METHODS FOR QUANTITATIVE DETERMINATIONS. 413 



It is for this reason that the basis in preparing a volumetric normal 

 acid solution is oxalic acid, a substance which can be readily obtained 

 in a pure crystallized condition. 



Normal acid solution. Crystallized oxalic acid has the com- 

 position H 2 C 2 O 4 .2H 2 O and a molecular weight of 125.1. Being 

 dibasic, only half of its weight is taken for the normal solution, 

 which is made by placing 62.55 grammes of pure crystallized oxalic 

 acid in a liter flask, dissolving it in pure water, filling up to the 

 mark at the temperature of 25 C. (77 F.) and mixing thoroughly. 



Normal solutions of sulphuric or hydrochloric acid are, for various 

 reasons, often preferred to oxalic acid. These solutions are best 

 made by diluting approximately the acids named, titrating the solu- 

 tion with normal sodium hydroxide, using phenolphthalein as an 

 indicator, and adding water until equal volumes saturate one another. 

 For instance, if it should be found that 10 c.c. normal alkali solution 

 neutralize 7.6 c.c. of the acid, then 24 c.c. of water have to be added 

 to every 76 c.c. of the acid in order to obtain a normal solution. 

 Normal sulphuric acid contains 48.675 grammes of H 2 SO 4 , and normal 

 hydrochloric acid 36.18 grammes of HC1 per liter. 



These normal solutions can be made conveniently by diluting either 30 c.c. 

 of pure, concentrated sulphuric acid of sp. gr. 1.84, or 130 c.c. of hydrochloric 

 acid of sp. gr. 1.16 to 1000 c.c. The solutions thus obtained are yet too con- 

 centrated and are adjusted as described above. 



Other methods of determining the exact standard of normal acids depend 

 upon the precipitation of 10 c.c. of the sulphuric acid solution by barium 

 chloride, or of 10 c.c. of the hydrochloric acid solution by silver nitrate, and 

 weighing the precipitated barium sulphate or silver chloride. Ten c.c. of 

 normal sulphuric acid give 1.1587 grammes of barium sulphate, and 10 c.c. of 

 normal hydrochloric acid 1.423 grammes of silver chloride. 



A third method depends on the formation of, and the weighing as, an 

 ammonium salt. Ten c.c. of either acid are neutralized (or slightly super- 

 saturated) with ammonia water. The solution is evaporated in a previously 

 weighed platinum dish over a water-bath, the dry salt is repeatedly moistened 

 with alcohol, and finally dried in an air-bath at a temperature of 105 C. 

 (221 F.) for about half an hour. Ten c.c. of normal sulphuric acid give of 

 ammonium sulphate 0.65605 gramme, and 10 c.c. of normal hydrochloric acid 

 of ammonium chloride 0.5311 gramme. 



Normal alkali solution. A normal solution of sodium carbonate 

 may be made by dissolving 52.655 grammes (one-half the molecular 

 weight) of pure sodium carbonate (obtainable by heating pure sodium 

 bicarbonate to a low red-heat) in water, and diluting to one liter. 

 This solution, however, is not often used, but may serve for standard- 



