Analysis of Kjeldahl digests by the salicylate method: Optimizing pH and buffering improves both sensitivity and precision

Digest acidity can interfere strongly with ammonium analysis using salicylate-indophenol colorimetry, but many methods over-neutralize acids so that buffering capacity and sensitivity are reduced. To optimize analysis of acidic digests, we replaced the phosphate buffer with salicylate. Base was added stoichiometrically to neutralize acids and dissociate one-half the salicylate to reach the phenolic pKa (approx. 13). Sample flow was 20% of total flow and a 60 degrees C water bath was used. This gave optimal sensitivity and minimized interferences due to acidity fluctuations. The limit of detection was 1 mu M in digest solution.