抄録
The micro-determination of hydrogen sulfide accomplished by I. St. Lorant which utilized the transference of sulfur into methylene blue (MB) molecule in Caro's reaction, as well as a modification by Freudenberg and Münch, was found to have the upper limit of its range of determination at about 5μM of sulfide. By addition of excess sulfide, precipitates, which may be composed of sulfur compound, were formed without formation of MB. Better condition for the formation of MB was obtained by adjustment of the reagents. This phenomenon was elucidated stoichiometrically and a method for micro-somimicro determination of sulfide was described.
The standard analytical curve for electrophotometry obeyed the Beer's law only in the range of the sulfide amount as small as 1μM, beyond which it flew away to the right finally to reach a plateau at 40μM. The deviation of the standard analytical curve from the Beer's law was interpreted as the effect of association of MB molecules. Whereas the concentration-absorbancy curve prepared from aqueous solutions of MB (pure powder) declined continuously as an exponential curve does, the analytical curves prepared from standard sulfide solutions after Caro's synthesis showed on themselves a series of linear stretches, or a trough in some favorable conditions. Moreover, the evolution in time course of MB, as measured in absorbancy of the reaction mixture, also progressed stepwise in linear relationships. Mechanisms of these phenomena were discussed.
