TIN AND LEAD
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| | | | |
| SYMBOL | ATOMIC | DENSITY | MELTING | COMMON OXIDES
| | WEIGHT | | POINT |
_____|________|________|_________|_________|________________________
| | | | |
Tin | Sn | 119.0 | 7.35 | 235° | SnO SnO_{2}
Lead | Pb | 206.9 | 11.38 | 327° | PbO Pb_{3}O_{4} PbO_{2}
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~The family.~ Tin and lead, together with silicon and germanium, form a
family in Group IV of the periodic table. Silicon has been discussed
along with the non-metals, while germanium, on account of its rarity,
needs only to be mentioned.
The other family of Group IV includes carbon, already described, and a
number of rare elements.
TIN
~Occurrence.~ Tin is found in nature chiefly as the oxide (SnO_{2}),
called cassiterite or tinstone. The most famous mines are those of
Cornwall in England, and of the Malay Peninsula and East India Islands;
in small amounts tinstone is found in many other localities.
~Metallurgy.~ The metallurgy of tin is very simple. The ore, separated as
far as possible from earthy materials, is mixed with carbon and heated
in a furnace, the reduction taking place readily. The equation is
SnO_{2} + C = Sn + CO_{2}.
The metal is often purified by carefully heating it until it is partly
melted; the pure tin melts first and can be drained away from the
impurities.
~Properties.~ Pure tin, called block tin, is a soft white metal with a
silver-like appearance and luster; it melts readily (235°) and is
somewhat lighter than copper, having a density of 7.3. It is quite
malleable and can be rolled out into very thin sheets, forming tin foil;
most tin foil, however, contains a good deal of lead.
Under ordinary conditions it is quite unchanged by air or moisture, but
at a high temperature it burns in air, forming the oxide SnO_{2}. Dilute
acids have no effect upon it, but concentrated acids attack it readily.
Concentrated hydrochloric acid changes it into the chloride
Sn + 2HCl = SnCl_{2} + 2H.
With sulphuric acid tin sulphate and sulphur dioxide are formed:
Sn + 2H_{2}SO_{4} = SnSO_{4} + SO_{2} + 2H_{2}O
Concentrated nitric acid oxidizes it, forming a white insoluble compound
of the formula H_{2}SnO_{3}, called metastannic acid:
3Sn + 4HNO_{3} + H_{2}O = 3H_{2}SnO_{3} + 4NO.
~Uses of tin.~ A great deal of tin is made into tin plate by dipping thin
steel sheets into the melted metal. Owing to the way in which tin
resists the action of air and dilute acids, tin plate is used in many
ways, such as in roofing, and in the manufacture of tin cans, cooking
vessels, and similar articles.
Many useful alloys contain tin, some of which have been mentioned in
connection with copper. When tin is alloyed with other metals of low
melting point, soft, easily melted alloys are formed which are used for
friction bearings in machinery; tin, antimony, lead, and bismuth are the
chief constituents of these alloys. Pewter and soft solder are alloys of
tin and lead.
~Compounds of tin.~ Tin forms two series of compounds: the stannous, in
which the tin is divalent, illustrated in the compounds SnO, SnS,
SnCl_{2}; the stannic, in which it is tetravalent as shown in the
compounds SnO_{2}, SnS_{2}. There is also an acid, H_{2}SnO_{3}, called
stannic acid, which forms a series of salts called stannates. While this
acid has the same composition as metastannic acid, the two are quite
different in their chemical properties. This difference is probably due
to the different arrangement of the atoms in the molecules of the two
substances. Only a few compounds of tin need be mentioned.
~Stannic oxide~ (SnO_{2}). Stannic oxide is of interest, since it is the
chief compound of tin found in nature. It is sometimes found in
good-sized crystals, but as prepared in the laboratory is a white
powder. When fused with potassium hydroxide it forms potassium stannate,
acting very much like silicon dioxide:
SnO_{2} + 2KOH = K_{2}SnO_{3} + H_{2}O.
~Chlorides of tin.~ Stannous chloride is prepared by dissolving tin in
concentrated hydrochloric acid and evaporating the solution to
crystallization. The crystals which are obtained have the composition
SnCl_{2}·2H_{2}O, and are known as tin crystals. By treating a solution
of stannous chloride with aqua regia, stannic chloride is formed:
SnCl_{2} + 2Cl = SnCl_{4}.
