硫酸錳催化靛藍法測氨之研究

Abstract

本文主要在研究硫酸錳催化靛藍法測氨的方法，並探討溫度、final pH及鹽度對硫酸錳催化靛藍呈色的影響。溫度和final pH 都不會影響靛藍呈色的光譜（最大吸收波長630 nm）。但是final pH會影響硫酸錳催化靛藍呈色之反應速率及莫耳吸光係數， final pH越高，靛藍呈色的反應速率越快。淡水的靛藍呈色在final pH = 9.89有最大莫耳吸光係數約8000 M-1cm-1，而海水靛藍呈色則是在final pH = 9.74有最大莫耳吸光係數約6400 M-1cm-1。由於不同的鹽度對final pH影響太大，會影響氨氮的測定，為了方便實驗操作的進行，所以本文建議：不論是測定海水或淡水，所使用的試劑皆相同，但是都必須以final pH對吸光值之校正公式計算「淨吸光值」，校正公式為：Abs(corr.)=（Abs（actuality）-blank）× (0.4704×pH2 -9.935×pH +53.442) ，校正後之淨吸光值再依莫耳吸光係數為7600 M-1 cm-1計算原樣水中的氨濃度。本法的精密度約為1% ( at 20 mM level)，偵測極限約0.5 mM， 線性範圍可至100 mM)，雖然本法的試劑空白偏高，但如能精準控制二氯異三聚氰酸（DIC）試劑的加入量，仍可保持相當可信的再現性。由於硫酸錳催化之反應時間非常快(常溫下約7分鐘可達平衡)，很容易將操作改為自動分析，非常適用於近岸河口海洋環境中高濃度氨樣水之測定。

The feasibility of using MnSO4 as a catalyst for the determination of ammonia in natural water based on the Indophenol blue reaction is studied. The salt interference on pH and color formation rate, the temperature effect on the kinetics of the formation reaction are examined in details. It was found that, using the MnSO4 catalyst, the reaction rate and the final molar extinction coefficient of the Indophenol blue complex are not affected by temperature but by the final pH, which is in oppose to the traditional Indophenol blue method using nitroprusside as a catalyzing agent. The maximum extinction coefficient of the freshwater samples is about 8000 M-1cm-1 at final pH=9.89 but it of the seawater samples is about 6600 M-1cm-1 at final pH=9.74. Samples of different salinities which would result in different final pH and different molar extinction coefficients. It is suggested the samples of different salinities should be added the same reagents to make the operation of determining ammonia in water easier. An empirical equation is nominated to correct for such salt error: Abs(corr.)=（Abs（actuality）-blank）× (0.4704×pH2 -9.935×pH +53.442) Although the molar extinction coefficient by the proposed procedure is comparatively low, to be 7600 cm-1M-1, it takes advantage that the reaction time is much shorter (can be completed within 7 minutes under room temperature) than that of the traditional Indophenol blue method. The precision was 1 % at a concentration level of 20 mM, and the detection range is 0.5∼100 mM. The rapid reaction time also render this method to be readily adopted by auto-analysis, thus becomes a useful tool for the study of ammonia in estuarine and coastal environment.