254 ANALYTICAL CHEMISTRY. 



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 

 Mohr's burette with glass stopcock, or 

 Gay Ltissac's burette (Fig. 35) is generally 

 used. 



Standard solutions. The test solu- 

 tions used in volumetric analysis are ad- 

 justed according to a uniform system, so 

 that each solution contains in a liter 

 (1000 c.c.) the weight of one atom or one 

 molecule of the active reagent expressed 

 in grammes. This rule refers to all cases 

 of univalent elements (Ag, Cl, I), or 

 monobasic acids (HC1, HNO 3 ), or monacid 

 bases (KOH,NH 4 OH). In case a bivalent 

 element (O, S), or di-basic acids (H 2 SO 4 , 

 H 2 C 2 O 4 ), or diacid bases [Ca(OH) 2 ], are 

 used in volumetric solutions, only one- 

 half of the atomic or molecular weight in 

 grammes is used per liter, so as to have the 

 saturating or neutralizing power the same 

 for an equal number of cubic centimeters 

 of univalent and bivalent substances. 



To illustrate why this is done, if we 

 were to take the molecular weight of 

 hydrochloric acid, 36.4, and of sulphuric 

 acid, 98, in grammes, diluted to 1000 c.c., 

 the saturating power of 1 c.c. of the 

 diluted sulphuric acid would be equal to 

 that of 2 c.c. of hydrochloric acid solution^ 

 because 36.4 parts by weight of hydro- 

 chloric acid saturate 40 parts by weight of sodium hydroxide, and 

 98 parts by weight of sulphuric acid saturate 80 parts by weight of 

 sodium hydroxide. 



The solutions thus obtained are known as normal solutions. For 

 some operations these normal solutions are too concentrated, and are 

 diluted to one-tenth of their strength, and are then called deci- 

 normal solutions. 



