Solvent Extraction of Transition Metal Ions by an in situ Extractant Formation Method. Diethyldithiocarbamate-Forming System

1997 Analytical Sciences  
By mixing carbon disulfide and diethylamine in chloroform prior to use, diethyldithiocarbamic acid forms. Such metal ions as Cue, Cot, Zn2+, Nit, Cd2+, Fe3+, and Mn2+ were quantitatively extracted from an aqueous sulfate medium as a diethyldithiocarbamate complex. The optimum extraction conditions, such as the pH, mixing ratio of reagents, and shaking time for equilibrium, were established. Keywords Solvent extraction, in situ extractant formation method, diethyldithiocarbamate It has
more » ... te It has previously been reported"2 that during in situ extractant formation in which the extractant, 3-methylbutylxanthic acid, was formed in the organic phase by mixing organic solvents, such as carbon disulfide (CS2), pyridine, and 3-methylbutyl alcohol, the divalent transition metal ions, such as Zn2+, Cot, Nit, Cd2+, and Cue+, could be extracted quantitatively. Since this mixed solvent was characterized by having a wide mixing range of solvents for the quantitative extraction of metal ions, it could be prepared by simply a rough measurement of the volumes using a beaker. The conversion of the mixed solvent to 3-methylbutylxanthic acid, however, was not complete. Consequently, the extractability of metal ions with this system could not be compared well with that using the ordinary alkylxanthate extraction method. With the objective of finding a more effective extraction system, a diethyldithiocarbamic acid-forming system was investigated in this study. A CHC13 solution containing CS2 and diethylamine was found to be capable of extracting some divalent transition-metal ions over a wide pH range; it was considered that diethylammonium diethyldithiocarbamate (DEDDTC) formed in the solution. In the conventional diethyldithiocarbamic acid(DDTC)-extraction method, a DEDDTC CHC13 solution is generally recommended for use in metal extraction instead of a sodium diethyldithiocarbamate (NaDDTC) aqueous solution, since NaDDTC rapidly decomposes in an acidic aqueous solution, even though DEDDTC is more than 5-times more expensive than NaDDTC. It was found that the method described in this paper has the following advantages over the conventional DDTC-extraction method: (1) ability to obtain the same effect as that gained with the extractant in a high concentration by simply changing the amount of CS2 and diethylamine, (2) it is not necessary to measure the extractant solution volume precisely on preparation, and (3) precautions for preserving the unstable extractant solution are unnecessary, since diethyldithiocarbamic acid (HDDTC) is produced by mixing CS2 and diethylamine prior to use. Experimental Reagents and apparatus A Hitachi Model 508A atomic absorption spectrophotometer was used to measure the metal-ion concentration with a flame atomizer, and a Shimadzu GDU-20C double-beam spectrophotometer to obtain absorption spectra with 1.0 cm pathlength quartz cells. Metal standard solutions were prepared as follows: metals of X99.9% purity were dissolved in nitric acid and fumed with sulfuric acid almost to dryness, followed by dissolution of the residue in 0.05 M sulfuric acid to a definite volume. The metal solutions were standardized by EDTA titration and stored in polyethylene bottles. A buffer solution of 0.033 M diammonium hydrogencitrate and 0.017 M citric acid was used to adjust the pH as well as to mask the metal ions. The other reagents were of analytical grade. Preparing a DDTC forming CHCl3 solution A DDTC-forming CHC13 solution having a composition of 0.5 : 0.5 : 99 (v/v%) for CS2 : (C2H5)2NH : CHC13 was mainly used for the metal extraction in this study, and was prepared as follows: after 1 ml of CS2 was added to ca. 100 ml of CHC13 in a 200 ml volumetric flask using a pipette, 1 ml of diethylamine was added to it
doi:10.2116/analsci.13.225 fatcat:45h6bcczyja7bep7bdtow67dj4