Sulfamic acid, also known as amidosulfonic acid, amidosulfuric acid, aminosulfonic acid, sulphamic acid and sulfamidic acid, is a molecular compound with the formula H3NSO3. This colourless, water-soluble compound finds many applications. Sulfamic acid melts at 205 °C before decomposing at higher temperatures to water, sulfur trioxide, sulfur dioxide and nitrogen. Sulfamic acid (H3NSO3) may be considered an intermediate compound between sulfuric acid (H2SO4), and sulfamide (H4N2SO2), effectively replacing a hydroxyl (–OH) group with an amine (–NH2) group at each step. This pattern can extend no further in either direction without breaking down the sulfonyl (–SO2–) 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(NH2)2 + SO3 → OC(NH2)(NHSO3H) OC(NH2)(NHSO3H) + H2SO4 → CO2 + 2 H3NSO3 In this way, approximately 96,000 tonnes were produced in 1995. Structure and reactivity The compound is well described by the formula H3NSO3, not the tautomer H2NSO2(OH). The relevant bond distances are 1.44 Å for the S=O and 1.77 Å for the S–N. The greater length of the S–N is consistent with a single bond. Furthermore, a neutron diffraction study located the hydrogen atoms, all three of which are 1.03 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: H3NSO3 + H2O → [NH4]+[HSO4]− Its behaviour resembles that of urea, (H2N)2CO. 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 CO2 while sulfamic acid releases sulfuric acid. Acid–base reactions Sulfamic acid is a moderately strong acid, Ka = 0.101 (pKa = 0.995). Because the solid is not hygroscopic, it is used as a standard in acidimetry (quantitative assays of acid content). H3NSO3 + NaOH → NaH2NSO3 + H2O Double deprotonation can be effected in ammonia solution to give the anion . H3NSO3 + 2 NH3 → + 2 Reaction with nitric and nitrous acids With nitrous acid, sulfamic acid reacts to give nitrogen: HNO2 + H3NSO3 → H2SO4 + N2 + H2O while with nitric acid, it affords nitrous oxide: HNO3