Factors important in the selection and description of liquid-liquid extraction procedures used in inorganic analytical chemistry (Technical Report)
The report deals with liquid-liquid distribution systems, in which one phase is aqueous, and the distribution of the component can be influenced in many cases with chemical reactions. The formulas and equations are given, by which the actual distribution ratio of the analyte at given conditions and concentrations can be predicted. The conditional extraction constants, which are essential for the prediction, can be calculated from the real extraction constants and side reaction functions involving the competing chemical reactions, and their equilibrium constants. The kinetic aspects of the distribution process, the current analytical techniques, the analytical methods involving liquid-liquid distribution, the fields of applications of liquid-liquid extraction are briefly introduced. A classification scheme of the extraction systems used in analytical chemistry is proposed in which the basic reaction equations are given, from which the extraction constant, the distribution ratio of the analyte can be formulated, and using the formulas, and the necessary equilibrium constant values, the attainable extent of extraction (fraction extracted) can be estimated. INTRODUCTION Liquid-liquid extraction procedures used in analytical chemistry are controlled by distribution processes of the components between two liquid phases. This report deals with systems of analytical interest in which The concentration of the component of interest distributed between the liquid phases is low; One liquid phase is aqueous. This phase favours ionization processes, and shows preference for ionic compounds in contrast to the other, less polar, organic phase; The distribution of the component in most cases can be influenced by chemical reactions (by protonation, complex or adduct formation, etc.); No distinction is made, whether a solvent with solvating properties (with donor or acceptor group content) or an inert solvent is used, the solvent always being in large excess compared to the distributed species. (Thus, the effect of coordination by solvent molecules is included in the extraction constant.) DESCRIPTION OF DISTRIBUTION EQUILIBRIA According to fundamental phy sico-chemical considerations the distribution of a component between two immiscible solvents in equilibrium is completely defined by the partition constant at given temperature and pressure. The partition constant is a ratio of the activities of the component in the two liquid phases. If the concentration of the component is low (< 10” mol 1-l) in both phases and the composition of the liquids is not changed (i.e. in the aqueous solution the ionic strength is constant), the partition ratio (distribution constant) in terms of concentrations maybe used in place of the partition constant, and accepted as practically constant.