32 report— 1877'. 



suits, the sources of error are too numerous to be wholly disregarded. 

 Titration of the precipitate with uranium appears preferable to direct 

 weighing. 



Precipitation ivith " Magnesia Mixture." 



Repeated experiments having shown that the employment of sulphate of 

 magnesium for the precipitation of ammonio-magnesium-phosphate is at- 

 tended with considerable tendency to error, the Committee is of opinion that 

 it should bo definitely abandoned in favour of the chloride. 



The volume of " magnesia mixture " employed for the precipitation should 

 only be in moderate excess of the amount necessary to completely precipitate 

 the phosphate present. 



The use of a large excess of the precipitant causes a more rapid separation 

 of the double phosphate, but is attended with such a serious tendency to 

 error that any advantage gained is more than counterbalanced. The pre- 

 cipitant should be added slowly. 



The precipitation should be conducted in the cold, and solution should 

 not be too concentrated. The proportion of free ammonia in the liquid 

 should be large. The minimum amount of ammonia water should be em- 

 ployed for washing. 



If the above precautions are duly observed, and silica, fluorine, iron, and 

 aluminium be previously removed, it will rarely be necessary to purify the 

 precipitate by solution in acid and reprecipitation with ammonia. In re- 

 precipitating, some "magnesia mixture" should be added, as its presence 

 tonds to reduce the solubility of the precipitate in the ammoniacal liquid. 

 Any correction for solubility of the precipitate should be applied to the 

 ammoniacal washing, and not to the original filtrate. 



In igniting the precipitate the heat should be very gentle at first and 

 afterwards be raised as high as possible. If citric acid has been employed, 

 the ignited precipitate is often discoloured. This may be remedied by 

 cautious treatment in the crucible with strong nitric acid followed by 

 reignition. 



If the precipitate of ammonio-magnesium-phosphato be titrated by 

 standard solution of uranium instead of being weighed, many of the above 

 precautions are rendered superfluous. 



Estimation by Uranium. 



The removal of iron and aluminium by addition of an alkaline acetate in 

 the cold, with determination of the phosphoric acid in the filtrate by means 

 of a standard solution of uranium, is a method which, in the opinion of the 

 Committee, deserves extended employment. The use of an acetate in a 

 slightly acid solution brings the liquid into just the condition required for 

 the use of the uranium process. The proportions of acetic acid and alkaline 

 acetate employed, and the volume of the solution, should bo approximately 

 constant. The uranium nitrate should bo standardized with an acetic-acid 

 solution of pure precipitated ammonio-magnesium-phosphate or tricalcic 

 phosphate, instead of with phosphate of sodium. 



The titration should be converse, the solution of the phosphate being added 

 to that of the uranium. The latter should be mixed with a constant pro- 

 portion of acetic acid, and heated on a bath of boiling water. The indicator 

 should bo powdered potassium ferrocyanide on a white porcelain slab. 

 Owing to the reversal of the usual process, the brown colour of the ferro- 

 cyanide of uranium becomes gradually fainter till the end of the titration. 



