What's the proton content of phosphorous acid?

Phosphorous acid, with its molecular formula of H3PO3, is an important inorganic acid with reducing properties. Its structural formula reveals that it contains one phosphorus, three hydrogen atoms, and three oxygen atoms, with a phosphorus content of 37.4%. Common dibasic acids include H2SO4, H2CO₃, and H2C2O4, while common monobasic acids include HNO3and HCl. So, what's the proton content of phosphorous acid? Is it a monobasic or dibasic acid? Some even say that because it contains three hydrogen atoms, it should be a tribasic acid. What's the truth? Let's explain this from the basic ionization mechanism of inorganic acids.

First, it's important to understand that determining the proton content of an inorganic acid depends primarily on how many hydrogen atoms it can ionize in aqueous solution. Hydrogen atoms that cannot ionize cannot be used to calculate the acid content of an organic acid. The actual structural formula of H₃PO₃ is O=P(OH)2-H. From this structure, we can see that two hydrogen atoms are attached to oxygen atoms, both of which are ionizable. The other hydrogen is attached to a phosphorus atom and is non-ionizable. The ionization equations are:H3PO3⇌H++H2PO3−，H2PO3−⇌H++HPO32−. These two steps ionize, releasing a total of two hydrogen ions. Therefore, phosphorous acid is a dibasic acid, not a monobasic or tribasic acid.

In short, determining the acidity of an acid primarily depends on analyzing its molecular structure to determine the number of ionizable hydrogen atoms. The number of ionizable hydrogen atoms represents the number of inorganic acids. Non-ionizable hydrogen atoms cannot be used to determine the acidity of an acid.