Phosphorus acid

Phosphorus acid is a chemical compound with the formula H₃PO₃, more descriptively written as HPO(OH)₂. It is also known as phosphonic acid or orthophosphorous acid. This compound is an intermediate in the preparation of other phosphorus compounds. It is not to be confused with phosphoric acid (H₃PO₩), which contains one more oxygen atom than phosphorus acid.

Properties

Phosphorus acid is a diprotic acid, meaning it can donate two protons (H⁺ ions), unlike phosphoric acid, which is triprotic. In its pure form, it exists as white, crystalline solids at room temperature. It is moderately soluble in water, with its solutions being weakly acidic. Phosphorus acid decomposes into phosphoric acid and phosphine (PH₃) upon heating.

Synthesis

Phosphorus acid can be synthesized by hydrolysis of phosphorus trichloride (PCl₃) with water or steam: \[ PCl_3 + 3 H_2O \rightarrow H_3PO_3 + 3 HCl \] This reaction produces phosphorus acid and hydrochloric acid. The hydrochloric acid can be neutralized in a subsequent step.

Uses

Phosphorus acid and its derivatives, known as phosphonates, are used in various applications. One of the primary uses is in the field of agriculture, where it serves as a source of phosphorus for plants. It is also employed in the manufacture of pesticides and fungicides, exploiting its ability to inhibit the growth of certain types of fungi and bacteria.

In the industrial sector, phosphonates derived from phosphorus acid are used as water treatment agents. They act as scale inhibitors and corrosion protectors in water systems by forming a protective layer on metal surfaces.

Safety

Phosphorus acid is considered to be of low toxicity, but it can cause irritation to the skin, eyes, and mucous membranes upon contact. Appropriate safety measures, such as wearing protective gloves and goggles, should be taken when handling the chemical.

Environmental Impact

While phosphorus acid is used beneficially in agriculture and industry, its derivatives can contribute to environmental concerns. Phosphonates are persistent in the environment and can lead to eutrophication of water bodies, promoting excessive growth of algae which can harm aquatic ecosystems.

See Also

• Phosphoric acid
• Phosphine
• Water treatment
• Pesticides
• Fungicides