Hypophosphorous acid

The chemical structure of hypophosphorous acid contains only one ionizable hydroxyl group (-OH), so it is a monoprotonic acid. It is an inorganic strong acid with an effective concentration of 50%, a decomposition temperature of 130°C, and strong reducing properties. It is used in industries such as alcohol-based resin decolorizers, electroplating, chemical plating, and pharmaceutical intermediates. Although H₃PO₂ is an inorganic acid containing phosphorus and oxygen, its acidity strength is inversely proportional to the oxidation state of its main elements. The acidity strength of hypophosphorous acid, phosphorous acid (a diprotic acid), and phosphoric acid (a triprotic acid) follows the order: H₃PO₂ > H₃PO₃ > H₃PO₄. Hypophosphorous acid is relatively stable at room temperature but can undergo vigorous acid-base reactions and redox reactions when exposed to oxidizing agents or strong bases, forming corresponding hypophosphite salts.

Chemical Parameters

Chinese Name: Hypophosphorous acid, hypophosphite, hypophosphorous acid, HPA

English Name: Hypophosphorous acid

Chemical Formula: H3PO2

Structural Formula: HOP(O)H2

Molecular Weight: 66

CAS Number: 6303-21-5

HS Code: 2811199090

UN Number: UN 3264

Appearance: Colorless transparent liquid

Export Tax Rebate Rate: 0%

Hazard Class: Class 8

Export Requirements: Commodity inspection, dangerous goods packaging, and some ports prohibit consolidated shipments.

Main Specifications

H₃PO₂ ≥ 50.0%

H₃PO₃ ≤ 0.3%

Sodium (Na) ≤ 0.25%

Iron (Fe) ≤ 0.0005%

Chlorine (Cl) ≤ 0.015%

Color: 30 APHA

Density: 1.210–1.265 g/ml

Product Features

1. Strong Reducing Power

Hypophosphorous acid, a low-valent form of phosphorus, can be gradually oxidized to +3 and +5 valences, making it significantly more effective than ordinary phosphorus-containing reducing agents. During the reaction, electron transfer occurs, reducing the target substance. For example, it can reduce metal ions such as copper, iron, nickel, and cobalt to their metallic states. It enables dense and uniform chemical nickel plating without external current, suitable for complex-shaped components. It features a dense coating, strong corrosion resistance, and high hardness. It is widely used in precision molds, aerospace materials, electronic circuits, automotive parts, and precision machinery.

2. Has certain corrosivity

As a monoprotonic strong acid, hypophosphorous acid has strong corrosivity toward skin and metals, requiring special protective measures. Its strong acidity makes it suitable for cleaning and chelating various metals, particularly in the electronics industry, where its performance surpasses that of ordinary phosphoric acid.

3. Good water solubility

It is miscible with water, various organic acids, ethanol, propanol, surfactants, dispersants, etc. In the formulation of chemical plating solutions, cleaning agents, chelating agents, etc., the proportion can be adjusted within a wide range, making it an excellent inorganic acid.

Reduction mechanism

1. The phosphorus in hypophosphorous acid has a valence of +1. In the presence of oxygen or oxidizing agents, it easily loses electrons and is oxidized to hypophosphorous acid (H₃PO₃) with a valence of +3 or phosphoric acid (H₃PO₄) with a valence of +5, releasing electrons and thereby reducing other substances. The reaction equation is: H₃PO₂ + H₂O → H₃PO₃ + 2H⁺ + 2e⁻. Further oxidation-reduction: H₃PO₃ + 2H₂O → H₃PO₄ + 4H⁺ + 4e⁻. The rate of reduction can be controlled based on the strength of the oxidizing agent.

2. Hypophosphorous acid exhibits strong reducing properties toward metals such as Cu, Au, Fe, Ni, and Cr. It donates electrons, reducing metal ions to their elemental forms while oxidizing itself to hypophosphorous acid or phosphoric acid. For example, in nickel sulfate, the nickel ion Ni²⁺ gains electrons and is reduced to metallic nickel, which precipitates on the surface: Ni²⁺+ 2e → Ni↓. This reaction is the basic reaction mechanism for chemical nickel plating using NiSO₄, NiCl₂, and sodium hypophosphite..

Main applications

1. One of the primary raw materials for chemical nickel plating, copper plating, and zinc plating solutions. Due to its absence of sodium ions, it performs better than sodium hypophosphite or sodium phosphite as a reducing agent in certain surface treatment industries, avoiding interference from sodium ions in the reaction.

2. An important intermediate in organic synthesis and a chain transfer agent, it exhibits strong reducing effects on functional groups such as nitro (-NO₂), azo (-N=N-), and aldehyde/ketone (-CHO/-C=O). It has minimal impact on unsaturated bonds like double or triple bonds, demonstrating excellent reaction selectivity. It produces virtually no polluting byproducts, making it highly environmentally friendly. It has widespread applications in the synthesis of vitamins and antibiotics.

3. It can be used to synthesize high-purity hypophosphites, such as aluminum hypophosphite flame retardants, calcium hypophosphite, manganese hypophosphite, potassium hypophosphite, zinc hypophosphite, and copper hypophosphite. It significantly reduces the generation of sulfates, chlorides, and heavy metals. It is suitable for high-purity electronic-grade and analytical-grade applications, but the synthesis cost is high, making industrial-scale production impractical.

Packaging and Storage

Net weight: 25 kg, 250 kg, or 1,000 kg per IBC drum. Store in a cool, dry warehouse, away from high temperatures and sources of ignition. Keep separate from oxidizers, metal salts, and strong alkaline chemicals.

The concentration of hypophosphorous acid, pH value, and temperature have different effects on the speed of chemical nickel plating, coating quality, and the deposition rate of metallic nickel. Sodium hypophosphite can generally be used as raw material, produced via cation exchange method, resulting in sodium-free or low-sodium finished products. The resin is reusable, suitable for continuous production, and has lower production costs.