Hello all - sorry to be slow in responding here. Keith is correct on all counts. It is important not to mix any of these three clearing solutions, if for no other reason than the first two mixed will, as Mike Ware puts it, " tend to release sulphur dioxide gas SO2… - producing an even worse choking smell."
Sodium metabisulfite does have a longer working life, but many find the smell to be irritating. It seems to be easier to find than sodium sulfite, since it is used extensively in sterilizing cooking utensils, and in home brewing.
Sodium sulfite is easier to work with as it does not generate an irritating / smelly vapor. However, it has a very short working life. I recommend preparing it fresh, and that it be used within two to three hours, and replaced after processing the equivalent of one 8˝x10˝ point per liter. the stuff is cheap, and can be purchased from Photographer’s Formulary, B&H Photo and others.
Disodium EDTA may be prepared exactly as Keith describes above. It is also cheaply available from dietary/health supplement providers (!) like bulk supplements.com. See my list of resources for Pt/Pd printing on my web site
Intermediate fresh water rinses between each of the three baths helps
a) reduce smelly sulfur dioxide being released from carryover of EDTA disodium into bath 2, and
b) extend the life of the final, tetrasodium EDTA.
The final wash is very important, which should be between 65–75°F in gently running water for at least 30 minutes. If water conservation is important, then do six changes of fresh water for five minutes each, with gentle/occasional agitation.
Finally, I have found it easiest, when making large prints or working with delicate paper, to pour liquids in and out of a tray rather than move the print around across multiple trays.
hope this helps. Thanks.