Elimination of interference from sulfur dioxide in the determination of ammonia by indophenol blue absorptiometry

According to the present JIS method, which describes the determination of a mmonia(NH3) in flue gas, the presence of more than 10 times as much sulfur dioxide(SO2) as NH3 results in interference with the determination of NH3 b y indophenol blue absorptiometry. The magnitude of interference from the co existing sulfur dioxide and its elimination of described. The magnitude of interference was investigated using NH3 and SO2 standard gases. Both standa rd gases were bubbled through a 0.5% boric acid solution independently and simultaneously; 0 similar to 200 times as much SO2 as NH3 by mole was bubbl ed through a boric acid solution. The absorbed NH3 and SO2 were determined by coulometry and alkalimetry, respectively. The magnitude of interference was dependent upon the amount Of SO2 absorbed in the boric acid solution, n ot the molar ratio Of SO2 to NH3. More than 100 mu mol SO2 led to a serious decrease in the absorbance. In the case of a 20 liter sample, it was found that more than approximately 110 volppm Of SO2 gave a lower analytical val ue of NH3. The interference could be eliminated by bubbling 400 ml of oxyge n at a flow rate of about 70 ml min(-1) through a boric acid solution conta ining the absorbed gas sample.