What is the pseudo-first Order Rate Constant?

When studying reaction kinetics of second order reactions, control of all the variables is difficult and it is sometimes helpful if one reactant is present in large excess. It can then be umed to a good approximation that the concentration of that reactant remains constant and the kinetics you observe are those of a first order reaction. It is then said to be pseudo first order reaction and the k you observe will be the true k times the concentration of the reagent in excess. I don't quite understand your figures but it seems that your H2S is in large excess.