METHODS FOR QUANTITATIVE DETERMINATIONS. 407 



tion is excluded whenever the test solution is chemically affected by 

 rubber, as in the case of solutions of silver, permanganate, and a few 

 other substances. For such solutions p IQ 66 



Mohr*s burette with glass stopcock, or 

 Gay Lussac's burette (Fig. 66) is generally 

 used. 



Standard solutions are solutions con- 

 taining a known and definite quantity of 

 some reagent employed in volumetric 

 analysis. A standard solution may be 

 normal, or it may be an empirical solu- 

 tion. In the latter case it contains in 

 a liter some arbitrarily chosen weight of 

 reagent. As an instance may be men- 

 tioned Fehling's solution, used for the 

 determination of sugar. This solution 

 is so adjusted that 1 c.c. decomposes or 

 indicates 0.005 gm. of grape-sugar. 



Normal solutions. The solutions gen- 

 erally used in volumetric analysis are 

 known as normal solutions, and are 

 chemically equivalent to each other be- 

 cause of the standard adopted in their 

 preparation. This standard is one gm. 

 of hydrogen, or the weight of one atom 

 of hydrogen expressed in grammes, or 

 the chemical equivalent of one gm, of 

 hydrogen, contained in one liter of solu- 

 tion. For the sake of convenience the terms 

 gram-atom and gram-molecule are often 

 used in connection with volumetric work, Gay Lussac's burette. 

 and refer, of course, to the atomic or molecular weight of the substance 

 considered, expressed in grammes. 



A consideration of the application of these principles to practical work is 

 volumetric analysis may assist the student in understanding them fully. 



Thus, a normal add solution may be defined as one containing in a liter 

 as much acid as contains one gram-atom of replaceable hydrogen. In such 

 acids as HC1, HBr, HN0 3 , a liter solution containing the gram-molecule would 

 be normal, since each solution would contain one gram-atom of replaceable 

 hydrogen. In order to make a normal solution of such acids as H 2 SO 4 , H 2 C 2 O 4 , 



