1988 Volume 37 Issue 2 Pages 72-75
The suppressed ion chromatography of anions for the electrochemical suppression of water-dip caused by H+ from carbonic acid in the eluent was investigated with coulometric detector, and applied to the determination of NO3- in biological denitrification process water and PO43- in crystalization process water for phosphorus removal. The detection principle is based on the electrochemical reaction of the H+ of acid eluted from the suppressor column (H+-form) with p-benzoquinone at a constant applied potential. In the chromatograms of anions such as Cl-, NO3-, SO42-and PO43- obtained by the electrochemical reaction at pH 7 under the applied potential of 0.47 V vs. Ag-AgI, although a good chromatogram was obtained, the water-dip was observed due to the H+ from carbonic acid eluent. At pH 4.4, the dissociation of carbonic acid is only 1%, whereas that of the strong acid is almost complete. Therefore, the electrochemical reaction at pH 4.4 under the applied potential of 0.62 V was used to detect the strong acid only. By the proposed method, the water-dip could be removed, and reproducible chromatograms of anions were obtained (R.S.D. =2.4%). The linear calibration curves were obtained in the concentration range of 10 to 100 ppm for each anion. The detection limits(S/N=2) were 0.03 ppm for Cl-, 0.07 ppm for NO3-, 0.11 ppm for PO43- and 0.10 ppm for SO42- using the injection volume of 0.5 ml. The method was applied to the determination of NO3- and PO43- in several actual samples with satisfactory results.
