Flow Injection Spectrophotometric Method for the Quantitative Determination of Humic Acid (HA) in Treated and Natural Waters

A double channel flow injection (FI) spectrophotometric method was established for humic acid (HA) determination in milligram levels in treated and natural waters. FI method was developed for the determination of humic acid by injecting 100 mu L of humic acid into a carrier stream of sodium citrate phosphate buffer (0.05 M, pH 7.0) followed by the injection of 50 mu L, toluidine-blue (TB) solution. Both reagents were allowed to mix at T-junction and passed through a 25 cm reaction coil tubing before passing through the flow through cell (10 mm light path, 30 mu L volume). Change in absorbance was measured at 630 nm. The method was calibrated with standard solutions of HA from 0.5-60 mgL(-1). The calibration curve showed two linearity trends one ranging 0.5-20 mgL(-1) and the other ranging 20-50 mgL(-1). The detection limit was 0.5 mgL(-1) of HA, and relative standard deviation (rsd) of 5 replicate measurements was 0.294%. The developed method was successfully applied for the HA determination in water samples from drinking water plants, rivers, lakes and streams. The sampling rate was more than 80 samples h(-1). The method proves to be simple, rapid, feasible and reproducible.