NaOH prevents the zinc sulphide( ZnS) precipitate, which is produced on addition of H2S, forming and the chelating agent forms attractions with the zinc ions increasing the stability of the zinc sulphide in solution thus preventing the loss of S-2 ions by volatilization of H2S(g).
You can do quantitative estimation of Sulphide ion by UV spectrophotometric method ( Plz. find attachment- zoom if needed]
The best approach is going to depend upon the volume of gas to be cleaned and the concentration of H2S. If the number of moles of H2S is not very large, you can pass it through a solution of NaOCl, which will oxidize the H2S into suspended elemental sulfur. Since the NaOCl reacts with the H2S, if you have a large number of moles of H2S, you might want to switch to a solution of FeCl3. The ferric ions will also react with the H2S, but the resulting ferrous ions in solution will also react to form a suspension of the mackinawite form of FeS. To get an idea of how all this might be done, you can check our the test equipment described in: