This acid may exist in wine, not merely as dissolved sulphur dioxide, but also in combination with various constituents - e.g., aldehydes, sugars, colouring matters, and tannins. Hence the direct determination by means of iodine in the cold does not give accurate results; but the sulphur dioxide may be distilled off completely in a current of carbon dioxide, oxidised by iodine, and the resulting sulphuric acid precipitated as usual with barium chloride and weighed.

The French official process describes a preliminary test to ascertain whether the amount of sulphur dioxide present exceeds 0 03 gram per 100 c.c. If it does, the distillation method is used for the estimation, as follows.

A flask of about 400 c.c. capacity is fitted with a two-holed rubber stopper carrying an entrance-tube reaching to the bottom of the flask, and connected with an apparatus for generating carbon dioxide. The exit tube is attached to a large Peligot absorption tube, of which the bulbs are each of about 100 c.c. capacity. The air is first expelled from the apparatus with a current of CO2 gas, and then 30 to 50 c.c. of iodine solution (5 grams of 1 + 7.5 grams of KI per litre) are introduced into the Peligot tube. The stopper of the flask is raised, and without interrupting the current of carbon dioxide, 100 c.c. of the wine are passed into the flask from a pipette, followed by 5 c.c. of phosphoric acid (sp. gr. 169). After re-closing, the flask, at the end of a little time, is heated until about one-half of the wine has distilled over into the bulb-tube, which it is well to immerse in cold water during the operation. The contents are then transferred to a beaker, and the sulphuric acid precipitated with barium chloride in the usual manner.

Weight of BaSO4 obtained X 027468 = gram of SO2 per 100 c.c.

The German official method is similar, except that a Liebig condenser is inserted between the flask and the Peligot tube.

Gayon and Laborde1 dispense with the current of carbon dioxide and the large bulb-tube, proceeding as follows.

A flask of 250-300 c.c. capacity is filled with carbon dioxide to replace the air; 100 c.c. of the wine and 2 c.c. of syrupy phosphoric acid are introduced, and the flask is connected with a spiral condenser to which is attached a delivery tube drawn out to a very narrow bore at its lower end. This dips into a special absorption tube, the lower part of which is similar to a test-tube and the upper portion expanded into a bulb of about 80-100 c.c. capacity. 225 C.c. of iodine solution (of such strength that 1 c.c. = 0002 gram of SO2) are placed in the absorption tube, and the wine is distilled, gently at first and afterwards more rapidly, until all the alcohol has passed over. The sulphur dioxide is absorbed without loss, as the bubbles issuing from the fine delivery tube are very small, and have to traverse a column of iodine solution more than 10 cm. long. If at the end of the distillation there is still some iodine remaining, the quantity of SO2 present is less than 0 045 gram, and the operation can be terminated if this is all that it is required to know; otherwise the liquid is removed to a beaker and precipitated with barium chloride as usual. Should the iodine show signs of becoming all used up during the distillation, a further quantity is at once added.

1 "Viris," p. 153.