海水中矽鉬複合物呈色反應機制研究

Abstract

矽鉬黃法常用以測定水中溶解態可反應矽酸鹽，但該法之反應生成物與反應速率受到試劑濃度與用量影響甚鉅。本研究藉由呈色反應曲線的觀察，反推最佳的反應條件並嘗試找出最方便的試劑組合方式。結果發現黃色矽鉬反應最佳條件須要鉬濃度30 mF以上，酸用量應在75 mM以上，且應遵循氫鉬比在2.5左右。然而鉬酸試劑配製不易，在微酸性中不穩定，本研究發現如事先將鉬酸銨溶於氨水中，使氫氧根用量大於鉬酸根當量之一半以上，即可得到穩定的高濃度試劑([Mo]675 mF)，可在常溫下長期保存。測定時樣水先加適量之酸，再加入此試劑，使最後pH達到1.5，最後鉬濃度30 mF，則反應可在常溫下，於120秒達到大於99%的完成率，吸光值可穩定十分鐘。因此在人手操作時只要三分鐘後即可得到測定數據。在測定條件在溫度25 ℃，波長400 nm時，使用1 cm光槽，吸光係數2175 M-1cm-1，鹽度干擾約2%，磷酸鹽的呈色干擾約為矽鉬黃呈色之一半(吸光係數1170 M-1cm-1) 以上兩者均可用經驗公式修正之。這個條件適合改為自動分析，不過傳統的自動分析均為流動式的設計，本研究則採用間斷式設計，即將樣水與試劑混合後送入流動式光槽中後停滯。於上樣三分鐘後在定溫及流動靜止狀況下測量其吸收光值。此種半自動之設計可此方法的整體表現更接近於人手操作，在250 μM濃度時相對標準偏差在(r.s.d.)小於0.2%，偵測極限為0.5 μM(使用1 cm光徑)，如用5 cm光徑可達0.1 μM，測定速度為每小時20個樣品。

Dissolved reactive silicate in natural water is usually determined colorimetrically based on the formation of a yellowish silicomolybdenum complex. The product and reaction rate can be affected significantly by the amounts of reagents added, and the optimal conditions are optimized by observing the color formation kinetics. Several criteria have been found: the final Mo concentration requires at least 30 mM, the final pH needs to be adjusted to 1.5, and the H+/Mo ratio should be around 2.5. To keep the above condition one needs to prepare a high Mo concentration reagent at [Mo]675 mF. A prior mixing with ammonia at a concentration covering more than half of Mo concentration allows the reagent stable at room temperature. In real measurement the sample is added with this reagent and acid to give a final [Mo] of ca. 30 mF. The reaction can reach >99% within 120 s at 25 ℃,. The molar extinction coefficient of silicomolybdum complex is 2175 M-1cm-1. The salinity interference is around 2%. The interference by phosphate is equivalent ot half of silicate concentration (molar extinction coefficient =1170 M-1cm-1 ). The reaction time in manual operation requires 3 min, which can be readily converted to autoanalysis. However, to improve the precision of measurement, a stepwise concept has been adopted in this study. The sample mixed with reagents is allowed to fill into a flow cuvette and the liquid is trapped in a spectrophotometer. The measurement was made 180 s after sample loading. At 250 μM the relative standard deviation reaches 0.2%, the detection limit is 0.5 μM using 1 cm cuvette, and can be further lowered to 0.1 μM if a 5 cm cuvette is used. The analysis throughput is 20 samples per hour.