Right. That's what led me to ask the question. Citrates are mentioned as an aside in Young's and Anderson's manuals. Both advise adding 20gm/L. I'm adding the citric acid to the salting solution rather than the silver sensitizer. I've got sodium citrate, so I'll try that before ordering potassium citrate.
Thanks to all
Hold on a second!!!!
You say that you want to add citric acid to the salting solution and then you go on to talk about sodium or potassium citrate.
Citric acid and citrate are not the same thing.
These species are related but have they
very different properties.
Citric acid is a tri-protic organic acid, i.e. it has three acidic hydrogens. Citrate is the conjugate base of citric acid. Citrate is citric acid with the three acidic hydrogens removed. Citrate is a tri-anion and is a weak base. Citric acid and citrate react with other materials in
very different ways.
In this case, at least part of the reason to include citric acid in the salting solution is to remove (some) of the alkaline carbonate 'buffer' present in many modern papers. Neither of the citrate salts will perform this function.
I note that Wynn's recipe cited above calls for potassium citrate in the salting solution. I don't know who Wynn is nor what his rationale for this addition is.
However, I just went back and re-read Christina's book where she describes Marek's idea of including 50 g /L of citric acid in the salting solution. It is clear from the text that this addition is meant to remove carbonates from the paper.
That said, my practice is to remove the buffer from paper that needs this using dilute (1%) hydrochloric acid (HCl). I soak the paper in the dilute HCl until the bubbling stops (roughly 2 minutes, but it is paper dependent). Then I hang the paper to dry. (Note that I do not wash the paper in plain water before drying.) I then salt the paper separately using just salt.
I do it this way because I also need pre-acidified paper for Pt/Pd printing and this allows me to use the same stock of paper for both processes. Although I must admit that the bulk of my printing (for both salted paper and Pt/Pd) these days is done on paper made for alt processes (HPR or Revere Pt). I need a very good reason to stray these days.
Using a strong acid such as HCl allows one to confirm that the carbonate is completely removed; when the bubbling stops all of the carbonate is gone. With a weak acid such as citric acid, the bubbling is less vigorous and thus harder to observe. Furthermore, you are more likely reach the capacity of the solution for neutralizing the carbonate base without knowing it.
Concentrated HCl is inexpensive and readily available (at least in US). One does have to be careful when handling the concentrated acid (remember to add acid to water, and not the other way round) but it is not particularly dangerous.
All of that said, I do follow what Wynn says about mixing citric acid with silver nitrate just before using them. This avoids the precipitation often seen when these two components are premixed and the solution stored.
My guess was based on experimenting with chlorides. 
