t!8 GENERAL METHODS 



is better therefore to make up a stronger solution than the one 

 planned and to let it stand for some two weeks until all the impuri- 

 ties have been oxidized, and then to standardize it against a stable 

 reagent, oxalic acid for example. However, when in a hurry 

 the solution can be standardized at once if the following is observed: 

 The glassware should be cleaned in dichromate cleaning mixture. 

 The water should be boiled with a few crystals of KMnO4 in it, to 

 oxidize the organic matter. After the solution is finally made 

 up, it should be filtered through asbestos that has been previously 

 treated with aqua regia to remove the organic matter. This 

 filtration removes the insoluble MnC>4. 



Weigh out 1.75 gms. KMnO 4 (for \ liter of N/10 solution) and 

 dissolve in the specially prepared water. Then filter the solution 

 through the specially prepared asbestos and dilute to the mark. 

 This solution is ready for standardization against N/10 oxalic acid. 



(6) Iodine. It is impracticable to weight out iodine accurately 

 because it sublimes so readily. It is therefore dissolved in strong 

 KI solution, in excess of the theoretical amount, diluted to volume 

 and then standardized against sodium thiosulphate solution. 



Weigh out 7 gms. Iodine (for \ liter of N/10 solution). Dis- 

 solve 10 gms. KI in 5 c.c. of water (no more) and then dissolve the 

 iodine in the concentrated KI solution. Use only glass stoppers 

 for the bottles. 



(c) Oxalic Acid. KMn(>4 is standardized against oxalic acid 

 as has been said. It will be necessary therefore to prepare an 

 N/10 solution. This can readily be made by diluting the N/2 

 oxalic acid solution that you have already prepared. 



(d) Sodium Thiosulphate. An N/10 solution of Na^Os is 

 used to standardize the Iodine solution according to the equation 



2Na 2 S 2 O 3 + 12 = Na 2 S 4 O 6 + 2NaI. 



Since 2 molecules of the thiosulphate is equal to 2 atoms of iodine a 

 normal solution of Na2S 2 O3 is equal to 



2Na 2 S 2 O 3 -5H 2 2(248.20) 

 = -5 - - = 248. 20 gms. per liter. 



i 2i 



Weigh out 13 gms. Na2S 2 O3 and dissolve in 500 c.c. C0 2 free 

 water. The carbonic acid attacks the thiosulphate weakening the 



