AESENICALS. 7 
They have essentially the same composition except for the presence 
of water in the paste. 
CALCIUM ARSENATES. 
It is not known who made the first sample of calcium arsenate. 
Pickering (31) in 1907 stated that calcium arsenate had already been 
used in the United States as an insecticide. He gave the proportions 
of a calcium salt and an arsenate to be united in preparing calcium 
arsenate, recommending the use of an excess of lime in order to pro- 
duce a calcium arsenate with all the arsenic precipitated and there- 
fore containing no appreciable amount of water-soluble arsenic. 
As many of the early commercial samples of calcium arsenate 
contained excessive amounts of water-soluble arsenic, frequent 
scorching of foliage resulted from its use, thus retarding its general 
introduction. Since 1907, many experiments to devise a method 
for making a commercial calcium arsenate have been performed. 
It is now being produced by many American manufacturers and its 
sale is constantly increasing. The quality of the commercial pro- 
duct has been much improved during the past few years, but its 
course of manufacture has not yet been standardized as has that of 
lead arsenate. 
Dicalcium arsenate (CaHAs0 4 (H 2 0)) contains theoretically 28.3 
per cent of calcium oxid and 58 per cent of arsenic oxid. It breaks 
down easily in water, yielding a large quantity of water-soluble 
arsenic and is not suitable for commercial spraying purposes. 
Calcium meta-arsenate (Ca(As0 3 ) 2 ) was prepared according to 
directions obtained from C. M. Smith, of the insecticide and fungicide 
laboratory. Because of its extreme insolubility, it can not be used 
for insecticidal purposes. 
All the commercial calcium arsenates are made more basic than 
tricalcium arsenate; that is, the molecular ratio of calcium oxid to 
arsenic oxid is 4 to 1, rather than 3 to 1. The additional lime is 
used in their manufacture in order to produce compounds relatively 
free from water-soluble arsenic. 
The following simple method of preparing calcium arsenate com- 
mercially, as outlined by Haywood and Smith (18), calls for the 
direct mixing of calcium hydroxid and arsenic acid, the only 
by-product being water: Slake the lime to a smooth paste by using 
from 3 to 3 J times as much warm water (by weight) as lime, and 
allow it to stand until the lime is completely slaked. Then mix it, 
add the cold arsenic-acid solution at room temperature as rapidly 
as possible, and stir the mixture well until the liquid becomes alka- 
line to phenolphthalein. Lastly, filter, dry, and grind the resulting 
compound. 
The lime and arsenic acid should be mixed in such proportion 
that the actual weight of calcium oxid used will be equivalent to 
that of the arsenic oxid employed. This method produces a reason- 
ably light (bulky) material, which is easily pulverized. The finished 
product should contain approximately 44 per cent of calcium oxid, 
from 40 to 42 per cent of arsenic oxid, and from 14 to 16 per cent of 
water and impurities, which approaches the ratio, 4 CaO: 1 As 2 5 . 
The excess of lime is used to keep any soluble calcium arsenate from 
remaining in the product. 
