Spectrophotometric determination of ammonia: a study of a modified Berthelot reaction using salicylate and dichloroisocyanurate

A reaction scheme for the spectrophotometric determination of ammonia by means of a modified Berthelot reaction is proposed, in which salicylate, dichloroisocyanurate and complex cyanides are the principal reagents. The experimental results presented are consistent with, and support, the reaction scheme proposed. It is suggested that the complex cyanides act on two stages of the reaction, firstly to stabilise monochloramine at pH values (12–13) at which it is normally unstable and hence facilitate the formation of 5-aminosalicylate from salicylate (this step is the rate determining step of the reaction), and secondly, to accelerate the oxidative coupling of 5-aminosalicylate with salicylate to form the indophenol dye possibly via a hexacyanoferrate(III) intermediate. It is shown that the optimum pH of reaction is a result of a complex inter-relationship of a number of equilibria and it thus remains necessary to optimise the pH value for each combination of reagents used. The implications of this study on the choice of reagents for the determination of ammonia by the Berthelot reaction are noted.