Re-examination of the cadmium reduction method and optimisation of conditions for the determination of nitrate by flow injection analysis

The present paper re-examines the widely used flow injection method for nitrate which is based on heterogeneous reduction of nitrate with copperized cadmium followed by spectrophotometric detection of nitrite formed. The thorough investigation presented here has shown that the reduction step is very critical as far as conversion rate, possible further reduction of nitrite and long-term stability of the reductor column is concerned. The reasons for inconsistent and irreproducible results mentioned in original papers and often obtained in routine applications could be traced back to the way the reductor material had been prepared, the chemical conditions under which reduction takes place (i.e. pH and concentration of complexing agents) and the poisoning of the cadmium surface by sample constituents. The concomitant reduction of dissolved oxygen has been identified as a potential problem causing loss of pH control (due to in-situ generation of hydroxide). De-oxygenation of the carrier solution was found to be a means to overcome this problem with the additional advantages of increasing the lifetime of the reductor column and significantly reducing the concentration of cadmium ions in the waste stream. The role of particle size and column dimension as well as sample residence time within the reductor column on reduction efficiency and sample dispersion has also been investigated.