LEAD ARSENATE 639 



then with a drop or two of sulfuric acid. Ignite, weigh, and 

 add this weight to the weight of the precipitate previously re- 

 moved from the paper for amount of the lead sulfate. 



Water-soluble lead oxid. Place two grams of lead ar senate in 

 a flask with 2,000 cubic centimeters of carbon dioxid free water 

 and let stand ten days, shaking eight times a day. Filter through 

 a dry filter and use aliquots of this for determining soluble lead 

 and arsenic oxids and soluble solids ; determine lead as described 

 above for total lead oxid, using the same relative proportions of 

 sulfuric acid, water, and alcohol, but keeping the volume as 

 small as possible. 



Total arsenic oxid. Transfer 100 cubic centimeters of the 

 nitric acid solution of the sample, prepared as in the above de- 

 termination of lead, to a porcelain dish, add six cubic centime- 

 ters of concentrated sulfuric acid, evaporate to a sirupy consis- 

 tency on the water bath and then on a hot plate to the appear- 

 ance of white fumes of sulfuric acid. Wash into a 100 cubic 

 centimeter flask with water, make up to mark, filter through dry 

 filters, and use 50 cubic centimeter aliquot parts for further 

 work. Transfer this to an erlenmeyer flask of 400 cubic centi- 

 meters capacity, add four cubic centimeters of concentrated sul- 

 furic acid and one gram of potassium iodid, dilute to about 100 

 cubic centimeters and boil until the volume is reduced to about 

 40 cubic centimeters. Cool the solution under running water, 

 dilute to about 300 cubic centimeters, and exactly use up the io- 

 din set free and still remaining in solution with a few drops of 

 approximately tenth-normal sodium thiosulfate solution. Wash 

 the mixture into a large beaker, make alkaline with sodium car- 

 bonate, and slightly acidify with dilute sulfuric 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 arsenic oxid. For this determination use 200 

 to 400 cubic centimeters of the water extract obtained under the 

 determination of soluble lead oxid. Add 0.5 cubic centimeter 

 of sulfuric acid and evaporate it to a sirupy consistency, then 

 heat on a hot plate to appearance of white fumes. Add a very 



