Quantitative reduction and inclusion of plant tissue nitrates in Kjeldahl digestion.

(...) A system for quantitative reduction of NO 3 − to NH 4 + using the Zn metal and Cu 2+ in 2 M H 3 PO 4 was developed and was used as a predigestion treatment. Using four different acids in a Zn metal and Cu ion system, the efficiency of NO 3 reduction was found to be dependent on the acid type and its concentration. The Cu ions catalyze the Zn-acid reaction and improved reproducibility comparted to treatments with no Cu 2+ added. (...)