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(seb) wrote in message . com>...
> (Ben Rotter) wrote in message . com>...
>
> > It means you'll get ~58% *total* SO2. Some of that will then become
> > free, and some will become bound (total = bound + free).
>
> Ben, here a quote from the winemakermag :
>
> "The key question now is: How much free SO2 is really added for any
> given amount of sulfite? Potassium metabisulfite and Campden tablets
> consist of approximately 57 percent and 48 percent SO2, respectively.
> This means that roughly half of the sulfite actually becomes free SO2
> when a solution is prepared and added to wine."
>
> Thanks for your link, I had already read it and many more about SO2
> but I read different things about the 57% explanation. That's the
> only part i can't be sure of.

If you read the section titled "3. Sodium and Potassium Salts" you'll
see why K meta is quoted as 57% SO2. Potassium metabisulphite has the
chemical formula K2S2O5 and a molecular weight of 222.4, and sulphur
dioxide has the chemical formula SO2 and a molecular weight of 64.1.
Each mole of the potassium meta salt will give 2 moles of SO2, thus
the SO2 content of K meta is 2 x 64.1/222.4 = 57.6%.

This, of course, is the theoretical maximum SO2 yield from K meta. In
practise, it can be lower as Dave has mentioned.

Dave: I've had similar experiences as you have with the strength of my
meta. It just depends on the supplier (i.e. how it's been stored,
etc). These days I always check my SO2 stock solutions to make sure
they're up to strength.

Ben