This section is from the book "Alcohol, Its Production, Properties, Chemistry, And Industrial Applications", by Charles Simmonds. Also available from Amazon: Alcohol: Its Production, Properties, Chemistry, And Industrial Applications.
Beyond the sulphates normally present, the quantity may be increased through the oxidation of sulphites added for preservative purposes, and by sulphate used for "plastering." Up to a maximum of 2 grams per litre (calculated as potassium sulphate), the latter addition is recognised in certain countries - e.g. Spain and Italy; the object being the clarification of the wine. The precipitate of calcium tartrate, produced when calcium sulphate is mixed with the wine, carries down suspended solid matters as it settles, and the potassium sulphate resulting from the reaction between the calcium sulphate and the cream of tartar remains in solution. It is chiefly in connection with this question of "plastering ' that the estimation of the sulphates is important in the analysis of wine. Normally, wine contains about 002 to 006 gram of sulphates per 100 c.c., calculated as potassium sulphate.
In general, the sulphates are precipitated in the wine itself, though the ash can be used in ordinary cases. Fifty c.c. are acidified with 1 c.c. of hydrochloric acid, raised to boiling, and if any sulphite is present boiled down to about 30 c.c. to expel sulphur dioxide. Then to the still boiling liquid 5 c.c. of a 10 per cent. solution of barium chloride are added drop by drop, the boiling is continued for a few minutes, the whole diluted with hot water to about double the original volume, and digested for some hours on the steam-bath. It is well to let the precipitate stand all night, and to filter through a double filter. The precipitate is washed carefully, dried, ignited, and weighed. The result is usually expressed in terms of potassium sulphate.