Indian Journal of Chemistry Vol. 22A. November 1983. pp. 962-964 Solvent Extraction of Potassium : Crown Ethers as Extractants for Potassium from Picrate Solution B S MOHITE & S M KHOPKAR· Department of Chemistry, Indian Institute of Technology, Bombay 400076 Received 27 May 1983; revised and accepted II July 1983 Extractive separation and flame photometric determination of potassium are described. Potassium in the concentration range oflOO JIgto 700 Jlg/ml is extracted quantitatively between pH 1.0and 2.5 with 0.0 1M 18-crown-6 or dibenzo-18-crown-6 or dicyclohexyl-18-crown-6 in methylene chloride in the presence of 0.0 I M picric acid as a counter ion. Potassium from the organic phase is stripped with 6M hydrochloric acid and determined flame photometrically at 767 nm. It is possible to quantitatively separate potassium in the presence of I:I0 of alkali and alkaline earth elements, and in large ratios of common anions. Large amounts of organic acids are tolerated in such extraction. The method has been extended to the analysis of potassium in blood serum, milk, soft soap and optical glass. The method is simple, rapid and selective. The use of crown ethers as extractant for the analytical lithium hydroxide or 0.01M picric acid. The total separation of alkali metals has been widely volume of the solution was made upto 10 ml, investigated 1-11. However, no systematic in- transferred into a separating funnel and a solution (10 vestigation has been carried out for the solvent ml) of appropriate crown ether (0.01M) in methylene extraction and flame photometric determination of chloride added to it. The mixture was shaken on a wrist potassium with various crown ethers. Such studies are action flask shaker for 10 min, the two layers were reported in this paper with 18-crown-6 (18-C-6), allowed to settle and separate. The organic layer dibenzo-18-crown-6 (DB-18-C-6) and dicyc1ohexyl- containing potassium was separated and equilibriated 18-crown-6 (DC-18-C-6) as the extractants for with 6M hydrochloric acid (10 ml) to strip out potassium with picrate as the counter ion. These potassium in the aqueous phase. From the aqueous studies have provided some novel methods for the phase potassium was determined flame photometri- separation of potassium from associated elements. cally at 767 nm. The amount of potassium from each fraction was computed from the calibration curve. Materials and Methods A digital pH-meter type 822 (ECIL, India) with Results and Discussion glass-electrode assembly, a digital flame photometer Extraction as a function ofpH (C-ISOD AIMIL, India), a wrist action flask shaker The pH for the extraction of potassium was (Toshniwal, India) and a centrifuge (Remi T8C, India) ascertained by carrying out its extraction between pH were used. 1.0 and 6.0 with 0.01 M solutions of various crown A stock solution of potassium chloride was prepared ethers in methylene chloride. The pH range for by dissolving accurately weighed potassium chloride quantitative extraction (IOO~.,)of potassium was 1.0- (1.907 g) in deionised water (l litre) and standardised 2.5 with 18-crown-6, 1.0 with dibenzo-18-crown-6 and gravimetrically. It was found to contain I mg 1.0-2.0 for dicyc1ohexyl-18-crown-6. The results potassium/ml. A solution containing 100 J.Lg/ml indicate that 18-crown-6 extracts potassium quanti- potassium was prepared by appropriate dilution. The tatively over a wide pH region. concentration of picric acid (BDH, AR) solution used was 0.05M. 18-Crown-6, dibenzo-18-crown-6 and Effect of varying concentration of 18-crown-6 dicyc1ohexyl-18-crown-6 (Aldrich Chemicals) were The extraction of potassium at pH 2.0 from I used as supplied. About 0.01 M solutions of all these x 10 -2 M picric acid was carried at different crown ethers in methylene chloride were used. concentrations of 18-crown-6 in the range of 1.0-10 x 10 -3 M with methylene chloride as the diluent. The General procedure extraction was 92% with 3.0 x 10 -3 M 18-crown-6 but To an aliquot of solution contammg 100 J.Lg/ml it was quantitative with 7.0 x 10 -3 M crown ether. potassium was added picric acid to give a Therefore, the optimum concentration of 18-crown-6 concentration of 0.01 Min a total volume of 10m I.The required for quantitative extraction of potassium was pH of the solution was adjusted to 1.0-2.5 with 0.01M LOx 1O-2M. 962 MOHITE & KHOPKAR: CROWN ETHERS AS EXTRACTANTS Effect of varying picric acid concentration the picric acid solution. The results showed that it was Potassium was extracted atpH 2.0 with O.OIM 18- possible to extract potassium in the concentration crown-6 in methylene chloride at different con- range of 100 to 700 ug with 0.01M IS-crown-6 in centrations of picric acid in the concentration range of methylene chloride. 