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

The apparent dissociation constants, pK\u27, 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\u27 and measured absorbance ratios of the acidic and basic components of a particular indicator. The pK\u27 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\u27s 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 indicators were used to study the effect of pressure on the second dissociation constant of bisulfate, K 1/2, and the rate of Fe(II) oxidation in seawater. The effect of pressure on bisulfate dissociation was determined over the pressure range atmospheric to 1000 bars from the compressional pH change of acidic seawater solutions. The partial molal volume and compressibility change for bisulfate Dissociation was -13.9 cm3mol-1 and -6.54 x 10-3 cm3mol-1bar-1 respectively. An empirical equation provides values for K 1/2 over the full range of oceanic temperatures, pressures, and salinities. The rate of Fe(II) oxidation was established by monitoring the increase in the Fe(III) UV absorbance with time using a high pressure optical cell. Fe(II) concentrations were calculated from the iron mass balance. The pseudo first order rate constant for Fe(II), k\u27, exhibited a second degree [H+] dependence over the pH range 7.0-8.2. At constant pH, k\u27 increased by a factor of 6 with compression from atmospheric to 1000 bars. The change ink\u27 was due to the increase in [OH-] with pressure. The pressure dependence was eliminated when the rate data were transformed to a constant pOH scale. The net effect is a small temperature dependent decrease in the predicted Fe(II) oxidation rates from surface to deep ocean environments. The oxidation rate of Fe(II) in Narragansett Bay seawater was determined for both naturally occurring Fe(II) and for Fe(II) added at close to natural concentrations. The oxidation rate of added Fe(II) was in good agreement with the predicted rate based on rate constants for surface waters of Narragansett Bay was between 6-11 nmol kg-1. The oxidation rate of naturally occurring Fe(II) was slower than the predicted rate by an order of magnitude. Stabilization of Fe(II) by adsorption onto particles or by organic complexation could explain the reduced Fe(II) oxidation rate. The Fe(II) production rate required to maintain the observed quasi-steady state (Fe(II) concentration was 8 ± 6 nmol kg-1 hr-1. This production rate could be sustained by photochemical reduction of particulate Fe(III) to Fe(II)