414 ANALYTICAL CHEMISTRY. 



izing acid solutions, as it has the advantage of being prepared from a 

 substance that can be easily obtained in a pure condition, which is 

 not the case in preparing the otherwise more useful normal solutions 

 of potassium or sodium hydroxide, both of which substances contain 

 and absorb water. 



The solutions are made by dissolving about 70 grammes of potas- 

 sium hydroxide or 60 grammes of sodium hydroxide in about 1000 

 c.c. of water, titrating this solution with normal acid, and' diluting it 

 with water, until equal volumes of both solutions neutralize each 

 other exactly. 



The indicators used in alkalimetry are chiefly solution of litmus 

 or phenolphthalein, only a few drops of either solution being needed 

 for a determination. 



The method adopted by the U. S. P. for standardizing the caustic 

 alkali solution, prepared as above mentioned, depends on the use of 

 chemically pure potassium bitartrate which acts on the alkali thus : 



KHC 4 H 4 O 6 + KOH = K 2 C 4 H 4 O 6 + H 2 O. 



As the molecular weight of potassium bitartrate is 186.78 it follows 

 that this weight in grammes will neutralize one liter of normal alkali 

 solution. The Pharmacopoeia directs to dissolve 9.339 grammes of 

 potassium bitartrate in boiling water and titrating with a portion of the 

 caustic alkali solution, the remainder of which is then diluted until 50 c.c. 

 are required for neutralization. Phenolphthalein is used as indicator. 

 Whenever carbonates are titrated with acids, or vice versa, the 

 solution has to be boiled towaid the end of the reaction in order to 

 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. 



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 -f 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. 



Method for calculating results. Before one can calculate how 

 much, say, of an acid is in a solution which he is titrating with a 



