They won't dissolve in Glycol so must be dissolved in water, a little water won't affect the keeping properties. It's important to use fresh Metabisulphite as it's the SO2 it releases that stops oxidation of the Pyrocatechin. Once the Metabisulphite breaks down the developer collapses.
Originally Posted by pdeeh
Oxygen is absorbed into water but not into Glycol which helps prevent oxidation. In practice with fresh Metabisulphite and the right bottles I have Pyrocat HD without Glycol that's lasted from March 2011 and is still OK today, so over 2½ years.
Commercial developers made up in Glycol or similar need some water to dissolve some of the components.