Determination of Lead in Commercial Zinc.—Dissolve 0.1 gram of the metal in 1 c.c. of dilute nitric acid evaporates till a solid separates out, dilute to 100 c.c. with water, and take 20 c.c. for assay. A sample treated in this way required 2.4 c.c.; this multiplied by 5 gives 12.0 c.c., equivalent to 1.2 milligram of lead, or 1.2 per cent. By gravimetric assay the sample gave 1.10 per cent.
PRACTICAL EXERCISES.
1. Thirty grams of galena gave on dry assay 21 grams of lead; and this, on cupellation, gave 15 milligrams of silver. Calculate the results in per cents. of lead and in ounces of silver to the ton of ore.
2. How many ounces of silver to the ton would be contained in the lead got from this ore if the loss in smelting is equal to that of the assay?
3. Having given you a sample of white lead freed from oil by washing with ether, how would you proceed to determine the percentage of lead in it?
THALLIUM.
Thallium is a rare metal, found in small quantities in some varieties of iron and copper pyrites, and in some lithia micas. It resembles lead in appearance. Its compounds resemble the salts of the alkalies in some respects; and, in others, those of the heavy metals.
It is detected by the green colour which its salts impart to the flame. This, when examined with the spectroscope, shows only one bright green line.
It is separated and estimated by dissolving in aqua regia; converting into sulphate by evaporation with sulphuric acid; separating the second group of metals with sulphuretted hydrogen in the acid solution, boiling off the excess of the gas; nearly neutralising with carbonate of soda; and precipitating the thallium with an excess of potassic iodide. On allowing the liquid to stand for some time a bright yellow precipitate of thallous iodide separates out. This is collected on a weighed filter; washed with cold water, finishing off with alcohol; dried at 100° C., and weighed. The precipitate is thallous iodide TlI, and contains 61.6 per cent. of thallium.