Chloroform Extraction of Cadmium and Zinc with Methyl, Ethyl, Propyl, and Butyl Xanthates
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BUNSEKI KAGAKU Vol. 32, pp.E17-E23, 1983 CThe Japan Society for Analytical Cheraistry, 1983 CHLOROFORM EXTRACTION OF CADMIUM AND ZINC WITH METHYL, ETHYL, PROPYL, AND BUTYL XANTHATES Yoshiaki SASAKI Department of Chemistry, Faculty of Science, Yamaguchi University, Yoshida, Yamaguchi-shi, Yamaguchi 753 The extraction behavior of cadmium and zinc with methyl, ethyl, propyl, and butyl xanthates into chloroform was studied spectrophotometrically. It was found that the extracted species were the type of M(xanthate)2 in all cases. In addition, the over- all formation constants and the distribution constants for these complexes were determined. In the systems of methyl and ethyl xan- thates, the water-soluble complexes having a formula of M(xan - thate)-3 were formed in the presence of a large excess of xanthate. The stability of M(xanthate)2 increased in the order methyl< ethyl < propyl < butyl xanthate, and a linear free energy relationship was observed between the basicity(pK a) of xanthate and the stabil- ity(log ƒÀ2) of complex. INTRODUCTION Alkyl xanthates(O-alkyl dithiocarbonates, abbreviated as X) react with many metal ions to give chelates which are sparingly soluble in water but readily solu- ble in organic solvents such as chloroform1). This property of xanthates is of interest in separation analysis. Ethyl xanthate has especially been used as a re- agent for the solvent extraction of metal ions2). However, there are few thermo- dynamical investigations on the solvent extraction since xanthates are unstable in acid media. The author and co-workers have studied the solvent extraction of several metal-xanthate complexes, and reported the compositions and the formation con- stants of the extracted complexes3-7). In addition, it was confirmed that water- soluble complexes such as MX-3 or MX-42- were formed in the presence of a large ex- cess of xanthate in the systems of cadmium and zinc benzyl(BeX) xanthates5) and zinc ethyl xanthate3). This paper describes the composition of the extracted species and the forma- tion and distribution constants of the complexes formed in the chloroform extrac- tion of cadmium and zinc with methyl(MeX), ethyl(EtX), propyl(PrX), and butyl(BuX) xanthates. EXPERIMENTAL Reagents E18 BUNSEKI KAGAKU Vol. 32(1983) Potassium salts of the alkyl xanthates were synthesized according essentially to the method of Price et al.8) with corresponding alcohols, carbon disulfide, and potassium hydroxide. These salts were recrystallized three times from solutions of the corresponding alkyl alcohols, washed with ethyl ether, dried in vacuo, and stored in a refrigerator. All xanthate solutions were freshly prepared daily. Stock solutions of cadmium and zinc were prepared by dissolving cadmium ni- trate tetrahydrate and zinc nitrate hexahydrate in 0.1 mol dm-3 nitric acid, re- spectively. These solutions were standardized volumetrically with EDTA (Cd2+: 1.986 •~ 10-2 mol dm-3; Zn2+: 1.036 •~ 10-2 mol dm-3). Apparatus A Shimadzu Model UV-200 spectrophotometer with 1 cm quartz cells and a Toa Model HM-9A pH meter with a glass electrode were used for spectrophotometric and pH measurements, respectively. A thermostat equipped with a Taiyo thermo unit "Minder Junior" was used to maintain the temperature of the solution at 25 + - 0 .5 •‹C during the extraction period. Procedure All solutions were previously thermostated at 25 •‹C. The solution of cadmium or zinc was taken into a 50 cm3 measuring cylinder equipped with a Teflon stopper. An ionic strength of the solution was adjusted to 1.0 with potassium nitrate and desired pH values were obtained by the addition of potassium hydroxide solution. A proper amount of potassium xanthate solution was added, and the total volume of the solution was made up to 20.0 cm3 with water. After the addition of 15.0 cm3 of chloroform, the cylinder was shaken vigorously for 1 min and allowed to stand in the thermostat for 4 min. The shaking and standing was further repeated 3 times to ascertain complete equilibration. The chloroform phase was separated and dried with anhydrous sodium sulfate. The absorbance of the organic phase was then measured against the reagent blank at a fixed wavelength. The pH of the aqueous phase was also measured. RESULTS AND DISCUSSION Effect of pH on extraction The complexes extracted into the chloroform phase gave the absorption spectra having an absorption maximum in the ultraviolet region, the absorption maxima of the cadmium complexes of MeX, EtX, PrX, and BuX being found at 305, 306, 307, and 305.5 nm, respectively. Similarly, those of the zinc complexes of MeX, PrX, and BuX were found at 301.5, 300, and 