Speciation 98: Abstracts

Probably the most important application of stability constants is to predict the behaviour of mixtures on metal ions and potential ligands in solution. This is important in almost every branch of solution chemistry including biological, environmental, industrial and analytical chemistry. Speciation curves express the amount of each chemical species present as a function of some variable; generally pH but also of free concentration of another reactant or of total concentration of a reactant. Amounts of individual complexed species may be expressed as a percentage of total amount of a particular reactant present, as free concentrations (generally as log₁₀ conc. values) or as the total amount of a particular reactant in solution (i.e. excluding precipitates), S - generally expressed as log₁₀S plots.

In order to calculate reliable speciation curves it must be assumed that the reactatants are at equilibrium and that all significant stability constants, measured under the experimental conditions used, are known. This will be a demonstration lecture, using two different software approaches^1,2, which will highlight some of the strengths and weaknesses of using modelling in real-life situations. Both approaches are based on mass-balance calculations and exploit the ease of use and flexibility of the Windows interface. User-friendliness and flexibility are sadly lacking from the majority of powerful speciation programs!

Factors which should be considered include:

1. the likelihood of precipitation.
2. the presence of polynuclear species.
3. ternary complex formation.
4. ionic strength corrections.

When studying the fate of heavy metals in environmental systems (generally very dilute solutions) precipitation is the most important consideration. Accurate prediction is handicapped because:

1. equilibrium is often slow and accompanied by super-saturation.
2. precipitation must be preceeded by an almost infinite number steps of polynuclear complex formation.
3. reliable stability constants and solubility products, measured under real conditions, are difficult to find.
4. at the best, ionic strength corrections are poor - especially for multivalent metal ions.

Important metal ions to be considered include: Al³⁺, rare earths and UO₂²⁺. Various attempts to model systems involving these ions under environmental conditions (e.g. natural water, including sea water, effluent and soil) will be demonstrated and discussed.

References

1. L.D. Pettit in the SolEq Tutorial package. In Preparation (see: http://www.acadsoft.co.uk)
2. L. Alderighi, P. Gans, A. Ienco, D. Peters, A. Sabatini, A. Vacca. In Press (see: http://www.chim1.unfi.it/group/vacsab/index.htm)

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