AN EXPERIMENTAL-DETERMINATION OF FERROUS CHLORIDE AND ACETATE COMPLEXATION IN AQUEOUS-SOLUTIONS TO 300-DEGREES-C

The formation of the monochloroiron(II) complex, FeCl+, was studied potentiometrically from 125 to 295-degrees-C at 25 degree intervals at one molal ionic strength in aqueous solutions containing acetic acid, sodium acetate, and sodium trifluoromethanesulfonate. In this method, competition between chloride and acetate ions for the ferrous cation resulted in a change in solution pH, which in turn was monitored in situ in a hydrogen-electrode, concentration cell. A simple empirical approach was used to extrapolate these formation quotients to infinite dilution. The resulting constants proved to be in excellent agreement with previous spectrophotometric results obtained from 25 to 200-degrees-C. Thus, the present study confirms the validity of the conclusions made based on these earlier data concerning the solubility of Fe-containing minerals in hydrothermal brines. Formation constants at infinite dilution for FeCl+ are compared with the stability of ferrous acetate and hydroxide complexes. The original potentiometric titration data for ferrous acetate complex formation were combined in a new fit with values determined from the present study at unit ionic strength. Two empirical treatments (namely the isocoulombic method and the temperature/water density function) were considered for fitting and extrapolating the infinite dilution formation constants to 350-degrees-C.