166 THOMAS COOKSEY. 



depending on the liberation of iodine from potassic iodide 

 by a ferric salt gives very accurate results. The iodine is 

 estimated by thio-sulphate and one atom of iodine is 

 equivalent to one atom of iron. The method is very simple, 

 quick, and easily carried out, but appears, so far as my 

 knowledge goes, to be very rarely used. The only pre- 

 caution necessary is the avoidance of any oxidising com- 

 pounds or substances liberating iodine. This condition is 

 in most cases easily obtained. The method is equally good 

 in the presence of phosphates. 



A volumetric method for the estimation of alumiua, 

 which I have found accurate, depends on its precipitation 

 as phosphate, and estimation of the amount of the phosphate 

 left in the filtrate by means of the method previously 

 described. If a known quantity of phosphate solution be 

 added, the amount precipitated with the alumina is found 

 from the difference between the amount added and the 

 quantity left in the filtrate, and the amount of aluminium 

 is simply calculated from this loss of phosphate. The 

 orthophosphate AJ 2 (P0 4 ) 2 is formed. This is carried out 

 as follows: — To the slightly acid solution containing 

 alumina, monosodic phosphate is added in known quantity, 

 and a drop of methyl orange. Standard decinormal caustic 

 potash is now run in until the yellow stage is just reached. 

 The whole is warmed to promote the coagulation of the 

 precipitate, cooled, made up to a definite volume, and an 

 aliquot part filtered, the phosphate determined in the latter 

 and therefore in the whole filtrate. The difference between 

 this quantity and the total amount added gives the quantity 

 precipitated as A1 2 (P0 4 ) 2 from which the aluminium is 

 simply calculated. Or, if preferred, the precipitate of 

 aluminium phosphate may be washed and the whole of the 

 filtrate used in the estimation of phosphate by the pre- 

 viously described method. Iron, of course, must not be 



