EFFECT OF HCN GAS AFTER COPPER FUNGICIDES 13 



Indicating the influence of excess base as a contributing factor In the production 

 -of soluble copper. Injury occurred only when lime was added to the basic copper 

 sulfates, and the amount of soluble copper found increased in proportion to the 

 amount of lime added. Corona 18 per cent copper carbonate with gas gave positive 

 tests for copper in leaf residues in two instances and none in the third, but no 

 foliage injury resulted in either of the cases where soluble copper was found. 



There is sufficient evidence here to affirm that the fact that a copper spray is 

 neutral or approximately neutral in laboratory preparation is no insurance 

 against the production of soluble copper on the leaf and consequent injury. 

 Factory-prepared basic copper sulfates and carbonates are safe, apparently 

 because of their greater stability. 



On Glass 



In the previous section it was shown that the copper residue on the sprayed 

 leaf gives rise to a soluble copper salt in the presence of HCN. This may indicate 

 the participation of the plant itself in the reaction, for according to Crandall 

 (2), Holland, Dunbar and Gilligan (8), and Pickering (12), complex soluble cupric 

 compounds of copper may be produced from copper fungicides by alkali and 

 exudation products of the plant, or by osmosis of the cell contents such as sugars, 

 hydroxy-organic acids, and biuret compounds. It is also possible that certain 

 inconsistencies in the tests for soluble copper in the residues on leaves may be 

 correlated with leaf action, although other factors such as dosage, period of ex- 

 posure, atmospheric conditions, and any narrow differences arising in the com- 

 pounding of the sprays are also to be considered as contributing factors. As a 

 matter of investigation, panes of glass 16 by 30 inches were sprayed with Bor- 

 deaux mixtures of different ratios of lime, ranging from neutral to 4-24-50 mixtures, 

 and allowed to air dry. Exposure to gas was over night. Controls against the 

 action of gas were included in each experiment. The dry residues were removed 

 and examined for color, soluble copper, and HCN. 



A given weight of dry residue, generally 1/10 gram, was macerated in water 

 and tested for soluble copper. Another 1/10 gram of residue was macerated with 

 water and tested for hydrocyanic acid. 



Dry residues of Bordeaux, increasing in ratios of lime from neutral to 4-24-50 

 mixtures exhibited increasing intensities of blue color ranging from pale nile or 

 lumiere blue to pale king's blue, according to the color chart of Ridgway (14). 

 Parallel residues of the same mixtures exposed to gas exhibited colors of rejane 

 and deep lichen green (4-0.8-50 to 4-4-50); glaucous, pale glaucous green, cory- 

 dalis green, and yellowish glaucous (4-4-50 to 4-16-50); and pale dull glaucous 

 blue (4-24-50). Butler and Jenkins (1) obtained brown precipitates on passing 

 hydrocyanic acid gas into toxic Bordeaux mixtures and white precipitates with 

 non-toxic mixtures such as 1 :0.2 and 1 :6. In the experiments of the writers, brown 

 or white residues were never obtained when gas acted on Bordeaux on glass, 

 irrespective of the ratio of the base to copper sulfate. Therefore, if brown pre- 

 cipitates suggest cupric cyanide and white precipitates cuprous cyanide, neither 

 of these compounds was present. Bordeaux mixture increasing in ratios of lime 

 from 4-0.8-50 to 4-24-50 showed soluble copper and HCN in residues on glass 

 after exposure to gas in each of sixteen tests. The" neutral mixtures prepared with 

 milk of lime and chemical hydrated lime were no exception (Table 1). Parallel 

 residues not exposed to gas were in every instance free of soluble copper and 

 HCN. The change in color of the residue on exposure to gas, therefore, is corre- 

 lated with the production of soluble copper, although the color could not be con- 

 sidered a safe guide as to the amount present. Butler and Jenkins contend that 

 Bordeaux 1:0.2 and 1:6 ratios do not give off detectable amounts of cyanogen. 



