IV. METHODS FOR THE ANALYSIS OF INSECTICIDES AND 



FUNGICIDES. 



11. Lead Arsenate (Haywood). Provisional. 



[Page 34.] 

 (a) PREPARATION OF SAMPLE. 



In case the sample is in the form of a paste, as it usually is, dry the whole of it to 

 constant weight at the temperature of boiling water and calculate the result as total 

 moisture. Grind the dry sample (which will gain a small amount of moisture by 

 so doing) to a fine powder and determine the various constituents as follows: 



(6) MOISTURE. 



Weigh 2 grams of the sample and heat in the water bath for eight hours or in the 

 hot-air bath at 110 C. for five to six hours or till constant weight is obtained. 



(c) TOTAL LEAD OXID. 



Dissolve 2 grams of the sample in about 80 cc of water and 15 cc of concentrated 

 nitric acid on the steam bath; transfer the solution to a 250 cc flask and make up to 

 the mark. To 50 cc of the solution add 3 cc of concentrated sulphuric acid, evaporate 

 on the steam bath to a sirupy consistency and then on the hot plate till white fumes 

 appear and all nitric acid has been given off. Add 50 cc of water and 100 cc of 95 

 per cent alcohol. Let stand for several hours and filter off supernatant liquid, wash 

 about ten times with acidified alcohol (water 100 parts, 95 per cent alcohol 200 parts, 

 and concentrated sulphuric acid 3 parts) and then with 95 per cent alcohol till free 

 of sulphuric acid. Dry, remove as much as possible of the precipitate from the paper 

 into a weighed crucible, and ignite at low red heat. Burn the paper in a separate 

 porcelain crucible and treat the residue first with a little nitric acid, which is after- 

 wards evaporated off, and then with a drop or two of sulphuric acid. Ignite, weigh, 

 and add this weight to the weight of the precipitate previously removed from the 

 paper for amount of the lead sulphate. 



(d) TOTAL ARSENIC OXID (MODIFIED GOOCH AND BROWNING METHOD ). 



Transfer 100 cc of the nitric acid solution of the sample, prepared as in the above 

 determination of lead, to a porcelain dish, add 6 cc of concentrated sulphuric acid, 

 evaporate to a sirupy consistency on water bath and then on hot plate to the appear- 

 ance of white fumes of sulphuric acid. Wash into a 100 cc flask with water, make 

 up to mark, filter through dry filters, and use 50 cc aliquot for further work. Transfer 

 this to an Erlenmeyer flask of 400 cc capacity, add 4 cc of concentrated sulphuric 

 acid and 1 gram of potassium iodid, dilute to about 100 cc and boil until the volume 

 is reduced to about 40 cc. Cool the solution under running water, dilute to about 

 300 cc, and exactly use up the iodin set free and still remaining in solution with a 

 few drops of approximately tenth-normal sodium thiosulphate. Wash the mixture 

 into a large beaker, make alkaline with sodium carbonate, and slightly acidify with 

 dilute sulphuric acid; then make alkaline again with an excess of sodium bicar- 

 bonate. Titrate the solution with a twentieth-normal iodin solution to the appear- 

 ance of a blue color, using starch as indicator. 



oAmer. J. Sci., 1890, 40:66. 



