What causes Pyrocat HD to Oxidize so fast?

[Natenaaron]

Is it the exposure to oxygen or does the two parts mixed together work to oxidize it quickly as well?

[Michael R]

There are several things going on, but to make a complicated long story short and simple, oxidation of the developing agents is accelerated in an alkaline environment - which is what happens when you mix part B (the alkali) with part A. In the case of tanning/staining developers, there is also relatively little preservative present. Part A will still go bad over time due to oxidation, but it is a much slower process than when the working solution is mixed.

[Natenaaron]

I get it. Thanks.