Continuous spectrophotometric monitoring of chlorine in air

Abstract

Spectrophotometric monitoring of atmospheric free chlorine, based on the reaction with 4-nitroaniline, was assessed. A Beckman air quality Acralyzer Model K-1008 for nitrogen dioxide monitoring was modified and used for this purpose. Some reducing and oxidising compounds interfered with chlorine measurements, but these were removed efficiently using a scrubber consisting of chromic acid (prepared from phosphoric acid and chromium trioxide) absorbed in silica gel. This scrubber was found to be suitable for chlorine measurements in the presence of hydrogen chloride. Humid conditions resulted in some absorption of chlorine in the scrubber. At 69 °C no such absorption was observed. Ozone was removed either by a glass-beads scrubber heated to 60–65 °C or by the addition of sodium nitrite to the absorbing solution. The sensitivity achieved using this monitor was 0.25 p.p.m. full-scale with solution and air flow-rates of 1 ml min^–1 and 2 l min^–1, respectively.