Sorry this question comes a long time from submission but you have never answered the question about the TRIS. How much TRIS to add to the DMBB per litre?
I have solved the issue for people who cannot get this assay to work. Simply add 5mL 100% ethanol per litre to the mix OR, within 15 minutes of the assay, add 1 part 2M TRIS per 10 parts DMMB reagent. This is what the TRIS was for.