IMPROVING THE BERTHELOT REACTION FOR DETERMINING AMMONIUM IN SOIL EXTRACTS AND WATER

Colorimetric methods based on the Berthelot reaction are used widely f or 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 w ith 38 diverse N-containing organic compounds. Interferences differed markedly between phenol and salicylate. The possibility of a simple co rrection was precluded by the fact that interferences were both positi ve and negative, and depended on the temperature during color developm ent and the concentration of NH4-N. Fourteen compounds were evaluated as alternatives to phenol and salicylate, of which the Na salt of 2-ph enylphenol (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. Precip itation was controlled by chelation with citrate rather than ethylened iaminetetraacetic acid (EDTA), which suppressed color development by p reventing monochloramine formation. Compared with Berthelot methods th at use phenol or salicylate, interference by amino acids was decreased by up to 10-fold. Interference by other organic N compounds was virtu ally eliminated.