Quote Originally Posted by Gerald C Koch View Post
Hi John,

I have used D-76H in the past and the results appear to be identical to those of D-76. This formula also points out that at the low pH of D-76 the main purpose of the hydroquinone is to regenerate the metol as it is consumed. This can be seen from the rather small adjustment to the metol to compnsate for the lack of developing activity from the hydroquinone.

Jerry
Hi Gerald, sorry about that I must have been in one of my moods when I was reading the thread...

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.