The copper catalysed reduction of nitric oxide by ammonia in aqueous solution studied by membrane inlet mass spectrometry

Abstract

The copper(II)–ammonia–nitric oxide reaction system in aqueous solution was studied by the measurement of dissolved nitric oxide with membrane inlet mass spectrometry. When the reaction was initiated by the addition of ammonia and copper(II) chloride to a saturated solution of nitric oxide, nitric oxide was found to be consumed in a two-stage reaction. Approximately 2 moles nitric oxide per mole copper(II) was consumed during the initial rapid reaction stage. Acidification of the reaction mixture at the end of the rapid reaction caused the reappearance of most of the nitric oxide. Spectrophotometric studies showed that copper(II) ammine complex disappeared during the rapid reaction consuming 2 moles of nitric oxide per mole copper(II). Chemical analysis showed that nitrite was a product of the rapid reaction and membrane inlet mass spectrometry revealed that nitrous oxide was a product of the slow reaction. The rate of the initial rapid reaction decreased with increasing ammonia concentration, whereas the rate of the slow reaction increased with increasing ammonia concentration. We ascribe the rapid reaction to the reversible conversion of copper(II) ammine complex and 2 moles of nitric oxide to copper(I) ammine nitric oxide complex and nitrite. The apparent inhibition of the rapid reaction by ammonia may indicate that nitric oxide competes with ammonia for binding to a vacant site in a copper(II) ammine complex. We ascribe the slow reaction to the irreversible reduction of nitric oxide to nitrous oxide by ammonia catalysed by a copper(I) complex of unknown composition.