Spectrophotometric pH measurements of surface seawater at in-situ conditions: absorbance and protonation behavior of thymol blue

Abstract

The proton exchange behavior of thymol blue (HI− ⇔ I2− + H+) and the molar absorbance characteristics of unprotonated (I2−) and protonated (HI−) forms of this indicator make thymol blue particularly well suited to pH observations in the upper ocean. In-situ Spectrophotometric measurements of seawater pH using sulfonephthalein indicators require assessment of the influence of temperature on indicator molar absorbance characteristics and the influence of temperature and salinity on indicator equilibrium characteristics. Our investigation of the physical/chemical behavior of thymol blue indicates that seawater pH can be described using the following equation: pHT = pK2 + log(R − e1e2 − R · e3) where pHT = − log[H+]T, [H+]T = [H+] + [HSO4−], K2 − [H+]T[I2−][HI−]−, R =596A435A (where λA is absorbance at wavelength λ), and pK2 = 4.706 ST+ 26.3300 − 7.172181ogT − 0.017316 × Se1= − 0.00132 + 1.600 × 10−5Te2 = 7.2326 − 0.0299717T+ 4.600 × 10−5 T2e3 = 0.0223 + 0.0003917 T Using the equations given above, K2 and [H+]T are expressed in units of molkg-soln.