Phosphite ions are derived from phosphorous acid (H₃PO₃) by removing its acidic hydrogen atoms. Phosphorous acid is diprotic, meaning that only the hydrogens bonded to oxygen are acidic and can be deprotonated to form conjugate bases. As a result, the two most common phosphite ions formed in solution are:

• H₂PO₃^– (dihydrogen phosphite) with a charge of -1
• HPO₃^2- (hydrogen phosphite) with a charge of -2

Phosphite is significant because it combines distinctive chemical reactivity with valuable practical applications, especially in agriculture. A common agricultural phosphite is dipotassium phosphite (K₂HPO₃), which is widely used to manage plant diseases caused by oomycete pathogens.

Structure and Bonding ^[1,2]

In the HPO₃^2- ion, the phosphorus atom forms a double bond with one oxygen (P=O), single bonds with two oxygen atoms bearing negative charges (P–O^–), and a single bond with a hydrogen atom (P–H). Resonance among the various bonding arrangements imparts partial double-bond character to the P–O bonds. The negative charge is primarily localized on the oxygen atoms. Because phosphorus also carries a lone pair, the overall geometry around the atom is not perfectly tetrahedral; instead, it is slightly distorted, often described as pseudo-tetrahedral.

In both dihydrogen phosphite (H₂PO₃^–) and hydrogen phosphite (HPO₃^2-), phosphorus has an oxidation state of +3. This lower oxidation state influences the structure and contributes to the characteristic reactivity of phosphite ions.

Preparation of Phosphite Salts ^[3]

1. From Phosphorous Acid

The most common method for obtaining phosphorous acid is the hydrolysis of phosphorus trichloride:

PCl₃ + 3 H₂O → H₃PO₃ + 3 HCl

Neutralizing the resulting acid with a base such as KOH, NaOH, or K₂CO₃ yields hydrogen phosphite salts. For example, reaction with potassium carbonate produces dipotassium phosphite:

H₃PO₃ + K₂CO₃ → K₂HPO₃ + H₂O + CO₂

2. From Ammonium Hypophosphite

Ammonium hypophosphite undergoes disproportionation upon heating, meaning the phosphorus atom is simultaneously oxidized and reduced:

NH₄H₂PO₂ → HPO₃²⁻ + PH₃ + NH₄⁺ 

The hydrogen phosphite ion formed can then be neutralized with a base, like sodium hydroxide, to produce disodium phosphite or related salts:

HPO₃²⁻ ​+ 2 NaOH ⟶ Na₂​HPO₃​ + OH^–

Chemical Reactions

Because phosphorus is in the +3 oxidation state, phosphite ions act as effective reducing agents. They are readily oxidized to the +5 oxidation state, forming phosphate in the process. Under appropriate conditions, many oxidizing agents, such as iodine, bromine, and hydrogen peroxide, can convert phosphite to phosphate: ^[3]

i. HPO₃²⁻ + H₂​O + I₂​ ⟶ HPO₄²⁻ ​+ 2 I⁻ + 2 H⁺

ii. HPO₃²⁻​ + H₂​O₂ ​⟶ HPO₄²⁻ ​+ H₂​O

These reactions highlight the strong electron-donating, or reducing, ability of the phosphite ion.

Applications

The primary use of phosphite salts, often referred to as phosphonates in agricultural literature, is in crop protection. These compounds play a major role in plant disease management, not as conventional fertilizers but as fungicidal and biostimulant agents. Phosphite enhances a plant’s natural defense responses. It is especially effective against oomycetes, a group of fungus-like pathogens responsible for serious diseases such as Phytophthora root rot and downy mildew. As a result, potassium phosphite and related salts are widely used in commercial agriculture, horticulture, and nursery production. ^[4,5]

Although phosphites are valuable for disease control, they do not act as true phosphorus fertilizers. Plants cannot readily convert phosphite into the biologically usable phosphate form. Therefore, while phosphites support plant health by suppressing pathogens, understanding and managing phosphorus nutrition requires studying phosphate salts instead.