ИСТИНА

The aqueous two-phase systems (ATPS) are widely used for the separation and concentration of various organic substances and metals. These systems are of interest in terms of potential applications for extraction of platinum group metals, which is carried out at high acidity in the presence of chloride ions. The addition of acid and chloride ions affects the phase equilibrium in ATPS. This study has investigated the phase equilibria in ATPS water – [polyethylenglycole with M = 1500 (PEG1500)] – Na+, H+ || Cl–, SO42– at 25°C. The binodal curve position was determined by the turbidimetric method. For this purpose, PEG solution was titrated with sodium sulphate solution, both solutions containing the same concentrations of acid (HCl or H2SO4) and sodium chloride. It is found that increasing the concentration of sodium chloride increases the heterogeneous region in ATPS (Fig. 1,a). The same effect of sodium chloride on the position of the binodal was observed in the PEG600-water-sodium sulphate system at pH = 7 [1]. The effect of acidity on phase equilibria in ATPS is reverse: the addition of HCl increases the solubility of the system components (Fig. 1,b). The influence of concentration of HCl proved to be much stronger than that of concentration of Cl– ions. Replacement of HCl with an equivalent amount of H2SO4 weakens the influence of acidity on the position of binodale line. Several tie-lines (compositions of coexisting phases) were determined for each binodal (Fig.1,c). Concentrations of Cl– and SO42– were determined by ion chromatography; concentration Na+ – by optical emission spectrometry with inductively coupled plasma (EOS ICP). The polymer content was determined by refractometry. For this purpose, the dependence of refractive index on the content of Na+, H+, Cl– and SO42– ions and PEG was studied. For the thermodynamic analysis of the data obtained (including the position of the tie-lines) the modified Pitzer model [2] was tested.