Hi Gerald, sorry about that I must have been in one of my moods when I was reading the thread...
Originally Posted by Gerald C Koch
Indeed it seems in many low pH, dual-agent fine grain developers there is functionally one primary agent doing the direct development (Metol in the case of D76) while the secondary agent's function is to regenerate the primary agent. In the case of MQ developers like D-76 as you point out the pH is too low for HQ to be active enough as a primary developing agent.
Interestingly this appears to be the case with modern Metol-Pyrogallol developers too, where the working pH (below 10 in the cases of both WD2D and PMK) is too low for Pyro to do much direct developing.
What is your take on XTOL though? At its working pH it would appear the Phenidone derivative is the direct developing agent and the ascorbic acid regenerates the Phenidone. However I remember reading that Zawadzki believed the ascorbic acid was the primary developing agent. That might have been a misquote though.