Question
# Write the chemical equation for the preparation of phosphoric acid (${H_3}P{O_4}$) from orthophosphorus acid (${H_3}P{O_3}$) ?
The orthophosphorus acid is the common name. In IUPAC nomenclature, it is called Phosphonic acid. The ${H_3}P{O_3}$ is a diprotic acid i.e. it can donate only two protons. Thus structurally, the formula for orthophosphorus acid can be written as $HPO{(OH)_2}$.
${H_3}P{O_3} + \frac{1}{2}{O_2} \rightleftharpoons \;{H_3}P{O_4}$
The orthophosphorus acid gets converted to phosphoric acid in the presence of oxygen which is present in moisture. This reaction is reversible in nature. There is another alternative which is to heat orthophosphorus acid above ${180^0}$ C temperature. Even on heating at such a high temperature also, the orthophosphorus acid gets converted to phosphoric acid.