Abstract
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 CO 2 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 μmol 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 μmol C kg −1 for the coulometric method and 1978.8 μmol C kg −1 for the manometric analysis. The difference between these measurements (0.4 μmol 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.
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