200] Method of Analysis 125 



magnesium chloride mixture (see page 26), stirring vigorously 

 all the while. Allow the beaker to stand two hours, stirring 

 occasionally. At the end of this time decant the liquid off 

 through a filter paper. When as much of the liquid has been 

 removed as possible without getting the precipitate on to the 

 filter, remove the filtrate beaker, and place the one containing 

 the precipitate under the funnel. Wash the filter paper with 

 dilute hydrochloric acid until the washings are sufficient to 

 dissolve the precipitate in the beaker below. Then give the 

 paper one wash with dilute ammonia. Where the ammonia 

 touches the liquid in the beaker it will give a white precipitate. 

 Complete the precipitation by adding strong ammonia until 

 strongly ammoniacal. (The strong ammonia should form a 

 quarter of the whole bulk.) Allow to stand, with frequent 

 stirrings, for half-an-hour, then filter through the same paper as 

 before. Wash, ignite, and weigh exactly as in paragraphs 39 

 and 40. 



200. Corrections for Solubility.— It is unfortunate that 

 phosphoric acid, which is so important a constituent of manures, 

 should be one of the most difficult to estimate accurately. If 

 the above instructions be followed out carefully, the ammonium 

 oxalate and the citric acid being weighed out before adding to 

 the solution, then a result will be obtained which is considerably 

 below the truth. Several circumstances combine to bring this 

 about. In the first place, Mg 2 (NH 4 ) 2 (P0 4 )2 is slightly soluble 

 in water even when it contains a large quantity of ammonia. 

 Ammonium chloride and ammonium oxalate increase the 

 solvent power of the liquid, as also do citrates of iron and 

 aluminium. Ammonium citrate does not seem to have very 

 much influence, except that it makes the precipitation much 

 slower. On the other hand, MgCl 2 decreases the solubility of 

 the precipitate, but should large excess be used various im- 

 purities, including magnesia and magnesium oxalate, occur in 



