Powdered fixer is also ammonium thiosulfate. The Rapid Fixer is just more concentrated.
Most powdered fixers are in fact based on
sodium thiosulfate, not ammonium thiosulfate. Fixers based on the latter almost invariably ship as liquids. I don't know of any powdered commercial ammonium thiosulfate fixers offhand, although it's conceivable there are some that I don't know about. It's also possible to compound a rapid fixer from sodium thiosulfate and add ammonium ions via another chemical, but I don't know if any commercial fixers use this approach, and if they do whether they ship in powder form. In any event, those ammonium ions are important for the rapidity of rapid fixers' action; it's not just being more concentrated that makes a fixer a rapid fixer.
There are lots of APUG threads on fixers, many of which cover these issues in detail. I haven't perused all of these threads recently, but some or all of the below might be of interest:
- (there was a url link here which no longer exists)
- (there was a url link here which no longer exists)
- (there was a url link here which no longer exists)
- (there was a url link here which no longer exists)
- (there was a url link here which no longer exists)
- (there was a url link here which no longer exists)
I just did a Google search on APUG to find these threads.