以鎘銅還原法測定河口環境中硝酸鹽之鹽度干擾及解決之道

Abstract

當今常以鎘銅還原法作為測定自然水體中硝酸鹽的方法，但河口環境中廣鹽性的樣本，卻會對鎘銅還原法產生干擾。本文利用不同鹽度的標準品，對鎘銅還原法進行分段測試，得知改變樣本的鹽度會影響鎘銅管的還原率，且觀察出緩衝錯合試劑的成分與酸鹼值也會造成還原反應的差異，因而以氯化銨、乙二胺四乙酸的濃度、酸鹼值為試劑變化因子，進行緩衝錯合試劑、鹽度與還原率三者之間的系列實驗，得到最適合測定廣鹽性樣本的緩衝錯合試劑，其成分為4% w/v氯化銨混合0.5% w/v乙二胺四乙酸，並以氫氧化鈉調整酸鹼值至7.5。此緩衝試劑能大幅降低鹽度干擾，且使還原率隨鹽度呈現近似線性的變化。當分析全數樣本後，求得淡、海水標準品的還原率，算出鹽度修正公式( E( S ) = a•S + E( DDW std )， 其中a = ( E( SW std ) – E( DDW std ) ) /ΔS )，將校正後的還原率代入硝酸鹽的計算( [ NO3- ] = { ( Abs total / k NO2- ) - [ NO2- ] } / E( S ) )，即可得到準確的濃度值。

The precision and accuracy of the copper-cadmium reduction method for nitrate determination have been evaluated for estuarine waters, and the interference induced by salinity has been studied. An erratic decay trend on the efficiency of the reduction column was found as salinity increases. Further study indicates that the efficiency is closely related to the concentration and pH of the complexing reagents ( NH4Cl or EDTA ) add prior to allowing the sample to pass through the column. Application of a combination of 4% w/v NH4Cl and 0.5% w/v EDTA gives more stable results than using either reagent alone. However, for estuarine samples with a wide salinity, the raw readings still need correction. Empirical formulae are proposed for this purpose: E ( S ) = a•S + E ( DDW std ) a = ( E ( SW std ) - E ( DDW std ) ) /ΔS [ NO3- ] = { ( Abs total / k NO2- ) - [ NO2- ] } / E (S) where E, a, S, k represent efficiency of the column, slope, salinity, and slope for nitrite respectively. The above equation set provides a more accurate and convenient way to calculate the concentration of nitrate in estuarine environment with salinity ranging from 0 to 35.