Structure of Orthophosphoric Acid Hydrates and Equilibrium Composition of the H₃PO₄–H₂O System According to IR Spectroscopy Data and Quantum Chemical Calculations

Composition, structure, energy parameters, and regions of existence of hydrate complexes formed in the H₃PO₄–H₂O system are determined by IR spectroscopy and quantum chemical methods. It is established that dilute solutions up to the 1:5 molar component ratio contain hydrates bearing one orthophosphoric acid molecule. In solutions with H₃PO₄:H₂O ≤ 1:6, the main structural unit is the H₃PO₄·6H₂O complex with a completely filled first hydration shell. Concentrated solutions (1:4-1:0) contain H-bonded 2H₃PO₄·nH₂O complexes (n = 1-8), each having a cycle of two acid molecules. Pure phosphoric acid is composed of the fragments of polymer chains made up of 2H₃PO₄ dimers as the chain links. Changes in the IR spectra of H₃PO₄–H₂O solutions in the whole concentration range are explained using the calculated data for the structure of mH₃PO₄·nH₂O hydrates.