Moody — loclometric Determination of Basic Alumina. 485 



iron thus found deducted from the total weight of the deposit 

 to give the amount of zinc. From the zinc was calculated 

 the zinc oxide. 



The next step was to determine the amount of iodine set 

 free by the action of the iodide-iodate mixture. Of the alum 

 solution a portion of 25 cm3 was drawn from a burette into the 

 Yoit flask of the distillation apparatus, a solution (10 cm3 ) con- 

 taining 0*3 grm. of potassium iodate and 1 grm. of potassium 

 iodide was added, the mixture boiled, and the iodine, collected 

 in the receiver charged with water containing 3 grms. of potas- 

 sium iodide (and acidified with sulphuric acid in case the 

 substance contains ammonia), was titrated with sodium thio- 

 sulpbate. 



Aluminium sulphate undergoes complete hydrolysis, accord- 

 ing to the equation, 



A1 2 (S0 4 ) 3 + 5KI + KI0 3 + 3H 2 



= 2Al(OH) 3 + 3K 2 S0 4 + 6l 



Ferric sulphate, like aluminium sulphate, undergoes complete 

 hydrolysis according to the similar equation, 



F e 2 (S0 4 ) 3 + SKI + KI0 3 + 3H 2 



= 2Fe(OH) 3 + 3K 2 S0 4 + 6l 



Ferrous sulphate likewise undergoes complete hydrolysis, but 

 the ferrous hydroxide first produced is oxidized at the expense 

 of the potassium iodate to form ferric hydroxide. This sec- 

 ondary reaction, however, affects in no way the amount of 

 iodine liberated in consequence of the hydrolysis. 



3FeS0 4 + 5KI + KI0 3 + 3H 2 = 3Fe(OH) 2 + 3K 2 S0 4 + 61 

 6Fe(OH) 2 + K10 3 + 3H 2 = 6Fe(OH) 3 + KI 



Zinc sulphate, on the other hand, does not undergo complete 

 hydrolysis* but the insoluble product formed when a solution 

 containing zinc sulphate and the iodide-iodate mixture is boiled 

 in a one-fifth basic sulphate holding zinc and sulphuric anhy- 

 dride in proportions indicated by the symbol Zn 5 (OH) 8 S0 4 , 

 and the reaction may be expressed by the equation, 



15ZnS0 4 + 2KI + 4KIO, + 12H a O 



= 3Zn 5 (OH) 8 S0 4 + 12K 2 S0 4 + 241 



Ammonium sulphate is hydrolyzed on boiling with the iodide- 

 iodate mixturef according to the equation, 



3(NH 4 ) 2 S0 4 + 5KI + KI0 3 



= 3K 2 S0 4 + 2NH 3 + 6I 



But, as has been said, to collect and correctly measure the 

 iodine, the solution of potassium iodide in the receiver 

 must be acidified with a little sulphuric acid. 

 * This Journal, xxii, 182, 1906. % Ibid. 



