METHODS OF ANALYSIS 17 



square, with the diagonal corners bent in opposite directions to 

 prevent the plate from lying flat on the bottom of the beaker) and 

 boil until all the Cu is ppted. on the Al; filter, and test the 

 filtrate for Cu with H 2 S. Dissolve the Cu in as little nitric acid 

 as possible, add a few drops of water and evaporate until only a 

 pasty mass remains in the beaker. Cool, add 20 cc. water and 5 

 cc. ammonia, boil 3 to 4 minutes, add 5 cc. acetic acid, cool and 

 titrate* with sodium hyposulphite, adding a few drops of starch 

 solution. 



Before proceeding to titrate add £ gram KI crystals and shake 

 well. Starch solution is made by placing 1 gram pure starch in a 

 No. 4 casserole and pouring upon it 100 cc. cold water and letting 

 it stand until the lumps of starch have broken down into a fine 

 powder; when this has been accomplished, stir well and bring rap- 

 idly to a boil; boil until the solution becomes translucent. When 

 cold the solution is ready for use. 



Pb. — Put the filter paper (silica and all) in a beaker, add 20 cc. 

 ammonia and 10 grams of sodium acetate crystals, then half fill 

 the beaker with water and boil slowly. Titrate with ammonium 

 molybdate, using tannic acid indicator. 



This scheme works equally well on ores, slags, or mattes. 



In such ores as those of Aspen, Colorado, which contain cal- 

 cium carbonate, lead carbonate and lead sulphide, it is necessary 

 to get rid of the lime, hence the following scheme: To ^ gram of 

 ore add 5 cc. HC1 and evaporate to 2 cc. Dilute and filter; filtrate 

 contains the lead. Ppt. the Pb on Al foil. Filter, dissolve 

 the metallic lead in dilute nitric acid, add water, neutralize with 

 ammonia, acidify with acetic acid and titrate with molybdate solu- 

 tion as before. 



Sulphur. — On ores, essentially as in slag. 



Baryta. — The insoluble residue from the acid treatment is fused 

 with alkaline carbonates, and dissolved in hot water. Boil and ppt. 

 the barium as barium sulphate with sulphuric acid. Or, weigh the 

 residue after burning in a platinum crucible, add a few drops of 

 sulphuric acid and a few drops of HF (Baker & Adamson's), place 

 on the hot bath and evaporate to dryness. Weigh; the difference 

 in the two weights will be the true silica. 



*If there is any Bi present in the ore, it will interfere with the end point in the Cu titration, 

 bismuth iodide being yellow. 



