How do I know how much sodium thiosulfate to add to the sodium hydroxide absorbing solution?
The method calls for you to estimate the concentration of chlorine (ppm) and multiply this by the dry standard volume sampled. If you use standard volume in “cubic meters”, add 25 milligrams for each PPM – DSCM, or 0.7 milligrams for each PPM – DSCF. These values have a safety factor of about 5 so you don’t have to be real concerned that you know the PPM precisely. In fact, adding too much sodium thiosulfate may interfere with the analysis so in retrospect, it would probably be better to have a smaller safety factor. If the PH of the sodium hydroxide absorbing solution becomes acidic then the Cl2 capture efficiency drops off. When should I add the sodium thiosulfate? We intended that the sodium thiosulfate be added after sampling is complete. Can I use ASTM Method D6735-01 as an alternative? Not automatically unless the ASTM method is listed as an acceptable alternative within the regulation. The ASTM method was intended to be used at mineral calcining exhaust sources where there is re
