FILTRATION AND WASHING 137 



magnesia or the carbonate employed contains some phosphoric 

 acid, which is to be separated by filtration, and the solution can 

 then be preserved indefinitely. 



This solution is made by the formula given by Button as fol- 

 lows: Add 27 grams of pure magnesium carbonate by degrees 

 to a solution of 270 grams of citric acid in 350 cubic centimeters 

 of warm water. When all effervescence has ceased and the liquid 

 is cooled to room temperature, add 400 cubic centimeters of am- 

 monia of about 0.96 specific gravity, containing approximately 

 10 per cent, of NH 3 . The volume is made up to one liter and kept 

 in a well stoppered bottle. 10 



122. Time of Subsidence. When the phosphoric acid is precipi- 

 tated by the mixture above mentioned, it is necessary to allow it 

 to subside for a certain time under a bell jar in order to avoid the 

 evaporation of the ammonia. In order to give plenty of time 

 for this subsidence, it is well to make the precipitations in the 

 afternoon and the filtrations the following morning. There are 

 thus secured from 12 to 15 hours of repose, which is time amply 

 sufficient for all cases. 



123. Filtration and Washing. Filtration is performed easily 

 and rapidly upon a small filter without folds placed in a funnel 

 with a long stem of about two millimeters internal diameter. 

 Placed in a series of six or eight, they allow the filtration to take 

 place in regular order without loss of time, the first filter being al- 

 ways empty by the time the last one is filled. The supernatant 

 liquid from the precipitate should first be decanted on the filter, 

 avoiding the throwing of the filtrate on the filter, which would 

 greatly retard the process, especially if it should contain a little 

 silica, as often happens. 



When the clear liquid is thus decanted as completely as possi- 

 ble, the rest of the precipitate is treated with water to which one- 

 tenth of its volume of ammonia has been added, and the washing 

 is continued by decantation as at first, and afterwards by wash- 

 ing upon the filter until the filtered solution gives no precipitate 

 with sodium phosphate. Four washings are generally sufficient 

 to attain this result. 



10 Sutton, Volumetric Analysis, gth Edition, 1904: 299. 



