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IUPAC name
Sodium hydroxymethanesulfinate
Other names
Sodium formaldehydesulfoxylate, sodium oxymethylene sulfoxylate
CAS number 149-44-0 YesY

[6035-47-8], dihydrate
ChemSpider 8649 YesY
Jmol-3D images Image
PubChem 8999
RTECS number PB0380000
Molar mass 118.10 g/mol
154.14 g/mol, dihydrate
Appearance colorless crystals
Density 1.75 g/cm3, dihydrate
Melting point 64.5 °C (148.1 °F; 337.6 K) dihydrate
600 g/L, dihydrate (approximate)
Acidity (pKa) decomposes at low pH
Molecular shape pyramidal at S
Main hazards non-toxic
R/S statement R: 36/37/38 S:26-36
Related compounds
Related compounds
SO32−, CH2O
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Rongalite, also called Rongalit (registered trademark of BASF), is sodium hydroxymethylsulfinate, or Na+HOCH2SO2. The salt has many names, including also sodium formaldehyde sulfoxylate, and Bruggolite. It is listed in the European Cosmetics Directive as sodium oxymethylene sulfoxylate (INCI). It is water-soluble and generally sold as the dihydrate.


This salt is prepared from sodium dithionite and formaldehyde:

Na2S2O4 + 2 CH2O + H2O → NaHOCH2SO3 + NaHOCH2SO2

This reaction proceeds quantitatively, such that dithionite can be determined by its conversion to rongalite, which is far less O2-sensitive and thus easier to handle.

The hydroxymethanesulfinate ion is unstable in solution towards decomposition to formaldehyde and sulfoxylate ion, however addition of at least one equivalent of formaldehyde pushes the equilibrium towards the side of the adduct and reacts further to give the bis-(hydroxymethyl)sulfone. Such solutions are shelf-stable indefinitely.

Sodium hydroxymethanesulfinate was originally developed in the early 20th century for the textile industry as a sulfoxylate ion storage compound, where the latter can be generated at will. In use, when sodium hydroxymethanesulfinate is made acidic, the reducing sulfoxylate ion and formaldehyde are released in equimolar amounts. For safety reasons the generation of formaldehyde must be taken into consideration when used industrially.

NaHOCH2SO2 can essentially be considered to be a source of SO22−. As such it is used both as a reducing agent and as a reagent to introduce SO2 groups into organic molecules. Treatment of elemental Se and Te with NaHOCH2SO2 gives solutions containing the corresponding Na2Sex and Na2Tex, where x is approximately 2. As a nucleophile, NaHOCH2SO2 reacts with alkylating agents to give sulfones.

NaHOCH2SO2 + 2 C6H5CH2Br → [C6H5CH2]2SO2 + NaBr + CH2O + HBr

Occasionally, alkylation will occur also at oxygen, thus α,α'-dibromoxylene gives both the sulfone and the isomeric sulfinate ester.


The original use of the compound was as industrial bleaching agent and as a reducing agent for vat dyeing. The other dominating use today is the application as reducing agent in redox-initiator systems for emulsion polymerization. One of the typical redox pair examples is t-butyl peroxide. A niche use is its use as water conditioner for aquaria as it rapidly reduces chlorine and chloramine and reacts with ammonia to form the innocuous aminomethylsulfinate ion.[1] It is also used as an antioxidant in pharmaceutical formulation.

The compound has, in recent years, been used increasingly in commercial cosmetic hair dye colour removers despite the generation of formaldehyde, a known human carcinogen.

Zinc formaldehyde sulfoxylate[edit]

The zinc complex Zn(HOCH2SO2)2 is marketed under the trademarks Decroline, Decolin, and Safolin. This compound is an additive in polymers and textiles.[2]


  1. ^ EP patent 0278515, Günter Ritter, "Chloramine removing means", issued 1988-08-17, assigned to Tetra-Werke 
  2. ^ Masciocchi, N.; Rigamonti, C.; Maspero, A. (2005). "Poly[di-μ3-hydroxymethanesulfinato-zinc(II)]". Acta Crystallographica E 61 (12): m2683–m2685. doi:10.1107/S1600536805038237.