﻿ARSBNICALS. 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 propori Lons 

 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^ 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,0 5 . 

 The excess of lime is used to keep any soluble calcium arsenate from 

 remaining in the product. 



