In 1961, Geoffrey Crawley wrote that potassium and sodium ions in developers are not exactly equivalent, possibly due to conversion of the developing agents to sodium or potassium salts. For developers with low concentrations of developing agent, potassium carbonate gives lower contrast than sodium carbonate and, by the time the same contrast has built up, will give greater toe speed. Does he still consider that potassium carbonate gives higher speed than sodium carbonate? US chemists have suggested using propylene glycol as a solvent for the developing agents as it reduces oxidation. The solvent is typically present at about 2% concentration in the working solution. Is it likely to have any effect on film speed or sharpness?
Yes, in the type of developer quoted, the statement still applies. Also, being more soluble than the sodium salt, it is of use in concentrated liquid developers. This is the type likely to contain a low quantity of developing agent when diluted. Generally, I would hold to the opinions expressed in that article and several of the formulae remain in wide use.
With regard to glycol, its inclusion is now standard practice in concentrated liquid developer formulae, if of a type that needs to be protected from crystallisation in low temperature storage conditions. When this protection is necessary, I use mono-ethylene glycol at around 2.5%.
by Geoffrey Crawley
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