Chapter 87

LEAD AND TIN.

LEAD.

Examine galena, lead protoxide and dioxide, red-lead, lead carbonate, acetate, and nitrate. Note especially the colors of the oxides, the cubical crystallization and cleavage of galena, the specific gravity of the compounds, the softness of Pb, and the tarnish, Pb2O, which covers it,if long exposed.

274. Distribution of Pb.--Pb is widely distributed, occurring as PbS and PbCO3. PbS, galenite or galena, is its main source. By heating it in air, SO2 is formed, and Pb liberated and drawn off.

Pb is but little acted on by cold H2SO4, unless concentrated. Describe its use in making that acid. See page 65. To show that a little Pb has been dissolved, as PbSO4, in the manufacture of that acid, perform this experiment.

Experiment 124.--To 5cc. of water in a clean t.t. add the same volume of H2SO4, not C.P.; shake, and notice any fine powder suspended. PbSO4, being insoluble in water, is precipitated. What is the test for Pb? See Experiment 109.

275. Poisonous Properties.--Ph is very flexible and soft, and is much used for water pipes. In moist air it is soon coated with suboxide, Pb20, as may be seen by exposing a fresh surface. Some portion of this is liable to dissolve in water, and, as all soluble salts of Pb are poisonous, water that has stood in pipes should not be used fordrinking. Lead is employed as an alloy of tin for covering sheet-iron in "terne plate." T his plate is rarely used except for roofing. The "bright plate," used for tin cans and other purposes, scarcely ever contains any lead except the small portion in solder. In soldering, ZnCl2 is employed for a flux. Sn, Pb, and Zn are somewhat soluble in vegetable acids. If citric acid be present, as it usually is, citrates of these metals are formed, and all of them are poisonous. The action is far more rapid after opening the can, since oxidation is hastened. Hence the contents should be taken out directly after opening.

Lead poisons seem to have an affinity for the tissues of the body, and accumulate little by little. Painter's colic results from lead poisoning. Epsom salt, or other soluble sulphate, is an antidote, since with Pb it makes insoluble PbSO4.

276. Some Lead Compounds.--Lead salts form the basis of many paints. White paint is a mixture of PbCO3 and Pb(OH)2 suspended in linseed oil. It is often adulterated with BaSO4, ZnO, CaCO3. Other lead compounds are used for colored paints. The two chief soluble salts are Pb(NO3)2 and lead acetate, Pb(C2H302)2.

Red-lead, Pb3O4, and, to some extent, litharge, PbO, are employed in glass manufacture. Name the kind of glass in which it is used, describe its manufacture, and write a symbol for lead silicate. What is the characteristic of lead glass? See page 132.

Experiment 125.--Put a small fragment of Pb on a piece of charcoal, and blow the oxidizing flame against it for some time with a mouth blow-pipe. Note the color of the coating on the coal. PbO has formed.

Experiment 126.--Dissolve a small piece of lead in dilute HNO3. Pour off the solution into a t.t. and add HCl or other soluble chloride. Pb(NO3)2 + 2 HCl = ? What is the insoluble product?

Experiment 127.--Add to a solution of Pb(C2H3O2)2 some H2SO4. Give the reaction and the explanation. TIN.

Examine cassiterite, tin foil, "terne plate," "bright plate."

277. Sn occurs as the mineral cassiterite, tin stone, Sn02, and is found in only a few localities, as Banca, Malacca, and England. It does not readily tarnish, and is used to cover thin plates of copper and iron. Tin foil is generally an alloy of Pb and Sn.

Sn is sometimes a dyad, at others a tetrad. Write symbols for its two chlorides, stannous and stannic, also for its sulphides and oxides.