Underway determination of alkalinity in estuarine waters by reagent-injection gas-diffusion flow analysis

The development and application of a portable, hybrid reagent-injection gas-diffusion flow analysis technique is described for the underway measurement of total alkalinity in estuarine waters. Injection of pH 4.5 buffer into a continuously flowing sample stream produced gaseous CO2 that diffused across a microporous PTFE membrane into a weakly buffered acceptor stream containing bromothymol blue indicator. The resultant change in acceptor stream pH was detected photometrically using a super-bright LED with a multi-reflection flow cell and charge coupled device detector. This method gave a detection limit of 0.5 mg CaCO3 L−1, with reproducibility of 1.0% R.S.D. at 160 mg CaCO3 L−1, and a measurement rate of 71 injections h−1. The portable FIA system was used for underway analysis of estuarine waters with salinities ranging from that of freshwater to seawater, and there was close agreement between the results obtained by underway analysis and from a reference titration method.