An Introduction to Chemical Science Part 21

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 acc.u.mulate 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 gla.s.s manufacture. Name the kind of gla.s.s in which it is used, describe its manufacture, and write a symbol for lead silicate.

What is the characteristic of lead gla.s.s? 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 ca.s.siterite, tin foil, "terne plate," "bright plate."

277. Sn occurs as the mineral ca.s.siterite, 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.

CHAPTER L.

COPPER, MERCURY, AND SILVER.

COPPER.

Examine native copper, chalcopyrite, malachite, azurite, copper acetate, copper nitrate, copper sulphate.

278. Occurrence.--Copper occurs both native and in many compounds, being diffused in rocks and, in minute quant.i.ties, in soils, waters, plants, and animals. Spain, Chili, and the United States are the chief Cu producing countries. The extensive mines of Michigan yield the native ore. The Calumet and Heela mine alone produces 4,000,000 pounds per month. The most abundant compound of Cu is chalcopyrite, or copper pyrites, CuFeS2.

Malachite, which is green, and azurite, which is blue, are carbonates, the former being used for ornamental purposes.

Cu is, next to Ag, the best conductor of electricity and heat among the elements; it is very ductile, malleable, and tenacious.

Cu has two valences, 1 and 2. Symbolize and name its chlorides, iodides, sulphides, and oxides. Cupric compounds, as a rule, are more stable than cuprous.

279. Uses.--Thousands of tons of Cu find use in domestic utensils, ocean vessels, electric wires, batteries, and plating.

Name the chief alloys of Cu and their uses. See page 136. How may CuS be obtained? See Experiment 7. Cu2O, cuprous oxide, is used to color gla.s.s red. CUSO4 is employed in calico-printing, electric batteries, etc. It is called blue vitriol.

Paris green, used for killing potato-beetles, is composed chiefly of copper a.r.s.enite. Write the symbol for this compound. All soluble salts of Cu are poisonous; hence care should be taken not to bring any acid in contact with copper vessels of domestic use.

With acetic acid, what would be formed?

MERCURY AND ITS COMPOUNDS.

Examine cinnabar, vermilion, mercury, red oxide, mercurous and mercuric chloride.

280. Cinnabar, HgS, is practically the only source of mercury-- quicksilver. Austria, Spain, and California contain nearly all the mines. In these mines the metal also occurs native to a small extent. It is the only commonly occurring metal that is liquid at ordinary temperatures; it solidifies at about -40 degrees. What other common liquid element? See page 12. Hg is reduced from the ore by Fe, Hg being distilled over and collected in water. Heat regularly expands the metal.

281. Uses.--For uses see Reduction of Ag and Au, pages 165 and 170; amalgams, page 137; laboratory work, page 68. It is also employed for thermometers and barometers, and as the source of the red pigment vermilion, which is artificial HgS.

Compare the vapor density and the atomic weight of Hg, and explain. See page 12. Hg is either a monad or a dyad. Symbolize its ous and ic oxides and chlorides. Which of the following are is salts, and which are ous, and why? HgNO3, Hg(NO3)2, HgCl, HgCl2? Calomel, HgCl or Hg2Cl2, used in medicine, and corrosive sublimate, HgCl2, are ill.u.s.trations of the ous and ic salts. The former is insoluble, the latter soluble. All soluble compounds of Hg are virulent poisons, for which the antidote is the white of egg, alb.u.men. With it they coagulate or form an insoluble ma.s.s.

SILVER AND ITS COMPOUNDS.

282. Occurrence and Reduction.--Silver is found uncombined, and combined, as Ag2S, argenite, and AgCl, horn silver. It occurs usually with galena, PbS. It is abundant in the Western States, Mexico, and Peru. Silver is separated from galena by melting the two metals. As they slowly cool, Pb crystallizes, and is removed by asieve, while Ag is left in the liquid ma.s.s. The principle is much like crystallizing NaCl from solution and leaving behind the salts of Mg, etc., in the mother liquor. When, by repeating the process, most of the Pb is eliminated, the rest is oxidized by heating in the air. Pb + O = PbO. Ag does not oxidize, and is left in the metallic state.

