Chemical Properties of Water-Miscible Solvents Separated by Salting-Out and Their Application to Solvent Extraction

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Chemical Properties of Water-Miscible Solvents Separated by Salting-Out and Their Application to Solvent Extraction ANALYTICAL SCIENCES JUNE 1994, VOL. 10 383 Chemical Properties of Water-Miscible Solvents Separated by Salting-out and Their Application to Solvent Extraction Masaaki TABATAt, Midori KUMAMOTO and Jun NISHIM0T0 Department of Chemistry, Faculty of Science and Engineering, Saga University, Honjo-machi, Saga 840, Japan Fourteen water-miscible polar solvents were investigated for the separation from their aqueous solutions by salting-out using sodium chloride (4 mol dm-3). The following solvents showed the phase separation: acetone , acetonitrile, 1,4- dioxane, tetrahydrofuran, l-propanol, and 2-propanol. The chemical properties of the separated organic solvents were determined by measuring ET(30) (1.196X105/2 (kJ mol-')) and D11,1(=1.196X105 ()i'1) (kJ mol-')) values from the spectral change of 2,6-Biphenyl-4-(2,4,6-triphenylpyridinio)phenolate (DTP) and bis(1,3-propanediolato)vanadium(IV) (VO(acac)2), where 2, A,, and 2„ denote the absorption maximum wavelengths (nm) of DTP and VO(acac)2. Solvent properties of acetone, acetonitrile, 1,4-dioxane, and tetrahydrofuran were dramatically altered by the salting- out. Acceptability of the phase-separated solvents increased due to the dissolution of water molecules having large acceptor numbers. The ion-pair complex of tris(1,10-phenanthroline)iron(II) chloride was easily extracted into the phase-separated acetonitrile by the salting-out. Some metal chelates of 1-(2-pyridylazo)-2-naphthol (Hpan) and 8-quinolinol (Hox), 5,10,15,20-tetraphenylporphyrin (H2tpp), and ionic species (H2ox+, ox, and H4tpp2+) were also extracted into 1,4-dioxane. The raised donor and acceptor abilities of the phase-separated solvents allowed application to solvent extraction. Keywords Salting-out, water-miscible solvents, ET(30), solvent extraction The salting-out technique has long been used for by the salting-out have not been studied. extraction of metal-chelates, ion-pairs, or organic mate- In this paper we describe the chemical properties as rials, prior to atomic absorption spectrophotometry1, donor and acceptor abilities of the phase-separated high-performance liquid chromatography2'3, polarog- solvents by salting-out, i.e. ET(30) (=1.196X105/, raphy4, and absorption spectrophotometry.5 More (kJ moL1)) and D11,1(=1.196X105(211-1-21-1) (kJ mol-1)) recently, the salting-out using a surfactant of a water- values12 were determined from the spectral change of 2,6- soluble polymer has been used for preconcentration of diphenyl-4-(1,2,4-triphenylpyridinio)phenolate (DTP) porphyrins.6'' Application of the salting-out to solvent and bis(1,3-propanediolato)vanadium(IV) (VO(acac)2), extraction of ionic species will become easier when we where 2, 21,, and 2, denote the absorption maximum understand the chemical properties of the phase- wavelengths (nm) of DTP and VO(acac)2 (ET(30) is separated solvents, because the solvents will have high abbreviated as ET for simplicity). We also applied the polarity resulting from dissolution of water and phase separation to extraction of ion-pair complexes electrolyte into the solvents by salting-out, in addition to of tris(1,10-phenanthroline)iron(II) (Fe(phen)32+) and water-miscible solvents themselves. cationic ligands of 8-quinolinol (Hox) and 5,10,15,20- Several mechanisms have been proposed for the effects tetraphenylporphyrin (H2tpp), and metal-chelates of 8- of electrolytes on the salting-out of water-miscible sol- quinolinol and 1-(2-pyridylazo)-2-naphthol (Hpan). A vents. For example, some electrolytes have been clas- large change in the chemical properties of the solvents sified as having either salting-out or salting-in effects and by the salting-out was observed for acetonitrile, 1,4- ranked according to their salting strength. Most of dioxane, and tetrahydrofuran, but the change was small theories concerned with the salting-out effect have used for 1-propanol and 2-propanol. Thus, acetonitrile and salting-out coefficient (Setschenow constant) defined as 1,4-dioxane were used to examine the effect of the ks=1/m(log So/S), where So and S are the solubilities of salting-out on the extraction of metal chelates and ion- the organic solvent in water and in an electrolyte solution pair complexes. Possible mechanisms responsible for of molality m, respectively.g-10 However, microscopic change in ET values and the solvent extraction using the properties like ET(30)11of the solvents phase-separated salting-out are discussed. t To whom correspondence should be addressed . 