Effect of Copper Compounds 19 



minimum weight of salt, which, dissolved in 100 parts of water, kills 

 the seedling," the results were as follows : 



Toxic equivalent Containing copper 



Copper bromide (CuBr 2 ) -004875 -001387 



Copper chloride (CuCl 2 . 2 aq.) -005000 -001865 



Copper sulphate (CuS0 4 . 5 aq.) -005555 '001415 



Copper acetate (Cu{C 2 H 3 02} 2 .aq.) -005714 -001820 



Copper nitrate (Cu{N0 3 } 2 . 6aq.) -006102 -001312 



These numbers appear to be very close, so Coupin considered that it 

 might be permissible to regard the differences as due to the impurities 

 in the salts, and to the water of crystallisation which may falsify the 

 weights, so that under these conditions one may believe that all these 

 salts have the same toxicity. This is considerable, and is evidently due 

 to the copper ion, the electro-negative ion not intervening with such 

 a feeble dose. A recalculation of these toxic equivalents to determine 

 the actual amount of copper present in each, gives results that are fairly 

 approximate, but it is difficult to accept this hypothesis in view of other 

 work in which different salts of the same poison are proved to differ 

 greatly in their action on plant growth. 



Kahlenberg and True (1896), working with Lupinus albus, found 

 that the various copper salts, as sulphate, chloride and acetate, were 

 similar in their action upon the roots. Plants placed in solutions of 

 these salts of varying strengths for 15 24 hours showed that in each 

 case 1/25,600 gram molecule killed the root, while with 1/51,200 gram 

 molecule the root was just alive. These workers discuss their results 

 from the standpoint of electrolytic dissociation, and concur in the opinion 

 that the positive ions of the toxic salt are exceedingly poisonous. 



The toxicity of the positive ion was again set forth by Copeland and 

 Kahlenberg (1900). Their water culture experiments were carried on 

 in glass vessels coated internally with paraffin to avoid solution of glass, 

 and in tests with seedlings of maize, lupins, oats and soy beans it was 

 found that such metals as copper, iron, zinc and arsenic were almost 

 always fatal to the growth of plants. As a general rule those metals 

 whose salts are toxic, themselves poison plants when they are present in 

 water. The assumption made was that the injury to plants when 

 cultivated in the presence of pure metals depends on the tendency of 

 the metal to go into solution as a component of chemical compounds 

 and on the specific toxicity of the metallic ion when in solution. 



22 



