Knox unflavored gelatin also contains a silicate dessicant (IIRC) and a few other ingredients to make it more palatable and 'easy to dissolve'.
It would be best to use photo grade gelatin for this purpose than food grade gelatin. The photo grade gelatins are much more highly purified and have no additives. Use 175 bloom or higher.
Look on the side of the package for the extra ingredients in the Knox gelatin.