THE MONTHLY BULLETIN. 135 



SOLVENTS FOR ARSENIC. 



"White arsenic is not very soluble in water, although sufficiently so 

 to prohibit its use on plants as an insecticide. In order to make a 

 strong solution of arsenic, it is customary to dissolve white arsenic in 

 water with either sal soda, soda ash, caustic soda, or concentrated lye. 

 Similar potash or amuionia compounds could also be used, but they are 

 more expensive and would be no better than the soda compounds, 

 except for special purposes. 



The following table may be of use in indicating the approximate 

 weights of the commoner solvents required for the solution of white 

 arsenic (arsenic trioxide) : 



Solvent 



Solvent parts 

 by weight 



Arsenic tri- 

 oxide parts 

 by weight 



Sal soda or washing soda (Na2CO3.10H2O) (crystallized sodium 

 carbonate) 



Soda ash (XazCOs and impurities) (crude sodium carbonate) 



Caustic soda (NaOH) (sodium hydroxide). 



Concentrated lye (mixture of NaOH and NasCOs) (may also contain 

 similar potash compounds) 



If sal soda or soda ash is used, it is necessary to boil the mixture 

 to dissolve the arsenic. If caustic soda is used, little or no boiling is 

 required. In either case, however, a corrosive chemical is formed, 

 known as sodium arsenite (or arsenite of soda), which is readily 

 soluble in water and is even more poisonous than the original white 

 arsenic. 



COMMERCIAL SODIUM ARSENITE. 



Sodium arsenite may be purchased ready made as a white powder. 

 The chemistry of arsenites is rather complex, and there are many 

 recognized forms of sodium arsenite, among which may be mentioned 

 by way of illustration, the ortho, meta, and pyro, and both neutral and 

 acid types, each one normally containing a different percentage of 

 arsenic. The commercial article frequently has a large excess of 

 sodium carbonate and other impurities. So far as known, the poison- 

 ous quality of a soluble arsenite is dependent solely upon the amount 

 of arsenic which it contains, the soda, potash, or ammonia serving only 

 as a solvent for the arsenic. Quite variable results are, therefore, to 

 be expected from the use of commercial sodium arsenite. It can not 

 be recommended as a dependable material on account of the great 

 variation in the percentage of arsenic as found by analyses made in 

 the insecticide and fungicide laboratory. It appears, however, that the 

 purity of the white arsenic of commerce can be depended upon. Many 

 samples have been analyzed and in no case was a sample found to be 

 much less than 99 per cent pure. It is recommended, therefore, that 

 the soluble arsenicals be made up with white arsenic whose purity may 

 be depended upon with reasonable certainty, or that sodium arsenite 

 be purchased from a reliable firm with a definite guarantee of the 

 amount of arsenic which it contains. 



*A soluble arsenical can also be made by using one part of caustic soda to four 

 parts of arsenic trioxide. 