The salt which crystallizes from such a solution has the composition
SnCl_{4}·5H_{2}O, and is known commercially as oxymuriate of tin. If
metallic tin is heated in a current of dry chlorine, the anhydrous
chloride (SnCl_{4}) is obtained as a heavy colorless liquid which fumes
strongly on exposure to air.
The ease with which stannous chloride takes up chlorine to form stannic
chloride makes it a good reducing agent in many reactions, changing the
higher chlorides of metals to lower ones. Thus mercuric chloride is
changed into mercurous chloride:
SnCl_{2} + 2HgCl_{2} = SnCl_{4} + 2HgCl.
If the stannous chloride is in excess, the reaction may go further,
producing metallic mercury:
SnCl_{2} + 2HgCl = SnCl_{4} + 2Hg.
Ferric chloride is in like manner reduced to ferrous chloride:
SnCl_{3} + 2FeCl_{3} = SnCl_{4} + 2FeCl_{2}.
The chlorides of tin, as well as the alkali stannates, are much used as
mordants in dyeing processes. The hydroxides of tin and free stannic
acid, which are easily liberated from these compounds, possess in very
marked degree the power of fixing dyes upon fibers, as explained under
aluminium.
LEAD
~Occurrence.~ Lead is found in nature chiefly as the sulphide (PbS),
called galena; to a much smaller extent it occurs as carbonate,
sulphate, chromate, and in a few other forms. Practically all the lead
of commerce is made from galena, two general methods of metallurgy being
in use.
~Metallurgy.~ 1. The sulphide is melted with scrap iron, when iron
sulphide and metallic lead are formed; the liquid lead, being the
heavier, sinks to the bottom of the vessel and can be drawn off:
PbS + Fe = Pb + FeS.
2. The sulphide is roasted in the air until a part of it has been
changed into oxide and sulphate. The air is then shut off and the
heating continued, the reactions indicated in the following equations
taking place:
2PbO + PbS = 3Pb + SO_{2},
PbSO_{4} + PbS = 2Pb + 2SO_{2}.
The lead so prepared usually contains small amounts of silver, arsenic,
antimony, copper, and other metals. The silver is removed by Parkes's
method, as described under silver, and the other metals in various ways.
The lead of commerce is one of the purest commercial metals, containing
as a rule only a few tenths per cent of impurities.
~Properties.~ Lead is a heavy metal (den. = 11.33) which has a brilliant
silvery luster on a freshly cut surface, but which soon tarnishes to a
dull blue-gray color. It is soft, easily fused (melting at 327°), and
quite malleable, but has little toughness or strength.
It is not acted upon to any great extent by the oxygen of the air under
ordinary conditions, but is changed into oxide at a high temperature.
With the exception of hydrochloric and sulphuric acids, most acids, even
very weak ones, act upon it, forming soluble lead salts. Hot,
concentrated hydrochloric and sulphuric acids also attack it to a slight
extent.
~Uses.~ Lead is employed in the manufacture of lead pipes and in large
storage batteries. In the form of sheet lead it is used in lining the
chambers of sulphuric acid works and in the preparation of paint
pigments. Some alloys of lead, such as solder and pewter (lead and tin),
shot (lead and arsenic), and soft bearing metals, are widely used. Type
metal consists of lead, antimony, and sometimes tin. Compounds of lead
form several important pigments.
~Compounds of lead.~ In nearly all its compounds lead has a valence of 2,
but a few corresponding to stannic compounds have a valence of 4.
~Lead oxides.~ Lead forms a number of oxides, the most important of which
are litharge, red lead or minium, and lead peroxide.
1. _Litharge_ (PbO). This oxide forms when lead is oxidized at a rather
low temperature, and is obtained as a by-product in silver refining. It
is a pale yellow powder, and has a number of commercial uses. It is
easily soluble in nitric acid:
PbO + 2HNO_{3} = Pb(NO_{3})_{2} + H_{2}O.
2. _Red lead, or minium_ (Pb_{3}O_{4}). Minium is prepared by heating
lead (or litharge) to a high temperature in the air. It is a heavy
powder of a beautiful red color, and is much used as a pigment.
3. _Lead peroxide_ (PbO_{2}). This is left as a residue when minium is
heated with nitric acid:
Pb_{3}O_{4} + 4HNO_{3} = 2Pb(NO_{3})_{2} + PbO_{2} + 2H_{2}O.
It is a brown powder which easily gives up a part of its oxygen and,
like manganese dioxide and barium dioxide, is a good oxidizing agent.
