68 METHODS OF ANALYSIS [Chap. 



Method 11. ^0— Tentative. 

 (Not applicable in the presence of calcium.) 



25 REAGENT. 



Acidified alcohol.— Mix water 100 parts; 95% alcohol 200 parts; and concentrated 

 sulphuric acid 3 parts by volume. 



26 DETERMINATION. 



Heat, on a hot plate, 0.73G0 gram of the dry powdered sample with about 25 cc. 

 of dilute nitric acid (1 to 4) in a porcelain evaporating dish or casserole. Remove 

 any insoluble residue by filtration. Add 3 cc. of concentrated sulphuric acid and 

 evaporate on the hot plate to the appearance of white fumes. It is important that 

 all nitric acid be expelled. Cool, add 50 cc. of water and about 100 cc. of 95% alco- 

 hol, let stand several hours (preferably over-night) and filter through a weighed 

 Gooch crucible, previously washed with water, the acidified alcohol and 95% alco- 

 hol, and dried at 200°C. Wash the precipitate of lead sulphate in the crucible about 

 10 times with the acidified alcohol and then with 95% alcohol until free from sul- 

 phuric acid. Dry at 200°C. to constant weight, keeping the crucible covered to pre- 

 vent loss by spattering. The weight of the lead sulphate multiplied by 100 gives 

 the per cent of lead monoxid (PbO) in the dried sample. 

 TOTAL ARSENIC. 



27 Method I. ^—Official. 



Proceed as directed under 5, using an amount of the sample equal to the arsenic 

 oxid equivalent of 500 cc . of the standard iodin solution and titrating a 200 cc . aliquot 

 of the distillate. The number of cc. used of the standard iodin solution represents 

 directly the total per cent of arsenic in the sample expressed as arsenic oxid (AS2O6) . 



Method 11.^^— Official. 

 (Not applicable in the presence of antimony.) 



28 REAGENTS. 



The reagents and solutions used are described under 3. 



29 DETERMINATION. 



Dissolve an amount of the powdered sample equal to the arsenic oxid equivalent 

 of 400 cc. of the standard iodin solution, in dilute nitric acid in a porcelain casserole 

 or evaporating dish. Add 5 cc. of concentrated sulphuric acid and heat on the 

 hot plate to copious evolution of white fumes. Wash into a 200 cc. graduated flask 

 with water, cool, make up to the mark and filter through a dry filter. Transfer 

 100 cc. of the filtrate to an Erlenmeyer flask and proceed as directed under 22, 

 beginning with ''add 1 gram of potassium iodid," to "Subtract from the corrected 

 titration reading." The number of cc. of the standard iodin solution used, divided 

 by 2, represents directly the per cent of total arsenic in the sample expressed as 

 arsenic oxid (As205)- 



WATER-SOLUBLE ARSENIC OXID. -TENTATIVE. 



30 REAGENTS. 



The reagents and solutions used are described under 3. 



