pH, alkalinity and total CO2 in coastal seawater by potentiometric titration with a difference derivative readout

A method for measuring three components of the CO2 system, pH, alkalinity (A(t)) and total CO2 (TCO2) in coastal seawater is presented. The measurements are sufficiently precise to register CO2 changes of biological origin in surficial and coastal waters or in culture media. The method is based on a modified potentiometric titration of seawater with acid in a custom-built cell with the data plotted as a difference derivative, giving two peaks from which total carbonate and alkalinity can be computed. pH is calculated directly from the initial millivolt reading of the sample. One important aspect of this technique is that, unlike a Gran titration, the measured values of pH, A(t) and TCO2 are independent of any pre-conceived seawater model. In this work we demonstrate that the relative position of the two peaks (used for determining TCO2) is relatively insensitive to interferences from dissolved organic matter to about 1 x 10(-4) M, while peak height is sensitive to it. This last observation permits the detection of organic bases which might be included in the measurement of alkalinity. (C) 1999 Elsevier Science B.V. All rights reserved.