Sulphuric Acid.—100 c.c. are acidulated with hydrochloric acid, the liquid heated to boiling, and the sulphuric acid precipitated by barium chloride. The precipitate is well washed, dried, and weighed. 100 parts represent 42·49 parts H2SO4. Pure wine contains from 0·109 to 0·328 gramme of monohydrated sulphuric acid per litre (corresponding to 0·194 to 0·583 gramme potassium sulphate). The presence of an excess of this maximum amount indicates that the wine has been plastered.
Phosphoric Acid.—100 c.c. of the wine are evaporated, the residue ignited, dissolved in a little water, acidulated with nitric acid, and then added to an excess of solution of ammonium molybdate. After standing over night the separated precipitate is dissolved in ammonia and the phosphoric acid determined by means of an ammoniacal solution of magnesium sulphate. 100 parts of the precipitate thus obtained correspond to 63·96 parts of phosphoric acid. The former belief that the best qualities of wine contain the largest proportion of phosphoric acid does not appear to be invariably correct.
Salicylic Acid.[103]—The determination of this acid is accomplished as follows:—100 c.c. of the sample are repeatedly agitated with chloroform, which is subsequently separated and evaporated to dryness. The residue is re-crystallised from chloroform and weighed; its identity can be established by dissolving it in water and adding solution of ferric chloride (see p. [149]).
Sulphurous Acid.—For the detection and estimation of sulphurous acid, the following methods have been recommended:—500 c.c. of the wine are placed in a flask, the exit-tube of which dips into a test-tube which is suitably cooled, and subjected to distillation. When about 2 c.c. have distilled, a few drops of a neutral solution of silver nitrate are added to the distillate: in presence of sulphurous acid, a white curdy precipitate will be formed, which differs from silver chloride in being soluble in nitric acid. According to Haas,[104] this test is not invariably decisive, as pure wine may cause the precipitation under certain conditions; moreover, acetic acid is said to render silver nitrate turbid in strong alcoholic solutions. Sulphurous acid can be quantitatively determined by adding phosphoric acid to 100 c.c. of the wine, and distilling it in an atmosphere of carbonic acid gas. The distillate is received in 5 c.c. of normal iodine solution. When one-third of the sample has passed over, the distillate (which should still contain an excess of free iodine), is acidulated with hydrochloric acid, and the sulphuric acid formed precipitated with barium chloride.
Colouring matters.—Very numerous processes have been published for the detection of foreign and artificial colouring matters in wine. Among those suggested are the following:—
1. A few drops of the sample are placed in succession on the smooth surface of a piece of white calcined lime, and notice taken of the tint produced. The following colours are stated to occur with pure and artificially coloured wine:—
| Natural red wine | yellowish brown. |
| Wine coloured with fuchsine | rose colour. |
| „ „ „ Brazil wood | „ „ |
| „ „ „ logwood | reddish violet. |
| „ „ „ black hollyhock | yellowish brown. |
| „ „ „ poke-berries | yellowish red. |
2. If ammonium hydroxide be added to the suspected sample to distinct alkaline reaction, then a little ammonium sulphide and the liquid filtered, the filtrate from genuine wine will possess a green tint, whereas that obtained from artificially coloured wine will exhibit other colours, such as red, blue, violet, or brown.
3. 100 c.c. of the wine are evaporated to about one-half of the original volume, ammonium hydroxide added to alkaline reaction, and the liquid thoroughly shaken. Ether is then added, and the mixture again well shaken. It is next introduced into a separator, and allowed to stand at rest until the ether has risen to the surface, when the lower stratum is drawn off, and the residual ether washed by agitation with water, which is subsequently removed. The ethereal solution is now transferred to a flask connected with a Liebig’s condenser, a piece of white woollen yarn introduced into the liquid, and the contents of the flask distilled at a gentle heat: in presence of the smallest amount of fuchsine, the wool will acquire a very perceptible reddish hue.