Nitrate Plus Nitrite Analysis By Nitrate Reductase

By William Lipps and Libby Badgett, OI Analytical

Methods for the determination of nitrate plus nitrite nitrogen employ granular cadmium columns, or tubular cadmium coils reducing NO₃-N to NO₂-N by the following equation:

Cd + NO₃^- + 2H⁺ NO₂^- + Cd ⁺² + H₂O

Although the cadmium catalyst is a solid, the cadmium ion formed as a result of the reduction of nitrate is soluble and becomes part of the waste stream. Dissolved oxygen, present even in degassed reagents, can exceed the nitrate concentration by a factor of 100. The reaction of cadmium metal with dissolved oxygen proceeds at a rate 30 times faster than the reduction of nitrate to nitrite. Also, because hydrogen ions are consumed in the reduction reaction, the pH increases during reduction (1). Without proper degassing of reagents the amount of cadmium in the waste solution can be as high as 200 ppm. Even with degassed reagents the dissolution of cadmium by oxygen cannot be prevented (2). Cadmium is a Toxicity Characteristic Leaching Procedure (TCLP) target analyte; it makes sense to consider the use of nitrate reductase for nitrate measurement. Nitrate reductase is an enzyme derived from upper level plants that together with the natural reducing agent nicotinamide dinucleotide (NADH) specifically catalyze the reduction of nitrate to nitrite nitrogen.