Sodium Metabisulphite is a sulphur based compound that is used primarily to sterilise wine and cider making to kill bacteria and to inhibit the growth of most wild yeast.
It can also be used to eliminate both free chlorine and the more stable form, chloramine, from water.
One gram of metabisulphite will impart approximately 60 parts-per-million of sulphur dioxide (SO2) to each litre of wine.
As a guide, to sanitise wine must add 0.2 g of metabisulphite per litre of juice. Pre-dissolve it in a small quantity of water before adding to the must. Allow the must to ventilate in an open container for 24 hours before adding yeast.
Here is a link to an excellent article on the use of metabisulphate.
