III. Mixed with Organic Liquids, &c.
In tea, coffee, or beer the mode of applying the tests are the same, the mixture being previously filtered, or the acid separated from the organic mixture by dialysis, or the following cautions are necessary:
Objection A.—Alum, or any acid sulphate, would give all the reactions with the nitrate and chloride of barium.
Answer A.—Evaporate a portion of the doubtful liquid; if pure acid, there will be no residue, sometimes only the slightest trace of sulphate of lead.
Objection B.—Erroneous estimation of free sulphuric acid, in consequence of the presence of some saline or neutral sulphates.
Answer B.—Evaporate as before. The free sulphuric acid separated by warming the liquid is then added to a known weight of powdered carbonate of baryta until effervescence ceases. The resulting precipitate when weighed, represents the free sulphuric acid present.
Calculate in the following manner—
- BaCO₃ + H₂SO₄ = BaSO₄ + H₂O + CO₂.
- BaCO₃ = 197.
- H₂SO₄ = 98.
- CO₂ = 44.
In the above equation, 98 parts of H₂SO₄ take the place of 44 parts of CO₂. If, therefore, 100 grains of BaCO₃ renders the liquid neutral, the amount of free SO₄ present will be represented by the increased weight of the precipitate in the proportion of 54 to 98, the difference between 44 and 98 the equivalent of CO₂ + SO₄.
To extract the acid from organic mixtures, digestion with alcohol is required: filter and neutralise the filtrate with caustic potash, evaporate to dryness, and dissolve the residue in distilled water acidulated with hydrochloric acid; the previous tests may then be applied. Extraction with alcohol leaves behind combined acid in the form of sulphates. The presence of free acid in the filtrate or original mixture may be detected by the change of tropæolin solution from yellow to crimson or ruby colour when added to it; or on adding a few drops of the mixture or filtrate to a solution of potassio-tartrate of iron B.P. in water, so as to make it of a yellow tinge, and to which potassium sulphocyanide has been added, the liquid changes from a yellow to a port-wine colour.