282 PHYSIOLOGICAL CHEMISTRY 



chloric acid gas equal to its molecular weight, since this amount, 

 36.46 g., will contain the desired amount of hydrogen. If sul- 

 phuric acid is used, the amount required will be % the molec- 

 ular weight, since the formula is H 2 S0 4 , and if this weight were 

 taken, it would contain twice as much hydrogen as required. A 

 normal sodium hydrate solution is of such strength that it will 

 correspond exactly to a normal acid solution, that is, it will 

 exactly neutralize an equal volume of normal acid. Since one 

 molecule of sodium hydrate will neutralize one molecule of 

 hydrochloric acid, a liter of normal sodium hydrate must con- 

 tain the same number of molecules as a liter of normal hydro- 

 chloric. This result will be obtained if the solution contains an 

 amount equal to the molecular weight of sodium hydroxide, or 40 

 grams, since 40 grams of sodium hydrate will neutralize 36.46 

 grams of hydrochloric acid. If barium hydrate were used Ba 

 (OH) 2 , then as was the case with sulphuric acid, i/2 the molec- 

 ular weight should be contained in a liter of solution if it is to be 

 exactly normal. 



From the amounts required for a normal solution, other 

 strengths such as N/10, N/5, etc., may be calculated. 



The term ' ' normal ' ' also is used in connection with oxidizing 

 and reducing agents. Thus in the estimation of uric acid a 

 N/20 potassium permanganate solution is used. A "normal" 

 reducing solution must yield one gram equivalent of reducing 

 hydrogen per liter, and a normal oxidizing solution must furnish 

 per liter enough oxygen to oxidize this amount of hydrogen. A 

 normal potassium permanganate solution contains a weight in 

 grams per liter equivalent to 1/5 the molecular weight. 



(a) PREPARATION OF STANDARD ACID AND ALKALI. In order 

 to prepare an acid or alkali of known strength it is necessary to 

 start from a substance of known composition and purity. Var- 

 ious methods are used for standardizing, the two most frequently 

 used being the oxalic acid, and the sodium carbonate methods. 



(1) Oxalic acid method. Oxalic acid combines with varying 

 amounts of water of crystallization. By drying the oxalic acid a 

 day or two in a desiccator over sulphuric acid of specific gravity 



