ACIDIMETRY AND ALKALIMETRY 13 



The acid dissolves rather slowly. The solution is therefore best 

 made in a beaker with the aid of gentle heating with about 250 c.c. 

 water. Transfer every drop of the solution to a measuring flask 

 (500 c.c.), carefully rinsing the last traces from the beaker into the 

 flask by means of successive small amounts of cold distilled water. 

 Cool the flask in running tap water until the contents of the flask 

 have reached the room temperature. (If a thermometer is used 

 it must be rinsed carefully before it is removed from the flask.) 

 Fill up with water until the lower side of the " meniscus " is exactly 

 even with the 500 c.c. mark. Stopper the flask, and invert several 

 times (30 to 40) so that the solution is thoroughly mixed. Trans- 

 fer to a clean, dry bottle; label and preserve. 



Using a strong base like sodium hydroxide and a sensitive 

 indicator like phenolphthalein for the titration, it is possible to 

 obtain quite reliable and accurate results with oxalic acid. The 

 volumetric determinations involved in metabolism studies and 

 urine analyses are, however, extensively based on titrating 

 ammonia, which is a very weak base. Phenolphthalein, because 

 of its high degree of sensitiveness to weak acids and its lack of 

 sensitiveness to weak bases, is useless in titration of ammonia. 

 The oxalic acid and the phenolphthalein are therefore used only 

 for the purpose of securing a standard alkali solution. 



2. Preparation of .5 N Sodic Hydrate (1000 c.c.). The sodic 

 hydrate used for standard solutions must be as free as possible 

 from carbonates, because otherwise the solutions will not have the 

 same titrating value with all the common indicators. Sodic 

 hydrate absorbs rapidly carbonic acid from the atmosphere. 

 The carbonic acid should first be removed from the alkali, and the 

 solutions must afterwards be protected from too much exposure 

 to the carbonic acid of the air. As the carbonates are almost 

 insoluble in very strong sodic hydrate solutions, 40 per cent solu- 

 tions in which the carbonates have settled can be used as a starting 

 point. 



Transfer about 80 c.c. clear 40 per cent sodic hydrate solution 

 to a large flask or bottle, and add 1000 to 1200 c.c. water. Dis- 

 solve about 2 gms. barium hydroxide in about 100 c.c. hot water, 

 and without filtering pour this solution into the sodic hydrate 

 solution. Cover with a watch-glass, and set aside over night. 

 By means of a siphon or compressed air remove the clear, super- 

 natant solution to another flask. 



