ALKALIMBTRIC ESTIMATION 163 



tity of substance used be so chosen that it contains not more than 

 50 milligrams of phosphoric acid. If more than 40 cubic centi- 

 meters of the acid mixture have been used in the incineration, then 

 the quantity of water added in the dilution and the quantity of 

 ammonium nitrate therein should be proportionately increased. 



The flask containing the precipitate is vigorously shaken for a 

 half minute, by which process the precipitate becomes more 

 granular, and it is then allowed to stand at rest for 15 min- 

 utes. The filtering and washing are carried on by decantation. 

 Thin, ash-free filter paper is used, which on the subsequent solu- 

 tion of the precipitate in dilute soda-lye is easily torn and the par- 

 ticles of which are evenly divided throughout the liquid. The 

 filter paper, which has a diameter of from five to six centimeters, 

 may be used either as a folded filter or as a smooth filter in a 

 fluted funnel. Before filtration the filter is filled with ice-cold 

 water, in order to draw the filter pores together and to prevent 

 the first portions of the solution, which is still warm, from run- 

 ning through cloudy. In order to conveniently decant the super- 

 natant liquid, the flask is laid upon the ring of the stand some- 

 what higher than the filter and the neck is drawn down in such 

 a way that the clear liquid runs without intermission upon the 

 filter. In this way only a very small quantity of the precipitate 

 is collected upon the filter, which is always kept about two-thirds 

 full. The washing of the precipitate is conducted with about 

 150 cubic centimeters of ice-cold water, which is thoroughly in- 

 corporated with the precipitate, and after standing is poured 

 through the filter in the way described. The decantation is con- 

 tinued with repeated washing with water until the wash-water 

 no longer gives an acid reaction with litmus paper. The washed 

 filter and its contents are then put back in the flask containing the 

 principal part of the precipitate, 150 cubic centimeters of water 

 added, the filter broken up by vigorous shaking, and the precipi- 

 tate dissolved by adding a measured portion of one-half normal 

 potash lye with constant shaking and without warming. It is then 

 advisable to add an excess of from five to six cubic centimeters 

 of one-half normal soda-lye and to boil the solution until no 

 longer any ammonia is evolved, which usually requires about 



