Determination of Free Chlorine Content of the Atmosphere (Methyl Orange Method)

Near a pH of 3.0 the color of a methyl orange solution ceases to vary with acidity. The dye is quantitatively bleached by free chlorine, and the extent of bleaching can be determined colorimetrically. The optimum concentration range is 0.05 to 1.0 ppm Cl₂ in ambient air (145 µg to 2900 µg /m³ at 25°C and 101.3 kPa (1,2,3).