VII. INSECTICIDES AND FUNGICIDES. 

 GENERAL METHOD. 



1 PREPARATION OF SAMPLE.— TENTATIVE. 



Mix thoroughly all samples before analysis. Make water-soluble arsenic deter- 

 minations on samples as received without further pulverization or drying. In 

 the case of lye, sodium cyanid or potassium cyanid, weigh large quantities in weigh- 

 ing bottles and analyze aliquots of the aqueous solutions. 



PARIS GREEN. 



2 MOISTURE.— TENTATIVE. 



Dry 2 grams at 105°-110°C. for 5 hours and express the loss in weight as moisture. 



TOTAL ARSENIC.!— OFFICIAL. 



(Arsenic, present as arsenate, is titrated as arsenious oxid.) 



3 REAGENTS. 



(a) Starch indicator . — Mix about 0.5 gram of finely powdered potato starch with 

 cold water to a thin paste; pour into about 100 cc. of boiling water. 



(b) Standard arsenious oxid solution. — Dissolve 2 grams of pure arsenious oxid 

 in a beaker by boiling with about 150-200 cc. of water containing 10 cc. of concen- 

 trated sulphuric acid, cool, transfer to a 500 cc. graduated flask and dilute to the 

 mark. 



(C) Standard iodin solution. — Prepare an approximately N/20 solution as fol- 

 lows: Mix intimately 6.35 grams of pure iodin with twice its weight of pure potas- 

 sium iodid. Dissolve in a small amount of water, filter and dilute the filtrate to 



1 liter in a liter graduated flask. Standardize against (b) as follows: Pipette 50 

 cc. of the arsenious oxid into an Erlenmeyer flask, dilute to about 400 cc, neutralize 

 with sodium bicarbonate, add 4-5 grams in excess, and add the standard iodin solu- 

 tion from a burette, shaking the flask continuously, until the yellow color disap- 

 pears slowly from the solution, then add 5 cc. of the starch indicator and continue 

 adding the iodin solution, drop by drop, until a permanent blue color is obtained. 

 Calculate the value of the standard iodin solution in terms of arsenious oxid \As2Os) 

 and arsenic oxid (AS2O5). Occasionally restandardize the iodin against freshly 

 prepared arsenious oxid solution. 



4 APPARATUS. 



The apparatus used is shown in Fig. 5. The distillation flask rests on a metal 

 gauze which fits over a circular hole in a heavy sheet of asbestos board. The first 



2 Erlenmeyer flasks are of 500 and 1000 cc. capacity and contain about 40 and 100 

 cc. of water, respectively. Both of these flasks should be placed in a pan and kept 

 surrounded with cracked ice and water. The third flask, containing a small amount 

 of water, is used as a trap. 



