Moody — Iodometric Determination of Basic Alumina. 483 



Art. XLI. — The Iodometric Determination of Basic Alu- 

 mina and of Free Acid in Aluminium Sulphate and 

 Alums ; by Seth E. Moody. 



[Contributions from the Kent Chemical Laboratory of Yale University — cli.] 



Ix the exact analysis of aluminium, sulphate and commercial 

 alums the determination of the total alumina (soluble) and of 

 the free- acid (the sulphuric acid actually free or present 

 in an acid sulphate in excess of that amount needed to form 

 neutral salts with the bases found) or of the " basic alumina " 

 (that amount of base reckoned as Al a 3 , left over after all sul- 

 phuric acid has been combined with the bases found to form 

 neutral salts) is of importance. 



This paper is an account of an attempt to apply an iodomet- 

 ric process to the determination of the total alumina and the 

 free acid and basic alumina in aluminium sulphate and alum. 



It is well known that sulphuric acid reacts immediately 

 with potassium iodide and potassium iodate in mixture, w T ith 

 liberation of iodine in definite amount, according to the equa- 

 tion, 



3H 2 S0 4 + 5KI + KI0 3 = 3K 2 S0 4 + 3H 2 + 61 



The determination of the free iodine by titration with so- 

 dium thiosulphate gives a measure of the sulphuric acid entering 

 into the reaction. 



It has been shown in previous papers* that various sulphates 

 undergo hydrolysis when boiled with water, and that in pres- 

 ence of the iodide-iodate mixture iodine is liberated propor- 

 tionately to the sulphuric acid thus formed. Aluminium 

 sulphate undergoes complete hydrolysis when boiled with the 

 iodide-iodate mixture for a sufficient length of time, and, as 

 Stockf has shown, the aluminium hydroxide precipitated in the 

 process is especially adapted to filtration and gravimetric deter- 

 mination. 



The decomposition of an aluminium sulphate by this hydro- 

 xy tic process in presence of the iodide-iodate mixture makes 

 possible the gravimetric estimation of the total alumina and 

 the iodometric estimation of the sulphuric acid formed in the 

 hydrolysis. From the results thus obtained it is easy to calcu- 

 late the basic alumina (the amount of alumina in excess of 

 that required to form the neutral sulphate) or the free acid 

 (the sulphuric acid, free or in an acidic sulphate, in excess of 

 that needed to form the neutral sulphate), as the case may be. 



*This Jour, xx, p. 181, 1905. 



fBsr. Dtsch. Chem. Ges., xxxiii, i, p. 548, 1900. 