Another mode of reduction is to change the silver salt to its chloride, and then remove the Cl with Fe. Roasting with NaCl makes the first change, 2 NaCl + Ag2S = Na2S + 2 AgCl, and with Fe the second, 2 AgCl + Fe = FeCl2 + 2 Ag. Ag is separated from the other products by adding Hg, with which it forms an amalgam.

By distilling this, Hg pa.s.ses over and Ag remains. This is the amalgamating process.

283. Salts of Silver are much employed in organic chemistry, and AgCl, AgBr, and AgNO3 are used in photography. AgNO3 is a soluble, colorless crystal, and is the basis of the silver salts.

It blackens when in contact with organic matter. Stains on a photographer's hands are due to this substance, and the use of AgNO3 in indelible inks depends on the same property. This may be due to a reduction of AgNO3 to Ag4O. Stains can be removed from the skin or from linen by a solution of Kl, or of CuCl2 followed by sodium hyposulphite. Lunar caustic is made by fusing AgNO3 crystals, and is used for cauterizing (burning) the flesh. Much AgCN finds use in electroplating.

Experiment 128.--Put 5 cc. AgNO3 solution in each of three t.t.

To the first add 3 cc. HCl, to the second 3cc.NaCl solution, and to the third 3 cc. KBr solution. Write the reaction for each case, and notice that the first two give the same ppt., as in fact any soluble chloride would. Filter the second and third, on separate filter papers, and expose half the residue to direct sunlight, observing the change of color by occasionally stirring.

Solar rays reduce AgCl and AgBr, it is thought, to Ag2Cl and Ag2Br. Try to dissolve the other half in Na2S2O3, sodium thiosulphate solution. This experiment ill.u.s.trates the main facts of photography.

CHAPTER LI.

PHOTOGRAPHY.

284. Descriptive.--The silver halogens, AgCI, AgBr, AgI, are very sensitive to certain light rays. Red rays do not affect them; hence ruby gla.s.s is used in the "dark room."

Photography involves two processes. The negative of the picture is first taken upon a prepared gla.s.s plate, and the positive is then printed on prepared paper. The negative shows the lights and shades reversed, while the positive gives objects their true appearance.

Few photographers now make their own plates, these being prepared at large manufactories. The gla.s.s is there covered on one side with a white emulsion of gelatine and AgBr, making what are called gelatine-bromide plates. This is done in a room dimly lighted with ruby light. The plates are dried, packed in sealed boxes, and thus sent to photographers. The artist opens them in his dark room, similarly lighted, inserts the plates in holders, film side out, covers with a slide, adjusts to the camera, previously focused, and makes the exposure to light. The time of exposure varies with the kind of plate, the lens, and the light, from several seconds, minutes, or hours, to 1/250 part of a second in some instantaneous work. In the dark room the plates are removed and can be at once developed, or kept for any time away from the light. No change appears in the plate until development, though the light has done its work.

To develop the plate, it is put into a solution of pyrogallic acid, the developer, and carbonate of sodium, the motive power in the process. Other developers are often used. The chemical action here is somewhat obscure, but those parts of the plates which were affected by the light are made visible, a part of the AgzBr being reduced to Ag by the affinity which sodium pyrogallate has for Br. Ag2Br = 2 Ag + Br. Br is dissolved and Ag is deposited.

When the rather indistinct image begins to fade out, the plate is dipped for a minute into a solution of alum to harden the gelatine and prevent it from peeling off (frilling). It is finally soaked in a solution of sodium thiosulphate (hyposulphite or hypo), Na2S208. This removes the AgBr that the light has failed to reduce. The processis called fixing, as the plate may thereafter be exposed to the light with impunity. It must be left in this bath till all the white part, best seen on the back of the plate, disappears. 2AgBr + 3Na2S2O3 = Ag2Na4(S2O3) + 2 NaBr.

Both products are dissolved. It is then thoroughly washed. Any dark objects become light in the negative, and vice versa. Why?

For the positive, the best linen paper is covered on one side with alb.u.men, soaked in NaCl solution, dried, and the same side laid on a solution of AgNO3. What reaction takes place? What is deposited on the paper, and what is dissolved? This sensitized paper, when dry, is placed over a negative, film to film, and exposed in a printing frame to direct sunlight till much darker than desired in the finished picture. What is dark in the negative will be light in the positive. Why? The reducing action of sunlight is similar to that in the negative. Explain it.