Flow-injection spectrophotometry of nitrites based on the diazotization reactions of azine dyes

The diazotization reactions of azine dyes (safranin, Neutral Red, ethacridine, and trypaflavine) were studied under flow conditions. These reactions are accompanied by the formation of intensely colored diazo compounds with a high contrast (more than 100 nm). The formation, stability, and optical properties of these compounds in solutions were supported by quantum-chemical calculations. The kinetic stability of diazo compounds was found to be sufficient for the flow-injection detection of an output peak. Trypaflavine was found to be the best and most sensitive reagent; in this case, the diazotization reaction in 0.2 M HCl occurred especially rapidly and almost completely with a degree of conversion of 0.98. A procedure for the determination of nitrite ions based on the reaction with trypaflavine was developed. The combination of on-line preconcentration on an EDE-10P anion-exchange resin and the reaction at the instant of elution in a flow system allowed us to determine selectively low nitrite concentrations in water with the limit of detection equal to 2 ng/mL, as calculated according to Kaiser (3s) or IUPAC. The procedure developed was used for the monitoring of nitrite concentrations in various water samples; the throughput was 100 samples per hour.