PROCEDURE FOR CALIBRATION OF A COULOMETRIC SYSTEM USED FOR TOTAL INORGANIC CARBON MEASUREMENTS OF SEAWATER

A calibration procedure is described for measurements of total inorganic carbon (C(T)) in seawater using a coulometric system. In contrast to a previously published protocol which is based on pure CO2 gas as a calibration standard, the present procedure is based on measurements of standard solutions treated similarly to seawater samples. We show that standard solutions for C(T) measurements can be easily prepared both in the laboratory and at sea from distilled water and preweighed sodium carbonate salt. With reasonable care, 1 l of standard solution can be used regularly over a period of a week. The precision of this calibration procedure ranges from +/-0.02 to +/-0.06%, allowing measurements of seawater samples to within +/-0.3 to +/-1.3-mu-mol C kg-1; such precision is necessary for detection of any increase in C(T) as a result of anthropogenic input. A preliminary comparison to assess the accuracy of these measurements when compared with an established manometric method performed on the same seawater shows close agreement, with a mean of 1978.4+/-1.3-mu-mol C kg-1 for the coulometric method and 1978.8-mu-mol C kg-1 for the manometric analysis. The difference between these measurements (0.4-mu-mol C kg-1) is within the precision of either method. These standard solutions are simple, practical and inexpensive to prepare and measure, making them ideal for measurements at sea.