Section III, 1916 [67] Trans. R.S.C. 
The Occlusion of Iron by the Ammonium Phospho-Molybdate Precipitate. 
By E. H. ARCHIBALD and H. B. KEEGAN 
Presented by Dr. R. F. RUTTAN, F.R.S.C. 
(Read May Meeting, 1916.) 
Apart from any theoretical considerations—which are not without 
considerable interest—the extent of the occlusion of iron by am- 
monium phospho-molybdate, and the conditions which influence the 
amount of iron occluded, must be known before the direct estimation 
of phosphorus can be made from solutions containing iron. 
The chemicals used in this research were carefully selected and 
where necessary were treated for the elimination, as completely as 
possible, of any impurity. 
The weights used were carefully standardized. All pipettes and 
burettes were re-calibrated. 
The analysis of the precipitate involved the determination 
both of the phosphoric acid formed and of the iron carried down by 
the precipitate. For these purposes a modified method of Pemberton’s 
was adopted. In the place of the usual filter for collecting the pre- 
cipitate, a platinum Gooch crucible—carefully selected with the object 
of having the perforations somewhat smaller than usual and as regular 
in size as possible—was used to great advantage. 
For dilute solutions the amount of iron occluded is independent of 
the concentration of the iron in the solution. For the more concen-. 
trated solutions, the amount of iron carried down increases with the 
concentration of the iron. In the case of a precipitate containing 
0-0314 grams of P.O; the iron occluded amounts to -0050 grams. 
For solutions containing the same amount of iron but different 
amounts of phosphoric acid the amount of iron occluded increases 
somewhat more rapidly than the concentration of the phosphoric 
acid increases. 
With solutions containing equal amounts of iron and of phos- 
phoric acid, increasing the volume before precipitation did not di- 
minish, appreciably, the occlusion of iron; which would tend to show 
that it is not mechanical inclusion that accounts for the carrying down 
of the iron. 