384 ANALYTICAL SCIENCES JUNE 1994, VOL. 10 dimethylformamide, and N,N-dimethyl sulfoxide. The Experimental volumes of the separated phases and the concentrations of water and chloride in the phase-separated organic Reagents solvents are summarized in Table 1, together with donor The purification of 1,4-dioxane was performed as numbers (DN) and acceptor numbers (AN) of the pure follows: solid potassium hydroxide was added to 1,4- solvents. The volumes of the organic phases recovered dioxane; the solution was left for 1 day and then refluxed from the salting-out were smaller than the initial volumes under metal sodium; this was followed by distillation. of the organic solvents for acetone, acetonitrile, 1,4- Acetonitrile was purified by distillation in the presence of dioxane, and tetrahydrofuran, but the volumes of 1- molecular sieves. All other organic solvents and propanol and 2-propanol were larger than their initial chemicals were analytical reagents and were used without volumes. Propanol interacts strongly with water mole- further purification. cules through hydrogen-bonding and the alkyl group is hydrophobic. Thus, aqueous propanol solution is Procedure easily salted-out by the addition of sodium chloride and Into a stoppered graduated tube, a 5-cm3 volume of yields a large volume of the separated organic phase aqueous sample solution was poured, then and 1.169 g containing a lot of water. The phase-separated solvents (2.00X102 mol) of sodium chloride was added to the contain water and chloride ion to a great extent and the solution. The sodium chloride was dissolved suffici- concentration of chloride in the organic phase increases ently and a 5-cm3 volume of organic solvent was added to with the concentration of water in the organic phase. the aqueous solution; then the mixed-solvent was shaken Two theories have been used to explain salting-out for about 1 min. Two phases were allowed to stand for phenomena. Long and McDevit8 have considered the a few minutes. The volume of each of the two phases salting-out as arising from electrostriction of the solvent was measured, and the concentration of chloride ion and caused by addition of electrolyte, where the Setschenow water in the separated-organic phase were determined constant (ks) has been expressed by the partial molar by argentmetry and Karl Fisher titration on a Karl volumes of the electrolyte and the organic solvent and Fisher Moisture Titrator (MKL-200, Kyoto Electronics, the compressibility of water.8-10 Another quantitative Japan). Absorption spectra of metal-chelates, ion-pair description has been made based on Scaled-Particle complexes, and the porphyrin were recorded on a theory.13"4 The parameters used for the electrolyte in Shimadzu UV-VIS spectrophtometer 2100. The pH of this theory are the diameters and polarizabilities of the the aqueous phase after the salting-out was measured on cation and anion, and the apparent molar volume of the a Radiometer Ion 85 Ion Analyzer. All experiments electrolyte at infinite dilution in water. The scaled- were carried out at 25° C. particle theory is useful in understanding solute-solvent interactions, but quantitative description by the theory is not possible for the relatively high concentrations used in Results and Discussion this study. Furthermore, the theory does not give any information about change in the chemical properties of Phase separation of polar solvent from aqueous solutions the solvents after the salting-out. Thus we measured ET Fourteen water-miscible polar solvents were examined and D11,1values to elucidate the chemical properties of the for salting-out from 4 mol dm 3 of sodium chloride. Of phase-separated organic solvents. the water-miscible polar solvents, acetone, acetonitrile, 1,4-dioxane, tetrahydrofuran, 1-propanol, and 2-pro- Donor and acceptor abilities of phase-separated solvents panol were easily separated from aqueous solutions, It is expected that the dissolution of sodium chloride whereas the phase separation was not observed for the and water into the phase-separated solvents would alter following solvents: methanol, ethanol, 1-butanol, form- the chemical properties of the solvents. Thermodynam- amide, N-methylformamide, propylenecarbonate, N,N ic quantities which represent a measure of the donor and Table 1 Salting-out data for the phase-separation by sodium chlorides a. Five cubic centimeters of each solvent and of water were taken initially and 1.169 g (2.00X 10.2 mol) o f sodium chloride was added. b. Donor number and acceptor number of water are 18.0 and 54.8, respectively (ref. 15). ANALYTICAL SCIENCES JUNE 1994, VOL. 10 385 the red part of the visible spectrum composed of two bands, 21 and 211in non-polar solvent (Fig. 2). With an increase of donor number of the solvent, the 21 is notably shifted to the red, while the 211is shifted a little to the blue, so that the difference in their frequency values (D11,1) increases with the donor number of the aprotic solvent used. However, a protic solvent which can form hydrogen bonds generally yields much higher D11,1values than those expected from their DN alone.16 Such effects have been ascribed to the solvation of oxygen atom of the V=0 group by solvent molecules which behave as acceptors. It is easy to see that such a solvation weakens the V=0 bond, making it more polar (i.e.
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