Introduction to Geochemical Modelling - University of Bristol

Physical Chemistry of Minerals and SolutionsDM Sherman, University of Bristol2008/2009Method..•Define the unique (“basis” ) species that define thecomposition (i.e., the components).•Consider all of the species that might form from thatbasis.•Set up the mass-action law for each species.•Set up the mass or charge balance constraint.•Solve the simultaneous equations...Example: ZnS in 0.1m NaCl, pH = 3•Define a set of “basis” species (components) thatdefine the compositional variations of the system:Zn +2 , H 2 S, Cl - , H + , Na +•Consider all of the other species that might form fromthat basis:ZnS(s) + H + = Zn +2 + HS-Zn +2 + Cl - = ZnCl-Zn +2 + 2Cl - = ZnCl 2-HS- + H + = H 2 S(Note: each equilibrium must be unique so that onereaction cannot be a sum of two others.)Page ‹#›