~Soluble salts of lead.~ The soluble salts of lead can be made by dissolving
(Pb(C_{2}H_{3}O_{2})_{2}·3H_{2}O), litharge in acids. Lead acetate
called sugar of lead, and lead nitrate (Pb(NO_{3})_{2}) are the most
familiar examples. They are while crystalline solids and are poisonous
in character.
~Insoluble salts of lead; lead carbonate.~ While the normal carbonate of
lead (PbCO_{3}) is found to some extent, in nature and can be prepared
in the laboratory, basic carbonates of varying composition are much more
easy to obtain. One of the simplest of these has the composition
2PbCO_{3}·Pb(OH)_{2}. A mixture of such carbonates is called white lead.
This is prepared on a large scale as a paint pigment and as a body for
paints which are to be colored with other substances.
~White lead.~ White lead is an amorphous white substance which,
when mixed with oil, has great covering power, that is, it
spreads out in an even waxy film, free from streaks and lumps,
and covers the entire surface upon which it is spread. Its
disadvantage as a pigment lies in the fact that it gradually
blackens when exposed to sulphur compounds, which are often
present in the air, forming black lead sulphide (PbS).
~Technical preparation of white lead.~ Different methods are used
in the preparation of white lead, but the old one known as the
Dutch process is still the principal one employed. In this
process, earthenware pots about ten inches high and of the
shape shown in Fig. 89 are used. In the bottom A is placed a
3% solution of acetic acid (vinegar answers the purpose very
well). The space above this is filled with thin, perforated,
circular pieces of lead, supported by the flange B of the
pot. These pots are placed close together on a bed of tan bark
on the floor of a room known as the corroding room. They are
covered over with boards, upon which tan bark is placed, and
another row of pots is placed on this. In this way the room is
filled. The white lead is formed by the fumes of the acetic
acid, together with the carbon dioxide set free in the
fermentation of the tan bark acting on the lead. About three
months are required to complete the process.
[Illustration 1: Fig. 89]
~Lead sulphide~ (PbS). In nature this compound occurs in highly
crystalline condition, the crystals having much the same luster as pure
lead. It is readily prepared in the laboratory as a black precipitate,
by the action of hydrosulphuric acid upon soluble lead salts:
Pb(NO_{3})_{2} + H_{2}S = PbS + 2HNO_{3}.
It is insoluble both in water and in dilute acids.
~Other insoluble salts.~ Lead chromate (PbCrO_{4}) is a yellow substance
produced by the action of a soluble lead salt upon a soluble chromate,
thus:
K_{2}CrO_{4} + Pb(NO_{3})_{2} = PbCrO_{4} + 2 KNO_{3}.
It is used as a yellow pigment. Lead sulphate (PbSO_{4}) is a white
substance sometimes found in nature and easily prepared by
precipitation. Lead chloride (PbCl_{2}) is likewise a white substance
nearly insoluble in cold water, but readily soluble in boiling water.
~Thorium and cerium.~ These elements are found in a few rare
minerals, especially in the monazite sand of the Carolinas and
Brazil. The oxides of these elements are used in the
preparation of the Welsbach mantles for gas lights, because of
the intense light given out when a mixture of the oxides is
heated. These mantles contain the oxides of cerium and thorium
in the ratio of about 1% of the former to 99% of the latter.
Compounds of thorium, like those of radium, are found to
possess radio-activity, but in a less degree.
EXERCISES
1. How could you detect lead if present in tin foil?
2. Stannous chloride reduces gold chloride (AuCl_{3}) to gold. Give
equation.
3. What are the products of hydrolysis when stannic chloride is used as
a mordant?
4. How could you detect arsenic, antimony, or copper in lead?
5. Why is lead so extensively used for making water pipes?
6. What sulphates other than lead are insoluble?
7. Could lead nitrate be used in place of barium chloride in testing for
sulphates?
8. How much lead peroxide could be obtained from 1 kg. of minium?
9. The purity of white lead is usually determined by observing the
volume of carbon dioxide given off when it is treated with an acid. What
acid should be used? On the supposition that it has the formula
2PbCO_{3}·Pb(OH)_{2}, how nearly pure was a sample if 1 g. gave 30 cc.
of carbon dioxide at 20° and 750 mm.?
10. Silicon belongs in the same family with tin and lead. In what
respects are these elements similar?
11. What weight of tin could be obtained by the reduction of 1 ton of
cassiterite?
12. What reaction would you expect to take place when lead peroxide is
treated with hydrochloric acid?
13. White lead is often adulterated with barytes. Suggest a method for
detecting it, if present, in a given example of white lead.