Development of a monitoring system for vinyl chloride gas in air by using an HCl monitoring tape and pyrolyzer

A continuous monitoring system for vinyl chloride gas in air has been developed using an HCl monitoring tape and pyrolyzer consisting of a heater around a quartz tube. It is based on the color change of the tape by reaction with HCl gas produced by decomposition of vinyl chloride gas in the heated quartz tube. The conversion efficiency of vinyl chloride into HCl depends on the temperature of the pyrolyzer. The tape impregnated with a coloring solution that includes Metanil Yellow (pH indicator; pH 1.2-2.3, red-yellow), glycerin and methanol is a highly sensitive means of detecting HCl gas. When vinyl chloride gas was passed through the heated quartz tube (910 degrees C) and the HCl gas produced was passed through the tape, the color of the tape changed from yellow to red. The degree of color change was proportional to the concentration of vinyl chloride gas with a constant sampling time and flow rate. The degree of color change could be recorded by measuring the intensity of reflecting light (555 nm). This method is scarcely affected by other gases with the exception of chlorinated hydrocarbons such as trichloroethylene and chloroform or strong acids such as HCl gas. Reproducibility tests showed that the relative standard deviation of the relative intensity (n = 10) was 4.5 for 5 ppm vinyl chloride. The detection limit was 0.4 ppm for vinyl chloride with a sampling time of 40 s and a flow rate of 300 ml min (-1).