Sulfamic acid, also known as amidosulfonic acid, amidosulfuric acid, aminosulfonic acid, sulphamic acid and sulfamidic acid, is a molecular compound with the formula H<sub>3</sub>NSO<sub>3</sub>. This colourless, water-soluble compound finds many applications. Sulfamic acid melts at 205&nbsp;°C before decomposing at higher temperatures to water, sulfur trioxide, sulfur dioxide and nitrogen.

Sulfamic acid (H<sub>3</sub>NSO<sub>3</sub>) may be considered an intermediate compound between sulfuric acid (H<sub>2</sub>SO<sub>4</sub>) and sulfamide (H<sub>4</sub>N<sub>2</sub>SO<sub>2</sub>), effectively replacing a hydroxyl (−OH) group with an amine (−NH<sub>2</sub>) group at each step. This pattern can extend no further in either direction without breaking down the sulfonyl (−SO<sub>2</sub>−) moiety. Sulfamates are derivatives of sulfamic acid.

Production

Sulfamic acid is produced industrially by treating urea with a mixture of sulfur trioxide and sulfuric acid (or oleum). The conversion is conducted in two stages, the first being sulfamation:

:OC(NH<sub>2</sub>)<sub>2</sub> + SO<sub>3</sub> → OC(NH<sub>2</sub>)(NHSO<sub>3</sub>H)

:OC(NH<sub>2</sub>)(NHSO<sub>3</sub>H) + H<sub>2</sub>SO<sub>4</sub> → CO<sub>2</sub> + 2&nbsp;H<sub>3</sub>NSO<sub>3</sub>

In this way, approximately 96,000&nbsp;tonnes were produced in 1995.

Structure and reactivity

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thumb|150px|[[Ball-and-stick model of a sulfamic acid zwitterion as it occurs in the crystal state Furthermore, a neutron diffraction study located the hydrogen atoms, all three of which are 1.03&nbsp;Å distant from the nitrogen. In the solid state, the molecule of sulfamic acid is well described by a zwitterionic form.

Hydrolysis

The crystalline solid is indefinitely stable under ordinary storage conditions; however, aqueous solutions of sulfamic acid slowly hydrolyse to ammonium bisulfate, according to the following reaction:

: H<sub>3</sub>NSO<sub>3</sub> + H<sub>2</sub>O → [NH<sub>4</sub>]<sup>+</sup>[HSO<sub>4</sub>]<sup>−</sup>

Its behaviour resembles that of urea, (H<sub>2</sub>N)<sub>2</sub>CO. Both feature amino groups linked to electron-withdrawing centres that can participate in delocalised bonding. Both liberate ammonia upon heating in water, with urea releasing CO<sub>2</sub>, while sulfamic acid releases sulfuric acid.

Acid–base reactions

Sulfamic acid is a moderately strong acid, K<sub>a</sub>&nbsp;= 0.101 (pK<sub>a</sub>&nbsp;= 0.995). Because the solid is not hygroscopic, it is used as a standard in acidimetry (quantitative assays of acid content).

: H<sub>3</sub>NSO<sub>3</sub> + NaOH → NaH<sub>2</sub>NSO<sub>3</sub> + H<sub>2</sub>O

Double deprotonation can be effected in liquid ammonia to give the anion .

: H<sub>3</sub>NSO<sub>3</sub> + 2&nbsp;NH<sub>3</sub> → + 2&nbsp;

Reaction with nitric and nitrous acids

With nitrous acid, sulfamic acid reacts to give nitrogen:

: HNO<sub>2</sub> + H<sub>3</sub>NSO<sub>3</sub> → H<sub>2</sub>SO<sub>4</sub> + N<sub>2</sub> + H<sub>2</sub>O

while with concentrated nitric acid, it affords nitrous oxide:

: HNO<sub>3</sub> + H<sub>3</sub>NSO<sub>3</sub> → H<sub>2</sub>SO<sub>4</sub> + N<sub>2</sub>O + H<sub>2</sub>O

Reaction with hypochlorite

The reaction of excess hypochlorite ions with sulfamic acid or a sulfamate salt gives rise reversibly to both N-chlorosulfamate and N,N-dichlorosulfamate ions.

: HClO + H<sub>2</sub>NSO<sub>3</sub>H → ClNHSO<sub>3</sub>H + H<sub>2</sub>O

: HClO + ClNHSO<sub>3</sub>H Cl<sub>2</sub>NSO<sub>3</sub>H + H<sub>2</sub>O

Consequently, sulfamic acid is used as hypochlorite scavenger in the oxidation of aldehydes with chlorite such as the Pinnick oxidation.

Reaction with alcohols

Upon heating, sulfamic acid reacts with alcohols to form the corresponding organosulfates. It is more expensive than other reagents for doing this, such as chlorosulfonic acid or oleum, but is also significantly milder and does not sulfonate aromatic rings. Products are produced as their ammonium salts. Such reactions can be catalyzed by the presence of urea.

Cleaning agent

Sulfamic acid is used as an acidic cleaning agent and descaling agent, either pure or as a component of proprietary mixtures, typically for metals and ceramics. For cleaning purposes, there are different grades based on application such as GP&nbsp;grade, SR&nbsp;grade and TM&nbsp;grade. It is frequently used for removing rust and limescale, replacing the more volatile and irritating hydrochloric acid, which is cheaper. It is often a component of household descalant, for example, Lime-A-Way Thick Gel contains up to 8% sulfamic acid and has pH&nbsp;2.0–2.2, or detergents used for removal of limescale. When compared to most of the common strong mineral acids, sulfamic acid has desirable water descaling properties, low volatility, and low toxicity. It forms water-soluble salts of calcium, nickel, and ferric iron.

Sulfamic acid is preferable to hydrochloric acid in household use, due to its intrinsic safety. If inadvertently mixed with hypochlorite-based products, such as bleach, it does not form chlorine gas, whereas the most common acids would; the reaction (neutralisation) with ammonia produces a salt, as depicted in the section above.

It also finds applications in the industrial cleaning of dairy and brewhouse equipment. Although it is considered less corrosive than hydrochloric acid, corrosion inhibitors are often added to the commercial cleansers of which it is a component. It can be used as a descalant for descaling home coffee and espresso machines and in denture cleaners.

Other uses

  • Catalyst for esterification process
  • Dye and pigment manufacturing
  • Herbicide, as ammonium sulfamate
  • Descalant for scale removal
  • Coagulator for urea-formaldehyde resins
  • Ingredient in fire extinguishing media. Sulfamic acid is the main raw material for ammonium sulfamate, which is a widely used herbicide and fire-retardant material for household products.
  • Pulp and paper industry, as a chloride stabilizer
  • Synthesis of nitrous oxide by reaction with nitric acid
  • The deprotonated form (sulfamate) is a common counterion for nickel(II) in electroplating.
  • Used to separate nitrite ions from mixture of nitrite and nitrate ions (NO<sub>3</sub><sup>−</sup> + NO<sub>2</sub><sup>−</sup>) during qualitative analysis of nitrate by brown ring test.
  • Obtaining deep eutectic solvents with urea
  • As a treatment for removal of nitrite (NO<sub>2</sub><sup>−</sup>) interference in Cyanide analysis.

Silver polishing

According to the label on the consumer product, the silver cleaning product Tarn-X contains thiourea, a detergent, and sulfamic acid.

References

Further reading