METHODS OF ANALYSIS 41 



complete decomposition is effected, the liquid nearly neutralized 

 with HN0 3 , and filtered. The filtrate is then mixed with Zn(N0 3 ) 2 , 

 dissolved in NH 4 OH and H 2 S added gradually until a perfectly 

 white ppt. begins to appear. The ppt. is then allowed to settle, 

 filtered, washed with very dilute NH 4 OH, and the filtrate titrated 

 with AgN0 3 , with addition of KI as indicator. 



9. Estimation of zinc by decomposition with HN0 3 and HC1, 

 and proceeding by Low's method, volumetric. 



10. Estimation of Copper. — Volumetrically by iodide titra- 

 tion. Method of A. H. Low. (See chapter on Lead Smelting.) 



Qualitatively, the presence of copper may readily be detected, 

 even in very small quantities, by acidulating the liquid with any 

 mineral acid, and adding a few drops of dilute ferro-cyanide solu- 

 tion, which gives the characteristic reddish brown color. 



11. Estimation of Gold. — Where the gold or silver are pres- 

 ent in quantities sufficient to be easily weighed up from one assay 

 ton of solution, 30 cc. are placed in a lead foil tray 3"x2"xl", evap- 

 orated to dryness, the lead tray rolled up and placed on the hot 

 cupel and the resulting bead parted and weighed in the usual man- 

 ner. Due care must be exercised to prevent loss from spitting 

 during the last few moments on the sand bath or hot asbestos 

 plate. The following method, while not so rapid, is accurate, and 

 is preferable in all cases where large quantities of solution must 

 be taken for assay: Argall's Method.* — Take 500 cc. of the solu- 

 tion, or, for greater convenience, prepare a pipette to hold 20 A.T. 

 of the solution. Take a tall beaker, add 7 grams of zinc dust and 

 pour in the 20 A.T. of solution; next add 10 cc. of commercial 

 H 2 S0 4 , stir well with a glass rod and cover with a watch glass. 

 When the action begins to fall off, add another 10 cc. of acid. The 

 precious metals will be completely ppt. in from 10 to 20 minutes, 

 but the solution had best be left in the beaker till the Zn is prac- 

 tically dissolved, usually occupying 30 minutes. A smaller quan- 

 tity of Zn should not be used, and if the gold and silver is over 0.05 

 ozs. per ton, 10 grams should be taken. 



When action is completed, filter, add 3 grams of Si0 2 to the 

 residue on the filter paper, fold, place in a 10 gram crucible, incin- 

 erate in the muffle, remove crucible, and when cool add 10 grams 

 each of flux and litharge, thoroughly mix in the crucible, fuse, 



* The Mineral Industry, volume X, page 361. 



