METHODS FOR QUANTITATIVE DETERMINATIONS. 

 Sodium thiosulphate (Hyposulphite). From the equation : 



2Na. 2 S 2 O 3 .5H 2 O + 21 Na 2 S 4 O 6 + 10II 2 O + 2NaI. 



2X246.46 2X125.9 



we see that 246.46 grammes of crystallized sodium thiosulphate are 

 equivalent to e., will exactly decolorize 125.90 grammes of iodine ; 

 hence, to make a f solution of this compound, 246.46 grammes must 

 be taken in a liter, and for a ^ solution 24.646 grammes are used. 

 If the salt should not be absolutely pure, a somewhat larger quan- 

 tity (30 grammes) should be dissolved in 1000 c.c. of water, and 

 this solution titrated with deci-normal solution of iodine and diluted 

 with a sufficient quantity of water to obtain the deci-normal solu- 

 tion. 



If a decinormal iodine solution is not at hand, and perfectly pure sodium 

 thiosulphate cannot be obtained, the method adopted by the U. S. P. may be 

 followed. 30 grammes of the ordinary thiosulphate are dissolved and made up 

 to 1000 c.c. To a solution of about 1 gramme of potassium iodide in 10 c.c. of 

 dilute sulphuric acid in a flask, 20 c.c. of decinormal potassium dichromate 

 solution are slowly added from a burette, and the solution shaken after each 

 addition. The flask is then covered with a watch-glass and allowed to stand 5 

 minutes, after which about 250 c.c. of pure water are added, and the thiosul- 

 phate solution dropped in from a burette slowly, with constant shaking, until 

 most of the iodine is decolorized. Finally, a little starch solution is added and, 

 cautiously, more thiosulphate until the blue color changes to a light green. 

 After noting the volume used, the thiosulphate is diluted so that it is exactly 

 equivalent to the deci-normal dichromate solution. 



Potassium dichromate can be obtained pure, and the decinormal solution 

 easily made by weighing the exact quantity needed. The solution, moreover, 

 is perfectly stable. 



The article to be tested, containing free iodine, either in itself or 

 after the addition of potassium iodide, is treated with this solution 

 until the color of iodine is nearly discharged, when a little starch 

 liquor is added, and the addition of the solution continued until the 

 blue color has just disappeared. 



The titration of iron in ferric salts by thiosulphate is based on 

 the liberation of iodine from potassium iodide by all ferric salts : 



2FeCl 3 + 2KI = 2FeCl 2 + 2KC1 + 21. 



The reaction shown in the above equation requires a temperature 

 of 40 to 50 C. (104 to 122 F.), and at least half an hour's time 

 to make sure of its completion. The digestion should be performed 



