METHODS OF ANALYSIS 



heat on a hot plate, with occasional additions of small portions of concentrated 

 nitric acid, until the mixture no longer blackens when evaporated to the point at 

 which white fumes of sulphur trioxid appear. Cool, dilute with 20 cc. of water, 

 warm until the ferric sulphate goes into solution, cool and then add 40 cc. of 95% 

 alcohol by volume. Allow to stand overnight, filter on a Gooch and wash with 

 95% alcohol. Dissolve the lead sulphate remaining on the filter by washing with 

 20 cc. of 25% ammonium acetate solution, rendered slightly alkaline with ammo- 

 nium hydroxid, collect the filtrate in a small beaker, passing it through the filter 

 3^ times. Finally wash the filter with hot water, acidify the combined filtrate 

 and washings with acetic acid, add an excess of potassium dichromate solution and 

 allow to stand overnight. Filter on a small, tared Gooch, wash, dry for 30 minutes 

 at 125°-150°C. and weigh as lead chromate. Calculate the metallic lead. 



34 ARSENIC— TENTATIVE. 



Introduce 5 grams of the sample directly into the generator described under 

 XII, 2 (Fig. 7), add 10 cc. of water, a little at a time to prevent foaming over, 

 and then 15 cc. of concentrated, arsenic-free hydrochloric acid, introducing it drop 

 by drop until foaming ceases. Heat on a steam bath until a drop of the mixture, 

 when diluted and treated with iodin solution, shows no blue color. Then dilute to 

 about 30 cc. with water, add 4 cc. of potassium iodid solution and continue from 

 this point as directed under XII, 4, beginning with "Heat to about 90°C.", except 

 that the blank and the standards for comparison are made by the use of the arsenic- 

 free hydrochloric acid of the same concentration as that used in the determination. 



BIBLIOGRAPHY. 



1 J. Am. Chem. Soc, 1899, 21: 396. 



2 Ibid., 1896, 18:1. 



3 Conn. Agr. Exp. Sta. Kept., 1900, (11), p. 169. 



* Inland Revenue Dept., Canada, Bull. 68, p. 31. 



'" Catlin. Baking Powders: A Treatise on Their Character, Method for Deter- 

 mination of Their Values, etc. p. 20. 



6 Ann. chim. anal., 1899, 4: 263. 



7 Conn. Agr. Exp. Sta. Rept., 1900, (11), p. 180. 



8 Ibid., p. 174. 



5 Rept. Mass. State Board of Health, 1899, p. 638. 

 1" Conn. Agr. Exp. Sta. Rept., 1900, (II), p. 178. 



11 U. S. Bur. Chem. Bull. 13 (V), p. 596; Conn. Agr. Exp. Sta. Rept., 1900, (II), 

 p. 179. 



12 J. Assoc. Official Agr. Chemists, 1915, 1: 249. 



