Apr. 29. i9i8 The Calcium Arsenates 283 



hydrogen-arsenate solution, the solution being constantly stirred. A 

 heavy voluminous precipitate formed immediately and slowly settled; 

 the clear supernatant liquid was decanted ofif. It was further 

 washed by decantation several times and finally was brought upon a 

 Buchner filter and washed with hot distilled water until free of chlorids. 

 The pure white amorphous powder was then dried at 100° C. 



The combined washings were evaporated down to a small volume, 

 when crystals of calcium hydrogen arsenate separated out. After 

 removing the mother liquor by suction, the crystals were washed several 

 times with small quantities of boiling water and dried at 100°. 



Analyses made of the samples thus obtained are as follows : 



Total 

 Powder: percentage. 



Calcium as CaO 28. 14 



Arsenic as arsenic pentoxid 57- 91 



Crystals: 



Calcium as CaO 28. 40 



Arsenic as AsjOj 58-15 



Theoretical composition of CaHAs04.H20: ' 



Calcium as CaO 28. 24 



Arsenic as AsjOj 58. 1 1 



Comparing the results with the theoretical composition of 

 CaHAsO^.HjO as noted above, we conclude that both samples are pure 

 calcium hydrogen arsenate, containing one molecule of water of crystal- 

 lization. 



When these samples were further dried at 175'^ C, both lost their 

 water of crystallization, as shown by the following analytical results: 



Total 

 Powder: percentage. 



Calcium as CaO 31-10 



Arsenic as As^Oj 63. 80 



Crystals: 



Calcium as CaO 31-20 



Arsenic as AsjOg 63. 83 



Theoretical composition of CaHAsO^: 



Calcium as CaO 3^^- '^3 



Arsenic as As^Oj 63. 86 



A comparison of these results with the theoretical composition of 

 anhydrous calcium hydrogen arsenate shows each of the samples to be 

 a pure calcium hydrogen arsenate. 



When the heating is continued at 230°, the tendency to form the 

 pyroarsenate (Caj^SsOy) is observed as shown by the analysis: 



Total 

 percentage. 



Calcium as CaO 32- 01 



Arsenic as AsoO^ 65-78 



