Solvent Extraction of Alkali Metal(I) and Cobalt(Ii) from Aqueous Alkali Metal Thiocyanate Solutions Into 1,2- Dichloroethane with Nonionic Surfactant, Triton X-100

BUNSEKI KAGAKU Vol. 32, pp.E367-E370, 1983 C The Japan Sociery for Analytical Chemistry, 1983

NOTES

SOLVENT EXTRACTION OF ALKALI METAL(I) AND COBALT(II) FROM AQUEOUS ALKALI METAL THIOCYANATE SOLUTIONS INTO 1,2- DICHLOROETHANE WITH NONIONIC SURFACTANT, TRITON X-100

Masaru NIITSU R and Yoshio MUKAI

Faculty of Pharmaceutical Sciences, Josai University, 1-1, Keyakidai, Sakado-shi, Saitama 350-02

The solvent extraction of alkali metal(I) and cobalt(II) from 1 mol dm-3 solutions of five alkali metal thiocyanates into 1,2-dichloroethane with Triton X-100 was examined. The extractability of cobalt (II) was closely dependent on that of the alkali metal thiocyanate. The reason for this was considered.

For the spectrophotometric determination of nonionic surfactants, solvent extraction with thiocyanatocobaltate(II) complex has been employed1)-4) Recently, nonionic surfactants have also been used as the extractants for various metal ions5)-10). The extracted species of cobalt(II) from ammonium thiocyanate solutions into 1,2-dichloroethane with Triton X-100 (polyethylene glycol alkylphenyl ether) was estimated by Sotobayashi et al5) to be (NH4)2Co(SCN)4(Triton X-100)2.

However, the effect of co-extracted cations on the extraction of thiocyanatocobaltate(II) complex with nonionic surfactants has not been examined.

In the present paper, the extraction behavior of alkali metal(I) from its thiocyanate solution (1 mol dm-3) and cobalt(II) from a weakly acidic aqueous solution containing 1 mol dm-3 alkali metal thiocyanate and a small amount of cobalt

(II) into 1,2-dichloroethane containing Triton X-100 was studied and the extraction curves were compared. On the basis of the results obtained, the effect of alkali metal cations on the extraction of cobalt(II) was considered.

EXPERIMENTAL

All the procedures were carried out at 25•Ž. Triton X-100 was obtained from

New England Nuclear Co., and thiocyanates of lithium, rubidium, and cesium were from Soekawa Rikagaku Co.. The other reagents were of analytical grade.

One mol dm-3 aqueous solutions of each alkali metal thiocyanate containing none or 2•~10-4 mol dm-3 cobalt(II) nitrate were prepared. When the solution con- tained cobalt(II), hydrochloric acid was added and the acid concentration was adjusted to be 1•~10-3 mol dm-3. A 10-ml portion of this aqueous solution and the same volume of 1,2-dichloroethane containing a certain amount of Triton X-100 were placed in a stoppered glass tube and agitated mechanically until the distribution equilibrium was achieved. After centrifuging, the alkali metal(I) and cobalt(II) E368 BUNSEK I KAGAKU Vol. 32(1983)

in the organic phase were back-extracted into 0.1 mol dm-3 hydrochloric acid. The amounts thus back-extracted and that of cobalt(II) which remained in the aqueous phase were determined by an atomic absorption method. The distribution ratio was calculated by the following equation: where M is the alkali metal(I) or cobalt(II) and CM( org) and CM are the total concentrations of M in the organic and aqueous phases, respectively.

RESULTS AND DISCUSSION In the preliminary experiments, the extraction of alkali metal(I) into 1,2-dichloroethane was found to be very poor in the absence of Triton X-100; the distribution ratio of potassium(I) was 10-5'3 and that of rubidium(I) or cesium(I) was about 10-6. The distribution ratios of sodium(I) and lithium(I) could not be determined quantitatively by the present method and were estimated to be less than 10-6. However, the extraction in the presence of Triton X-100 was markedly en- hanced. Figure 1 shows the extraction of alkali metal(I) into 1,2-dichloroethane with Triton X-100.

Fig. 1 Distribution ratio of alkali metal(I) as a function of concentration (w/v%) of Triton X-100 in 1,2-dichloroethane.

In Fig. 1, the plot of each alkali metal thiocyanate is a straight line in all cases and the distribution ratios increase in the following order: LiSCN < NaSCN < CsSCN RbSCN

In the preliminary experiments, it was found that the distribution ratio of cobalt(II) was not affected by aqueous hydrochloric acid concentrations less than 2x10-3 mol dm-3 but it decreased with an increase in the acid concentration at higher concentration ranges. Figure 2 shows the extraction of cobalt(II) from its acidic aqueous thiocyanate solution into 1,2-dichloroethane with Triton X-100.

Fig. 2 Distribution ratio of cobalt(II) as a function of concentration (w/v%) of Triton X-100 in 1,2-dichloroethane.

Each extraction curve is a straight line except that from the potassium thiocyanate solution; the curve for potassium thiocyanate deviates from a straight line at the concentration range of Triton X-100 less than 0.02 %. As is seen from Fig. 2, the extraction of cobalt(II) is better in the following order of the salts in the aqueous phase: LiSCN

RbSCN

{M・Triton X-100} +2 {Co (SCN) 4} 2-. This assumption reasonably explains the results in

Figs. land 2. Although the slope of the extraction curves of alkali metal ions in

Fig. 1 is not always unity, still it is seen that the slope of the extraction curve of cobalt(II) in Fig. 2 is nearly twice that of the alkali metal ions. Further- more, the order of the extraction of cobalt(II) as ion-pairs agrees quite well with that of alkali metal ion as thiocyanate. This close dependence of the extraction

of cobalt(II) on that of the alkali metal ion may be explained as follows. The extractability of the cobalt(II) complex is almost entirely dependent on that of the counter complex cation ({M•ETriton X-100}+) in the ion-pair and the extractability of the counter complex cation should be just parallel to that of alkali metal thiocyanate in Fig. 1. The much better extraction of the cobalt(II) complex anion than thiocyanate ion should be due to the larger volume of the former and the difference in the slope should be due to the difference in the charge of these anions.

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Keyword phrases extraction of alkali metal(I) and cobalt(II) as thiocyanate complex; effect of alkali metal ion on extraction of cobalt(II); Triton X-100.

(Received April 30, 1983)