I may need to adjust the pH of my fix...

I have an AGFA Rapiline processor I use for RC paper, and I use Ilford RT 2000 Developer and Fixer. There are three tanks, one each for the developer, the fix and a wash tank. As there is no tank for stop bath, I would like to follow the Ilford recommendations for adjusting the pH of the fix, using acetic acid.

Ilford recommends a few drops of 50% acetic acid to bring the pH into line. That sounds a bit vague to me, and I would prefer to calculate how much acid is required to correct the pH. I have an inexpensive pH meter on order from Amazon, but as I pretty much slept through chemistry class 40+ years ago, I haven't a clue how to calculate the proper volume of what percentage of acetic acid is required to effect any changes which might be needed.

If I remember correctly (and I probably do not), the adjustment is based on the number of moles of the acid which is to be added. So, what I require from any APUGer who is a chemist/engineer/more knowledgeable in chemistry than I am, is: what is the proper method to make the calculation? That is to say, once I find the pH of the tank solution (and assuming that it is outside of the tolerance specified by the manufacturer), how do I calculate what per cent of acetic acid should I use, and how much do I add, per litre of tank solution?

And thanks in advance to any APUGer willing to bestow this vital information to me!

Unless the fixer is a hardening one its pH is not that critical. Adding 50% acetic acid could cause sulfurization to start. I would suggest leaving it alone. I don't use a processor but never adjust the pH of the fixer.

Depending on buffering strength of the components in your fixer, widely different amounts of acid will be needed to create a desired pH shift. Simple acid/base calculations will not get you anywhere, because ionic strength is a big issue in fixers due to their high concentration of multivalent ions. If you have a pH meter, and want your fixer to reach some pH, then leave the pH electrode in your fixer while you slowly add 10-20% Acetic Acid and stir. Don't go below pH 4.5 or you risk precipitation of Sulfur.

Some development agents work at pH as low as pH 6.0, but they are extremely slow at pH 6. Therefore your fixer will provide adequate stopping action at any pH between 4.5 and 6.0. As fixer ages, it typically gets lower in pH, therefore the lower you set fixer pH, the shorter its working solution life will be.