0.012 ± 0.010 + ph (spec., 20°C, free H + scale) - pH (potent., 25°C, NBS scale). The ±0.010 S.E. obtained in our at-sea comparison suggests that the imprecision associated with either type of measurement was <±0.010. Spectrophotometric procedures involving dissolved sulfonephthalein indicators appear well suited to in situ analysis of both pH and alkalinity.