Pool-supply "chlorine neutralizer" works great
Pool-supply "chlorine neutralizer" works great
Sodium thiosulfate from a pool-supply store is the only fixing agent that I use for negative film and prints. I live in a rural area where darkroom chemicals have to be ordered by mail. I got tired of waiting for shipments.
I buy "Thio-Trine Chlorine and Bromine Neutralizer" made by AB (Applied Biochemists, Alpharetta, Georgia, USA). It's not cheap. Hopefully you can find a locally produced brand instead. Check hot tub supply stores as well as pool-supply stores.
It's the penhydrate form (crystals). So I have to use more (by weight) when using a recipe that call for the anhydrous form.
Washed crystals should be reasonably pure, I would think. Obviously, I haven't had it analyzed... But consider that no pool supply or photo manufactuer actually makes hypo -- they buy it in bulk from a chemical supplier. Only certain grades are available, and no facdtory buys a higher grade than it needs Contaminants are usually byproducs of preparation/synthesis, and may or may not matter in a given application.
A second question is whether *any* slow fixer mixture (a fixer mixed from hypo = sodium thiosulfate) will meet your needs. That depens on what film/paper you are developing and on your total darkroom process.
Slow fixer is not recommended for tabula-grain film such as Kodak T-MAX or Ilford Delta. According to Kodak, it will not completely fix tabular-grain film--at least not in a feasible time.
Another drawback with any film is increased wet-time. This matters more at higher temperatures. It can be addressed if necessary by using "tropical" mixtures (with an inert salt such as sodium sulphAte added to produce a more hypertonic solution that won't cause the emulsion to swell).
So there's trade-off: slow fixer is odorless, and has a longer shelf life (especially in acidic, hardening fixers). Rapid fixer is faster, but it's also smelly, more expensive, and tends to have a shorter shelf-life (all else being equal). Lots of people use plain hypo (just hypo and water) for fixing prints--especially if they are going to be toned. In general, there seem to be more people using slow fixer for paper than for film.
All fixer solutions (fast or slow) will have a shot shelf-life unless an oxygen scavenger (almost always sodium sulfIte) is added in liberal amounts. That doesn't matter if you always mix them fresh and discard any leftovers.
The fixer I use for both film and paper is hypo plus sodium sulfIte. I make the stock solution with distilled water, and dilute with tap water to make the working solution. Since sodium sulphIte is a mild bass, this is generally called "alkaline fixer". An acid stop bath is usually a good idea when using an alkaline fixer, IMHO.
Just for the record, here are all the types of slow fixer you can mix by adding other ingredients to hypo:
* "plain hypo" (sodium thiosulfate and water only)
* a alkaline fixer (non-hardening, non-acid)
* an acid non-hardening fixer (rare)
* an acid, hardening fixer (toxic)
* a formalin/formaldehyde non-acid hardening fixer (very toxic and gives off toxic vapors)
BTW, it is possible to make ammonium thiosulfate from sodium thiosulfate by reacting it with ammonium chloride (with sodium chloride as a byproduct). I haven't tried it--I don't like the smell of rapid fixer.
Salt in a fixer mixture usually doesn't do any harm, but it doesn't do any good, either. It was tried in the early days of photography, but never worked well (according to various sources)--and film and paper were *much* less light-sensitive in those days. As soon as Herschel discovered that sodium thoisulfate (then known as sodium hyposulfate) would fix silver halogen photographic media, the use of salt was abandoned.
I have no idea how pure this is, but I know suppliers of pool chemicals may have sodium thiosulfate.
As for "fixing" with table salt (NaCl) well ... no. There is no scientific evidence that it works as a long term solution, as far as I have been able to ascertain.
