Studies in solvent extraction of inorganic complexes

• Main Author: Pierce, T. B.
• Published: University of Oxford 1959
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.734651

The solvent extraction of inorganic complexes has been studied in connection with two distinct problems, (i) the determination of small quantities of silver at a level of 1 part in 10⁸ in trade effluent, and (ii) the measurement of the stability constants of metal completes which are very sparingly soluble in water, though soluble in an organic solvent. (i) The method finally proposed for the determination of small quantities of silver in trade effluent containing organic material in addition to copper, iron, lead, mercury, and calcium, etc. was arrived at after detailed study of a) The concentration of the silver; b) The separation of the silver from elements likely to interfere in the final determination; c) The absorptiometric determination. a) Concentration of silver by co-precipitation was investigated using Ag^110m as a radioactive tracer. It was found that silver was quantitatively co-precipitated as sulphide with mercury, copper, and lead. If organic material was present in the solution it was more satisfactory to co-precipitate the silver with elementary tellurium. Although efficient as a means of concentration, co-precipitation did not provide a satisfactory separation of silver from the metals likely to interfere in the final determination. The silver complex of dithizone (diphenyl thiocarbazone) is soluble in organic solvents, and its considerable stability to acid solutions suggested that it would provide a very useful method of concentration of silver by a factor of 10 or better, since only mercury and some copper are likely to be co-extracted. These predictions-were verified experimentally and, in addition to concentration, a preliminary separation of silver was thus effected. b) It was shown that a solution of dithizone in carbon tetrachloride would retain copper and mercury even in the presence of aqueous thiocyanate, but that the superior stability if the thiocyanate complex of silver over that of the dithizone complex, permitted its quantitative back extraction into an acidic aqueous phase. Selective extraction was thus obtained, After the thiocyanate had been destroyed by wet ashing the whole of the silver was obtained free from metals which could interfere in the final determination. c) p-Dimethylaminobenzylidene rhodanine, tetraethylthiouram disulphide and dithizone were all considered for the absorptiometric determination of silver. Dithizone was finally chosen as it was considered to provide the best compromise between overall accuracy and general convenience for the determination of the concentration of silver after destruction of the thiocyanate with sulphuric acid. Both visual and photoelectric determinations were found to be accurate to ± 0.1 μg in the range from 0-10 μg. It was found that traces of silver are absorbed upon glassware very readily. If, however, all surfaces which are likely to come into contact with the element are given a coating of silica by treatment with a commercial silicone preparation, it was found that the absorption could be reduced to negligible proportions. (ii) The determination of the stability constant of the metal complex ML_n by the normal titration technique is not possible if it is insoluble in an aqueous phase. If the complex is soluble in an organic phase, however, it is often possible to determine a mixed stability constant K′_f, where K′_f = [ML_n]_o/[Mⁿ⁺]_w[L^-]ⁿ_w (the subscripts o and w referring to the organic and aqueous phases respectively) which involves a partition coefficient. This is not altogether satisfactory, since comparison of the changes in this stability constant observed going from one compound to another in a series of closely related complexing agents, involve concomitant changes in the partition coefficients p_r and p_c, of the reagent HL and the complex ML_n respectively. A method is derived from theoretical principles which enables the true formation constant K_f of ML_n K_f = [ML_n]_w/[Mⁿ⁺]_w[L^-]ⁿ_w to be obtained if the metal M is labelled with a radioactive isotope. An added complication is the possible presence of lower complexes ML, ML₂ ... in the aqueous phase. A full theoretical study of equilibrium between a solution containing metal complexes ML_j, (j = 0 to N) in the aqueous phase, in equilibrium with an immiscible organic phase in to which only the formally neutral species ML_n is extracted, shows that a) the relative concentrations of the intermediate species is lower than for a single phase system, and b) the overall degree of formation is higher. It has also been shown that Job's Method of Continuous Variations applies equally to a two-phase system and enables the composition of the extracted species to be determined with greater reliability than in the case of a one-phase system. If the radioactive isotope of M is available so that the total concentration of metal in the aqueous phase (^{j = N}_{j = O} Σ[ML_j]) and in the organic phase ([ML_n]_o) is known, the stability constants β_n of all the intermediate complexes may be calculated, where β_n = [ML_n]_w/[Mⁿ⁺]_w[L^-]ⁿ_w. By use of radioactive silver f the solubility of some substituted dithizones in water and several organic solvents is obtained, in order to calculate the partition coefficients. Also the two-phase method of continuous variants is applied to the indium-dithizone system and the extinction coefficient and the overall formation constant of indium dithizonate calculated.