Industrial Grade 98.5% 99% Phosphorous acid CAS 13598-36-2

What is phosphorous acid? How does it work?

Phosphorous acid, also known as phosphite (chemical formula H3PO3), is a trivalent oxyacid of phosphorus. It is a white crystalline acid with both strong reducing properties and pH bond reactivity. It is the primary raw material for PVC plastic stabilisers, organophosphonic acid water treatment agents and agricultural phosphite formulations. Industries like plastic pipes, drinking water treatment, crop disease prevention, and electroless nickel plating use phosphorous acid widely.

Basic identification information: Molecular formula H3PO3 (actually a binary HP(O)(OH)2 with the 3rd hydrogen directly bonded to phosphorus) Molecular weight 82.00 CAS number 13598-36-2 EC number 237-066-7

Physical properties of phosphorous acid are: phosphorous acid is white crystalline solid; it is hygroscopic; density is 1.651 g/cm 3 ; melting point is 73.6℃; boiling point is about 200℃ (decomposes); it is extremely soluble in water (310 g/100 mL) and ethanol; aqueous solution is acidic.

Chemically, phosphorous acid is a diprotic moderately strong acid (pKa 1.3, 6.7), and phosphorus is in a +3 oxidation state, thus acting as both a strong reducing agent (easily oxidized to +5 phosphoric acid/phosphate) and capable of disproportionation (disproportionating to phosphoric acid and phosphine PH3 upon heating to approximately 200℃). The pH bond in the molecule is reactive, participating in free radical addition (preparation of phosphite esters/phosphonic acids) and reactions with aldehydes and ketones (preparation of organophosphonic acids), making it an important raw material for the synthesis of organophosphine compounds. Storage must be airtight and moisture-proof, away from oxidants, and care must be taken to prevent the generation of phosphine gas when heated.

From the perspective of its mechanism of action, industrial-grade phosphorous acid plays an irreplaceable role due to its "+3 valence reducing property + pH bond activity," and its mechanism of action in different applications is as follows:

1. Reduction mechanism (+3 valent phosphorus is oxidized)

Phosphorus in phosphorous acid is in a +3 valence intermediate oxidation state, which is easily oxidized to +5 valence phosphoric acid/phosphate and releases electrons. It is the basis of reducing agent in processes such as electroless nickel plating, metal reduction, and antioxidant. Its reduction potential is also used to remove impurity metal ions.

2. pH bond reaction mechanism (synthesis of phosphonic acid and phosphite)

The pH bond in the phosphorous acid molecule can undergo free radical addition and electrophilic addition: it reacts with formaldehyde and amines to produce organophosphonic acids such as aminotrimethylenephosphonic acid (ATMP), and reacts with olefins/alcohols to produce phosphite esters. The pH bond is the reaction center for the synthesis of organophosphorus compounds (water treatment agents, stabilizers, flame retardants).

3. Chelating scale inhibition mechanism (phosphonic acid groups complexing metal ions)

Organophosphonic acids derived from phosphorous acid contain multiple phosphonic acid groups and have a strong chelating/complexing ability for metal ions such as Ca2+, Mg2+, and Fe3+. They can inhibit scale crystal growth and disperse scale particles, which is the core chemical basis for scale inhibition and corrosion inhibition in water treatment.

4. PVC stabilization mechanism (capturing HCl and chelating metal chlorides)

Lead phosphite (basic lead phosphite) and phosphite ester stabilizers: the former absorbs HCl released from PVC degradation and blocks ultraviolet light, while the latter chelates metal chloride catalysts such as ZnCl2 and prevents "zinc burning". The two work together to extend the thermal stability time of PVC and are an irreplaceable stabilization system for PVC processing.

5. Agricultural disease resistance induction mechanism (phosphite-based acquired resistance)

When phosphorous acid (in the form of potassium phosphite) is applied to crops, phosphorus is absorbed by the plants in a +3 valence form, which can induce the plants to produce systemically acquired resistance (SAR) and directly inhibit the growth and development of oomycetes such as downy mildew and Phytophthora, thus achieving the dual function of "nutrition + disease resistance".

In summary, industrial-grade phosphorous acid is the "cornerstone of organophosphorus chemistry." Due to its reducing properties and pH bond activity, it is used in a wide range of industrial fields, from PVC stabilization to scale inhibition in water treatment, from crop disease prevention to chemical nickel plating, including plastics, water treatment, agriculture, and surface treatment.

 What are the uses of industrial-grade Phosphorous acid?

Industrial-grade phosphorous acid has applications in plastics, water treatment, agriculture, surface treatment, fine chemicals , scientific research, and other fields.

1.Raw materials for PVC heat stabilisers (Key use in the plastics industry)

Use areas: Processing of plastics and PVC products.

Matching products: Dibasic Lead Phosphite (heat stabiliser for PVC pipes/wires/cables/artificial leather), phosphite chelating auxiliary stabiliser (anti-"zinc burning"), and composite calcium-zinc stabiliser components.

Why it works for this: PVC decomposes at high temperature and produces HCl gas which accelerates degradation. Dibasic Lead Phosphite absorbs HCl and blocks ultraviolet light. Phosphite chelates can chelate with metal chloride catalysts such as ZnCl2. The synergistic effect of both greatly prolongs the heat stability time of PVC; it is an indispensable stabilising system for PVC pipes, cables, artificial leather and other products.

