This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
A simple test for this purpose is of great advantage, and the following method will be of some use in places where no chemist is employed: A small portion of the sulphuric acid is evaporated on a platinum sheet, which is subsequently brought to a red heat. Good sulphuric acid should not leave any residue; if there is any, it is generally sulphate of potash, or soda, or even lead. These are derived from the manufacture, and cannot be classed among adulterations. A little sulphuric acid is diluted with water, and a few drops of concentrated muriatic acid added; if the solution, which was clear before, becomes milky, it indicates the presence of lead. On pouring the acid into 4 volumes of alcohol, no precipitate should be formed (lead).
Another ingredient which is often found in sulphuric acid, particularly such products as are made from pyrites, is arsenic. The sulphuric acid manufactured in the United States, is made from sulphur and generally free from the contamination of arsenic. For the manufacture of carbonic acid gas, which requires pure sulphuric acid, it is especially required that the acid be entirely free from arsenic, and also nitric acid and nitrous acid. 'Arsenic is detected by mixing with water and granulated zinc, when hydrogen gas is liberated, which should not contain any trace of arsenic. The hydrogen gas is ignited, and the flame allowed to strike a cool porcelain plate, on which, if arsenic is present, metallic arsenic is deposited. Nitrous or nitric acid may be detected by throwing a small piece of copperas in the questionable acid; if it shows a brown coloration where it touches the liquid, the presence of the above impurities are indicated.
Warington recommends: "For the detection of sulphurous acid the suspension in a quart bottle, half filled with the sulphuric acid, of a strip of paper colored blue by iodide of starch, which will be bleached by the sulphurous acid; a paper covered with starch and iodide of potassium, suspended in a similar manner, becomes blue in the presence of nitrogen oxides".
Probably not one in a hundred carbonators thinks to investigate the quality of his sulphuric acid, as he generally takes the word of the supplier or manufacturer. When the latter is a reputable house it is sufficient, but "tricks of the trade" are resorted to by irresponsible parties, who may, by persistence, dispose of a lot of "cheap acid" to an unsuspecting bottler, and, if not carefully examined, may be the cause of heavy loss in his trade.
A most important impurity is the addition of saline matters to weaker acids to increase the specific gravity, but this is recognized by the fixed residue left on evaporating a small quantity in a platinum dish or crucible.
Sulphuric acid unites with water and alcohol in all proportions, forming transparent liquids; a white turbidity indicates lead sulphate, which becomes black with hydro-sulphuric acid. Arsenic would be indicated by a yellow precipitate occurring in the diluted acid with sulphuretted hydrogen, or more rapidly by a solution of nitrate of silver (arsenite of silver). When sulphuric acid is wanted for any purpose in diluted form (for cleansing, etc.), it must be observed to add the acid to the water in all cases, and more particularly if large quantities of acid are to be diluted. During the mixing of the two liquids, which is best performed in a porcelain dish or beaker-glass, a considerable elevation of temperature takes place, causing a portion of the water to evaporate. Filtration through paper (when far diluted only), through asbestos or decantation, will remove any deposits formed by the dilution.
The Nordhausen oil of vitriol or fuming sulphuric acid is different from the afore-described, and of no use in the mineral-water trade. This name was used officinally in Germany. It is an oily liquid, emitting white suffocating fumes, containing the volatile sulphuric anhydride, and having a specific gravity of from 1.860 to 1.900. It is usually of a brownish color, due to organic matter, and in commerce used to dissolve indigo and to prepare artificial alizarine. A variety of acids has been experimentally employed in the trade, but for practical purposes sulphuric acid has been found superior to any other.
Sulphuric acid is, next to hydrofluoric acid, the strongest, and displaces most other acids from their combination, such as carbonic acid; therefore its use in the trade.
A nitrous taste imparted to carbonated beverages can frequently be detected in the beverages of inferior makers, proving that common vitriol or impure sulphuric acid has been used; therefore much pains should be taken in procuring acid free of nitric acid, to save trouble and insure purity of the beverages. According to Hirsh, sulphuric acid of 66 degrees may displace 41 to 41.5 per cent. of its weight on carbonic acid gas and double the quantity from bicarbonates.