COMPOSITION OF LEAD ARSENATES ON THE MARKET. 7 



METHODS OF ANALYSIS. 



The samples were analyzed according to the provisional methods 

 of the Association of Official Agricultural Chemists , as follows: 



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 results 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: 



MOISTURE. 



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

 bath at 110 C. for from five to six hours, or till constant weight is obtained. 



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, evapo- 

 rate 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 sepa- 

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

 afterwards evaporated off, and then with a drop or two of dilute 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. If preferred, the lead sulphate may be 

 filtered and weighed in a porcelain Gooch crucible. 



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 filter, and use a 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 bicarbonate. 

 Titrate the solution with a twentieth-normal iodin solution to the appearance of a 

 blue color, using starch as indicator. 



WATER-SOLUBLE LEAD OXID. 



Place 2 grams of the sample in a flask with 2,000 cc of carbon-dioxid-free water and 

 let stand ten days, shaking eight times a day. Filter through a dry filter (being sure 

 a clear filtrate is obtained) and use aliquots of this for determining soluble lead and 

 arsenic oxida and soluble solids; determine lead as described above for total lead, 



U. S. Dept. Agr., Bureau of Chemistry Bui. 107, Revised, p. 239. 

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



