However, as is the case with the copperised cadmium reduction procedure, the mechanism of the Zn reduction of nitrate to nitrite is still not clearly understood. Zn columns require activation with a higher concentration (e.g. 500 μg L−1 NO3–N) for ca. 30 min until a stable response is achieved [139] (cf. Fig. 8a). Thereafter, columns perform quantitative reduction of nitrate to nitrite for many hundreds of injections (Fig. 8b). If a column is conditioned with nitrite rather than nitrate (Fig. 8a), the same trend is observed. Since the photometric detection system is specific for nitrite, this implies that initially both nitrite and nitrate are reduced, probably to ammonia, until some quasi-equilibrium state is reached where quantitative nitrate to nitrite reduction is achieved.