Automated simultaneous determination of nitrate and nitrite by pre-valve reduction of nitrate in a flow-injection system

Nitrate is reduced to nitrite by using the pre-valve in-valve reduction technique prior to the sampling system. One loop of a two-position sampling valve is replaced by a copperised cadmium column. Nitrite from the samples as well as nitrite formed in the reduction procedure is sampled by a second valve and introduced into the flow system. The two sampling valves are synchronised in such a way that two peaks are obtained, one corresponding to the nitrate plus nitrite and the other to the nitrite only. The method is suitable for the simultaneous determination of nitrate and nitrite at a sampling rate of up to 72 determinations per hour with coefficients of variation better than 1.96% for nitrate and 0.83% for nitrite.