I suspect that the 12-month shelf-life given by PF for TF-5 is conservative. However, I'd be a bit uncomfortable keeping the stock solution for four years without more testing than a clearing-time test. It would be interesting if you used your old TF-5 on a fiber-base print or two and then did the ST-1 (or KRST) test for residual silver on it.
Ron Mowrey (PE) developed TF-5 and I suspect that the 12-month storage life caveat comes from him. Also, the recommendation to store smaller amounts of stock TF-5 in smaller bottles points to degradation due to oxidation. Maybe these recommendations are overly-cautious, but seeing some test results to really refute them would be reassuring
Best,
Doremus
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