Industrial Preparation of Hydrochloric Acid

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Hydrochloric acid is produced along with the industrial preparation of caustic soda (sodium hydroxide). During the electrolysis of sodium chloride, large quantities of hydrogen and chlorine gas are obtained as by-products. These two gases are burnt to form hydrogen chloride gas.

The hydrogen chloride gas so formed is dissolved in water to form hydrochloric acid. A saturated solution of the acid has a density of 1.2 g cm^-3. It contains about 40% by mass of hydrogen chloride.

Funnel Arrangement for Dissolving Hydrogen Chloride in Water

Due to the high solubility of hydrogen chloride gas, it cannot be passed through water like other gases. If the delivery tube is placed in water as shown in figure 5.9. (a) Water will get sucked up into the apparatus and cause damage.

To prevent this from happening, the delivery tube is attached to a wide mouthed funnel and placed in such a way that it touches the level of water [Fig.5.9 (b)]. The gas coming through the delivery tube fills the mouth of the funnel and then dissolves in the water. Hence a low pressure is created and water gets sucked up into the wide mouth of the funnel.

As soon as the suction occurs, the level of water in the beaker gets lowered and the funnel is no longer in contact with the water. Thus, the water in the funnel comes down into the trough (and the wide mouth of the funnel will once again be in touch with the water level). As soon as this happens the space inside the funnel gets filled with gas again and the process gets repeated.

Advantages of this arrangement

a) It prevents the back suction of water into the apparatus.

b) It provides a larger surface area for dissolution of hydrogen chloride gas.