An improved procedure for flow-based turbidimetric sulphate determination based on a liquid core waveguide and pulsed flows

Abstract

An improved flow-based procedure is proposed for turbidimetric sulphate determination in waters. The flow system was designed with solenoid micro-pumps in order to improve mixing conditions and minimize reagent consumption as well as waste generation. Stable baselines were observed in view of the pulsed flow characteristic of the systems designed with solenoid micro-pumps, thus making the use of washing solutions unnecessary. The nucleation process was improved by stopping the flow prior to the measurement, thus avoiding the need of sulphate addition. When a 1-cm optical path flow cell was employed, linear response was achieved within 20–200 mg L −1, described by the equation S = −0.0767 + 0.00438 C (mg L −1), r = 0.999. The detection limit was estimated as 3 mg L −1 at the 99.7% confidence level and the coefficient of variation was 2.4% ( n = 20). The sampling rate was estimated as 33 determinations per hour. A long pathlength (100-cm) flow cell based on a liquid core waveguide was exploited to increase sensitivity in turbidimetry. Baseline drifts were avoided by a periodical washing step with EDTA in alkaline medium. Linear response was observed within 7–16 mg L −1, described by the equation S = −0.865 + 0.132 C (mg L −1), r = 0.999. The detection limit was estimated as 150 μg L −1 at the 99.7% confidence level and the coefficient of variation was 3.0% ( n = 20). The sampling rate was estimated as 25 determinations per hour. The results obtained for freshwater and rain water samples were in agreement with those achieved by batch turbidimetry at the 95% confidence level.