28 Estimations occurring in Agricultural Analysis [40-42 



should appear. If this test shows that the precipitate is not yet 

 free from chlorides, it should receive further washing. 



40. Ignition and Weighing.— After the filter has been 

 dried (paragraph 16), transfer as much as possible to a platinum 

 dish. Burn the filter paper completely in the platinum spiral 

 (see fig. 19). Place the ash in the dish, and remove it to an 

 Argand which is turned down to the smallest possible flame. 

 The flame is increased gradually until, in about forty minutes, 

 the dish is at a dull red heat. Now watch it until a bright red 

 glow passes suddenly over the precipitate and dies away again. 

 Turn up the Argand until the dish is as hot as possible, and 

 ignite for about five minutes. Cool in the desiccator and weigh. 



41. Calculation. — One molecule of Mg 2 P 2 7 corresponds 

 to one of P 2 5 , hence the quantity of P 2 5 in the quantity of 

 Na 2 HPO i .i2H 2 taken may be calculated, and from this the 

 percentage. The factor for converting Mg 2 P 2 7 into P 2 5 is 

 •64: Thus : 



Wt. of Mg 2 P 2 7 x 100 



Wt. of Na 2 HP0 4 .i2H 2 X * 64 ~ % of F ' 2 ° h - 



N.B. If this exercise has been carefully worked, the percentage of 

 P 2 5 found will be lower than that required by theory, as the magnesium 

 ammonium salt is slightly soluble in dilute ammonia. For further dis- 

 cussion of this error see paragraph 200. 



ESTIMATION OF CALCIUM 



42. Substance used. — Calcium carbonate, CaC0 3 . 



Method employed. — The calcium is precipitated as 

 calcium oxalate, and either heated gently and weighed as 

 CaC0 3 , or ignited strongly and weighed as CaO. 



Weigh out about *5 gram powdered Iceland spar, wash into 

 a 1 2-oz. beaker, and cover with a clock glass. Insert the jet of a 



