Cyanuric Acid, (CONH)₃

Cyanuric Acid, (CONH)₃, is produced by the condensation of cyanic acid vapour above 150° C.,by the action of ammonia on phosgene, COCl₂, and by heating urea above its melting-point as long as ammonia is evolved (Wohler). The reaction

3CO(NH₂)₂ = (CONH)₃ + 3NH₃

depends upon the formation of ammonium cyanate, followed by the loss of ammonia and polymerisation of three molecules of cyanic acid thus set free. The use of anhydrous zinc chloride promotes the formation of cyanuric acid. Cyanuric acid is also produced by chlorinating or brominating urea, and hydrolysing the resulting cyanuric halide. The acid crystallises from concentrated sulphuric acid in anhydrous quadratoctahedra, and from water in dihydrated monoclinic prisms.

The formation of this acid from urea suggests that it possesses an imidic rather than a hydroxylic constitution, and this view is borne out by the character of its absorption spectrum. Cyanuric acid is therefore, probably, tricarbonimide:



Anhydrous cyanuric acid has a density of 1.768 at 0° C.; it does not melt, but sublimes above 150° C., producing cyanic acid vapour; at 350° C. it decomposes.

The heat of formation of cyanuric acid from liquid cyanic acid is 37,000 calories, and from its elements 166,400 calories; its heat of combustion is 220,000 calories. One hundred parts of water dissolve 0.125 parts of cyanuric acid at atmospheric temperature, and 4 parts at 100° C.; the acid is readily soluble in alcohol.

Cyanuric acid is a very weak tribasic acid whose aqueous solution does not affect indicators; its tri- and di-alkali salts are much hydrolysed by water, and are converted by carbon dioxide into the mono-salt.