Fig. 4From: Online zinc reduction-sequential injection analysis for the determination of nitrogen species in extracts of riverine sediment a Three-factor search of the optimum colorimetric conditions for nitrate SI analysis using 1.0 mg N L−1 solution. b Three-factor variation of nitrate extraction conditions in search of the optimum. c Optimization of sediment-to-potassium persulfate ratio for quantitative recovery of TNBack to article page