GRAVIMETRIC AND VOLUMETRIC ANALYSIS 521 



Normal Solutions. A normal or standard solution (N sol.) may be 

 defined as one which contains one molecular weight of the reagent 

 in grams dissolved in enough distilled water to make exactly 1000 c.c. 

 at a temperature of 16 C. (60 F.). If, for instance, a normal solution 

 of caustic soda or sodium hydrate (used in standardizing our culture 

 media) is to be prepared the molecular weight of sodium hydrate, 

 which has the chemical formula NaHO, must first be ascertained. The 

 atomic weight of sodium (Na) is 23, that of hydrogen (H) is 1, and 

 that of oxygen (O) is 16. Hence, NaHO has a molecular weight of 40. 

 In preparing a standard or normal solution of NaHO, therefore, 

 40 grams of the chemically pure sodium hydrate (NaHO chemically 

 pure) are weighed out, placed in a 1000 c.c. volumetric flask and dis- 

 solved in several hundred c.c. of distilled water; the solution (which 

 has become warm) is allowed to cool and then enough distilled water 

 is added to make up exactly 1000 c.c. at 16 C. Sodium hydrate is 

 a hygroscopic substance, that is, one which will draw water from the 

 atmospheric air, and in order to obtain an accurate normal solution 

 it must be weighed out rapidly. In fact, in exact chemical work it is 

 impossible, in preparing normal solutions, to begin with a normal 

 solution of sodium hydrate, but another normal solution must first be 

 prepared from a substance which is not hygroscopic and which can be 

 weighed out very accurately. Dry, normal sodium carbonate, Na 2 CO 3 , 

 is generally used for this purpose. Another factor must be considered 

 in the preparation of normal solutions. The molecular weight in grams 

 (in the above case of NaHO, 40 grams) is to be taken only in case of 

 univalent substances, which contain one hydrogen atom in the molecule, 

 while in the case of bivalent substances, which contain two hydrogen 

 atoms, one-half of the molecular weight in grams is taken. If a 

 normal solution of sulphurie acid, H 2 SO 4 (the molecular weight of 

 which is 98), is to be prepared, 49 grams are taken and diluted with 

 enough distilled water to make 1000 c.c. at 16 C. The amounts of 

 the reagents to be taken in the preparation of the few normal solutions 

 used in bacteriologic work are as follows : 



Grams per 

 1000 c.c. 



Crystallized oxalic acid 63.00 



Sulphuric acid 49.00 



Hydrochloric acid 36.37 



Nitric acid 63.00 



Water-free sodium carbonate 53.00 



Sodium hydrate 40.00 



Potassium hydrate 56.00 



The student, however, must not suppose that all of these normal 

 solutions can be obtained by simply weighing out the amount indicated 

 and dissolving it in sufficient water to make 1000 c.c. Several of the 

 above chemicals, sulphuric acid and sodium and potassium hydrate, 

 for instance, are very hygroscopic ; others, like nitric acid and hydro- 

 chloric acid, evaporate and change during manipulation; in fact, the 



