Article — From the August 1872 issue
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Article — From the August 1872 issue
SODA-WATER, strange to say, contains no soda at all in any form. As made at present, it is simply a solution of carbonic acid in water of the ordinary purity. It is true that a small proportion of the bicarbonate of soda is sometimes added, but no satisfactory reason is assigned for the practice, and it is believed to be a rare exception to the general rule.
Our readers who have not cared to remember much of their chemistry may need to be reminded that carbonic acid is, at ordinary temperatures and pressures, a colorless gas of slightly pungent odor and pleasant acid taste; and that it exists in abundance in nature, both free and locked up in combination with basic elements, forming carbonates. These carbonates can be easily decomposed, and the carbonic acid obtained from them at will in a pure state.
Water impregnated with this gas is a grateful and, according to the medical authorities, a wholesome drink, although the gas itself, when respired, is fatally poisonous. All fermented beverages which are “brisk,” or sparkling, owe that property to the presence of carbonic acid.
The simplest way of carbonating water is to dissolve in it a soluble carbonate and a decomposing acid. The old-fashioned soda-powders are an application of this principle, the blue paper containing bicarbonate of soda, and the white one tartaric acid, which being dissolved in separate portions of water and the solutions mixed, a brisk effervescence ensues from the escape of liberated carbonic acid. It is reasonable to infer that the carbonated water now known as soda has its name from this extemporaneous method of preparing it. The tartrate of soda formed by the reaction is necessarily present in the liquid, and in order to make pure carbonated water the gas must be allowed to escape from the liquid in which it is set free, and then conducted into pure water contained in a separate vessel. This is essentially what is done in the manufacture of the soda-water of the present day.
At the ordinary pressure of the atmosphere water will absorb carbonic acid only to the extent of a volume equal to its own bulk, which is not nearly a sufficient proportion to give agreeable “soda.” An increase of pressure, however, causes further absorption in a direct ratio, and by taking advantage of this fact water may be acidified to almost any degree desired.
The original method of making soda was to generate the gas and conduct it to a gasometer or gas-holder in the ordinary way, and then force it into the water by means of a condensing air-pump. This method is still in almost exclusive use in England.
In this country the pump is usually dispensed with. The tremendous elasticity of the gas itself when freed from the bonds of chemical union furnishes ample power for compression, and by generating it in a closed vessel of sufficient strength an indefinite number of volumes may be condensed into one, and water impregnated accordingly.
The essentials of an apparatus for making soda-water by the last-mentioned process are a generator and fountain. The generator consists of a strong metallic vessel, divided into two compartments by a diaphragm, or of two separate chambers, mounted one over the other. The upper chamber is a receptacle for the acid to be used, and the lower one receives the carbonate to be decomposed. An opening between the two is furnished with a valve which can be opened or closed at pleasure by an outside connection. The external openings of both vessels are closely fitted with screw plugs, so that after the chemicals are introduced the generator can be rendered gas-tight. The fountain is a cylindrical vessel, into which the water is introduced, and in which it is impregnated with the gas under pressure, and retained until it is drawn for use.
To the generator is attached a small cylinder, which is partially filled with water, through which the gas is conducted to free it from impurities before it reaches the fountain. A gauge, constructed to show the amount of expansive power or pressure exerted by the liberated gas, completes the apparatus. The capacity of the fountains varies from ten to thirty gallons, and that of the generators in like proportion.
The material of which the apparatus is usually constructed is copper, which, from its great toughness and ability to resist expansive pressure, is admirably adapted to the purpose. The acid chamber has a lining of lead, and the fountain one of tin, both of which are necessary to protect the copper from the chemical action of the liquids these vessels are to contain. Fountains of iron lined with glass or enamel are also in use. They afford absolute protection of the water against any metallic contamination, but have the disadvantage of being more weighty, and consequently more troublesome to handle, than copper ones, and are also less secure against explosion.
The chemicals usually employed by the soda makers for the evolution of carbonic acid are whiting or marble dust — both native carbonates of lime, varying only in purity or in aggregation of particles — and sulphuric acid, the oil of vitriol of commerce.
The manufacture of soda-water by the apparatus just noted is exceedingly simple. The process is conducted in the following manner: The whiting, previously mixed with water enough to form a thin, pasty liquid, is introduced into the whiting chamber of the generator through an opening made for the purpose, and the vitriol is in like manner introduced into the “acid chamber.” The washer and fountain each being filled to about two-thirds of its capacity with water, the necessary connection of pipes being made, and the openings secured with well-adapted screw plugs, the acid is allowed to trickle slowly down into the whiting by opening the valve at the bottom of the chamber. The mixture of vitriol and whiting is then thoroughly stirred together by means of an “agitator” operated by a crank on the outside of the chamber, and carbonic acid is rapidly evolved. When the pressure gauge shows a sufficient amount of gas to be present, the cock communicating with the fountain is opened, and the gas flows into it, passing first through the water in the washer, which absorbs impurities that may be brought over from the generator. When the pressure has been equalized in all the vessels, the fountain is disengaged and agitated to promote absorption of the gas by the water; it is then reconnected with the generator, and a fresh charge of gas formed and passed into it, which operation is repeated until the gauge finally shows a uniform pressure against the inner surface of the apparatus of 120 to 160 pounds to the square inch. Sometimes it is carried even higher.
It will be perceived that there is considerable danger attending the manufacture of soda-water. Frightful explosions sometimes occur from the carelessness of the operator, or unnoticeable defects in the apparatus. The water being sufficiently charged with gas, the fountain is connected by a suitable pipe with the draught stand on the counter, where the soda is to be served to customers. As the gas absorbed under pressure rapidly escapes from the water when restraint is removed, and in its efforts to do so forcibly carries with it the water in which it is entangled, the soda is readily “drawn” from the delivery pipe, whatever may be the level or position of the reservoir or “fountain.”
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