ガス拡散/フローインジェクション分析法による水中のアンモニア態窒素の定量

水中のアンモニア態窒素のガス拡散/吸光光度FIAについて検討した.キャリヤー液として水酸化ナトリウム溶液を用いることにより,試料中のアンモニウムイオンをガス状のアンモニアとし,多孔質ポリテトラフルオロエチレンチューブを使用したガス拡散装置で分離した.アンモニアガスはチモールブルー溶液に吸収させ,pHの変化を596nmで測定する吸光度の変化に変えて定量した.ガス拡散装置はフランジ型の接続方法を採用し,液漏れしにくいものを製作した.本法によれば5ppm以下のアンモニア態窒素を1時間当たり25試料の速さで定量できた.又,検出限界は2ppbであり,20及び200ppbのアンモニア態窒素の9回繰り返し定量の相対標準偏差はそれぞれ3.8及び1.6%であった.環境水をFIA装置に直接注入してアンモニア態窒素を定量した結果とミクロケルダール蒸留法で不純物を取り除いた後本法で定量した結果は良く一致した.

In this paper, the authors present an assembly of a gas-diffusion unit with a tubular microporous PTFE (polytetrafluoroethylene) membrane, constructed without any glue, and also present the determination of ammoniacal nitrogen in water. In this method, the sample was injected into an alkaline stream (carrier stream) in which the ammonium ions injected were converted to ammonia molecules, then the carrier stream flowed into the gas-diffusion unit. Reagent solution, which contained Thymol Blue and was adjusted to pH 8.4, was also fed to the gas-diffusion unit, and flowed through the microporous PTFE membrane tubing. Ammonia passed through the PTFE membrane and reacted with the acidic form of Thymol Blue to change it to the basic form. The color change to the basic form was measured at 596 nm in a flow-through cell. The detection limit was about 2 ppb as ammoniacal nitrogen, the response was linear up to 5 ppm, the sampling rate was 25 h(-1), and the relative standard deviations obtained by nine injections of 20 ppb and 200 ppb solutions were 3.8% and 1.6%, respectively. The results obtained by directly injecting water samples were in good agreement with that obtained by injecting the solutions prepared according to the Micro-Kjeldahl's distillation method.