ANALYSIS OF KJELDAHL DIGESTS BY THE SALICYLATE METHOD - OPTIMIZING PHAND BUFFERING IMPROVES BOTH SENSITIVITY AND PRECISION

Digest acidity can interfere strongly with ammonium analysis using sal icylate-indophenol colorimetry, but many methods over-neutralize acids so that buffering capacity and sensitivity are reduced. To optimize a nalysis of acidic digests, we replaced the phosphate buffer with salic ylate. Base was added stoichiometrically to neutralize acids and disso ciate 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 us ed. This gave optimal sensitivity and minimized interferences due to a cidity fluctuations. The limit of detection was 1 mu M in digest solut ion.