STABILITY OF COMPLEXES OF Ni(II) AND 18-CROWN-6 ETHER IN WATER-ETHANOL SOLVENTS

The stability constants of the nickel(II) complex with 18-crown-6 in aqueous and aqueous-ethanol solutions containing 0.20 and 0.30 mol fractions of ethanol at 298.15 K were determined by spectrophotometry. The electronic absorption spectra of the solutions were recorded in the wavelength range of 250-900 nm and optical densities of 0-4 rel. units. 2 ml of nickel(II) nitrate solution with an initial concentration of 0.13-0.15 mol/l were placed in a standard quartz cuvette (10 mm). A solution of 18-crown-6 with an initial concentration of 0.30-0.40 mol/l was used as a titrant. 25 injections of 50 mu l of titrant were added to the cell. Water or an aqueous-ethanol solvent was used as a reference solution. The extinction coefficients of nickel (II) ions for wavelengths in the range from 250 to 900 nm in the corresponding solvent were preliminarily determined and used in further calculations. With increasing ethanol concentration, a tendency toward a decrease in the stability of the [Ni18K6] (2+) complex particle is observed . An analysis of the solvation contributions of the reagents to the change in the Gibbs energy of the reaction of [Ni 18K6] (2+) formation in water-ethanol solutions is carried out. The obtained results are analyzed from the standpoint of the solvation approach. Changes in the Gibbs energy of 18K6 solvation upon transfer from water to water-ethanol solutions are insignificant. Changes in the solvation state of [Ni18K6] (2+) are determined by a change in the solvation state of Ni (2+) (Delta (tr) G degrees([Ni18K6] (2+) )approximate to Delta (tr) G degrees(Ni (2+) )) and demonstrate a slight weakening of solvation upon moving from aqueous solutions to a solution containing X(EtOH) = 0.10 mol d. A further increase in the ethanol content in the mixture leads to increased solvation of Ni (2+) . As a result of the change in the Gibbs energy of transfer, the reactions have almost zero values. The obtained results are compared with the thermodynamics of complexation of the silver (I) ion with 18K6 in a water-ethanol solvent.