DECOMPOSITION MECHANISM AND DISSOCIATION CONSTANTS OF BICARBONATE IONS
Keywords:bicarbonate ions dissociation, carbonic acid, derivation of constant, dissociation mechanism, pH changes
Using carbonic acid to explain pH changes in aqueous solutions is invalid due to its undetectability under normal temperature and pressure conditions. Instead, two reversible reactions involving the decomposition of HCO3- ions into OH- and CO2 or H+ and CO32- should be employed. The second reaction (H-mechanism) is well known as the basis for the second dissociation constant of “carbonic acid”. For the first reaction (OH-mechanism), the formula has been derived from the first imaginary constant of the same acid. That is, the researchers experimentally determined the proposed constant based on the results of pH value, CO2 and HCO3- concentrations, but calculated the imaginary constant from these values. The pH increase observed following filtration on the cationic resin in Na+ form is attributed to the weakened HCO3- decomposition via the H-mechanism, resulting in decreased H+ and CO32- concentrations. A significant decrease in Ca2+ concentration (from 5.0 to 0.05 mmol/dm3) is the main reason for the observed phenomenon, as it leads to a decrease in the driving force of calcium carbonate formation. The mechanism of bicarbonate ions decomposition based on two reactions has been confirmed experimentally. An increase in the pH of the mixture of CaCl2 and NaHCO3 solutions (both with the same pH and concentration) indicates the H-mechanism, while a decrease in the pH indicates the OH-mechanism decomposition of bicarbonate ions. The pH value at which the pH does not change indicates a change in the decomposition mechanism. The change in the HCO3- dissociation mechanism depends on hardness and alkalinity, and the pH of this change decreases from softened water (pH 8.30) to seawater (pH 7.5).
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