Muralikrishna, U

Rao, V M

Suryanarayana, M V S

English

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IndianJournalof ChemistryVol.22A,February1983,pp. 179-180Effect of Diluents on the Extraction ofMercury(II) by n-Butyl AcetateU MURALIKRISHNA*, VM RAO &MVS SURYANARAYANADepartmentof Chemistry,AndhraUniversity,Waltair530003Received 9 June 1982;revised and accepted 4 October 1982The influenceof isooctane, n-hexane, lI-heptane,cyclo-hexane, CCl4 and CHCls as diluentson the extraction ofmercuric chloride by n-butylacetatehas beenstudied. It hasbeenfound that the theoryof regular solutions is applicableto thesesystems. The solubility parameter of the extractedspeciescomputed from the extraction studiesishigherthanthat calculated from the latent heat of vaporization data.This is attributedto the solvationof the extractedspecies.As a part of our programme! on the extraction ofmercury(II) using organic esters as extractants, wereport here the results of studies on the effect ofsome diluents on the extraction of Hg(II) by n-butylacetate.A stock solution (0.05M) of mercuric chloride(E. Merck), was prepared and diluted appropriately.The tracer 203mercuryin the form of mercuric nitratein nitric acid solution (specific activity 30 m Cilg,mercury) was supplied by the Isotope Division,BARC, Eombay. n-Butyl acetate (BDH, India) wasused after purification. AR grade samples of theorganic solvents were fractionated immediatelybefore use.A single channel analyser (SC 604 ECIL, India)coupled with a 3 inch well-type scintillation detector(Nuclear, Chicago) was used for y -activity .measure-ments of 203Hg.Procedure-An aqueous phase (15 ml) containing1.0 x IO-iM of mercuric chloride and tracer wasequilibrated with an equal volume of n-butyl acetatesolution in the organic solvent (5 ml in 15 ml) forabout 5 min and the two phases were separated, afterallowing the mixture to stand for 5 min. Equalaliquots (IO ml) of each phase were used to measurethe activity, and the distribution coefficient (Q) wascalculated using the equation,Q _ counts/min in the organic phase- counts/ruin III the aqueous phaseThe extractions were repeated for checking repro-ducihility.Due to the limitation on the availability of diluentsinto which there is no extraction of mercury, thestudies were carried out only with isooctane, n-hexane,n-heptane, cyclohexane, carbon tetrachloride andchloroform as diluents. In each case, the extractedspecies was found to be HgC12.2Sas revealed by theTableI-Influence of theOrganicSolventson the Extractionof the SpeciesHgCl2byn-ButylAcetate[HgCI21= 1.0 x 10-4MtlogKx /lorg lier----, r----, ...----..0.3112 6.85 15.10.4942 7.30 15.30.4632 7.45 15.30.3103 8.20 15.7Solvent Q Eorgr---.~0.2220 1.940.4255 1.900.3557 4.300.3370 2.02r---------,Isooctanen-Hexanen-HeptaneCyclohexaneCarbontetra-chlorideChloroform15.90.24300.21402244.800.1177 8.600.0172 9.30Averagea, = 15.5tValuesof Kx were obtained from the expressionKx =Q (VoN aq) wherethemolar volumesare calculatedfromdensi-tiesvalidat 25°C.procedure of Hesford et al.2• The extraction effi-ciencies were in the order (dielectric constants aregiven in the parentheses) : n-hexane (E = 1.9) >nheptane (e = 4.3) > cyclohexane (E = 2.02) >carbon tetrachloride (E = 2.24) > isooctane (E =1.94) > chloroform (EO = 4.80).The results show a poor correlation between theextraction efficiency of the solvent and its dielectricconstant (E). The solvents, however, can be corre-lated better by the application of the theory ofregular solutions+",If K« is the distribution coefficient expressed inmole fraction ratio,- /::" Fpart = 2.303 RT log Kx= Vc (8c-3aq)2_(8c- 8org)2 ... (1)where /::"Fpart is the free energy of partition of thesolute (HgCI2.2S); 8c, 80rg and 8aq are the solubilityparameters of the extracted species, the organicsolvent and the aqueous phase respectively. Vc isthe molar volume of the extracted species. SinceVc, 3c and 8aq are constants, a plot of log Kx