The following was taken from Fining with Bentonite, Purdue Extension FS-53-W
Should I use sodium or calcium bentonite?
It doesn’t really matter, as long as the oenologist performs an ethanol titration or one of the arbitrary heat stability bench tests on each wine and determines the smallest effective dose to satisfy the test.
Suppliers of sodium bentonites argue that this form has a protein fining capacity twice as high as its calcium cousin. Suppliers of calcium bentonites argue that their form swells less in water, and it creates fewer lees and a smaller loss of wine when racking.
The quantity of sodium added to wine by a heavy bentonite treatment can double the amount of sodium naturally present in grape juice (10 to 20 mg/L), but even then wine is still considered a “very low sodium” beverage. On the other hand, excess release of calcium into a wine from bentonite via exchange with grape proteins may increase the risk of calcium tartrate instability. Since calcium tartrate does not respond as readily to cold stabilization as potassium bitatrate, this may mean the difference between a stable wine and a wine throwing a glass-like precipitate that may worry consumers.