Words of wisdom from Richard Knoppow, patron saint of USENET rec.photo.*:(posted 8422 days ago)Details can be found in the _Kodak Black-and-White Darkroom Dataguide_ along with a testing method for the two baths.
Start with two freshly mixed fixing baths. Fix the film or paper in the first for half the usual fixing time. Drain and finish fixing in the second bath for the other half of the time. For prints a water bath can be used between the two fixing baths so that all prints from a session are given the final bath at the same time, sometimes a convenience. The chemistry has been described in detail in a couple of postings by Mike Gudzinowicz to this news group. You can find them by doing an author search on Google.com Basically, the fixing process consists of converting the unused silver halide in the emulsion into a form which is water soluble and will wash out. When you wash film or prnts you are washing out not only the residual hypo but also some of the converted halide (some comes out in the hypo). This reaction is not a simple one, it goes in steps, each step a little more soluble than the previous one. At some point a single hypo bath can no longer complete the series of reactions so leaves some of the halide in an insoluble condition. If the print is left this way the insoluble silver complexes left in the emulsion will begin to decompose after a time and will also begin to attack the silver image. For this reason its very important to complete the fixing process if prints or films are expected to have a long life. It turns out that the capacity of a single bath is very limited, around 10 8x10 prints per _gallon_ for archival quality for prints. If two successive baths are used the first bath does most of the work and the second bath stays fresh enough to complete the reactions so that the silver complexes are in water soluble form. The capacity of a double bath system is four to ten times that of a single bath, which makes it an economy as well as insuring complete fixing. With this system when the first bath reaches exhaustion (as determined by an iodide test) it is replaced with the second bath, which is still relatively fresh. The second bath is replaced with a fresh bath. Since there is some carryover both baths should be replaced by fresh baths about every five times. Two bath fixing has a further advantage for film. Film emulsions invariably contain Silver Iodide. The Iodide ion comes out in the fixing bath and acts to slow down the fixer. The rather large amount of Iodide in T-Max films is one reason they take longer to fix than other films. Most, if not all, of the Iodide comes out in the first bath, leaving the second bath to work at normal speed. It is generally considered good practice to use separate fixing baths for film and papers due to the iodide left by film. While this is still good practice with a two bath fixer, it is not nearly as necessary since the second bath remains relatively iodide free. A note. I am not a fan of alkaline fixing baths. They really serve no useful purpose except where a dye image is concerned. While it is claimed that they have an advantage over conventional acid fixing baths in washing out faster, this advantage is present only in comparison with a hardening fixer containing aluminum sulfate hardener in a narrow range of pH. When a sulfite wash aid, like Kodak Hypo Clearing Agent, is used, the wash rates of films and papers treated in hardening fixing baths is no different than those treated in a non-hardening acid fixer or in an alkaline fixing bath. The acid in acid fixers has an advantage in that it prevents activation of carried over developer with resultant staining. The hardener is advantageous in preventing damage to the emulsion while fixing and washing. Wash aid is buffered to a point where it does not undo the hardening, as does a carbonate bath. Wash aid is also an ion exchange medium for thiosulfate so considerably accelerates washing, even over an emulsion treated in an alkaline bath. A wash aid is also effective in removing the residual sensitizing dye in T-Max films, generally it all comes out in the wash aid which turns a light purple color, once popular for ladies foundation garments (which dates the color and me).
Here is Kodak's version of fixer tester and instructions. Kodak Fixer Test Solution FT-1
Water at 80F 750 ml Potassium Iodide 190 gm Water to make l liter
This is about a hundred year supply, scale it down to something more reasonable. The instructions are: Testing a single bath fixer: To 5 drops of the test solution add 5 drops of the fixing bath and 5 drops of water. Discord the fixer if a yuellow-white precipitate forms instantly. Disregard any slight milkiness. Two-Bath Fixer: First bath--/test as descrubed abivce fir a single-bath fixer. Second bath--To 5 drops of test solution, add 5 drops of the fixing bath and 15 drops of water. If both tests produce a yellow-white precipitate, replace both baths with fresh baths. If ony the first bath forms a precipitate, replace the first bath with the second and the second with a fresh bath. The stock solution can be stored in a closed bottle for one year.
Edwal Hypo Check is the same stuff but probably at a different strength.