448 PRINCIPLES OF CHEMISTRY 



vapour , and therefore the residue will contain a mixture of a certain 

 quantity of the sodium chloride taken and of the sodium nitrate formed. 

 If a fresh quantity of nitric acid be then added, reaction will again set 

 in, a certain portion of hydrochloric acid is again evolved, and on hea> 

 ing is expelled together with nitric acid. If this be repeated several 

 times, it is possible to expel all the hydrochloric acid, and to obtain 

 sodium nitrate only in the residue. If, on the contrary, we take 

 sodium nitrate and add hydrochloric acid to it in an aqueous solution, 

 a certain quantity of the hydrochloric acid displaces a portion of the 

 nitric acid, and on heating the excess of hydrochloric acid passes away 

 with the nitric acid formed. On repeating this process, it is possible 

 to displace the nitric acid with an excess of hydrochloric acid, just as 

 it was possible to displace the hydrochloric- acid by an excess of nitric 

 acid. The influence ol the mass of the substance in action and the 

 influence of volatility are here very distinctly seen. Hence it may be 

 affirmed that sulphuric acid does not displace hydrochloric acid 

 because of an especially high degree of affinity, but that this reaction is 

 only carried on to the end because the sulphuric acid is not volatile, 

 whilst the hydrochloric acid which is formed is volatile. 



The preparation of hydrochloric acid in the laboratory and on a 

 large scale is based upon these data. In the first instance, an excess 

 of sulphuric acid is employed in order that the reaction may proceed 

 easily at a low temperature, whilst on a large scale, when it is 

 necessary to economise every material, equivalent quantities are -taken 

 in order to obtain the normal salt Na 2 SO 4 and not the acid salt, which 

 would require twice as much acid. The hydrochloric acid evolved is 

 a gas which is very soluble in water. It is most frequently used 

 in practice in this state of solution under the name of muriatic acid. 31 



31 The apparatus shown in fig. 46 (Chapter VI., Note 12) is generally employed for the 

 preparation of small quantities of hydrochloric acid. Common salt is placed in the retort ', 

 the salt is generally previously fused, as it otherwise froths and boils over in the apparatus. 

 When the apparatus is placed in order sulphuric acid mixed with water is poured down 

 the thistle funnel into the retort. Strong sulphuric acid (about half as much again as 

 the weight of the salt) is usually taken, and it is diluted with a small quantity Of water 

 (half) if it be desired to retard the action, as in using strong sulphuric acid the action 

 immediately begins with great rigour. The mixture, at first without the aid of heat and 

 then at a moderate temperature (in a water-bath), evolves hydrochloric acid. Commercial 

 hydrochloric acid contains many impurities; it is usually purified by distillation, the 

 middle portions being collected. It is purified from arsenic by adding FeCl 2 , distilling, 

 and rejecting the first third of the distillate. If free hydrochloric acid gas be required, 

 it is passed through a vessel containing strong sulphuric acid to dry it, and is collected 

 over a mercury bath. 



Phosphoric anhydride absorbs hydrogen chloride (Bailey and Fowler, 1888 ; 

 2P 2 O 5 + 3HCl=POCl3+SHP0 3 ) at the ordinary temperature, and therefore the get 

 cannot be dried by this substance. 



