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2Pb(N08), = 2PbO + 4N0, + 0,.

This method rarely, if ever, gives a pure oxide, as the last traces of oxides
of nitrogen are very difficult to remove ; nevertheless it is a useful laboratory
method.

(iv) The hydroxide of the metal is heated strongly. It is obtained by
adding the required amount of sodium or ammonium hydroxide to a solution
of a metallic salt, filtering, washing, drying and igniting. (Ammonium
hydroxide is used only in the cases of the trivalent metals, iron(ic) chromium
and aluminium ; sodium hydroxide is used in other cases. The reason for
this is that ammonium hydroxide either does not precipitate, or does so very
incompletely, the hydroxides of the divalent metals. (See Qualitative
Analysis under each metal.) «

8



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METALLIC OXIDES 9

(v) In a few cases none of the above methods are available. These are
discussed in detail below.

Preparation of Metallic oxides by above Methods.

Method I.

1. Preparation of copper oxide, CaO, from Oopper.—A known weight of
precipitated copper (from the action of zinc on copper sulphate solution) is
placed in a wide crucible and heated by a bunsen burner, at first gently,
being constantly stirred by a stout copper wire. If this is not done, it quickly
forms a hard eake, and complete combustion is impossible. After a time
raise the temperature to a red-heat, and stir frequently, until the theoretical
gain in weight is obtained. The heating may be finished in a small crucible
furnace or a muffle. The product should be completely soluble in dilute
sulphuric acid.*

If copper turnings, filings, or clippings of copper wire are used, combustion
will be incomplete. This, however, is inamaterial for most purposes. (See
p. 10).

Mercuric oxide is also prepared on the large scale by heating mercury
in air under pressure. It is not, however, a useful laboratory method.

When iron wire is burnt in oxygen the oxide Fe304 is produced.

Method II.

2. Preparation of quick-lime from marble. — Some clean pieces of marble
are placed in a crucible and heated for several hours in a crucible furnace or
a muffle. The M^ker furnace (Fig. 4c) is also excellent for this purpose. The

reaction CaCOj ;;: CaO + CO,

is reversibje, and unless the CO, formed is removed (by air current, etc.) the
action will not be complete except at a very high temperature. Continue
the heating until the theoretical loss of weight is obtained.

3. Magnesiun oxide is prepared similarly from magnesite. The decom-
position in this case takes place much more readily.

4. Copper oxide. — Weigh out convenient quantities of copper sulphate
CUSO4, 5H2O, and sodium carbonate NajCOj, lOHgO. Dissolve them separately
in water, filter if necessary, raise to a boil and mix gradually with constant
stirring. A basic carbonate of copper is precipitated, which is filtered, washed
with boiling water till 25 c.cs. of the filtrate no longer react with barium
chloride solution (t.«. till the soluble sulphate is washed out), and dried in
the steam oven. It is transferred to a crucible and heated strongly to convert
it to the oxide.

CuS04(5H20) + Na2CO3(10HaO) = CuCO, + Na^SO^,
249 286

and then CuCOa + HjO goes partly into Cu(OH)2 + C02. The higher the
temperature and the more dilute the solution, the more basic will be the
carbonate. (See under basic salts.)

Thus we see that 25 gms. of copper sulphate crystals and 29 gms. of soda



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10 TEXT-BOOK OF PRACTICAL CHEMISTRY

crystals are convenient quantities to take. The yield of CuO should be
theoretical.

Cadmium oxide, CdO, may be prepared similarly.

5. Bariun oxide from witheriie, BaCOs.— Since barium carbonate is
completely decomposed by heat only with great difficulty, it is reduced by
mixing with carbon and heating strongly.

BaCOs + C = BaO + 2CO.

Grind up very finely 50 gms. of the mineral, mix intimately with the
theoretical weight of carbon (lamp-black should be used, as other forms of
carbon leave considerable ash), place in a clay crucible and heat strongly for
2-3 hours in a muffle. Bottle up the BaO whilst still warm, and seal the cork
with wax.

The BaO should dissolve in dilute hydrochloric acid without evolution
of carbon dioxide. On exposure to the air it absorbs carbon dioxide and
moisture very readily.

6. Strontiun oxide. — SrO (strontia) may be similarly prepared from
strontianite, SrCOj.

Method III.
By strongly heating the nitrate of the metal.

7. Oopper oxide from copper. — A known weight of copper is dissolved in
the least possible amount of strong nitric acid, and the solution evaporated
to dryness on a sand-bath in a good draught cupboard. Care must be taken
towards the end to prevent the substance spirting.

