Measurement of total alkalinity of surface waters using a continuous flowing spectrophotometric technique

Abstract

Methodology and instrumentation are described which were used for continuous measurement of total alkalinity (TA) in surface seawater during a Joint Global Ocean Flux Study (JGOFS) cruise in the Indian Ocean. The procedure involves a single step acid addition of HCl to a known volume of seawater with rapid spectrophotometric measurement of pH (3 to 4) and calculation of the TA after stripping out the CO2. The flowing total alkalinity system (FLTA) consists of a calibrated volumetric pipette (∼100 cm3), a thermostated mixing cell (∼200 cm3), a computerized piston titrator, a platinum thermometer, and a Hewlett Packard diode array spectrophotometer with a flowing spectrophotometric cell as the detector. All the electronic components have interfaces which are easily connected to a personal computer and manipulated using a Quick Basic program. The FLTA system makes continuous measurements of TA from the shipboard flowing seawater line. Laboratory results (N=127) on Gulf Stream seawater (GSSW) have shown a precision within ±2–3 μmol kg−1 in TA. Certified reference materials (CRMs) measured at sea (N=14) gave precise total alkalinities to ±2.2 μmol kg−1. Intercomparison of the total alkalinity of underway surface samples (N=16) obtained in the Indian Ocean using potentiometric and spectrophotometric titration systems gives average differences of ±3 μmol kg−1 in TA. The agreement between the two methods is very good considering the temporal and spatial differences in the sampling. The continuous measurement of TA using the FLTA system may reflect small variations of TA in the surface waters of the oceans which would not be visible when measuring TA in samples taken from CTD casts at individual stations. These data can also be combined with flowing pH or pCO2 data to fully characterize the carbonate system in surface waters.