Sometimes bombs filled with liquid sulphur dioxide are purchased for this purpose. These are inconvenient to use, and this method is ordinarily more expensive than the usual one of burning the sulphur and producing the gas directly at the factory.
Sulphur dioxide is a heavy gas which is very readily absorbed in water, and at a temperature of zero C. nearly 80 per cent by volume of the gas will be taken up.
At 40° C. only about 18 per cent by volume of the gas will be absorbed. It may readily be seen that the percentage of gas contained in the juice when saturated will be determined by the temperature.
The following equation expresses the absorption of sulphur dioxide in water at ordinary temperature:
SO2 (sulphur dioxide) + H2O (water at low temperature)—>H2SO3 (sulphurous acid).
Another thing of very great importance is the cooling of the gases to condense any water that may be present so that no hot gas will reach the juice to be treated or combine with water in the pipes. The equation represented when high temperatures are used is as follows:
SO2 (sulphur dioxide) + H2O (water) + O (high temperature)—>H2SO4 (sulphuric acid).
This last-named acid is very corrosive and a powerful investing agent. It therefore has the property of rapidly destroying sucrose, especially at a high temperature.
In the burning of sulphur it is well that as thorough a combination as possible be obtained, else there will be a loss of sulphur, which will deposit in the tubes and choke them, and more time will be required for the process. The fumes from a well-regulated sulphur furnace should contain from 15 to 16 per cent sulphurous acid. The theoretical percentage obtainable is about 21 per cent of the acid.
Carbon dioxide.—In recent years carbon dioxide gas has found a very useful application in the cane-sugar factories, where a good grade of plantation sugar is desired.