388 Mr Sell, On a soluble colloidal form 



On a soluble colloidal form of ferric and of other phosphates. 

 By W. J. Sell, M.A, F.R.S., Christ's College. 



[Read 18 January 1904.] 



It is well known that ferric phosphate is soluble to a small 

 extent in an excess of disodium hydrogen phosphate in presence 

 of ammonia. In the course of some work on the ammonium 

 phosphates it was found that when the corresponding ammonium 

 phosphate was employed in presence of ammonia a larger amount 

 of the ferric phosphate was dissolved and that if the strengths of 

 various solutions were properly adjusted a clear reddish brown 

 solution was obtained. The change may be thus represented : 



n (NH 4 ) 3 P0 4 + FeCl 3 = FeP0 4 + (n-l) (NH 4 ) 3 P0 4 + 3NH 4 C1. 



Now since ferric phosphate as ordinarily produced by pre- 

 cipitation is only known as a jelly-like substance insoluble in 

 water, it seemed possible and desirable to ascertain if from the 

 solution produced as above the compound could be obtained in 

 solution in water by the process of dialysis. It seemed at least 

 probable that ferric phosphate would diffuse with extreme slow- 

 ness while the other ingredients are known to have high rates of 

 diffusion : accordingly the following experiments were carried out 

 with this object. 



A solution of ferric chloride was made by dissolving about a 

 hundred grams of the hydrated salt in 800 c.c. of water, the iron 

 determined in the solution and the strength subsequently adjusted 

 so as to contain 50 grams of the anhydrous salt in one litre. The 

 diammonium hydrogen phosphate contained 100 grams of the 

 crystallised salt in one litre. The ammonia used was the ordinary 

 •880 diluted with an equal volume of water. 



If to 100 c.c. of ferric chloride be now added 400 c.c. of the 

 diammonium hydrogen phosphate and, after shaking, sufficient 

 ammonia to make the whole smell decidedly of that reagent, the 

 large white precipitate turns slightly brown in colour and gradually 

 but completely dissolves to a clear brownish red solution. 



The solution was now dialysed through parchment paper in 

 the ordinary way until every trace of chloride had disappeared. 

 The dialysed solution had a somewhat lighter colour than before 

 dialysis and had to some extent increased in volume by osmosis 

 from the surrounding water. Positive tests having shewn the 



