IV. METHODS FOR THE ANALYSIS OF INSECTICIDES AND 



FUNGICIDES. 



1. General Directions. Provisional. 



Samples of Paris green, London purple, soft soap, copper carbonate, and 

 tobacco extracts should be thoroughly mixed before analysis, taking care in 

 the case of the first two substances that they are not further pulverized. Weigh 

 Lirw quantities of lye and potassium cyanid in weighing bottles and analyze 

 aliquots of the water solutions. 



2. Paris Green. 

 ( a ) MOISTUEE.O PBOVISION AL. 



Dry from 1 to 2 grams at 105 to 110 C., and calculate the loss as moisture, 

 (b) TOTAL AESENIOUS OXID,*> METHOD I (SMITH). OFFICIAL. 



(1) SOLUTIONS BEQUIKED. 



(a) Starch solution. Use a starch solution which is prepared by boiling 2 

 grams of starch with 200 cc of distilled water for about 5 minutes. 



(&) Standard iodin solution. Prepare a standard iodin solution in the follow- 

 ing manner : Dissolve 12.7 grams of powdered iodin in about 250 cc of water 

 to which has been added about 25 grams of chemically pure potassium iodid, 

 and make up the whole to a volume of 2 liters. To standardize this solution, 

 weigh 1 gram of chemically pure dry arsenious oxid, transfer to a 250 cc flask 

 by means of about 100 cc of a solution containing 2 grams of sodium hydroxid 

 in each 100 cc, and boil until all arsenious oxid goes into solution. Make up 

 to a volume of 250 cc and use 50 cc for analysis. Concentrate this portion of 

 50 cc, by boiling in a 250 cc flask, to half its volume and allow to cool to about 

 80 C. Add an equal volume of concentrated hydrochloric acid and 3 grams of 

 potassium iodid, mix, and allow the whole to stand for 10 minutes (to reduce 

 the arsenic oxid formed by boiling the alkaline arsenite to arsenious oxid). 

 Dilute the solution with cold water and add an approximately tenth-normal 

 solution of sodium thiosulphate, drop by drop, until the solution becomes 

 exactly colorless. (The end point is easy to read without the aid of starch.) 

 Make this solution slightly alkaline with dry sodium carbonate (using a drop 

 of methyl orange to read the change) and then make slightly acid with hydro- 

 chloric acid, taking care that all lumps of sodium carbonate on the bottom are 

 acted on by the hydrochloric acid. Add sodium bicarbonate in excess and run 

 in the solution of iodin drop by drop, using starch water to read the end 

 reaction. (Sometimes the solution becomes dark toward the end of the titra- 

 tion. This change must not be confused with the final dark-blue color given by 

 the iodin starch.) 



J. Amer. Chem. Soc., 1900, 22 (9) : 568. 

 " Ibid., 1899, 21 (8) : 769. 



25 



