Sandy King (sanking@hubcap.clemson.edu)
Tue, 25 May 1999 12:03:50 -0500
Dave,
I am skeptical of your conclusions about the effect of changing the amount
of carbonate in a working solution. For the past several months I have been
working on an experimental pyrocatechin/metol developer with sodium
carbonate as the accelerator. Working from concentrated A
(Metol+Pyrocatechin) and B (10% sodium carbonate) solutions the film tests
that I completed showed a signicant difference in developer activity (as
measured by develolpment to a specific CI) between the following working
dilution: 1) 1:2:100, 2) 1:5:100 and 3) 1:10:100
(developer:carbonate:water). The actual pH varied very little as I recall,
from about 10.3-10.5. Unfortunatley my notes are somewhat deficient in this
regard because although I took pH readings of all dilutions I had to change
calibration in mid-stream because of operational difficulties.
Sandy King
>I think we are over emphasizing the accuracy of carbonate. It is used only as
>the accelerator, and it is used basically only to set the pH.
>
>For a strong acid or base (such as Sodium Hydroxide), even if you double the
>amount, the pH change is at most 0.30 (pH is in log scale, so we get the
>familiar one stop, 2 times, 0.30).
>
>But for weak acid or weak base (such as carbonate), the change for doubling
>the amount won't even be that high. The pH will remain about the same. What
>you get by giving more chemical is better buffering.
>
>
>Dave S
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