# Hypophosphorous acid

Hypophosphorous acid (H₃PO₂, CAS 6303-21-5), known systematically as phosphinic acid, stands at the lowest oxidation level among the familiar phosphorus oxyacids; it donates only one proton, acts as a powerful reductant, and is typically handled in water.<sup>[1](https://neemcco.com/blog/hypophosphorous-acid-reducing-agent/)</sup><sup> • </sup><sup>[2](https://en.alegsaonline.com/art/46207)</sup> It is the parent acid of the hypophosphite salts and is used industrially in electroless plating, pharmaceuticals, and chemical synthesis.<sup>[3](https://pubs.acs.org/doi/10.1021/acs.iecr.9b04710)</sup>

| Key fact | Detail |
|---|---|
| Formula and name | H₃PO₂; systematic name phosphinic acid; CAS 6303-21-5<sup>[1](https://neemcco.com/blog/hypophosphorous-acid-reducing-agent/)</sup> |
| Molar mass | 65.996 g/mol<sup>[4](https://pubchem.ncbi.nlm.nih.gov/compound/Phosphinic-Acid)</sup> |
| Physical form | Colorless deliquescent crystals or oily liquid with a sour odor; density 1.493 g/cm³; melts at 26.5 °C; boils at 130 °C<sup>[5](https://enky-afina.ru/f/10a7_handbook_of_inorganic_chemicals_pradyot_patnaik_phd.pdf)</sup> |
| Acidity | Monoprotic, pKₐ ≈ 1.24; the two other hydrogen atoms are bonded directly to phosphorus<sup>[6](https://www.wbnsou.ac.in/online_services/SLM_UGC_DEB/UG_4YR/CHEMISTRY/NEC_CH_03_E.pdf)</sup><sup> • </sup><sup>[1](https://neemcco.com/blog/hypophosphorous-acid-reducing-agent/)</sup> |
| Main uses | Reducing agent, especially for electroless nickel plating; preparation of hypophosphites; bleaching agent and catalyst<sup>[4](https://pubchem.ncbi.nlm.nih.gov/compound/Phosphinic-Acid)</sup><sup> • </sup><sup>[7](https://citeseerx.ist.psu.edu/document?doi=42fdc2cd5688c063409f39fe747abd27d6cd7e23&repid=rep1&type=pdf)</sup> |
| Regulation | Listed by the US Drug Enforcement Administration as a special surveillance chemical used in clandestine drug manufacture<sup>[4](https://pubchem.ncbi.nlm.nih.gov/compound/Phosphinic-Acid)</sup> |
| Transport | 50% aqueous solution ships as UN 3264, corrosive liquid, acidic, inorganic, hazard class 8, packing group II<sup>[8](https://assets.thermofisher.com/DirectWebViewer/private/results.aspx?LANGUAGE=d__EN&PLANT=d__ALF&SKU=ALFAA14142&SUBFORMAT=d__CGV4&page=NewSearch)</sup> |

## Structure and acidity

Judging only by its empirical formula H₃PO₂, one might expect the compound to be a triprotic acid comparable to phosphoric acid, yet its structure is more accurately represented as H₂P(O)OH: an acidic hydroxyl accounts for one hydrogen, while the other two are attached directly to phosphorus.<sup>[1](https://neemcco.com/blog/hypophosphorous-acid-reducing-agent/)</sup> Only the hydroxyl proton is ionizable, so the acid is monoprotic despite the three hydrogens in its formula.<sup>[2](https://en.alegsaonline.com/art/46207)</sup> Its pKₐ is about 1.24, compared with pKₐ(1) = 2.00 for phosphorous acid (H₃PO₃) and 2.21 for phosphoric acid (H₃PO₄).<sup>[6](https://www.wbnsou.ac.in/online_services/SLM_UGC_DEB/UG_4YR/CHEMISTRY/NEC_CH_03_E.pdf)</sup> [Phosphorus](https://www.edgepedia.org/phosphorus) in the acid is in its lowest common oxidation state among the phosphorus oxyacids, which is the basis of its reducing behavior.<sup>[1](https://neemcco.com/blog/hypophosphorous-acid-reducing-agent/)</sup>