A green basic nitrate of copper is left, which on being heated strongly
decomposes, leaving CuO. However strongly this is heated, it always contains
a little nitrogen (probably as a highly basic nitrate), and copper oxide prepared
in this way is useless for organic combustions in which nitrogen is being
determined (q.v,).

For such a purpose copper oxide made from wire clippings by heating
them strongly in air is used.

The normal action of heat on a metallic nitrate is represented as follows :

2X(N03)j = 2X0 + 4NO2 + Ojj.

This takes place more or less readily with all nitrates except those of
sodium, potassium and ammonium. But the action is not always as complete
as ii^the case of copper nitrate : e.g. barium nitrate requires an exceedingly
high temperature to effect anything like complete decomposition.

Furthermore, lead nitrate is the only common nitrate (except silver
nitrate, q.v.) which does not contain water of crystallisation. When such
nitrates are heated, partial hydrolysis takes place during dehydration, leaving
a basic nitrate (see under basic salts), somewhat as follows :

4 Cu(N03)„6H20 = Cu4(OH)e(N03)j + 6HNO3 + 18H,0.

Ferric oxide, Fe203, is prepared by the method of No. 7. (See also p. 12.)

8. Merenric oxide from mercury. — A known weight of mercury is dissolved
in the minimum quantity of strong nitric acid, and evaporated carefully to



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METALLIC OXIDES 11

dryness. (A basic nitrate is thereby formed.) This is cautiously heated on
a gauze, remembering that mercury oxide is readily decomposed by a high
temperature. The mass gradually darkens in colour till it is almost black.
This black substance is hot HgO (red when cold), and its decomposition into
mercury and oxygen is detected by the condensation of mercury on a test-
tube of cold water held over the basin. When this occurs, allow to cool.
Compare the yield with that demanded by theory.

It should here be noted that mercurous nitrate and mercuric nitrate are
both liable to be formed, but either salt when heated gives mercuric oxide,

thus : Hg2(N03)e = 2HgO + 2NO2,

2Hg(N08)8 = 2HgO + 4NO2 + O2.

Note that silver oxide, Ag^O, cannot be prepared thus, as its decomposition
temperature is below that of silver nitrate. On heating silver nitrate we

have : 2AgN03 = 2Ag + 2NO2 + Oj.

The action of strong nitric acid on tin and antimony is to form the oxides
(hydrated) directly. In the case of tin a white insoluble substance is formed,
having when dried in vacuo the empirical formula

Hj^nO, (t.e.SnO,.H20).

It is called meta-stannic acid, and is quite different in its properties from
the substance produced when stannic chloride is treated with ammonium
hydroxide, though this substance has the same empirical formula. (For
full discussion of these compounds see under Tin in Qualitative Analysis.)

9. Stannic oxide, SnOj. — Five grams of tin are treated in a basin, in the
draught-cupboard, with nitric acid (1:1). As the action slackens the liquid
may be g^^y warmed, and more acid added as required until the tin is
dissolved. Dilute considerably, allow to settle, decant, add boiling water,
filter, wash, dry and ignite. The product should be white, but as commercial
tin contains traces of lead, iron, etc., it is generally yellow as thus prepared.

The action may be represented :

Sn + 4HNO3 = HjSnOa + H^O + 4NO2.

Ignited SnOg is quite insoluble both in acids and alkalis. It has much
more markedly acidic than basic properties, and is really an acid anhydride.

Antimony pentoxide, 8b205, is similarly produced, but the product must
not be heated above 275'^ C, or Sb204 will be formed.

Method IV.
By the action of heat on the metallic hydroxide.

This method fails (for practical purposes) in the case of sodium and
potassium hydroxides, but will work in all other cases. The metallic hydroxide
is obtained by adding sodium hydroxide solution (the required amount only)
to a solution of a salt of the metal, the precipitate is filtered, washed and
dried, and heated.

If excess of alkali is used in the precipitation, it is retained in the precipitate
very tenaciously. Moreover, the hydroxides of several metals are soluble
in excess of alkali, e.g, zinc, lead, tin.



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12 TEXT-BOOK OF PRACTICAL CHEMISTRY

In the case of mercury and silver salts (also gold and platinum) the hydrox-
ides do not exist, and the oxide is therefore obtained at once, e,g.

Hg2(N03), + 2K0H = Hg,0 + 2KNO3 + H^O,
Hg(N03)2 + 2K0H = HgO + 2KNO3 + H,0,
2 AgN03 + 2K0H = AgjO + 2KNO3 + H,0.

