A quick question for my own information: Is the cation critical mainly because of its effect on complexation of silver ions? In particular, ammonia and amines can help solublize silver compounds (e.g. silver halides) by forming so called "complex ions" Neither sodium nor potassium can do this, but ammonium ions (in the form of the non-ionic form ammonia, which exists in equilibrium with the ammonium ion in water solutions) readily forms silver complexes. Would this be the reason that the ammonium form of hypo is a more rapid fixer than the sodium, and why the addition of certain amino-compounds act as grain reducers in developers, e.g. by the mechanism of silver halide solublization?
Originally Posted by Photo Engineer
If sodium and potassium act as "spectator ions" in the fixing process then I would expect the substitution of one for the other to have a relatively small effect. This is not to say that sodium and potassium salts would behave identically, but only that the differences would not be extreme.
Interestingly, high concentrations of chloride can also form complexes with silver ions and solublize silver compounds, which is probably the chemistry behind a discussion I recently read about using salt water as a weak fixing agent.