INTERNAL INSECTICIDES 285 



water, is used in the preparation of the spray from acid 

 arsenate, and when, furthermore, heavy dews or fogs keep 

 the foliage soaked with moisture part of the time, and so 

 dissolve enough arsenic to injure plant tissue. Rain would 

 wash off the dissolved arsenic and cause no harm. It is 

 possible to purchase lead arsenate at present which is guaran- 

 teed to be the neutral arsenate. 



(c) Paris Green, Schweinfurth Green, is made by 

 boiling arsenous oxide, As 2 3 , with basic copper acetate, 

 Cu(C2H 3 2 )2.CuO. The brilliant green precipitate is used 

 as a suspension in spraying. Its chemical name is copper 

 aceto-metarsenite, [Cu(As0 2 )2]3.Cu(C 2 H 3 02)2. The arsenic 

 content is the active poison for insects. 



Leaf burning is very apt to result from the use of Paris 

 green, due to water soluble arsenic. The national Insecticide 

 Act of 1910 prohibits the sale of Paris green containing 

 more than 3.5 per cent, water soluble As 2 3 . 1 In addition 

 to this, however, the action of water and carbon dioxide 

 will dissolve arsenic according to the following equation: 



[Cu(As02)2]3.Cu(C 2 H302) 2 + IOH2O+ C0 2 = 

 6H 3 As0 3 + CuC0 3 .Cu(OH) 2 + Cu(C 2 H 3 2 ) 2 .CuO. 



This danger is so great that it is usually customary to mix 

 Paris green with lime or Bordeaux, the latter containing 

 an excess of lime. The free arsenic is neutralized bv the 

 lime, thus: 



2H 3 As03+ 3Ca(OH)2 = Ca3(As0 3 ) 2 + 6H2O. 



Sometimes Paris green is adulterated with worthless 

 articles like gypsum. It is easy to test for such adultera- 

 tion by treating the material with strong ammonia. Paris 

 green dissolves according to the following equation: 



[Cu(As0 2 )2]3.Cu(C 2 H30 2 )2+ 36NH40H = 

 4[Cu.(NH 3 )4.(OH) 2 ] + 6(NH4) 3 As0 3 + 2NH4C2H3O2 + 22H:0. 



1 See footnote, page 284. 