against80rg should take the form of a parabola", and theresults now obtained (Table 1) are in agreement withthis expectation.The extraction of mercury by n-butyl acetatedissolved in the mixture of a good solvent (cyclo-hexane) and the poor solvent (chloroform) has beenalso studied. The theoretical treatment of the datahas been carried out according to Irving and Rossotti'sprocedure? for the purpose of calculation of the dis-tribution coefficient (Qcal) and the solubility para-meter (811) of the mixtures [the terms good and poorare considered with respect to the extraction efficiencyof Hg(II) by n-butyl acetate-organic solvent system].The results are dipicted in Fig. 1. Progressive dilu-tion of cyclohexane by chloroform produces the expe-cted decrease in the distribution coefficient Q leadingto a minimal extraction when its mole fraction ratiois 0.33, and thereafter a gradual increase in thedistribution coefficient is noticed due to the increasein the mole fraction of chloroform (Fig. 1, curve a).179INDIAN J. CHEM., VOL. 22A FEBRUARY 19839.3 9.16MB.9 B.7 B.S B.3a0.3Q0.1 L-._-L_--::'-:-_--=-"~-_!_::-__:_' 0.1o 0.2 Q.4 0.6 O.B 1.0MOLE FRACTION OF CYCLOHEXANE. XFig. 1.-The extraction of HgCI2{[HgClz] = 1.0 x 10-'M} bysolutions of n-butyl acetate in mixtures oj chloroform andcyclohexane. The variation of Q with the mole fraction ofcyclohexane is shown in (curve a) (lower abscissa). Valuescalculated by Irving and Rossotti's theory? are given as circles(curve b). The plot of Qcal against 8M (upper abscissa) isshown in curve c.The calculated curve (Fig. 1, curve b) deviatesmarkedly from the experimental curve confirmingthe importace of solvent-solvent and solute-solventinteractions in this system. On the other hand, aplot of Qcal against aM of the mixture (Fig. 1,curve c)· gives a better agreement with the experi-mental curve.Calculation of the solubility parameter of theextracted mercury species (HgClz.2S)-An attemptwas made to determine the solubility parameter ofthe extracted species (HgCI2.2S) as described below.In Eq. (1), the term Vc(ac-aaq)!! on the right hand180side represents the free energy of solution of thesolute in the aqueous phase, while the second termis the free energy of solution of the solute in theorganic phase.Substituting the values of log Ks, aorg, aaq and Vcin Eq. (1), ac can be calculated (Table 1). A valueof 23.4 was assigned to aaq for the purpose of thecalculation of 8c. In view of the fact that the con-centration of extracted mercury (lO-'M) was verymuch less than that of the extractant, n-butyl acetate(2.5M), Vc was calculated to be 571.7, assuming thedensity of the extracted species as the density of theextractant (0.88 glml at 25°C). 8c thus computedfrom the experimental data is greater than solubilityparameter of the mercuric chloride (aHgcIZ = 11.4)calculated from the latent heat of vaporization (2:, Hv)and molar volume data". The difference may pro-bably be due to the solvation of extracted mercuryspecies.Thanks are due to the UGC, New Delhi forfinancial assistance under the project entitled Desali-nation and Economic Recovery of ChemicalProducts.References1 Murlikrishna U, Rao V M & Suryanarayana M V S,Z anal Chern, 310 (1982) 429.2 Hesford E & McKay HAC, Trans Faraday Soc, 54(1958) 573.3 Hildebrand J H, Prausnitz J M & Scott R L, Regular andrelated solutions (Von Nostrand Reinhold Co,New York) 1970.4 Hildebrand J H & Scott R L, The solubility of nonelectro-lytes, 3rd Edn, (Dover Publishers Inc., New York)1964.5 Irving H M N H, in Ion exchange and solvent extraction,Vol, 6 (M Dekker Inc, New York) 1974.6 Siekierski S & Olszer R, J inorg nuc/ Chern, 25 (1963) 1351.7 Irving H & Rossotti F J C, J chem Soc, (1956) 2475.

Effect of Diluents on the Extraction of Mercury(II) by n-Butyl Acetate

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Effect of Diluents on the Extraction of Mercury(II) by n-Butyl Acetate

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