Campden tablets are either potassium or sodium metabisulfite (usually K-meta) held together with fillers that if not crushed bery well wont disslove in wine and thats why you really have to crush them and grind them well.
Is there any differences/benefits between using Potassium Metabisulphate K-Meta or Sodium Metabisulphate?
Are they used in exactly the same way, ie Prior to fermentation and adding yeast and prior to bottling?
When these products first came into general use (in the 60's), it was Sodium meta and my father-in-law used to send to some where in New York (from Pittsburgh) for a small 5" tube of it for $5.00. Highway robbery at its finest. Then, I believe, people who were watching their Sodium intake demurred and Potassium meta became available. Prices have decreased drastically (supply and demand) since then. Personally, I use Potassium but I got a 1 lb. bag of sodium by mistake (I ordered the wrong thing) so I used it for sterilizing and not as an additive to wine.