The recommended name phosphinic acid has not gained wide acceptance for inorganic compounds, though it is generally used for organophosphorus derivatives.<sup>[9](https://www.sciencedirect.com/topics/chemistry/phosphinic-acid)</sup>

## Preparation

Hypophosphorous acid is now produced by the acidification of sodium hypophosphite.<sup>[10](https://patents.google.com/patent/WO1996020890A1/en)</sup> The hypophosphite itself is made by reacting elemental white phosphorus with aqueous sodium hydroxide and lime, a reaction that also releases phosphine and hydrogen gases.<sup>[10](https://patents.google.com/patent/WO1996020890A1/en)</sup>

Several routes to the free acid have been described:

- **Sulfuric acid displacement.** Reacting sulfuric acid with sodium hypophosphite gives the acid plus sodium sulfate, but the sulfate must be precipitated by refrigeration to about −40 °C, and residual sulfate is removed with barium, generating barium sulfate waste.<sup>[11](https://patents.google.com/patent/US5368832A/en)</sup>
- **Hydrogen chloride displacement.** Treating sodium hypophosphite with hydrogen chloride precipitates sodium chloride, which is filtered off; the filtrate is passed through an anion exchange column to remove residual chloride. A concentration of about 75 to 85 wt% acid is preferred, because above 85 wt% evaporation at high temperature causes slow decomposition of the acid into phosphorous acid and phosphine.<sup>[11](https://patents.google.com/patent/US5368832A/en)</sup>
- **Ion exchange.** Passing sodium hypophosphite solution through a hydrogen-loaded cation exchange resin exchanges sodium for hydrogen; the drawback is the very large resin column required and a phosphorus-containing aqueous waste stream.<sup>[11](https://patents.google.com/patent/US5368832A/en)</sup>
- **Electrodialytic water splitting.** In bipolar membrane electrodialysis (BMED), sodium hypophosphite is split directly into the acid and sodium hydroxide. A patent on the method reports that at 80% current efficiency the acid reaches about 2.2 normal (14.3 wt%), roughly double what the acid's published dissociation constant would predict for a strong acid, reducing the evaporation needed downstream.<sup>[10](https://patents.google.com/patent/WO1996020890A1/en)</sup> A 2020 process-optimization study of BMED for H₃PO₂ production from NaH₂PO₂, aimed at minimizing phosphorous acid contamination, concluded the process has potential for industrial production of high-quality acid.<sup>[3](https://pubs.acs.org/doi/10.1021/acs.iecr.9b04710)</sup>

Free hypophosphorous acid cannot be isolated simply by evaporating acidified hypophosphite solutions, because of its ready oxidation to phosphorous and phosphoric acids and disproportionation to phosphine and phosphorous acid.<sup>[9](https://www.sciencedirect.com/topics/chemistry/phosphinic-acid)</sup> Commercially, the acid is commonly sold as a 50% w/w aqueous solution.<sup>[8](https://assets.thermofisher.com/DirectWebViewer/private/results.aspx?LANGUAGE=d__EN&PLANT=d__ALF&SKU=ALFAA14142&SUBFORMAT=d__CGV4&page=NewSearch)</sup>

