The effect of recrystallization of aqueous solutions of metal sulfates on the acid–base balance

Аннотация

A comparative study of pH values in recrystallized aqueous solutions of manganese, copper, and iron sulfates and the calculated values for the pH levels of similar solutions obtained directly from solid salts has been performed for the first time. The trend for the positive deviation of the pH_rec values from the pH_calc values has been established. The positive deviation was the strongest in the case of iron sulfate solutions. The sample of the recrystallized aqueous solution (recrystallized water) consisted of water obtained by melting of the initial aqueous solution after freezing of 60% of the solution (by volume). Four hypotheses were proposed to explain the positive deviation of pH_rec from pH_calc: (1) low СО₂ concentration; (2) more efficient adsorption of anions on the freezing front compared to that of cations; (3) mechanochemical and radiation chemical processes that occur upon the melting of ice; and (4) Fe²⁺-catalyzed decomposition of hydrogen peroxide formed in the recrystallized solution. Analysis of the putative causes showed that the fourth hypothesis provided the most adequate explanation of the difference between pH_rec and pH_calc. The formation of hydrogen peroxide in recrystallized water is proposed to occur due to the recombination of OH radicals formed during the radiation chemical processes.