This is an important issue. While it's true that chemicals are usually cheaper then film or paper, it's just waste to throw out chemicals that are still functional.
It's easy to test fixer whether using film or paper. With film, as has been suggested, you can test by putting some unexposed film in the fixer -- at whatever dilution you want -- until it clears, but it is better to be more exacting. That way, you can test not only whether the fixer is OK, but how long it needs to be used and at what dilution. And you can and should test for proper fixation with paper -- since difference films and papers exhaust the fixer at different rates. For example, microfilms need much less time in the fixer (or less dilution) than, for example, Kodak 2475 Recording film. So you really should run multiple tests for all your materials.
Here are some simple tests -- I'm surprised these have not been mentioned because they are well known -- that will save you time and money, and should be repeated occasionally -- especially if you re-use your fixer:
Test for proper fixation (the removal of all unexposed silver by the fixer), as follows
A. Fix, wash and dry an unexposed, undeveloped piece of the selected paper using any desired fixer, time and dilution. Normally, it is best to start with the manufacturer's recommended time and dilution rate.
B. On the emulsion side of the paper, apply one drop of Rapid Selenium Toner concentrate.
C. Wait two minutes.
D. Any yellowing indicates residual silver. Increase the amount of time in the fixer (or decrease dilution) and retest until no yellowing is noticeable..
E. Test for exhaustion of the fixer, as follows:
1. Take two oz of the used fixer.
2. Add two drops of test solution such as Hypocheck:
potassium iodide 0.01g
water 0.05ml
F. Shake.
G. Wait two minutes.
H. The solution should be clear. If it displays any milkiness, the fixer is exhausted -- decrease the dilution of the fixer and run the test again using a new piece of paper.
I. If the fixer clears, the test is passed; increase the dilution of the fixer and repeat the test until it fails.
With these simple, cheap tests -- using small pieces of film or paper -- I use a LOT less fixer than I otherwise would.
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