Gerald Koch said:
The concepts of a catalysis and super-additivity are being scrambled here.
I'm not going to go into a discussion here of each concept as they can be found in any good photochemistry book. However I will point out the following. Look at the composition of a typical MQ developer such as D-76 and its replenisher. If Metol were merely a catalyst the amount of Metol in the replenisher would be very small. Remember if it were a catalyst it should not be used up. What we find is that they are in the same ratio in the replenisher as in the developer. It is obvious that they are both contributing to development and BOTH being used up (not catalyst behavior). The same thing is found for a PQ developer but it is a little less obvious since Phenidone is more easily (and more efficiently) regenerated by hydroquinone than is Metol. The small amount of Phenidone in relation to hydroquinone may give the semblance of catalysis but that is NOT the case.
I am not confused. Before I made any such weird assertion, I looked up the definition of catalytic agent in the CRC Handbook:
"A substance which by its mere presence alters the velocity of a reaction, and may be recovered unaltered in nature or amount at the end of the reaction." Given that definition, how could we tell by appearances that the action of metol or phenidone on the reduction of silver halide by hydroquinone is not catalytic? Because someone told us?
The replenishment of D-76 is usually in the amount required to replace the volume lost. When that is the case, it is clear that neither the hydroquinone nor the metol is lost due to oxidation, or else both are equally lost, if the ratios are the same in replenisher as in the stock. The rule at one time was to put a certain volume of replenisher per unit of film area in the container and add the used developer until the original volume was regained, discarding the rest. This rule often led to increased developer activity with use. The replenisher, in addition to the same ratio of metol/hydroquinone, has a higher concentration of alkali.
My statement was not, or at least was not intended to be, that the process IS catalysis, but that it must be in some way similar, and not simply a replenishment of one agent by the other. I base this idea on my experience with a PQ developer that had only phenidone, hydroquinone and sulfite in its water, the hydroquinone being in fairly great excess of what was considered to be the "optimum" amount for superadditivity. This developer was reused without replenishment until I could no longer stand its color and sludge, with no apparent loss of activity.
Anyone who did not know that phenidone is a developing agent, upon seeing the effect that adding a minute amount of it to a solution of hydroquinone and sulfite, would, I think, suspect some form of catalysis. I think the catalysis that is present is well known to be the effect of metallic silver. What appears to me to be catalysis may well be the ability of the metol or phenidone to reduce these atoms of silver halides to metallic silver.
I have read that the replenishing action of hydroquinone on metol or phenidone is not sufficient to explain the synergism.
Hydroquinone alone at high enough pH is a very active developer. Why is it so unlikely that the catalytic action of metallic silver particles produced by the metol or phenidone, which are able to act at low pH, could stimulate the activity of the hydroquinone as does high pH or high temperature?