Flow Injection Analysis of industrial water samples

An automated version of Flow Injection Analysis (FIA) instrumentation has been designed and evaluated for the quantitative determination of chemical oxygen demand (COD) for industrial samples. The distinct advantages over conventional COD assays are the significantly reduced analysis time (FIA complete in 10 min, compared with conventional COD assay times of 120 min); simplified sample handling; significantly reduced sample volume (less than 1 cm3 for FIA); significantly reduced reagent consumption; and the avoidance of highly toxic reagents (such as CrVI+). The FIA system developed for this project included automation with a microcontroller, which enabled automatic sample introduction, which removed imprecision associated with the equivalent manual process. Glucose was used as a calibration standard, and to assess the day to day reproducibility of the apparatus. A direct comparison between the FIA assay and a commercial system made on the same samples of aqueous paint waste washings (kindly donated by Crown Paints Ltd. Darwen, Lancashire) showed that the FIA system gave close equivalent quantitation, but with a consistent over prediction which is due to the systematic error associated with using glucose as a standard. A scaling factor would be required to ensure equivalent COD values. The FIA system generated bubbles and required systematic deposit removal in the coiled oxidation reactor; effects of air bubbles in the analysis has been studied and resolved in this project. The FIA-based COD assay had a linear range over 0.1 mM to 1.1 mM glucose concentration – equivalent to 18 mg/l to 198 mg/l; a limit of detection of 0.1 mM and a response time of 5 min, this was based on permanganate chemistry monitored at 525 nm using an Agilent Technologies Cary 60 UV-Vis Spectrophotometer fitted with a 1 cm path-length flow cell