2. Raw material of organophosphonic acid water treatment agent (core application in water treatment industry).

Application Fields: Water treatment and circulating water systems.

Related Products: Aminotrimethylene Phosphonic Acid (ATMP), Hydroxyethylidene Diphosphonic Acid (HEDP), Polyphosphonic Acid Scale and Corrosion Inhibitors, Boiler/Cooling Water Scale Inhibitors.

Why it works for this application: The P-H bond of phosphorous acid reacts with formaldehyde, amines and ketones to form organic phosphonic acids containing a number of phosphonic acid groups. These strongly chelate metal ions such as Ca2+, Mg2+, Fe3+ and thus prevent scale crystal growth, disperse scale particles and inhibit corrosion. It is a key scale and corrosion inhibitor component in waterworks, power plants and factory cooling water systems.

3. Agricultural fungicides and phosphate fertiliser raw materials (Core application in agricultural field)

Application area: Fertiliser and plant protection in agriculture.

Related Products: Disease Resistance Inducer, Downy Mildew / Late Blight Control Fungicides, Seed Treatment Agents, Potassium Phosphite Disease Resistant Fertilisers, Calcium Phosphite Disease Resistant Fertilisers.

How it works for this purpose: Phosphite (in phosphite form) is absorbed by crops as trivalent phosphorus, inducing systemic acquired resistance (SAR) in plants, inhibiting oomycetes such as downy mildew/late blight, and having both phosphorus and potassium nutrition and disease prevention functions. It is a common raw material for the formulation of green agriculture that reduces the use of pesticides and increases its efficiency.

4. Raw material for electroless nickel plating reduction system (surface treatment industry)

Application domains: Surface Treatment & Electroplating Industry.

Related products: Electroless nickel plating bath reducing agent: Sodium hypophosphite (NaH2PO2) synthesis raw material; Electroless plating bath replenishing reducing agent.

Why it works for this purpose: Electroless nickel plating uses hypophosphite as a reducing agent to reduce Ni2+ and plate it on substrates. Phosphite (trivalent phosphorus) can be electrochemically reduced to hypophosphite. The phosphite in the plating bath also participates in the reduction/complexation balance and is an important source of phosphorus for both the electroless nickel plating process and the production of hypophosphite.

5. Raw materials for flame retardants and auxiliary antioxidants in plastics (field of plastics additives)

Uses: additive for plastics, flame retarding industry.

The corresponding products are phosphite flame retardants (e.g. triphenyl phosphite), auxiliary antioxidants (e.g. organic phosphite, Irgafos 168) and polymer processing stabilisers.

How it works for this purpose: Phosphite esterifies with phenols/alcohols to form organic phosphite. The phosphorus element promotes the formation of char, and inhibits the gas-phase combustion (flame retardancy) during combustion. It can also decompose peroxides in the process and protect the primary antioxidant (auxiliary antioxidant). It is an important auxiliary raw material of flame retardant and ageing resistance of plastic and rubber products.

6.Raw materials for the synthesis of fine chemicals (industry of fine chemicals)

Areas of Application: Fine Chemicals.

Corresponding Products: Organophosphorus compounds, phosphonate esters, other fine chemicals.

Why it is effective for this purpose: The P-H bond of phosphorous acid can participate in addition, esterification and phosphorylation reactions, forming the framework of fine chemicals. It is a common phosphorus source for the synthesis of fine chemicals containing organophosphorus compounds.

7.Chemical Reducing Agents and Chemical Analysis Reagents (Fine Chemicals and Scientific Research Testing Field)

Fields of application: Scientific research testing, fine chemicals.

Corresponding products: Industrial reducing agents (metal ion reduction, precious metal recovery), analytical grade/high-purity phosphorous acid reagent, phosphorus standard solution.

Why it can be used for this purpose: Phosphorous acid and trivalent phosphorus have reducing properties and can be used to reduce metal ions and recover precious metals. It has good solubility and can be made to high purity, so it is a commonly used reagent in analytical chemistry for phosphorus determination and synthesis research.

8.Other raw materials for the synthesis of phosphorus-based chemicals (phosphorus chemicals industry)

Application Areas: Chemical industry of phosphorus.

Related Products: Phosphite series (intermediate for plasticisers, stabilisers and flame retardants), phosphine/hypophosphite derivative systems, speciality phosphorus chemicals.

Reason for suitability for this application: Phosphite is an important raw material of trivalent phosphorus chemical system. It is an important intermediate in the chain of phosphorus chemical industry, and can produce various phosphorus chemical products such as phosphites, hypophosphites and phosphates by esterification, disproportionation, oxidation and other reactions.

To summarise, industrial phosphorous acid (H3PO3, CAS 13598-36-2) is the “corner stone of organophosphorus chemistry”. It has mature applications in eight major sectors including the PVC heat stabiliser (basic lead phosphite/phosphite esters), organophosphonic acid water treatment agent (ATMP/HEDP), agricultural fungicides and phosphite fertilisers, chemical nickel plating reduction system, plastic flame retardants and auxiliary antioxidants, fine chemicals, chemical reducing agents and analytical reagents and the synthesis of phosphorus-based chemicals, with its “+3 valence reducing properties, pH bond reactivity and phosphonic acid chelating ability. Phosphorous acid gets under our skin through downstream products from plastic pipes to drinking water to crop disease prevention to hardware plating.