Quantitative Reduction and Inclusion of Plant Tissue Nitrates in Kjeldahl Digestion
This study was prompted by controversy over the accuracy of the existing predigestion methods to include NO3-N in Kjeldahl digestion. A system for quantitative reduction of NO−3-N to NH+4 using the Zn metal and Cu2+ in 2 M H3PO4 was developed and was used as a predigestion treatment. Using four different acids in a Zn metal and Cu ion system, the efficiency of NO3 reduction was found to be depenndent on the acid type and its concentration. The Cu ions catalyze the Zn-acid reaction and improved reproducibility compared to treatments with no Cu2+ added. Four milliters 2M H3PO4 containing 40 mg Cu2+ as CnCl22H3O and 0.3 g Zn recovered 100% of the NO−3-N in plant materials containing as high as 2.51% NO−3-N (200 mg sample). Tissues containing higher levels of NO−3-N may be analyzed using smaller size sample, e.g. 150 mg. The digestion procedure includes acidified H2O2 treatment at low temperature to solubilize unused Zn and partially oxidize plant organic matter to control foaming and frothing. Small digestion tubes (50-mL) and a digestion block were used for greater efficiency. The method is simple, highly reproducible and convenient.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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