301.5 nm, respectively. The degree of the extraction of these metals was determined on the basis of the absorbance of the chloroform phase at each maximum wavelength. The relation- ships between the absorbance of chloroform phase and the final pH of aqueous phase were shown in Figs. 1 (for cadmium) and 2 (for zinc). A plateau was observed in each system except the zinc-MeX system. The decrease in absorbance on both sides of the plateau may be due to the acid decomposition of the xanthates in the lower pH region and the hydrolysis of metal ions in the higher pH region. The optimum E19 I: MeX(at 301.5 nm I: MeX(at 305 nm) II: PrX(at 300 nm) II: EtX(at 306 nm) DI: PrX(at 307 nm) DI: BuX(at 301.5 nm) IV: BuX(at 305.5 nm) Fig. 1. Effect of pH on extraction of cadmium. Fig. 2. Effect of pH on extraction of zinc. pH range for the extraction of cadmium and zinc increased in the order MeX < EtX < PrX < BuX. This indicates that the extractability of complex increases in the same order. Effect of concentration of xanthate on distribution ratio of metal The effect of the concentration of xanthate on the absorbance of the organic phase was investigated under the condition of the optimum pH for each system. The zinc-MeX system with no plateau pH region was not examined. The molar absorptivity(e) for each complex in chloroform at the maximum wave- length was calculated from a sum of the absorbances for the first and the second extracts. The distribution ratio(D) of metal can be calculated from the following equa- tion: (1) Where, CM is the total concentration of metal, A is the absorbance measured, while, Vorg and Vaq are the volumes of organic and aqueous phases, respectively. Figures 3 and 4 show the relationships between the concentration of xanthate and the distribution ratio of cadmium and zinc. E20 BUNSEKI KAGAKU Vol. 32(1983) Fig. 3. Relation between distribu- tion ratio of cadmium and concen- tration of xanthate ion. Fig.4. Relation between distribu- tion ratio of zinc and concentra- tion of xanthate ion. In these figures, the numerical values on the abscissa indicate the concen- tration of free xanthate ion in the aqueous phase at the extraction equilibrium, and this concentration is estimated as follows: 1) It is considered that potassium xanthate in aqueous solution is perfectly dis- sociated into ions, because the pH of the solution under the experimental condi- tions is in the range of 7.5 - 8.5 and the acid dissociation constants(pK a) of the xanthates used are less than 2. 2) Xanthates are unstable in an acidic aqueous solution and decompose into carbon disulfide and the original alcohols. The rate of the decomposition depends seri- ously on the acidity of solution but very slow above pH 7. For this reason, the decomposition of xanthate during the experiment can be neglected. 3) The quantity of free xanthate extracted into the chloroform phase can be ig- nored in comparison with that of free xanthate ion in the aqueous phase. 4) The extracted species is assumed to be only MX2. 5) On the basis of these considerations, the free xanthate ion concentration in the aqueous phase at the extraction equilibrium can be expressed by the following equation: E21 (2) Where, CXis the total concentration of xanthate and Kpthe distribution constant of MX2. Formation constant and distribution constant When it is assumed that the complexes formed in aqueous solution are MX+, MX2, and MXm-2+m and that the extractable species is only MX2, the distribution ratio of metal is represented by the following equation: (3) Equation (3) is transformed into eqn. (4) by the use of the formation constants( βn) and the distribution constant. (4) If the concentration of xanthate is sufficiently low, eqn. (4) can be simpli- fied to eqn. (5). (5) Equation (5) shows that a plot of log D vs. log [X-] gives a straight line with a slope of 2. The same relationships held in all extraction systems examined, as shown in Figs. 3 and 4. The assumption of MX2 to be extracted was amply justified by these results. If the concentration of xanthate is high enough, eqn. (4) can also be simpli- fied to eqn. (6). (6) Equation (6) shows that the linear relationship having a slope of minus m exists between log D and log [X-]. This relationship was only observed in the systems of cadmium-MeX and cadmium-EtX. In these systems the value of m was unity, and then the formation of Cd(MeX) and Cd(EtX)-3 was indicated. In the other systems in which no complex of the MXm-2+ m type was formed, it was expected that the distribu- tion ratio was nearly equal to the distribution constant being independent on the concentration of xanthate, as shown in Figs. 3 and 4. The formation constants and the distribution constants for each system could be determined from the plots of Figs.