STANDARDIZATION OF OXIDIZING SOLUTIONS 19 



solution. The water is therefore boiled and kept in bottles closed 

 by a soda lime tube. It is drawn off by a syphon. 



Standardization. The standardizations are carried on in this 

 order. 



KMn0 4 against N/10 oxalic acid. 



Na 2 S 2 O 3 against N/10 KMnO 4 . 

 Iodine against N/10 Na 2 S 2 C>3. 



(a) Potassium Permanganate. Measure out 25 c.c. N/ 10 oxalic 

 acid in a beaker and add 10 c.c. of diluted H2S04 (1:4). Then 

 dilute with 200 c.c. water at 70. The heat aids the destruction 

 of the oxalic acid. But a higher temperature may cause a volatili- 

 zation of the oxalic acid. Run in the permanganate solution from 

 a burette until the first drop in excess produces a permanent pink. 



5H 2 C 2 4 +2KMn0 4 -|-3H 2 S0 4 



= K 2 SO 4 +2MnSO 4 +10C0 2 +8H 2 O, 

 or 



5H 2 C 2 O 4 +5O = 10C0 2 +5H 2 O. 



From these equations it is apparent that 1 molecule of oxalic 

 acid is equivalent to 1 atom of oxygen. A normal solution of 

 oxalic acid is therefore equivalent to a normal solution of KMnO 4 , 

 hence the calculation is c.c. for c.c. From the calculated strength 

 figure out the amount of dilution necessary to make an exactly 

 N/10 KMnO 4 solution. 



(6) Sodium Thiosulphate Solution. Place 1 gm. of KI in a 

 beaker and dissolve it in 8 c.c. of water. To this add 5 c.c. HC1 

 solution (1 :5) and 25 c.c. of N/10 KMnO 4 . Dilute to 200 c.c. 

 The KMnO 4 oxidizes the HC1 to release chlorine which replaces 

 the Iodine in the KI. 



2KMnO 4 + 16HC1 = 2KCl+2MnCl 2 +3H 2 O+ 10C1. 

 10C1+ 10KI = 10KC1+ 101. 



Run in the thiosulphate solution from a burette until the 

 iodine color becomes pale yellow. Then add 2 c.c. of starch solu- 

 tion, The first drop that causes the blue color to disappear 



