以釩還原及格瑞氏反應測定海水中硝酸鹽之硏究：解析反應動力機制並設立最佳化條件

硝酸鹽(nitrate, NO₃⁻)為主要的氮營養鹽，是海洋浮游生物行光合作用合成有機物的原料，若海水中硝酸鹽不足將會導致藻類無法正常生長，進而影響基礎生產力乃至於整個海洋生物圈，是海洋化學的重要指標。傳統的測定法是利用鎘銅還原管(Cu-Cd column)將硝酸鹽還原成亞硝酸鹽後，再用格瑞氏反應(Griess reaction)測定，但鎘銅還原管的製備繁複且還原率不穩定。本研究嘗試另一種替代方式: 即採用三價釩當還原劑，並依粉紅偶氮呈色反應動力去尋找最佳的實驗條件，如試劑之添加順序、各種試劑濃度、加熱溫度、時間與冷卻步驟等。我們發現若將試劑預先混合會造成最後粉紅偶氮物的損失，但可以透過提高試劑比例將損失降到約6%。加熱溫度與時間控制也很重要，可避免粉紅偶氮物的快速褪色。經過各種條件調整比較，我們建議最佳的測定步驟是在樣水加入釩+格瑞氏的混合試劑後，置於50°C熱水浴中，加熱25-30分鐘，放入冷水浴中急速降溫，如此即可得到固定的最後吸光值。試劑濃度的條件為: SUL(sulfanilamide)試劑濃度約為30 mM，NED(N-1-naphtylethylenediamine)濃度約為0.5 mM，SUL/NED的莫爾比為60，VCl₃濃度為10 mM。在此條件下，硝酸鹽及亞硝酸鹽均會達到一個穩定且相同的莫爾吸光係數，約為50,000 M⁻¹cm⁻¹，且沒有顯著的鹽度效應。適用硝酸鹽濃度範圍0~50 μM，偵測極限為0.2 μM。本法操作簡便，結果精準，還原率為100%，非常適合於大量樣水的批次操作(batch operation)。

關鍵字

硝酸鹽；亞硝酸鹽；釩還原法；粉紅偶氮物；反應動力學

並列摘要

Nitrate is one of the most common nutrients limiting phytoplankton growth. The concentrations of nitrite and nitrate are important parameters in oceanography. Nitrite can be measured readily by the Griess reaction but nitrate requires a prior reduction process to convert it to nitrite. The copper-cadmium column is the most frequently used but it is tedious to prepare and its efficiency is inconsistent. The feasibility of using a vanadium reduction process to replace the cadmium process has been studied. A series of experiments have been conducted to optimize the reaction condition kinetically, including the reagent adding order, reagent strengths and heating condition. The results show that premixing of the Griess reagents can lead to loss of nitrite-yield due to a self-interference effect, which can be minimized by using a high reagent molar ratio. Heating at high temperature is favorable for the reduction reaction but it can also result in rapid fading of the pink azo dye. To solve this dilemma a ramping program controlling temperature not higher than 50°C for 25-30 min followed by a rapid cooling process can quantitatively reduce nitrate to nitrite with minimum loss to color fading. The optimal reagent strengths for SUL(sulfanilamide), NED(N-1-naphtylethylenediamine) and VCl₃ are 34.8, 0.58, and 10.17 mM, respectively. Both nitrite and nitrate samples produce consistent and equal molar absorptivity of ca. 50000 M⁻¹ cm⁻¹ and the color can be stable at least for one hour. The proposed method is easy to conduct, reliable and with a detection limit of 0.2 μM and a precision of less than 1% (r.s.d.). After 5 fold dilution the linear range can be extended to 50 μM which is suitable for most environmental studies.

並列關鍵字

Nitrate ； Nitrite ； Vanadium reduction ； Pink azo dye ； Reaction kinetics

參考文獻

Beda, N., Nedospasov, A. (2005). A spectrophotometric assay for nitrate in an excess of nitrite. Nitric Oxide, 13(2), 93-97.

Google Scholar

Bianchi, T. S. (2007). Biogeochemistry of estuaries. Oxford University Press.

Google Scholar

Brewer, P. G., Riley, J. P. (1965). The automatic determination of nitrate in sea water. Deep Sea Research, 12, 765-722.

Google Scholar

Cecchini, S., Caputo, A. R. (2012). A Direct Spectrophotometric Assay for Evaluating Nitrate-Nitrogen in Intensive Aquaculture Systems. Israeli Journal of Aquaculture-Bamidgeh, 64, 698-705.

Google Scholar

Chen, C.-T. A., Wang, S.-L., Wang, B.-J., Pai, S.-C. (2001). Nutrient budgets for the South China Sea basin. Marine Chemistry, 75(4), 281-300.

Google Scholar

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以釩還原及格瑞氏反應測定海水中硝酸鹽之硏究：解析反應動力機制並設立最佳化條件

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