In-situ Spectrophotometric PH Measurements: the Effect of Pressure on Thymol Blue Protonation and Absorbance Characteristics

Authors

Amanda E. Hopkins, Eckerd College
Karen S. Sell, Eckerd College
Alan L. Soli, Eckerd College
Robert H. Byrne, University of South FloridaFollow

Document Type

Article

Publication Date

2000

Keywords

Seawater pH, Spectrophotometric, Thymol blue, High pressure, Partial molar volume, Carbon dioxide system

Digital Object Identifier (DOI)

https://doi.org/10.1016/S0304-4203(00)00043-8

Abstract

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: ΔV_i=−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 aslogK2PK20=+[2.99(±0.02)×10−4]×P−[3.3(±0.3)×10−8]×P2,">logK2PK20=+[2.99(±0.02)×10−4]×P−[3.3(±0.3)×10−8]×P2, where K₂^P 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 (e_i) 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.

Was this content written or created while at USF?

Yes

Citation / Publisher Attribution

Marine Chemistry, v. 71, issue 1-2, p. 103-109

Scholar Commons Citation

Hopkins, Amanda E.; Sell, Karen S.; Soli, Alan L.; and Byrne, Robert H., "In-situ Spectrophotometric PH Measurements: the Effect of Pressure on Thymol Blue Protonation and Absorbance Characteristics" (2000). Marine Science Faculty Publications. 1724.
https://digitalcommons.usf.edu/msc_facpub/1724