Photo-induced reduction of nitrate to nitrite and its application to the sensitive determination of nitrate in natural waters

A simple, sensitive and less poisonous flow-injection method for the determination of nitrate using a newly developed activator for the reduction of nitrate to nitrite is presented. Nitrate in sample solutions injected in a buffered carrier containing the activator is first reduced to nitrite while flowing in a reduction coil installed in a photo-reactor; then nitrite is determined spectrophotometrically on the basis of a diazotization-coupling reaction. The photo-reactor is made of a low-pressure mercury lamp, a poly(tetrafluoroethylene) (PTFE) tubing and a reflection covering. The PTFE tubing is wound directly around the mercury lamp, and it is wrapped up in aluminium foil. By using the photo-reactor and EDTA analogues as an activator in carrier, the reduction efficiency reaches about 70–84%. In a simple flow system for nitrate determination, the manifold with double streams is adopted. The carrier stream, in which the sample solution is injected, is a buffered solution (pH 7.0) containing phosphate and EDTA analogues, and the reagent solution contains chromogenic reagents. In a flow system for simultaneous determination of nitrite and nitrate, the carrier stream is bifurcated after a sample injection valve. The detection limits for nitrate and nitrite are about 3 × 10 −8 M, and the sample throughput is about 30 samples per h for nitrate determination, and is about 10 samples per h for the simultaneous determination of nitrate and nitrite.