反應溫度對磷鉬銻藍法測水中磷酸鹽之影響與間歇式流動分析方法

Abstract

本研究利用自製之恆溫反應裝置探討了溫度對磷鉬銻藍法測定磷酸鹽反應之影響，發現溫度確實會影響磷鉬銻藍複合物之吸收光譜，不同溫度下已呈色磷鉬銻藍之光譜在波長 935 nm 時會有一結點，波長在此點以上時，吸光值與溫度呈正相關，此點以下則反之，以波長 880 nm 為例，每升高 1 ℃ 會使吸光係數降低約 0.22 %。因此當反應在加熱情況下進行時，雖反應速率加快，但吸光係數亦會降低，且若反應溫度超過 35 ℃ 時，吸光值還會出現先升後降之不可逆現象，即使降回常溫也無法回復，故建議測定時將反應溫度控制在 30 ℃ 以下。此外本研究亦證實提高抗壞血酸試劑濃度能加速反應進行以及在抗壞血酸中加入去氧劑 (亞硫酸鈉) 能有效延緩其氧化且不影響呈色結果，解決了保存不易之問題。綜合以上結論，本研究設計了一套間歇式磷酸鹽流動分析方法，採用停機測定之概念將樣品與試劑混合後送入恆溫光槽即停止進樣，使化學反應在光槽中完成。此方法之線性範圍為濃度 0~50 μM，靈敏度約 0.2 μM，相對精密度在測定濃度 10 μM 之樣品時小於 ±0.05 %，分析一個樣品約需時 150 s，雖停機測定增加了些許等待時間，但測定是在靜止且呈色完成之狀態下進行，測值非常穩定，亦不受樣品鹽度及溫度干擾，非常適合河口及海上作業。

This study investigates the influence of reaction temperature on the phosphoantimonylmolybdneum blue method for the determination of phosphate in natural waters. Results show that even though heating can effectively increase the reaction rate but it also changes the spectrum of the final blue color. At 880 nm, the change of the molar extinction coefficient is ca. -0.22 % for an increasing of 1 ℃, and it is a reversible process. If the temperature is elevated when the reaction is developing, the color formation curve would go erratic at higher temperatures and the final absorbance does not return to normal value even a later cooling procedure is carried out. Therefore, the traditional way of heating up the sample using a heating bath higher than 35 ℃ is not recommended. Instead, the measurement should be made at a constant temperature (e.g. 30 ℃). The reaction rate can be improved by using a higher concentration of ascorbic acid. The reagent was prepared in the presence of 1% (w/v) sodium sulfite which preserves the ascorbic acid from oxidation. According to the results mentioned above, an automated flow analyzer was developed based on a stepwise manner. The sample flow was mixed with reagents and filled a thermo-controlled flow cuvette then stopped to allow the color reaction to complete. The precision obtained was better than 0.05 % at a 10 μM level. This system suits well the on-board measurement especially when dealing a batch of sample of various temperatures collected from different depths.