Use of a single air segment to minimise dispersion and improve mixing in sequential injection: turbidimetric determination of sulphate in waters

In this work, we propose the use of an air segment in a sequential injection system to simultaneously improve the overlapping of the stacked zones and minimise dispersion. This strategy was developed for the determination of sulphate in natural and wastewaters. Barium chloride was used as a precipitating agent and the turbidity of the suspension formed was measured at 420 nm. Analysis was performed without sample pre-treatment and the system was able to monitor sulphate concentration at a rate of at least 20 determinations per hour. Slightly different analytical sequences were developed for natural and wastewaters in order to minimise specific interferences. Direct determination of sulphate was possible within a concentration range of 10–100 and 16–100 mg SO4^2- L^-1 for natural and wastewaters, respectively. Results obtained were comparable with those of the reference method with relative deviations lower than 5%. Relative standard deviations between 1.6% and 3.3% were found.