Determination of sulphur trioxide and sulphuric acid in flue gases in presence of oxygen, carbon dioxide, sulphur dioxide and nitric oxides

Abstract

q-he difficulties in the determination of small amounts of sulphur trioxide in gases in presence of sulphur dioxide, oxygen and nitric oxides are generally acknowledged. In order to overcome these interferences several precautions are necessary. To catch all sulphur trioxide and sulphuric acid the sampling with vacuum flask is most reliable. The determination of sulphur trioxide and sulphuric acid as difference (total sulphur-sulphur as dioxide) suffers from the inaccuracy due to the fact that low concentrations of sulphur dioxide are easily oxidized and further small amounts of nitric oxides interfere in the iodometric titration of sulphur dioxide (disturb the endpoint). The interference of nitric oxides can be avoided by means of sulphamic acid and thus a distinct endpoint is obtained. In order to avoid the interference due to oxidation the gas sample is sucked simultaneously through two parallel gas washing flasks. In one line the gas is oxidized by means of hydrogen peroxide 1 and the total sulphur is determined gravimetrically. In the other line SO2 is oxidized immediately with a measured excess of potassium dichromate solution in presence of sulphuric/sulphamic acid and determined iodometrieally. SO3 --}1-t2SO4 is obtained as difference. The apparatus consists of two 5-1 filtering flasks with ground stoppers with stopcocks. Before each flask are two gas washing bottles with sintered discs and before these gas washing bottles is an empty gas washing bottle with or without sintered disc. In one line each gas washing flask contains 50 ml of 5% hydrogen peroxide solution and the 5-1 filtering flask 100 ml of distilled water. In the other line each gas washing flask contains 50 ml of 0.02 N potassium dichromate solution containing 10 ml of sulphuric acid (D 1.4) and 50 g of sulphamic acid in 1 1. The total apparatus is evacuated, the stopcocks behind the filtering flask are closed and by regulating the stopcock behind the empty gas washing flask the gas is sucked in with a speed of 0.5-1 l/rain (about 5-10 rain). After this the 5-1 filtering flask in the hydrogen peroxide line is shaken and the contents of the flasks are combined, hydrochloric acid is added and boiled. The total sulphur is determined gravimetrically by precipitating with BaC12 solution. The potassium dichromate solutions are combined and transferred into a 300 ml Erlenmeyer flask. The excess of chromate is titrated with 0.05 N sodium thiosulphate solution using potassium iodide and starch. Sulphur dioxide is calculated from the amount of chromate consumed. Finally sulphur trioxide in the gas is obtained as difference (total sulphur-sulphur as dioxide). The empty gas washing flask is catching any water and sulphuric acid droplets in the tubing and these are neglected. If necessary this sulphur be determined separately, perhaps also by means of a Millipore-filter. Because the SO2 is oxidized immediately the interferences due to uncontrolled oxidation in later steps are excluded. Sulphamic acid reacts with nitric oxides according to the reaction HSO3NH2 + HNO2 = H2SO4 + H20 + N2.