A conventional SI system with peripheral UV digestor for determination of total nitrogen in wastewater using sodium persulphate in basic medium as oxidant operates at a sampling rate of 40s/hr.

Total N by On-line Digestion
The method is based on oxidation of nitrogen compounds to nitrate which is monitored spectrophotometrically at 226 nm. The assay protocol was designed to sandwich sample zone between zones of persulphate (A), while keeping the reaction mixture confined between two air segments. The stacked zones, were mixed by flow reversal, and held within the UV digester
by stopping the flow. This allowed digesting times to be extended up to 10 minutes, which of course reduced sampling frequency. A thorough investigation of pH, persulphate concentration, and digestion times on conversion of various organic N compounds, was carried out to establish conditions for optimized assay. The modified flow diagram (C) presented here suggests the use of three digester coils wound on the same UV lamp, for parallel mineralization, to increase sampling frequency for long digestion times. The debubler (B) was used to remove airsegment prior to absorbance monitoring.

M.T. Oms, A. Cerda`, V. Cerdá: Talanta 59 (2003) 319/326