Improving the Berthelot reaction for determining ammonium in soil extracts and water

Colorimetric methods based on the Berthelot reaction are used widely for quantitative determination of NH₄-N in biological and environmental samples. Studies to evaluate phenol and salicylate, the most commonly used chromogenic substrates, revealed minor interferences by metallic cations, whereas up to a threefold shift in absorbance was observed with 38 diverse N-containing organic compounds. Interferences differed markedly between phenol and salicylate. The possibility of a simple correction was precluded by the fact that interferences were both positive and negative, and depended on the temperature during color development and the concentration of NH₄-N. Fourteen compounds were evaluated as alternatives to phenol and salicylate, of which the Na salt of 2-phenylphenol (PPS) proved to be the most promising. Using PPS, macro- and microscale batch methods and an automated flow-injection method were developed. These methods are simple, convenient, and sensitive. Using the PPS microscale method, for which the limit of detection is 0.17 mg NH₄-N L^-1, recovery of NH₄-N added to soil extracts ranged from 98 to 104%, with a coefficient of variation of 1.4 to 2.7%. As with phenol and salicylate, precipitation of metal hydroxides was observed. Precipitation was controlled by chelation with citrate rather than ethylenediaminetetraacetic acid (EDTA), which suppressed color development by preventing monochloramine formation. Compared with Berthelot methods that use phenol or salicylate, interference by amino acids was decreased by up to 10-fold. Interference by other organic N compounds was virtually eliminated.