Phosphorous Acid

Phosphorous acid differs from phosphoric acid in the oxidation state of phosphorus. Phosphoric acid holds phosphorus at its highest oxidation state; phosphorous acid holds it one step lower, with a hydrogen bonded straight to the phosphorus atom. The P–H hydrogen is not acidic, so phosphorous acid gives up two protons rather than three, and it makes the compound a reducing agent rather than a stable end point. Most of it is made by hydrolysing phosphorus trichloride and sold as white, moisture-hungry crystals. Its main buyers are phosphonate makers, who turn it into ATMP and HEDP scale inhibitors, and agrochemical producers, for phosphite crop-protection salts and one route to glyphosate. It must not be overheated: near 200 °C it decomposes and releases phosphine. Ehlinar supplies crystal and solution grades for phosphonate, agrochemical and synthesis use.

Appearance: A white, deliquescent crystalline solid, usually supplied as crystals or flakes. It melts near 74 °C and is very soluble in water, more than 300 g per 100 mL. It pulls moisture from the air quickly and cakes or liquefies if left open. It is also sold as a concentrated aqueous solution.

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Also known as

Phosphonic acid · Orthophosphorous acid · Phosphorus(III) acid · Phosphorus trihydroxide · Phosphorus acid (trade spelling)

Frequently asked questions

Phosphorous, phosphoric, hypophosphorous: what is the difference?

Three different acids of phosphorus with names a letter or two apart. Phosphoric acid, H3PO4, holds phosphorus at +5, gives up three protons and is the fertiliser and food acid. Phosphorous acid, H3PO3, holds it at +3, gives up two and is a reducing agent used for phosphonates and phosphites. Hypophosphorous acid, H3PO2, holds it at +1, gives up one, and is a stronger reducing agent best known through sodium hypophosphite in electroless nickel plating. They differ in regulation too: hypophosphorous acid and its salts are on the US DEA List I of controlled chemicals, while phosphorous and phosphoric acid are not. 'Phosphorus acid' is not a real chemical name and turns up on trade documents meaning either of the first two, so confirm the CAS number before anything is quoted.

Why does phosphorous acid give up only two protons when it has three hydrogens?

Because only two of them are attached to oxygen. Its structure is HP(O)(OH)2: two hydroxyl hydrogens that ionise, with acid strengths around pKa 1.3 and 6.7, and one hydrogen bonded directly to phosphorus that does not. In practice that means neutralising a mole of phosphorous acid takes two moles of base, not three, which matters when making phosphite salts.

Is phosphite a phosphorus fertiliser?

Not in the way phosphate is. Plants cannot efficiently oxidise phosphite into phosphate, so it is a poor source of phosphorus nutrition, and in phosphorus-starved crops it can even suppress growth. Its well-established value is against oomycete diseases such as Phytophthora and Pythium. Whether a phosphite product is regulated as a fertiliser or as a plant protection product depends on the jurisdiction and the claims made, so check that before labelling.

How does it become a phosphonate scale inhibitor?

By building phosphorus–carbon bonds onto it. Reacted with formaldehyde and an amine it gives aminomethylene phosphonates such as ATMP; reacted with an acetyl source it gives HEDP. These phosphonates bind calcium and other metals and stop scale forming in cooling water, boilers, oilfield systems and detergents, at very low doses.

What does the rest of a 98.5% assay consist of?

Mostly phosphoric acid, from slow oxidation of the product, plus water and, from the phosphorus trichloride route, residual chloride. For phosphonate synthesis those impurities affect yield and colour, and for reduction chemistry the phosphoric acid content is effectively dead weight, so the certificate of analysis should state all three rather than the assay alone.

How should phosphorous acid be stored?

Sealed, cool and dry, away from oxidisers and heat. It is strongly hygroscopic, so an open bag will cake and then liquefy in humid air. Polyethylene, polypropylene or lined containers suit it; bare carbon steel does not. Never let it be heated dry or trapped against a hot surface, because it decomposes near 200 °C and releases phosphine.