STANDARDIZATION OF OXIDIZING SOLUTIONS. 17 



terms. The basis selected is oxygen and the strength of all stand- 

 ard solutions is expressed in terms of available oxygen. A normal 

 oxidizing solution is denned as one yielding 8 gms. of available 

 oxygen per liter. Since 8 gms. of oxygen is the equivalent of 1 

 gm. of H, this definition agrees with the general definition of a 

 normal solution (a solution yielding 1 gm. H per liter). 



(a) Potassium Permanganate. As observed above 2 molecules 

 of KMnC>4 yield 5 available atoms of Oxygen (equivalent of 

 10 H). By definition, therefore, a normal solution of KMn(>4 

 contains 



2KMn0 4 2X158.03 

 -^ = - -= -- =31.606 gms. per liter. 



(6) Iodine. As observed above 12 yields 1 atom of Oxygen 

 (equivalent of 2H). Hence by definition a normal solution of 

 Iodine contains 



21 2X126.92 



= -- - - = 126.92 gms. per liter. 



Indicators. It is of course necessary to know when the oxi- 

 dation is complete or in other words when the oxidizing agent has 

 been completely reduced. KMnO4 serves as its own indicator 

 because as it is reduced, its purple color disappears and the first 

 drop added in excess turns the solution a permanent pink. 



Iodine may also serve as its own indicator since its color ranges 

 from deep red to yellow, but a far better end-point is obtained by 

 employing starch as an indicator. As the last trace of iodine is 

 used up the blue of the starch iodine reaction disappears and the 

 solution becomes colorless. 



Preparation of Solutions, (a) Potassium Permanganate. 

 Whereas it is possible to weigh out the KMn04 accurately and 

 dissolve it in the proper amount of water, the practice is a poor one 

 unless certain precautions are observed, for there are several fac- 

 tors that tend to weaken the solution shortly after it is made up. 

 One is the presence of the impurity MnO4 even in the best prep- 

 arations on the market, which reduces the KMnO4 at an increas- 

 ing rate as the solution grows older. Others are the presence of 

 NHs in the purest distilled water, grease in the glassware, tc. It, 



