The keeping quality of most developers is poor, very poor. Developers oxidise rapidly with the air dissolved in the water used to dilute them and from air that permeates through the plastic container used for storage. They often discolour, particularly when diluted down to working strength. Working strength (film) developer solutions have to be discarded after use. This is not the case with paper developers, stop baths or fixers which can be kept and used many times. So how can you look after your developers better and insure against that 'failed' developer blank film look?
With film developer, dilute the stock solution immediately before you need it. This gives less time for deterioration to take place and keeps the agent active. In the bottle of stock solution the use of ‘Canned CO2’ or some other heavy inert gas helps to make it last a little longer.
The use of Tetenal Protectan, by giving a gentle 3 second burst of the gas into the neck of the bottle before closing, helps to dispel any oxygen that might otherwise be absorbed into the developer.