2 BULLETIISr 785, U. S. DEPAKTMEN'T OF AGRICULTUEE. 



tion than the standard spray calendar provides. It is difficult to 

 estimate the effects of the complicated set of local factors that deter- 

 mine the effective period of any spray appUcation. A suitable 

 chemical test of the spray coating would seem to be a desirable aid 

 in forming judgment as to the proper time for renewal. Such test 

 should be simple, rapid, and reasonably accurate. The more refined 

 methods of chemical analysis, while very exact, would not well serve 

 the practical purpose, because of the special requirements in profes- 

 sional training, apparatus, and time. 



A PRACTICAL METHOD OF FIELD TESTING. 



The following method has been used during two seasons and has 

 been found to be reasonably satisfactory in field practice. Tliis is 

 indicated by the consistent results obtained with varying strengths 

 of copper sprays, many of which have been checked by exact chem- 

 ical analyses of duplicate samples. 



A 200-gre>m fresh weight sample of leaves is washed at least three 

 minutes, with occasional stirring, in 1,000 c. c. of a 0.2 per cent 

 solution of chemically pure nitric acid in water, either distilled or 

 sufficiently pure to give correct results. A convenient amount of 

 this acidulated wash water is treated with a few drops of 2 per cent 

 solution of potassium-ferrocyanid solution, sufficient to precipitate 

 the copper. A color comparison is then made with a series of known 

 dilutions of a standard copper solution, similarly treated with the 

 potassium-ferrocyanid solution. From this comparison the copper 

 content of the wash water is determined. For more exact readings 

 a colorimeter may be used. 



The standard copper solution is inade by dissolving 3.928 grams of 

 pure crystals of copper sulphate in water to make 1,000 c. c. Each 

 cubic centimeter of such standard solution would then contain 1 

 milligram of copper, and by proper dilution with water stock solutions 

 may be readily prepared for the comparison series containing one-haK 

 to 10 mg. of copper per 100 c. c. of water, in one-haK or 1 mg. grades. 

 If the wash water gives readings above 8 or 10 mg. of copper per 100 

 c. c. it should be diluted for accurate color comparison and account of 

 this taken in figuring results. On the basis indicated above, any 

 direct reading multiplied by 5 will express the amount of copper in 

 the spray residue in milligrams per 100 grams fresh weight of leaves, 

 a convenient unit for tabulation purposes. The solutions of potassium 

 ferrocyanid and of nitric acid may be varied in strength for con- 

 siderable differences in the copper content of the spray coating, so as 

 to avoid using unnecessarily strong solutions, with consequent re- 

 duction in the delicacy of the test. There must always be a sufficient 

 amount of nitric acid to dissolve completely the copper compounds 

 and of potassium ferrocyanid to precipitate them completely. 



