238 PHYSIOLOGICAL CHEMISTRY 



gms. of the crystals so prepared are dissloved in water and made 

 up of 1 liter. This solution may be considered normal and used to 

 standardize an unknown solution of sodium hydrate, using 

 phenolphthalein as indicator. 



5. Sodium Carbonate Method, A more reliable method for 

 preparing a standard consists in heating pure sodium bicarbonate 

 in a platinum dish. The dish is placed in an air bath already 

 heated to 200 C. and the temperature raised to 270- 280 but 

 not above 300. It is heated at this temperature for half an 

 hour, then cooled in a desiccator, and before the sodium carbon- 

 ate is quite cool, transferred to a dry stoppered weighing bottle. 



To standardize an unknown acid solution, rapidly weigh 2-3 

 gms. of carbonate prepared as above, dissolve in 80-100 c.c. of 

 water, add 2 drops of methyl orange and titrate with the 

 unknown acid. 



An exactly normal acid should neutralize pure sodium car- 

 bonate in the ratio of 100 c.c. acid to 5.3 gms. carbonate. From 

 the results of the titration, which should be done in duplicate, 

 the strength of the unknown acid may be calculated, and the 

 acid diluted accurately to the required strength. After dilution, 

 the acid should be titrated again against sodium carbonate to 

 make sure that the dilution was accurate. 



After making an accurate solution of normal acid (H 2 S0 4 or 

 HC1) a solution of normal sodium hydrate may be prepared,- 

 using the normal acid as a standard and titrating with alizarine 

 red or methyl red as indicator. 



If time does not permit each student to start from oxalic acid 

 or sodium carbonate in the peparation of his normal solutions, 

 standard solutions of N/10 acid and alkali prepared as above 

 described should be furnished for the purpose of standardizing 

 the solutions made by the class. 



Note: In the author's classes it has been customary to 

 furnish to the class N/10 acid and alkali for the purpose 

 of standardizing solutions prepared by them as described 

 below, thus saving much time for the class ; students then 

 use their own N/10 solutions in their work. 



