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 po tentiometrically from 125 to 295-degrees-C at 25 degree intervals at o ne molal ionic strength in aqueous solutions containing acetic acid, s odium acetate, and sodium trifluoromethanesulfonate. In this method, c ompetition between chloride and acetate ions for the ferrous cation re sulted in a change in solution pH, which in turn was monitored in situ in a hydrogen-electrode, concentration cell. A simple empirical appro ach was used to extrapolate these formation quotients to infinite dilu tion. 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 mad e based on these earlier data concerning the solubility of Fe-containi ng minerals in hydrothermal brines. Formation constants at infinite di lution for FeCl+ are compared with the stability of ferrous acetate an d hydroxide complexes. The original potentiometric titration data for ferrous acetate complex formation were combined in a new fit with valu es determined from the present study at unit ionic strength. Two empir ical treatments (namely the isocoulombic method and the temperature/wa ter density function) were considered for fitting and extrapolating th e infinite dilution formation constants to 350-degrees-C.