266 ANALYTICAL CHEMISTRY. 



what larger quantity (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 solution. 



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 hyposulphite is based on 

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

 Fe 2 Cl 6 -f 2KI = 2FeCl 2 -f 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 

 in a closed flask. If iron be present in combination with organic 

 acids, the addition of some hydrochloric acid becomes necessary. 

 Before titration the solution is allowed to cool, and the titration 

 should be promptly finished, as otherwise errors by re-oxidation of 

 the ferrous salt may be made. 



One c.c. of deci-normal solution of sodium thiosulphate, containing 

 of the crystallized salt 0.024764 gramme, is the equivalent of: 



Gramme. 



Bromine, Br 0.007976 



Chlorine, Cl 0.003537 



Iodine, I 0.012653 



Iron, Fe, in ferric salts 0.005588 



Deci-normal bromine solution (Koppeschaar's solution). The 

 great volatility of bromine, even from aqueous solutions, interferes 

 very much with the stability of volumetric solutions. For this 

 reason a solution is prepared which does not contain free bromine, 

 but an alkali bromide and bromate, from which, by addition of an 

 acid, a definite quantity of bromine (7.976 grammes per liter) may 

 be liberated when required. The chemical change is this : 

 5NaBr + NaBrO 3 + HC1 = 6NaCl -f- 3H 2 O -f- 6Br. 



As the bromine salts are rarely chemically pure, a solution is made 

 which is stronger than necessary and is then adjusted to the titer of 

 hyposulphite solution. 



The solution is prepared as follows : Dissolve 3 grammes of sodium 

 bromate, and 50 grammes of sodium bromide (or 3.2 and 50 grammes 



