In Method 0050, why use sulfuric acid to trap HCl (and how does it work)? Why not use a base solution?
As specified in Method 0050, two impinger reagents are used to separate and trap HCl and Cl2 from the gas stream. Acidic and alkaline absorbing solutions collect gaseous HCl and Cl2, respectively. In the acidified water absorbing solution (i.e., 0.1 N H2SO4), the HCl gas is soluble and forms chloride ions by the following equation:
HCl + H2O = H3O+ + Cl-
The Cl2 gas present in the emissions has a very low solubility in acidified water and
passes through to the alkaline absorbing solution where it undergoes hydrolysis to form a proton (H+), Cl-, and hypochlorous acid (HClO) by the following reaction:
H2O + Cl2 = H+ + Cl- + HClO
As you recall, sodium thiosulfate solution is added to the contents of the recovered alkaline absorbing solution ( e.g., 0.1 N NaOH) to stabalize the ClO- and removes the possibility of partial reduction of ClO- to Cl- and the resulting high bias to the results.
The resulting Cl- ions in the separate solutions are measured by ion chromatography by SW-846, Method 9057. Those Cl- ions found in the acidified impingers are related to the HCl emissions and those in the alkaline impingers are related to the CL2 emissions. Thus, through the selection of absorbing solutions and solubilities, we are able to differentiate between HCl and Cl2.
To use only a base wouldn’t allow us the ability to speciate and we would also have to change our analytical finish because of the many possbile reactions in the base impinger with absorbing HCl along with Cl2.