CALCIUM ARSENATE. 3 



If arsenic acid of any strength is added slowly to lime water con- 

 taining phenolphthalein until the latter is just about to decolorize, a 

 gelatinous, slow-settling precipitate is formed. After the lapse of 

 an hour or so, this crystallizes and settles to about one-eighth of 

 the volume of the liquid used. The precipitate filters very readily, 

 leaving a cake which on the steam bath dries fairly rapidly to very 

 light lumps easily powdered between the fingers. Its extreme light- 

 ness is very noticeable, and it does not require grinding, simply a 

 disintegration of the lumps. One preparation, dried at about 70 °C. 

 on the steam bath, had the following composition : 



L'er cent. 



Calcium oxicl, CaO. 32.00 



Arsenic oxicl, As 2 5 42 . 96 



Water (by difference) (slight amount of C0 2 ) 25.04 



100.00 



The molecular ratio of CaO to As 2 5 is here 3.05, showing the com- 

 pound to be tricalcium arsenate. It approximates most closely 

 to the formula Ca 3 (As0 4 ) £ .SH 2 0. In an investigation on the chem- 

 istry of the calcium arsenates, the results of which will be published 

 later, it has been shown that such a compound exists. Drying at 

 110 °C. results in the loss of about seven of the eight molecules of 

 water of crystallization. The lightness of this material may be 

 judged from the fact that 2 ounces of it filled a graduated cylinder 

 to the 500 cubic centimeter mark, which is equivalent to 240 cubic- 

 inches per pound, this, of course, without any attempt at packing. 

 If jarred for a sufficient time the space occupied will be reduced to 

 about 60 per cent of this figure. 



The compound thus formed yields appreciable amounts of soluble 

 arsenic when treated with water. In one experiment 1.8 grams 

 were treated with 1 liter of water (equivalent in arsenic content to 

 1 pound of dilead arsenate in 50 gallons) for 24 hours, and 55 milli- 

 grams, or 3.04 per cent, of As 2 C 3 were rendered soluble. R. H. 

 Robinson 1 found 79 milligrams of As 2 5 per liter for somewhat 

 similar material. This, of course, is an excessive amount, and 

 would render the material unfit for use alone on any but the 

 hardiest foliage, such as will stand applications of paris green. 



If the manufacture of calcium arsenate by this process were at- 

 tempted commercially, the liquid left from the settling of one batch 

 of arsenate could be used again for the preparation of more lime 

 water, and thus the consumption of water might be greatly reduced. 

 However methods requiring the handling of as little material as 



1 Oregon Agr. Exp. Sta. Bui. 131, p. 



