MEASUREMENT OF TOTAL ALKALINITY OF SURFACE WATERS USING A CONTINUOUS FLOWING SPECTROPHOTOMETRIC TECHNIQUE

Methodology and instrumentation are described which were used for cont inuous measurement of total alkalinity (TA) in surface seawater during a Joint Global Ocean Flux Study (JGOFS) cruise in the Indian Ocean. T he procedure involves a single step acid addition of HCl to a known vo lume 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 pi pette (similar to 100 cm(3)), a thermostated mixing cell (similar to 2 00 cm(3)), a computerized piston titrator, a platinum thermometer, and a Hewlett Packard diode array spectrophotometer with a flowing spectr ophotometric cell as the detector. All the electronic components have interfaces which are easily connected to a personal computer and manip ulated using a Quick Basic program. The FLTA system makes continuous m easurements of TA from the shipboard flowing seawater line. Laboratory results (N = 127) on Gulf Stream seawater (GSSW) have shown a precisi on within +/-2-3 mu mol kg(-1) in TA. Certified reference materials (C RMs) measured at sea (N = 14) gave precise total alkalinities to +/-2. 2 mu mol kg(-1) Intercomparison of the total alkalinity of underway su rface samples (N = 16) obtained in the Indian Ocean using potentiometr ic and spectrophotometric titration systems gives average differences of +/-3 mu mol kg(-1) in TA. The agreement between the two methods is very good considering the temporal and spatial differences in the samp ling. The continuous measurement of TA using the FLTA system may refle ct small variations of TA in the surface waters of the oceans which wo uld not be visible when measuring TA in samples taken from CTD casts a t individual stations. These data can also be combined with flowing pH or P-CO2 data to fully characterize the carbonate system in surface w aters. (C) 1998 Elsevier Science B.V.