Spectrophotometric pH measurement in estuaries using thymol blue and m-cresol purple

Abstract

The conditional acid dissociation constants (p K a′) of two sulfonephthalein dyes, thymol blue (TB) and m-cresol purple ( mCP), were assessed throughout the estuarine salinity range (0< S<40) using a tris/tris–HCl buffer and spectrophotometric measurement. The salinity dependence of the p K a′ of both dyes was fitted to the equations (25 °C, total proton pH scale, mol kg soln −1): p K a ′ ( TB ) = 9.1427 − 0.3105 ⁢ S 1 / 2 + 0.0783 ⁢ S − 0.01135 ⁢ S 3 / 2 + 0.000684 ⁢ S 2 p K a ′ ( m CP ) = 8.6353 − 0.3238 ⁢ S 1 / 2 + 0.0807 ⁢ S − 0.01157 ⁢ S 3 / 2 + 0.000694 ⁢ S 2 The estimated accuracy of pH measurements using these calculated p K a′ values is considered to be comparable to that possible with careful use of a glass electrode (±0.01 pH unit) but spectrophotometric measurements in an estuary have the significant advantage that it is not necessary to calibrate an electrode at different salinities. pH was measured in an estuary over a tidal cycle with a precision of ±0.0005 pH unit at high ( S>30) salinity, and ±0.002 pH unit at low ( S<5) salinity. The pH increased rapidly in the lower salinity ranges (0< S<15) but less rapidly at higher salinities.