In-situ spectrophotometric pH measurements: the effect of pressure on thymol blue protonation and absorbance characteristics

Previous studies of the indicator thymol blue in seawater have characterized the temperature and salinity dependence of thymol blue protonation and molar absorbance. In this work, observations of thymol blue absorbance in synthetic solutions and natural seawater between 0- and 900-bar gauge pressure have been used to calculate the influence of pressure on absorbance ratios and the pK2 for the equilibrium HI− H++I2− (where I denotes thymol blue). The influence of pressure on indicator absorbance ratios is small but significant. The molar volume change and compressibility for the thymol blue dissociation reaction at 25°C in seawater are: ΔVi=−17.28±0.1 cm3 mol−1 and Δκ=(−38±4)×10−4 cm3 mol−1 bar−1. The pressure dependence of pK2 at 25°C is given as where K2P and K20 are the dissociation constants for HI− at a gauge pressure P (bars) and atmospheric pressure, respectively. The influence of pressure on thymol blue absorbance ratios (ei) in the expression pH=pK2+log(R−e1)/(e2−Re3) are given as e1=0.0035, e2=2.386−(2.7×10−6)×P, and e3=0.139+(6.6×10−6)×P. This indicator calibration in conjunction with previous studies of thymol blue equilibria at atmospheric pressure allows in-situ spectrophotometric measurements of seawater pH throughout the oceanic water column.