A SPECTROPHOTOMETRIC STUDY OF AQUEOUS IRON (II) CHLORIDE COMPLEXING FROM 25 TO 200-DEGREES-C

Ultraviolet absorption spectra of solutions in the system FeC12-HCl-H2O with chloride concentrations ranging from 0.01 to 3.4 m and Fe(II) concentrations ranging from 0.005 to 0.025 m have been measured at 25, 50, 100, 150, and 200°C and the equilibrium saturation vapor pressure of the system. Non-linear least squares regression was used to estimate cumulative thermodynamic association constants and molar absorptivities. At 25, 50, and 100°C, only Fe2+ and FeCl+ are detectable, whereas at higher temperatures a minor concentration of a third species, interpreted to be FeCl02, starts to contribute substantially to total absorbance. Recommended values of the association constants, β1 and β2, for the equilibria Fe2+ Cl-1 = FeCl+ and Fe2+ + 2C1- = FeCl02, respectively, (and their estimated maximum uncertainty range) are as follows: {A table is presented}. In combination with published thermodynamic data for the Fe-S-O-H system, the equilibrium formation constants are used to calculate solubilities of selected Fe mineral assemblages in NaCl-rich hydrothermal solutions. The results are in satisfactory agreement with published analytical data from basinal brines and geothermal fluids. © 1990.