Phosphorous acid (CAS 13598-36-2) is one of those quiet workhorse chemicals that sits upstream of several large industries: phosphite fungicides that protect high-value crops, phosphonate scale inhibitors that keep cooling towers running, and a family of polymer additives and synthesis intermediates. For procurement managers and formulators, however, buying it well is not trivial. Grades, specifications, the persistent confusion with phosphoric acid, and dangerous-goods logistics all affect both price and usability. This guide walks through what phosphorous acid is, where it is used, which specifications matter on the certificate of analysis, and how to source it reliably from China.
What Is Phosphorous Acid?
Phosphorous acid — also sold as phosphonic acid or orthophosphorous acid — is the inorganic compound described by the formula H3PO3 (more precisely written HPO(OH)2 to reflect its structure). Its identifiers are CAS 13598-36-2 and EC 237-066-7, with a molecular weight of 81.99 g/mol. It is a white, strongly hygroscopic crystalline solid that melts at about 74 °C and dissolves readily in water and alcohol. Despite the formula suggesting three acidic protons, it is diprotic: only two protons ionize readily. In air it slowly oxidizes to phosphoric acid, which is why moisture-proof packaging and tight specifications on phosphate content matter in trade.
Key Applications Across Industries
Agriculture and crop protection. The largest-volume downstream use is the manufacture of phosphite salts — potassium phosphite, ammonium phosphite, and calcium phosphite. Applied as foliar sprays, trunk injections, or fertigation inputs, these phosphites act as systemic fungicides and plant-defense activators, effective against oomycete pathogens such as Phytophthora (root rot), Pythium, and downy mildew. Phosphite-based foliar fertilizers are also marketed as disease-resistance nutrition products. Phosphorous acid is additionally an organophosphorus intermediate in the synthesis of herbicides such as glyphosate and the plant growth regulator ethephon.
Water treatment. Phosphorous acid is the key feedstock for phosphonate scale and corrosion inhibitors — including ATMP, HEDP, and PBTC — used in cooling towers, boilers, reverse-osmosis systems, and oilfield water treatment to control scale, corrosion, and metal fouling.
Chemical synthesis and reduction. As a reducing agent (milder than hypophosphorous acid), it is used to reduce noble-metal salts, as a nylon brightener, and as a general intermediate for phosphorus-containing fine chemicals, including pharmaceutical and agrochemical building blocks.
Polymer and metal applications. Phosphite derivatives serve as heat stabilizers in PVC and as stabilizing mixtures for plastics, while the acid’s chelating ability is used in corrosion inhibition for metalworking fluids and industrial cleaning formulations.
Grades and Specifications: What to Check on the COA
Export-grade phosphorous acid is typically offered at 98–99% assay, with 98.5% the most common commercial specification. When reviewing a certificate of analysis, buyers should verify: assay (acid content) and water-dissolution clarity (a clear, colorless solution); the impurity profile — iron and heavy metals (Fe commonly 20–50 ppm), chloride, sulfate, and water-insoluble matter; and phosphate content, since elevated phosphate indicates oxidation during storage and signals poor packaging or old stock. Appearance and moisture matter too: the solid is deliquescent, so lumps or caking suggest moisture ingress. Packaging is part of the specification — moisture-proof 25 kg bags or drums, palletized and shrink-wrapped for sea freight. Match the grade to the end use: fungicide and fertilizer formulators need consistent low-impurity material, while some synthesis applications tolerate technical grade.
Phosphorous Acid vs. Phosphoric Acid: A Costly Mix-Up
The names differ by two letters, and the confusion is one of the most common — and expensive — mistakes in phosphorus chemical procurement. Phosphorous acid (H3PO3, CAS 13598-36-2) is a diprotic reducing acid shipped as UN 2834, Class 8, PG III. Phosphoric acid (H3PO4, CAS 7664-38-2) is a triprotic, non-reducing acid shipped under UN 1805/3453. Their derivatives differ too: phosphites versus phosphates, with entirely different regulatory and performance profiles. Always confirm the CAS number on quotations, COAs, and shipping documents before committing.
Sourcing and Logistics: What B2B Buyers Should Know
Phosphorous acid is regulated for transport — UN 2834, Class 8 (corrosive solid), Packing Group III across ADR, IMDG, and IATA regimes — so compliant DG packaging, labeling, and declarations are non-negotiable, and freight forwarders should be briefed early. A practical sourcing checklist: batch-wise COA plus a current MSDS/SDS from the manufacturer; DG-compliant packaging certificates and proper shipping documentation; evidence of quality management (factory QC, third-party inspection such as SGS/BV where required); and clear Incoterms, realistic lead times, and moisture-controlled storage through the supply chain. Because the material is hygroscopic and corrosive, container stuffing conditions and warehouse handling deserve the same scrutiny as price.
Sourcing Phosphorous Acid from Unifarm Chemical
Hainan Unifarm Chemical exports agrochemicals and industrial chemicals to distributors and formulators in more than 30 countries, with phosphorous acid (CAS 13598-36-2) as a standing line in our agrochemical portfolio. Every shipment leaves with a batch COA and complete export documentation, backed by factory-direct pricing and an EcoVadis Gold sustainability rating. If you are qualifying a new supplier or benchmarking your current cost, contact our export team for specifications, packaging options, and a competitive quotation.