Spectrophotometric Determination of pH and Iron in Seawater: Equilibria and Kinetics

Abstract

The apparent dissociation constants, pK', of five sulfonephthalein indicators (thymol blue, bromophenol blue, bromocresol green, bromocresol purple, and phenol red) were determined in 35 o/oo salinity seawater at 25 °C and over a pressure range from atmospheric to 1000 bars. The indicators were used to measure seawater pH over the above pressure ranges using the pK' and measured absorbance ratios of the acidic and basic components of a particular indicator. The pK' of the indicators were determined without the use of potentiometric pH measurements. The indicators provide a thermodynamically consistent free hydrogen ion pH scale independent of the problems of electrode drift and liquid junction error common to pH electrodes. The pH indicators can be readily adapted for in situ pH measurements. The visible spectrum of each indicator was deconvoluted into four gaussian components using a nonlinear curve fitting approach. The spectrum of the basic form of the indicator was described by three components and the acid form by one peak. The gaussian components were defined in terms of their peak position, width, and height. Combining the gaussian parameters with the thermodynamic data for the indicators allowed quantitative modeling of an indicator's spectrum as a function of pH. A general equation was derived for the calculation of pH from two absorbance measurements of a solution which contains two or more indicators. The application of multiple indicators significantly expands the pH range over which pH measurements can be made with a single indicator. Using the modeled spectra for several indicators, optimal indicator combinations for specific pH ranges were determined. The combination of phenol red and bromocresol green allowed determination of seawater pH over the range 8.2-3.0 which is suitable for oceanic pH and alkalinity determinations. UV spectroscopy was used to determine the first hydrolysis constant of Fe(III), *1 B , at 25 °Cover a pressure range from atmospheric to 1000 bars. Based on these data the partial molal volume and compressibility change for the hydrolysis reaction were -13.1 cm3mol-1 and 9.3 x 10-4 cm3mol-1bar-1 respectively. The *1B values were determined independently of optical constants over the full pressure range. The results demonstrate that molal absorptivities of Fe3+ and FeOH2+ not independent of pressure as assumed by previous investigators. An empirical equation provides values of *1B as a function of temperature (273 ≤ T ≤ 300), ionic strength (0.1 ≤ T ≤ 1.0), and pressure (0 ≤ P ≤ 1000). The pH