Moody — Iodometric Determination of Aluminium. 181 



Akt. XXII. — The Iodometric Determination of Aluminium 

 in Aluminium Chloride and Aluminium Sulphate ; by 

 S. E. Moody. 



[Contributions from the Kent Chemical Laboratory of Yale Univ. — cxxxviii.] 



A process for the gravimetric determination of alumina in 

 salts of aluminium has been described by Stock,* who bases 

 the method upon the reaction represented by the following 

 equation : 



A US0 4 ) 3 + 5KI + KI0 3 + 3H 2 



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



This equation would show that iodine is liberated when 

 potassium iodate and potassium iodide are together added to 

 a solution of aluminium sulphate. It was found, however, that 

 in the action of the iodide-ioclate mixture upon a solution of 

 potassium alum, only about two-thirds of the iodine corre- 

 sponding to the aluminium salt is accounted for ; and this sug- 

 gests that the reaction is not completed according to the 

 equation, and that the precipitate formed is not the simple 

 hydroxide. Upon ignition the precipitate yields, however, the 

 total amount of alumina present ; and, since the character of 

 the precipitate is good, the process is easily managed and gives, 

 as Stock has said, an excellent gravimetric method for the 

 determination of alumina. 



Taking aluminium chloride, A1C1 3 . 6H 2 0, and proceeding 

 in the same manner, similar results are obtained, and after dis- 

 solving the precipitate in sulphuric acid and adding silver 

 nitrate to the dilute solution, a decided precipitate of silver 

 chloride is observed, which upon washing, drying and weigh- 

 ing is found to be about one-third of the amount of that sub- 

 stance corresponding to the original aluminium chloride. This 

 indicates that it is an oxychloride which is formed on the addi- 

 tion of potassium iodide and iodate ; moreover, upon removing 

 by sodium thiosulphate the iodine first set free in the action 

 and allowing the mixture to stand, progressive hydrolysis takes 

 place as shown by the return of color due to iodine, and this 

 change can be still further hastened by heating after adding 

 an excess of sodium thiosulphate to take up the iodine as lib- 

 erated. The attempt was made, therefore, to complete the 

 reaction between the iodide-iodate mixture and the aluminium 

 chloride, or alum, by heating the solution in a Yoit flask 

 through which steam or, still better, hydrogen was passed, as 

 an aid in the transfer of the iodine liberated to a receiver 

 *Ber. Dtseh. Chem. Ges., 1900, xxxiii, i, p. 548. ■ 



