ガス拡散/フローインジェクション分析法による水中の総炭酸の定量

水中の総炭酸(二酸化炭素,炭酸塩及び炭酸水素塩)のガス拡散/吸光光度FIA法について検討した.キャリヤー液として硫酸を用い,試料中の総炭酸をガス状の二酸化炭素にしてガス拡散装置で分離しガス状二酸化炭素のみ試薬溶液に吸収させる.試薬溶液として酸-塩基指示薬であるクレゾールレッドを含む水溶液を使用し,pHの変化による吸光度の変化から定量する.ガス拡散装置はフランジ型の接続方法を採用しており,接着剤を用いる必要がなく接続部から液漏れしにくい.本法は1.2×10(-3)M以下の総炭酸が定量でき1時間当たり15試料の分析が可能であった.又,検出限界は5×10(-6)Mであり,水道水(4.7×10(-4)M)の10回繰り返し定量の相対標準偏差は,0.8%と良好であった.環境水を本法で定量した結果と間接吸光検出イオンクロマトグラフィーで定量した結果はよく一致した.

In FIA, a gas-diffusion technique is highly selective, because there are only a few species which are generated as gases at room temperature. In this paper, the authors determined the total carbonate-carbon content in water by FIA with gas-diffusion unit assembly with a tubular microporous polytetrafluoroethylene (PTFE) membrane, which was reported previously. In this method, the sample was injected into a carrier stream of sulfuric acid in which carbonate and hydrogencarbonate ions injected were converted to gaseous carbon dioxide. The reagent stream which contained Cresol Red and was adjusted to pH 9.0, and the carrier stream, were fed to the gas-diffusion unit. Carbon dioxide passed through the PTFE membrane: the color change of Cresol Red to the acidic form was measured at 410 nm in a flow-through cell. The detection limit was 5×10(-6) M (S/N=3), the response was linear up to 1.2×10(-3) M, the sampling rate was 15 h(-1), and the relative standard deviation obtained by ten injections of tap water (4.7×10(-4) M) was 0.8%. The results obtained by the proposed method were in good agreement with those obtained by indirect photometric ion chromatography.