IN-SITU DETERMINATION OF THE APPARENT SOLUBILITY PRODUCT OF AMORPHOUS IRON SULFIDE

The apparent (pK(s)') and pK(sp)NBS solubility products of amorphous iron sulphide (FeS) were determined experimentally, in situ, in the Baltic Sea. The apparent and pK(sp)NBS equilibrium constants were 2.6 and 3.15, respectively, at a mean experimental temperature of 15.8-degrees-C. The temperature dependence of the apparent equilibrium constant (within the temperature range 12.8-18.8-degrees-C) is described by the equation: pK(s) = -0.068t + 3.70, where t denotes the temperature in centigrade. The composition of the amorphous precipitate was estimated to be FeS0.86. The charge of the non-stoichiometric iron sulphide compound is most likely charge balanced by some major sea salt anion, e.g. chlorine and/or fluorine. Such charge balance has been observed for colloidal amorphous iron (III) hydroxides, another common and kineticly fast-forming precipitate in natural waters.