424 ANALYTICAL CHEMISTRY. 



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 0.024646 gramme, is the equivalent of : 



Gramme. , 



Bromine, Br 0.007936 



Chlorine, Cl 0.003518 



Chromium trioxide, CrO 3 0.003311 



Iodine, I 0.012590 



Iron, Fe, in ferric salts 0.005550 



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 (7.936 grammes per liter) may 

 be liberated when required. The chemical change is this : 

 SNaBr -f NaBrO 3 -f 6HC1 = 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 

 thiosulphate solution. 



The solution is prepared as follows : Dissolve 3.2 grammes of potassium 

 bromate and 50 grammes of potassium bromide in 900 c.c. of water. Of this 

 solution, which is too concentrated, transfer 20 c.c. into a bottle of about 250 c.c., 

 provided with a glass stopper. Next add 75 c.c. of water and 5 c.c. of pure 

 hydrochloric acid, and immediately insert the stopper. Shake the bottle a few 

 times to cause liberation of the bromine, then quickly introduce 1 gramme of 

 potassium iodide, taking care that no bromine vapor escapes. Gradually an 

 equivalent quantity of iodine is liberated from the potassium iodide by the 

 bromine. When this has taken place add, from a burette, deci-normal thiosul- 

 phate solution until the iodine tint is discharged, using toward the end a few 

 drops of starch solution as indicator! Note the number of c.c. of sodium thio- 

 sulphate solution thus consumed, and then dilute the bromine solution so that 

 equal volumes of it and of g sodium thiosulphate solution will exactly corre- 

 spond to each other^ 



The use of bromine solution is directed by the U.S. P. in one case only, viz., 

 for the volumetric determination of phenol (carbolic acid). This substance 

 forms with bromine tribromphenol and hydrobromic acid : 

 C 6 H 5 OH + 6Br = C 6 H 2 Br 3 OH 4- SHBr. 



