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Dichromate Digestion and Simultaneous Colorimetry of Microbial Carbon and Nitrogen
Author(s) -
Doyle Allen,
Schimel Joshua P.
Publication year - 1998
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1998.03615995006200040013x
Subject(s) - kjeldahl method , colorimetry , chemistry , digestion (alchemy) , detection limit , colorimetric analysis , titration , chromatography , nitrogen , ammonium , inorganic chemistry , organic chemistry
Dichromate digestion is used frequently for analysis of organic C and is followed by manual titration. We sought to automate C detection and to include N in the analysis. We optimized digestion parameters (duration, temperature, acids, and catalysts), compared detection methods [manual and automated titration, colorimetric absorbance of Cr(III) and Cr(VI) and automated colorimetry of Cr(VI)], adapted salicylate‐indophenol colorimetry for N detection, and compared N digestion efficiency with Kjeldahl digestion. Optimal digestion conditions were 144°C internal temperature for 3 h with 2:1 H 2 SO 4 /H 3 PO 4 and Ag 2 SO 4 . Automated and manual titrations were reliable but the titrant (ferrous ammonium sulfate) precluded N detection. Colorimetric detection of Cr(VI) with s ‐diphenylcarbazide was fast and precise, but high blanks and steady decomposition of Cr(VI) necessitated several internal standards. Colorimetric analysis of N was possible after precipitating Ag and it was stable, precise, and accurate. Digestion recovery of yeast extract and soil extract N from birch ( Betula papyrifera Marsh.), alder ( Alnus tenuifolia Nutt.), and poplar ( Populus balsamifera L.) stands was low compared with Kjeldahl N (82, 79, 88, and 78%), but precision of the two digestions was the same. The detection limits were 25 µg C and 2 µg N per digestion (125 mg C and 10 mg N kg ‐1 dry soil). While this method is not suitable for work demanding high accuracy, automated C detection combined with N detection provides data acceptable for studies comparing field or laboratory soil treatments.