Determination of seawater pH from 1.5 to 8.5 using colorimetric indicators

Abstract

The apparent dissociation constants of five sulfonephthalein indicators (thymol blue, bromophenol blue, bromocresol green, bromocresol purple and phenol red) were determined in 35% seawater at 25°C. Measurements of seawater pH were made using the ratio of the optical absorbance of the acidic and basic components of a particular indicator and the apparent dissociation constant of the indicator. The suite of indicators allows quantitative determination of seawater pH over the pH range 1.5–8.5. For seven seawater samples ranging from pH 1.5 to 8.0 the average difference between pH measurements made with indicators and conventional electrodes was ± 0.005. Measurements of seawater pH with indicators is not limited to single indicator solutions. Simultaneous addition of phenol red and bromocresol green allows pH determinations from 3.0 to 8.2, which are suitable for alkalinity titrations. The p K′ a, values of the indicators were determined independently of potentiometric pH measurements. The pH indicators provide an absolute free hydrogen ion molality scale independent of the problems of electrode drift and liquid junction errors associated with potentiometric pH measurements.