INFLUENCE OF THE POLARITY OF THE ENVIRONMENT ON THE SOLUBILITY PRODUCT OF SULFATES OF ALKALI METALS IN WATER-ALCOHOLIC SOLUTIONS
Dilution of saturated aqueous solutions of lithium, sodium and potassium sulfates with distilled water remain homogeneous, and dilution with ethanol or isopropanol leads to the formation of salt precipitates. Such a fundamental difference of the effect of water and alcohol indicates a radical change in the physicochemical properties of aqueous-alcoholic solutions in comparison with the original aqueous solution. It is shown that dilution with ethanol or isopropanol of saturated aqueous solutions of lithium, sodium and potassium sulfates decreases the dielectric constant of the environment. It also changes the ionic strength of aqueous-alcoholic solutions. The coefficients of activity of alkali metal sulfate ions are calculated using the Debye-Gückel equation. The combined effect of the reduced dielectric constant due to the addition of less polar alcohols compared to water, the ionic strength of solutions and modified ion activities reduces the thermodynamic product of the solubility of lithium, sodium and potassium sulfates in water-ethanol and water-isopropanol solutions. A decrease of the product of solubility of salts leads to precipitation when diluting aqueous solutions of these salts with alcohols. If we add alcohols up to 90% by volume in aqueous solutions, the solubility of the salts decreases by 270-1280 times. Such a significant decrease in solubility is also explained by a change in the structure of the solvation shell of ions in aqueous-alcoholic solutions as a result of competition between hydrogen bonds in the systems water-water, water-ion, and water-dissolved organic matter. The calculation of the product of solubility of alkali metal sulfates in water-alcohol solutions was carried out. It is shown that when the ethanol content in the solution is 50-90% by volume, the salt solubility product decreases in the range from 3.8∙10-5 to 6.3∙10-9. Isopropanol additions in amount of 40-90% by volume reduce the product of salt solubility in the range from 1.6∙10-5 to 4.9∙10-9. Based on the values of the product solubility calculated for products of lithium, sodium and potassium sulfates in water-ethanol and water-isopropanol solutions, a conclusion can be made that salts in water-alcohol solutions become weak electrolytes with a high content of ethanol or isopropanol.