"seb" > wrote in message
m...
> I often read that the k-meta contain 57% sulphite and the campden
> tablets 48%. I would like a clarification on that.
>
> A) Is the 57% mean the % of sulphite contain in a given amount of
> k-meta from wich you will get some bound SO2 and free SO2
>
> OR
>
> B) The 57% mean that from any given amount of k-meta added you will
> roughly get 57% of free SO2 ( theorical of course )

Not quite either of A or B, but B is closer.

BTW, forget about Campden tablets if you do any serious amount of
winemaking. Buy meta in bulk and weigh it out yourself. You need a balance
with an accuracy of 0.1 g or better. You also need to reseal the meta
really well or it'll go dead on you.

K2S2O5 - straight out of a bag from the manufacturer - usually assays ~50%,
±2% available SO2. The stuff is very reactive to air - especially moist
air. By the time you get it the assay will be ~50% if you got a good batch.
Remember that part where I said to reseal the meta really well? I meant it.
Solutions don't last too long either. Make fresh if you're adding it to
your wine.

If you use this in a wine that has not been previously sulfited the free SO2
when you're done will be as if you used stuff that was only ~25%. A portion
of the sulfite becomes _bound_ in the wine.

Once the requirement for bound sulfite has been met, whatever you add beyond
that adds directly to the free SO2. That makes late adjustments really
easy - assuming you know the pH (which is what determines your target free
SO2).

This topic is not easy to grasp in its entirety, but it's very important to
know about the 0.8 ppm molecular free SO2 requirement and its interrelation
with pH and free SO2.

Tom S