1.0-10 x 10 -3 M. The extraction was more than 94% with 1.0 x 10 -3 M picric acid but it was quantitative in Nature of the extracted species the concentration range of 4.0-10 x 10 -3 M. Therefore The composition of the extracted species was the optimum concentration of picric acid for the confirmed from the plots of log D versus log [crown quantitative extraction of potassium was 0.01 M. ether] at picric acid concentration of 0.01 M and log D versus log [picric acid] at 18-crown-6 concentration of Effect of various diluents 0.01M. The slopes were 1.11 and 1.42respectively (Fig. Potassium Was extracted at pH 2.0 from 0.01 M 1). Therefore the probable composition of the picric acid with 0.01M crown ethers using various extracted species is 1:1:1, in confirmity with the earlier solvents as the diluents. The results indicate that the observation 12 . extraction was quantitative in the presence of methylene chloride as the diluent. The extraction was Effect of diverse ions more than 94% with either methylene chloride or Potassium was extracted in the presence of various ethylene chloride as diluents for dibenzo-18-crown-6. ions (Table 1). The tolerance limit was set as the The extraction was quantitative with dicyclohexyl-18- amount of foreign ion required to cause ± 2% error in crown-6 when methylene chloride or ethylene chloride the recovery of potassium. It was observed that the was used as the diluent. The extraction was incomplete alkali metals such as lithium, sodium, rubidium, when benzene, toluene and xylene were used as the caesium as well as alkaline earth metals such as diluents. The extraction was greater than 94% with beryllium, magnesium, calcium, strontium, barium chloroform as the diluent while with carbon were tolerated in the ratio of 1:10;either they were not tetrachloride as the diluent the extraction was extracted along with potassium or if extracted at all did incomplete. Since methylene chloride settles rapidly, not interfere in the flame photometric determination permits easy phase separation and effects quantitative of potassium. As regards the effect of anions, the extraction, it was preferred over ethylene chloride or halides were tolerated in the ratio of 1:40 while other chloroform as the diluent. oxyanions like nitrate, sulphate, phosphate were tolerated in a higher ratio. The anions of organic acids Choice of stripping agents were tolerated in the ratio of 1:50. After the extraction of potassium at pH 2.0 with 0.01M 18-crown-6 in methylene chloride from 0.01M Analysis of potassium in blood serum, milk, picric acid, it was stripped into aqueous phase with soft soaps and optical glasses various mineral acids. It was observed that the The proposed method was extended to the analysis stripping was quantitative with 3.0-8.0Mhydrochloric of potassium in real samples such as blood serum, acid, 1M nitric acid, 5.0-8.0M perchloric acid and 3.0- milk, soft soap and optical glass. 5.0M sulphuric acid. Acetic acid was ineffective as an An aliquot of blood serum (l ml) was mixed with 'stripping agent. In all practical work 6Mhydrochloric 1.5%aq. trichloroacetic acid (50 ml) and centrifuged to acid was employed as the stripping agent because it not remove proteineous matter. The centrifugate was only permitted stripping in the wide concentration evaporated almost to dryness, treated with cone. nitric range (3-8M) but also facilitated direct determination acid (5 ml) and evaporated again. The residual mass of potassium in the aqueous phase. was treated with-deinonised water (25 ml). From an Effect of period of equilibration aliquot (5 ml) of this solution potassium was extracted Potassium was extracted with O.OIM 18-crown-6 and determined as described in the general procedure. from 0.01M picric acid with methylene chloride as a . The amount of potassium found in quadruplicate diluent. The period of extraction was varied from 1to 30 determination was 4.6 mequiv/litre as against the min. It was observed that the potassium was extracted standard value of 4.8 mequiv/litre. to the extent of 94% in 1 min, 97% in 2 min, 99% in 4 For the estimation of potassium in milk sample, an min and 100% beyond 5 min. Therefore for all aliquot of milk (10 ml) was mixed with perchloric acid practical purpose the period of extraction was 10min. (4 ml) and nitric add (16 ml), the resulting solution evaporated to dryness, extracted with deionised water Effect of varying metal ion concentration and the final volume made upto 100ml with deionised Potassium, at different concentrations, was water.