Oxoacids of Phosphorus (Elementary Idea)

Phosphorus forms a family of oxoacids that differ systematically in their basicity (how many

protons they can donate) and in whether they act as reducing agents -- and both properties follow

directly from one simple structural rule: only the $-\text{OH}$ groups directly bonded to phosphorus ionise as acidic protons; any $\text{P-H}$ bond present does not ionise, because a

P-H bond is only weakly polarised and essentially non-acidic under ordinary aqueous conditions.

Hypophosphorous acid, $\text{H}_3\text{PO}_2$, has the structural formula

$\text{H}-\text{P(=O)}(\text{OH})\text{H}$: phosphorus bears just one $-\text{OH}$ group,

alongside two $\text{P-H}$ bonds and one $\text{P=O}$ bond. Only the single $-\text{OH}$

hydrogen is acidic, so $\text{H}_3\text{PO}_2$ is monobasic despite its formula suggesting

three ionisable hydrogens. Because it retains two P-H bonds -- readily oxidised, since phosphorus

here is in an unusually reduced $+1$ oxidation state -- $\text{H}_3\text{PO}_2$ is a strong reducing agent (for instance, it reduces silver nitrate to metallic silver).

Phosphorous acid, $\text{H}_3\text{PO}_3$, has the structural formula

$\text{H}-\text{P(=O)}(\text{OH})_2$: phosphorus in the $+3$ state bears two $-\text{OH}$

groups and one remaining P-H bond. It is therefore dibasic, and -- because it still

retains one P-H bond -- it too is a reducing agent, though a weaker one than

$\text{H}_3\text{PO}_2$ since it has one fewer reducing P-H bond.

Orthophosphoric acid, $\text{H}_3\text{PO}_4$, has the fully expected structural formula

$\text{P(=O)}(\text{OH})_3$: phosphorus in its highest, $+5$, oxidation state bears three

$-\text{OH}$ groups and no $\text{P-H}$ bond at all. It is fully tribasic, ionising in

three successive steps, and -- having no P-H bond left to oxidise -- it shows essentially no reducing character, unlike its two lower-oxidation-state relatives.

Putting these together gives a clean, structure-based ordering: basicity increases

$\text{H}_3\text{PO}_2 < \text{H}_3\text{PO}_3 < \text{H}_3\text{PO}_4$ (one, two, then three

ionisable $-\text{OH}$ groups respectively), while reducing power decreases in exactly the

same order, from strongest ($\text{H}_3\text{PO}_2$, two P-H bonds) to none at all

($\text{H}_3\text{PO}_4$, zero P-H bonds). A related condensed acid, pyrophosphoric acid, …