METHODS FOE QUANTITATIVE DETERMINATIONS. 259 



drive off the carbon dioxide, as neither of the two indicators men- 

 tioned gives reliable results in the presence of carbonic acid or an 

 acid carbonate. This boiling is unnecessary when methyl- orange is 

 used, because it is not influenced by carbonic acid. 



FIG. 36. 



Titration. 



The proper mode of performing the operation of titration is shown 

 in Fig. 36. 



When salts of organic acids with alkali metals are to be titrated with normal 

 acids, these salts are first converted into carbonates. This is accomplished by 

 igniting the weighed quantity of the salt in a crucible of porcelain or platinum. 

 The chemical action which takes place during the ignition of potassium acetate 

 may be shown thus : 



2KC 2 H 3 2 + 80 = K 2 CO 3 + 3H 2 O + 3CO 2 . 



In a similar manner the alkali salts of all organic acids are converted into 

 carbonates. Frequently some carbon is left unburned ; this, however, does not 

 interfere with the result of the titration. The titration is made with the liquid 

 obtained by dissolving in water the residue left after ignition. 



Neutralization equivalents. The normal solutions of acid and 

 alkali may be used for the determination of a large number of sub- 

 stances, either directly (as in the case of free acids, caustic and alka- 

 line carbonates and bicarbonates) or indirectly (as in the case of salts 

 of most of the organic acids, with alkalies, which are first converted 

 into carbonates by ignition). 



