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.
Deniges's process, official in France, is as follows: Ten c.c. of the wine are shaken with about 1 gram of lead dioxide, then mixed with 2 c.c. of a solution of mercurous sulphate (5 grams of HgO, 20 c.c. of H1SO4 cone, 100 c.c. of water), again shaken up, and filtered. Five or 6 c.c. of the filtrate are raised to ebullition in a test-tube, and one drop of a 1 per cent. solution of potassium permanganate is added. After the colour has disappeared another drop is added; and so on up to ten drops.
Normal wines give only a slight cloudiness under this treatment. With 0.01 gram of citric acid per 100 c.c. there is a distinct turbidity, and when the proportion reaches 0 04 gram and upwards, a flocculent precipitate is produced.
The presence of citric acid having been proved, comparative experiments with solutions containing known quantities of the acid serve to evaluate the proportion..