## Reducing agent and electroless nickel plating

The largest industrial use of hypophosphite chemistry is electroless nickel plating, in which nickel salts are reduced chemically on a surface without external current. The hypophosphite ion (H₂PO₂⁻) is the most commonly used reducing agent, and it produces a nickel–phosphorus alloy coating rather than pure nickel.<sup>[12](https://www.sciencedirect.com/science/article/pii/S0257897299004235)</sup> Deposits generally contain 3 to 15% codeposited phosphorus originating from the hypophosphite ion, and this phosphorus content gives properties different from electrodeposited nickel, including hardness, solderability, and corrosion resistance.<sup>[7](https://citeseerx.ist.psu.edu/document?doi=42fdc2cd5688c063409f39fe747abd27d6cd7e23&repid=rep1&type=pdf)</sup><sup> • </sup><sup>[13](https://www.mdpi.com/2079-6412/13/9/1490)</sup> The amount of phosphorus codeposited depends on bath variables such as pH and nickel and hypophosphite concentrations.<sup>[14](https://www.sciencedirect.com/science/article/pii/0376458381901400)</sup>

The ideal source of nickel ions for such baths is the nickel salt of hypophosphorous acid, nickel hypophosphite, Ni(H₂PO₂)₂, which would eliminate sulfate anion buildup and keep alkali metal ions to a minimum.<sup>[15](https://www.tau.ac.il/~chemlaba/Files/Electroless/12777_01.pdf)</sup> In practice, nickel hypophosphite's solubility limit in hypophosphorous acid, 35 grams per liter at saturation, complicates its use, since considerable solution must be added to maintain nickel ion concentration.<sup>[16](https://patents.google.com/patent/EP0861924A1/en)</sup> A Japanese study compared direct (ion-exchange) and double-decomposition routes to nickel hypophosphite and found that baths built on the direct-method salt gave films with less internal stress and longer bath life than conventional sodium hypophosphite and nickel sulfate baths, at a similar deposition rate.<sup>[17](https://www.jstage.jst.go.jp/article/sfj1989/50/12/50_12_1103/_article/-char/en)</sup>

Beyond plating, hypophosphorous acid serves as a bleaching agent, reducing agent, and catalyst, and is used in making polymer stabilizers and veterinary medicines and for precipitating metal ions from solution as elemental metals.<sup>[4](https://pubchem.ncbi.nlm.nih.gov/compound/Phosphinic-Acid)</sup><sup> • </sup><sup>[11](https://patents.google.com/patent/US5368832A/en)</sup>

## Hazards and regulation

Hypophosphorous acid is a strong reducing agent and a weak-to-moderate acid classified among reducing agents by CAMEO Chemicals.<sup>[4](https://pubchem.ncbi.nlm.nih.gov/compound/Phosphinic-Acid)</sup> It decomposes when heated into phosphoric acid and spontaneously flammable phosphine; it is oxidized by sulfuric acid with release of sulfur dioxide and sulfur, reacts explosively with mercury(II) oxide and violently with mercury(II) nitrate, and neutralizes bases in exothermic reactions.<sup>[4](https://pubchem.ncbi.nlm.nih.gov/compound/Phosphinic-Acid)</sup> Safety data sheets list strong oxidizing agents, strong bases, and metals as incompatible materials, with oxides of phosphorus and phosphine as hazardous decomposition products.<sup>[8](https://assets.thermofisher.com/DirectWebViewer/private/results.aspx?LANGUAGE=d__EN&PLANT=d__ALF&SKU=ALFAA14142&SUBFORMAT=d__CGV4&page=NewSearch)</sup>

The 50% aqueous solution is corrosive: it is classified skin corrosion category 1B and serious eye damage category 1, and a 50% solution was corrosive in a standardized in vitro corrosivity test.<sup>[8](https://assets.thermofisher.com/DirectWebViewer/private/results.aspx?LANGUAGE=d__EN&PLANT=d__ALF&SKU=ALFAA14142&SUBFORMAT=d__CGV4&page=NewSearch)</sup><sup> • </sup><sup>[4](https://pubchem.ncbi.nlm.nih.gov/compound/Phosphinic-Acid)</sup> Inhalation of vapors irritates or burns the respiratory tract, and liquid and vapors may irritate or burn eyes and skin.<sup>[4](https://pubchem.ncbi.nlm.nih.gov/compound/Phosphinic-Acid)</sup>

