Sequential injection determination of nitrate in vegetables by spectrophotometry with inline cadmium reduction

A sequential injection system for the determination of nitrate (NO ₃^-) in vegetables was developed to automate this determination, allowing for substantially reduced reagent consumption and generated waste using low-cost equipment. After extraction with water and filtration, the extracted nitrate is reduced inline to nitrite in a copperized cadmium (Cd) column and determined as nitrite. According to the Griess-Ilosvay reaction, nitrate is diazotized with sulfanilamide and coupled with N-(1-naphtyl)-ethylenediamine dihydrochloride to form a purple-red azo dye monitored at 538 nm. Nitrate can be determined within a range of 1.35-50.0 mg L^-1 of NO₃^- (corresponding to 0.270-10.0 g of NO₃^- per kg of vegetable), with a conversion rate of nitrate to nitrite of 99.1 ± 0.8%. The results obtained for 15 vegetable extracts compare well with those provided by the classical procedure, with a sampling throughput of 24 determinations per hour and relative standard deviations better than 1.2%.