CALCIUM OF THE BLOOD 161 



point for two or three hours. The heat is then increased and the 

 acid evaporated down to about 0.5 c.c. Sputtering must be guarded 

 against and the contents must not go completely to dryness. (If 

 charring occurs, more acid must be added and evaporation 

 repeated.) The sides of the beaker are washed down with 2 or 

 3 c.c. of water and a drop of phenolsulfonephthalein is added as 

 an indicator. Dilute ammonia is added drop by drop until alkaline. 

 (The indicator changes from an eosin pink to yellow and then 

 to reddish purple, the latter color being the alkaline end-point.) 

 The beaker is then replaced on the stove and heated until the 

 excess of ammonia is removed, as shown by a change in color 

 of the indicator and the fact that only a faint odor of ammonia 

 remains. Unless a very great excess of ammonia has been added, 

 two or three minutes at medium heat is usually sufficient. While 

 still hot 1 c.c. of a solution of 1.25 per cent oxalic acid in 0.25 N 

 hydrochloric acid is added drop by drop and with stirring. The 

 beaker is removed from the stove and when cool 0.5 c.c. of sodium 

 acetate solution (20 per cent) is added slowly and with stirring. 

 (This refers to the anhydrous salt. If crystalline sodium acetate 

 is used the solution should be made up to a strength of 35 per 

 cent.) The solution is allowed to stand over night. This method 

 of precipitation, which is essentially that advised by McCrudden, 

 results in a granular precipitate of calcium oxalate, that can be 

 readily filtered without loss. 



Filtration is carried out on a 10 c.c. Gooch crucible, the mat 

 being especially prepared as follows : A small disk of filter paper 

 is first placed in the bottom of the crucible, asbestos soup is poured 

 on to make a fairly thick mat, another disk of filter paper is laid 

 on and then a little more asbestos, and finally a suspension of 

 purified barium sulfate. This latter serves to make evident any 

 leaks in the crucible and also to close the pores. 



The calcium oxalate precipitate is washed into the crucible 

 and beaker and crucible are washed eight times, each time with 

 approximately 5 c.c. of 1 per cent ammonia (one part concentrated 

 ammonia in 100 of water), then once with 95 per cent alcohol, 

 containing just enough ammonia to be alkaline, and finally once 

 with ether. (Ether which contains acid should be redistilled over 

 sodium hydroxide before using.) The suction is best diminished 

 when the ether is poured on the crucible so as to prevent exces- 

 sively rapid filtration. After all the ether has passed through 



