aug2004 from Lloyd Erlick,

Calcium salts tend to be insoluble in water, or at least
soluble only with difficulty. If calcium and magnesium are
present in water, it is referred to as 'hard water'. In the
darkroom, substances such as Calgon (a brand-name for sodium
hexametaphosphate), and EDTA (available in two forms, with
either two or four sodium atoms) are used to react with the
calcium before other chemicals are added to the solution,
preventing insoluble salts of calcium from clouding the
solution.

Potassium salts tend to be highly soluble in water, and
hence suitable for concentrated solutions such as
commercially made stock solutions. Some people believe that
fiber base black and white papers develop with greater
warmth of tone in the presence of the potassium ion in the
print developer.

Sodium salts tend to be cheaper than their respective
potassium salts. Sodium salts tend to be less readily
soluble in water than potassium salts, but only slightly so.
Sodium salts are often much more readily available than
other forms.

regards,
--le

    Lower solubility and therefore a corresponding high risk of
leaving calcium deposits on the film.

Regards,

  John S. Douglas, Photographer -  http://www.darkroompro.com
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