Continuous determination of carbonate in sea water with gas-sensitive electrode.

NAGASHIMA Kunio, WASHIO Yukio, SUZUKI Shigetaka

doi:10.2116/bunsekikagaku.33.10_t108

The pCO₂ electrode (Denki Kagaku Keiki Co., Ltd. Tokyo) comprises carbon dioxide permeation membrane (Pore size 0.022 μm, diameter 10 mm, microporous Teflon), silver-silver chloride reference electrode and 0.01 M sodium hydrogen carbonate aqueous solution as internal solution. A sample is pumped at the rate of 5.2 ml min^-1 to the mixing coil (1 mm diameter, 18 cm length), where a pH 3.5 disodium hydrogen phosphate-citric acid buffer solution including 1 M sodium chloride is mixed at the rate of 2.6 ml min^-1. The mixture is fed to the pCO₂ electrode. The electrode response appeared within 1 min after the sample was introduced and reached within 1 mV of its steady-state value after 2.5 min.The determination results by this flow method agree well with those by the batch method in accuracy, reproducibility and the lower determination limit which is about 1 × 10^-4 M. In the range of 1 × 10^-24 × 10^-4 M, the Nernst equation is valid with a relative standard deviation of less than 2.5 %. Various ions such as sulfite ion and sulfide ion have no interference with this method. Since the nitrite ion is oxidized to nitrate ion with the pH 3.5 buffer solution including 5 % hydrogen peroxide and 0.03 % ammonium molybdate, nitrite ion up to 10^-4M does not obstruct the determination of 5 x 10^-3M carbonate. The pCO₂ electrode's response to carbonate standard solution prepared with artificial sea water is 2 mV higher than that prepared with pure water. This method is conveniently applicable to continuous determination of carbonate in sea water because of its easy and simple operation and low cost.

Continuous determination of carbonate in sea water with gas-sensitive electrode.

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