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.
Sulphuric acid is a chemical combination of sulphur and oxygen, the proportions being 63.2 sulphur and 36.8 oxygen. It is also called oil of vitriol. Its chemical formula is H3S04. Great quantities of sulphuric acid are used by the trade every year, and a few facts bearing on the article are timely, and may prove beneficial to many bottlers. Its consumption in the carbonated beverage business renders it necessary that it should be of a comparatively pure nature. Ingredients which happen to be found in the sulphuric acid during the process of manufacturing may not be of any consequence for some purposes, but it will in this trade.
The discovery of suphuric acid is ascribed to a monk during the middle ages, when it was first obtained by distilling green vitriol, or the sulphate of iron (pyrites), and as the liquid product had an oily appearance when poured out, it was called oil of vitriol. The fumes of sulphur are passed through a lead-lined chamber covered with a thin stratum of water, in which nitrous fumes are introduced, and the union of these elements forms sulphuric acid. Commercial oil of vitriol is an oily-looking, colorless and odorless liquid with a specific gravity of 1.842, and does not fume. It chars nearly all organic substances, in consequence of abstracting from them the element of water, leaving a carbonaceous residue, and the acid acquires a brown color. Its attraction for moisture is so great that, if exposed to the air for a few days in a shallow vessel, it frequently doubles its weight - a fact that bottlers should heed, and not leave their carboys of acid uncorked or exposed if they desire to preserve its strength. The freezing point of pure acid is at 29 degrees below zero Fahr., and when once frozen does not melt under 32° above, but when diluted it congeals at about 47° - 15 degrees above the freezing point of water or, strictly speaking, crystallizes in rhombic prisms.
The most common adulterant of sulphuric acid is water, which impairs its strength and value. Complaints have been made by carbonators in the colder portions of the country of their acid freezing, which is due to its adulteration with water. For use in the "soda" water factory its strength should stand at 66° on the acidimeter or hydrometer, an instrument that is as much a necessity in the bottling shop as a saccharometer. Every carboy of acid should be tested when received from the supplier, and if it registers below 65° it should be returned at once, or if this is not practicable, a reduction of price should be made, and the acid used before the temperature falls to 47 degrees.
In addition to water dilution, the oil of vitriol should be free from all impurities liable to contaminate the carbonic acid gas, such as sulphurous, or nitric acid, and arsenic, which ingredients may, more or less, act injuriously on the carbon dioxide. For the generation of pure carbonic acid gas the manufacturer of beverages requires, without doubt, an acid having none of the above elements; and although manufacturers may wish to deal fairly with the carbonator in every way, it may sometimes happen that one or more of the afore-mentioned impurities are found in it. Without special test they cannot be detected, and it is only found when beverages are injured by it; that is, when it is too late. It is therefore advisable to always test purchases of sulphuric acid for their purity, and to be sure and certain that it will not injure the beverage.