METHODS OP ANALYSIS 27 



and 50 cc. water, mix by shaking the flask, and allow it to stand 

 until the zinc has completely dissolved, all action has ceased, and 

 the black or red ppt. of copper has completely settled to the bot- 

 tom of the flask. Fill up the flask with hot water, shake, and allow 

 to settle completely. Carefully decant off as much of the liquid 

 as possible, leaving all the ppt. in the flask; repeat this washing 

 three times. Dissolve the ppt. Cu in 5 cc. strong nitric acid and boil 

 off the red fumes, cool, add 20 cc. cold water, then 10 cc. strong am- 

 monia, then 50 cc. cold water, mix, titrate with the volumetric so- 

 lution of KCN until the blue color has become faint, but still indi- 

 cates Cu. Filter rapidly through a 20 cm. Prat Dumas plaited 

 filter paper, using a funnel with a 2-inch stem, wash with a little 

 water, then finish the titration exactly as in standardizing the so- 

 lution. 



Multiply the number of cc. used by the standard previously 

 found, and the result by 100, gives the Cu.* 



Or, if the assay of the ore is known (since 2Ag=lCu, or 10 

 Ag=O.30 Cu), deduct .1$ Cu for each 100 oz. Ag present. Zinc 

 and nickel, which interfere with the titration by cyanide, not be- 

 ing ppt. by the zinc, have been removed. Lead has been converted 

 into insoluble sulphate, or subsequently that derived from the 

 sheet Zn used, has been ppt. as hydrate by ammonia. 



In both the Iodide method (given previously) and in this 

 method, when the Cu is over 200, a standard of C.P. Cu should be 

 run at the same time as the ore, and the ore should be run in du- 

 plicate, and the results should check. 



SPECIMEN SLAG ANALYSIS. 



* Only Ag interferes with this method; it can be removed by adding a few drops of HC1 to 

 the nitric acid solution of the copper before adding the ammonia. Shake well to make 

 the silver clot, then filter into another flask and wash well with cold water, add 10 cc. 

 ammonia and proceed as before. 



