§ 60. CARBONIC ACID. 79 



Unless the precipitated baric sulphate is washed, as 

 above directed, with a solution of cupric acetate, the re- 

 sult of the analysis may be very unreliable, particularly 

 if notable quantities of nitrates or alkaline salts were 

 present. 



CAKBONIC ACID OR ANHYDRIDE. CO2. 44. 



60. Carbonates of all except the alkaline metals are 

 insoluble, or sparingly soluble in water ; all carbonates, 

 without exception, are dissolved by dilute acids, with the 

 expulsion of carbonic anhydride, CO^. 



Reaction. — When dilute nitric or hydrochloric acid is 

 added to a carbonate, whether a solid or in solution, the 

 anhydride is expelled with effervescence, and if a drop 

 of lime-water, suspended on the end of a glass rod, is 

 held in the tube just above tlie liquid, it is made turbid 

 by the formation of insoluble calcic carbonate. 



Quantitative estimation. — Carbonic acid is usually 

 estimated by the loss of weight suffered by the carbonate 

 on treating it with a stronger acid, or by collecting and 

 weighing the expelled anhydride itself 



a. For the first method a convenient form of an ap- 

 paratus is represented in the adjoining figure. 



The carbonate is weighed in the flask A and water is 

 added. B is nearly filled with 

 nitric acid ; G contains fused calcic 

 chloride to absorb the^ moisture 

 from the carbonic acid as it passes 

 out, and so retain it in the appara- 

 tus. The apparatus being put to- 

 gether, with water enough in the 

 flask A to cover the mouth of the 

 tube leading from J5, close the 

 mouth of the tube at e with the finger, and suck a 

 very small quantity of air out at d; on letting air in 



Fig. 3. 



