Simultaneous determination of nitrite, nitrate, sulphate and phenolic compounds, by sequential injection analysis, in wastewaters

A sequential injection analysis (SIA) system for the simultaneous determination of nitrite, nitrate, sulphate and phenolic compounds, in wastewaters has been developed. The nitrite determination is based on the Griess-Llosvay reaction. Nitrate is previously reduced to nitrite in a copperized cadmium column and analyzed as nitrite. The sulphate determination is based on the turbidimetric determination using barium chloride as reagent. The phenolic compounds determination is based on oxidative coupling with 4-aminoantipyrine (4-AAP) in alkaline solution. The resulting products of each reaction were spectrophotometrically measured at 540 nm. The proposed system is fully automatized and is able to monitor nitrite, nitrate, sulphate and phenolic compounds, simultaneously, in samples at a frequency of about 12 samples per hour with a relative standard deviation (RSD) better than 2.0 %, 0.7 %, 1.5 % and 2.2 %, respectively. The calibration graph is linear between 0.05 and 25 mg.dm-3 for nitrite, 0.05 and 15 mg.dm-3 for nitrate, 5 and 200 mg.dm-3 for sulphate and, finally, between 0.5 and 25 mg.dm-3 for phenolic compounds.