Because it can serve as a precursor in illicit synthesis, hypophosphorous acid appears on the US Drug Enforcement Administration's Special Surveillance List of chemicals used in clandestine production of controlled substances.<sup>[4](https://pubchem.ncbi.nlm.nih.gov/compound/Phosphinic-Acid)</sup>

## References

1. [Hypophosphorous Acid as a Reducing Agent: Chemistry, Reactions and Industrial Uses, NeMcco](https://neemcco.com/blog/hypophosphorous-acid-reducing-agent/)
2. [Hypophosphorous acid (H3PO2), AlegsaOnline](https://en.alegsaonline.com/art/46207)
3. [Preparation of Hypophosphorous Acid by Bipolar Membrane Electrodialysis, Industrial & Engineering Chemistry Research](https://pubs.acs.org/doi/10.1021/acs.iecr.9b04710)
4. [Phosphinic Acid, PubChem CID 4124402](https://pubchem.ncbi.nlm.nih.gov/compound/Phosphinic-Acid)
5. [Handbook of Inorganic Chemicals, Pradyot Patnaik (Hypophosphorous acid entry)](https://enky-afina.ru/f/10a7_handbook_of_inorganic_chemicals_pradyot_patnaik_phd.pdf)
6. [Phosphorus oxyacids table with pKa values, UGC chemistry course material](https://www.wbnsou.ac.in/online_services/SLM_UGC_DEB/UG_4YR/CHEMISTRY/NEC_CH_03_E.pdf)
7. [Electroless Nickel Plating, CiteSeerX](https://citeseerx.ist.psu.edu/document?doi=42fdc2cd5688c063409f39fe747abd27d6cd7e23&repid=rep1&type=pdf)
8. [Safety Data Sheet, Hypophosphorous acid 50% w/w aqueous solution, Thermo Fisher/Avocado](https://assets.thermofisher.com/DirectWebViewer/private/results.aspx?LANGUAGE=d__EN&PLANT=d__ALF&SKU=ALFAA14142&SUBFORMAT=d__CGV4&page=NewSearch)
9. [Phosphinic Acid, ScienceDirect Topics overview](https://www.sciencedirect.com/topics/chemistry/phosphinic-acid)
10. [WO1996020890A1, Method of making hypophosphorous acid, Google Patents](https://patents.google.com/patent/WO1996020890A1/en)
11. [US5368832A, Zero discharge process for manufacturing of phosphorous acid and hypophosphorous acid, Google Patents](https://patents.google.com/patent/US5368832A/en)
12. [Electroless nickel plating from acid bath, Surface and Coatings Technology](https://www.sciencedirect.com/science/article/pii/S0257897299004235)
13. [Medium and High Phosphorous Ni-P Coatings Obtained via an Electroless Approach, Coatings (MDPI)](https://www.mdpi.com/2079-6412/13/9/1490)
14. [Phosphorus distribution in electroless nickel deposits, Surface Technology](https://www.sciencedirect.com/science/article/pii/0376458381901400)
15. [The Fundamental Aspects of Electroless Nickel Plating](https://www.tau.ac.il/~chemlaba/Files/Electroless/12777_01.pdf)
16. [EP0861924A1, Hypophosphite solutions and their use in nickel plating, Google Patents](https://patents.google.com/patent/EP0861924A1/en)
17. [Preparation of Nickel Hypophosphite for a Long-Life Electroless Nickel Plating Bath, Surface Finishing Society of Japan](https://www.jstage.jst.go.jp/article/sfj1989/50/12/50_12_1103/_article/-char/en)

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*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Organic substances › Alcohols, ethers, and organooxygen groups › Organosulfur, selenium, and heavier main-group organo derivatives › Heavier main-group organometaloids (B, Si, P, and neighbors) › Organophosphorus compounds › Phosphonates and phosphate esters › Phosphites, phosphinates, and hypophosphites*

*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*

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