Fig. 5. Comparison between the results of the coulometric determinations of total CO2 concentration in the SIO Reference Solutions (Batch #6) obtained during TUNES-2 expedition and the manometric determinations obtained by the staff of C.D. Keeling, SIO. The 158 measurements made during the expedition yeld 2303.2 ± 1.5 μmol/kg, which compares with the mean manometric value of 2304.6 ± 1.6 μmol/kg (N=9). The mean values for each set of measurements agree with each other within respective standard deviation.