Questions regarding the alkalinity of sodium phosphate hydrochloride
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This post was last edited by Hidden Pig on 2017-12-17 00:22. Trisodium phosphate is basic when dissolved in water; the principle behind this is PO4 3- + H2O → HPO4 2- + OH-, that is, the phosphate ions react with water to produce hydroxide ions, resulting in a basic solution. Disodium hydrogen phosphate is alkaline when dissolved in water as well. The mechanism behind this is: HPO4 2- → PO4 3- + H+, that is, the hydrogen phosphate ion dissociates to release hydrogen ions, which would suggest an acidic behavior; yet in reality it is alkaline. I’m not sure how to explain this. By extension, if hydrogen phosphate may undergo two ion reactions, namely HPO4 2- + H2O → H2PO4- + OH- and HPO4 2- → PO4 3- + H+, how can one determine which is the primary reaction? Is it thermodynamics?
Look up the third ionization constant of phosphoric acid, and use charge balance and mass balance to determine the acidity or alkalinity of the solution during its ionization. The ionization process does not require thermodynamic calculations, but it requires ionization equilibrium calculations.