> I often read that the k-meta contain 57% sulphite and the campden
> tablets 48%. I would like a clarification on that.

K-meta is about 58% SO2.

Campden tablets are designed to have about 0.44 grams of (often K-)
meta. Whether they are 48% SO2 or not really depends on the tablet
size, which tends to vary depending on the manufacturing process.

> B) The 57% mean that from any given amount of k-meta added you will
> roughly get 57% of free SO2 ( theorical of course )

It means you'll get ~58% *total* SO2. Some of that will then become
free, and some will become bound (total = bound + free).

See http://members.tripod.com/~BRotter/SO2.htm for more.

Ben