Spectral Sensitization of Photoreduction of Aqueous Ammonium Dichromate Solution by Methylene Blue

Abstract

Photoreduction of aqueous ammonium dichromate solution in the presence of ethanol or 2-propanol was found to be sensitized spectrally by Methylene Blue (MB) over its absorption region at about 700 nm. Efficiency of sensitized photoreduction of Cr(VI) was extremely increased by addition of diammonium ethylene diamine-N, N, N', N'-tetraacetate ((NH₄)₂H₂ edta)in the system. It was confirmed experimentally in the unaerobic condition that the Leuco methylene Blue (LMB) produced photochemically from MB reduces Cr(VI) to Cr(III) and it returns simultaneously to MB.
Supperadditivity was obs erved on the efficiencies of LMB formation and of MB-sensitized photoreduction of Cr(VI) in the system containing both ethanol (or 2-propanol) and (NH₄)₂H₂edta. It was concluded from the experimental results that the rate of the MB-sensitized photoreduction of Cr(VI) is of zeroth order with respect to the Cr(VI) concentration, and it depends on the intensity of light absorption of MB and on the kind and concentration of reducing agents.
A photo reduction scheme which well satisfied the experimental results was proposed.