Ammonium hydroxide gives a much cleaner precipitate than sodium
hydroxide in the case of the trivalent metak Fe(ic), Cr, Al and Bi.

10. Merouroiis oxide, EggO, from mercury. — Treat some mercury with
insufficient cold dilute nitric acid for complete solution. Add to the solution
the right amount of caustic soda solution. Filter, wash and dry the black
precipitate in the steam oven.

Note, — Mercuric oxide similarly prepared is yellow,

11. Chromic oxide, Cr203. — The mother-liquor from the preparation of
chrome alum is treated with ammonia till alkaline and boiled for several
minutes until the precipitate will settle well.

Avoid a large excess of anmionia, in which the Cr(0H)3 is somewhat
soluble. Filter, wash and dry the greenish-grey hydrate, and ignite strongly
in a muffle furnace.

Ferric oxide, Fe203, and alumina, AI2O3, may be similarly prepared, the
former from iron (dissolve in aqua regia, forming ferric chloride), the latter
from alum.

12. Ksmuth oxide, Bi203. — Dissolve 10 grams of bismuth nitrate,
Bi(N03)8, SHjO, in as little dilute nitric acid as possible. Add a consider-
able excess of ammonia solution, boil for some time, filter off the hydroxide,
wash, dry and heat to about 250*^ C. to convert to the oxide, a pale yellow
powder.

If potash be added to the boiling nitrate solution the trioxide is precipitated
at once in crystalline form.

Bi203 is also obtained by heating the carbonate or nitrate in the usual
way.

13. GrystaUine litharge, PbO.— To a boiling solution of 15 grams of caustic
soda in 100 c.cs. of water add a boiling solution of lead acetate (15 grams in
75 c.cs. of water). Continue the boiling imtil the litharge forms a crystalline
precipitate ; decant, add boiling water, filter, wash with boiling water till
free from alkali, and dry at lOO*' C.

Stronger alkali gives larger crystals of a darker colour.

V. Special Methods.

These are adopted when (a) they are more economical than the above ;
(6) when the above methods fail.

As an example of (a) may be taken the preparation of

14. Strontia from celestiie, 8rS04, which is reduced to SrS by ignition with
carbon, and converted to oxide by heating with CuO.

SrS04 + 4C=SrS + 4CO,
SrS + CuO + H2O = Sr(0H)2 + CuS.
An intimate mixture of celestite with slightly more than the calculated



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METALLIC OXIDES 13

amount of carbon is packed tightly into a crucible, covered with a layer of
powdered carbon and heated to white heat in a suitable furnace, until a
portion dissolves practically completely in dilute hydrochloric acid. It is
then suspended in plenty of water, excess of commercial copper oxide added,
and boiled till it no longer contains any soluble sulphide (test a portion with
lead acetate). Filter whilst hot, wash the residue with a little boiling water,
which is added to the main filtrate, and leave to crystallise. (Exclude air.)
Filter rapidly on the pump, dry and bottle as quickly as possible.

The product is Sr(0H)2, SHjO; it may be dehydrated by ignition if required.

Barsrta may be similarly made from barjrtes, BaS04.

As examples of (&) may be taken :

Cuprous oxide, CugO, which is readily converted to CuO by heat, and
no soluble cuprous salt is available for the wet methods.

Ferrous, manganous and stannous oxides, which are also readily oxidised,
even at ordinary temperatures. These are obtained by heating the oxalate
in a current of carbon dioxide.

FeC204 = FeO + CO + C02.

Note also that ferric oxide, FcgOj, is most economically made from
ferrous sulphate, FeS04, THgO, by ignition. It is thus obtained as a residue
in the manufacture of Nordhausen sulphuric acid. As thus obtained it is an
impalpable powder, called " jeweller's rouge." Several other preparations
follow.

15. Caproiu oxide, COsO, from copper sulphate. — 25 gms. of copper sulphate
and 15-20 gms. of glucose, C^Hj^Oe, *^^ dissolved separately in water and
filtered if necessary into a basin. A solution of sodium hydroxide (bench
strength) is now added. The precipitated copper hydroxide dissolves in the
glucose to form a blue solution. It is then heated, stirring constantly until
the blue colour has entirely gone and an orange-red precipitate forms. If
the blue colour persists, more caustic soda or glucose may be required. Filter,
wash very thoroughly with boiling water, and dry in a steam oven.

The glucose acts as a gentle reducing agent, thus :

2Cu(OH)2 - = Cu2(0H)2 -f- HjjO

gluconic acid, C^HijO,, being formed. The Cu2(OH)2 is dehydrated at 100° C,
forming CugO.

16. Manganous oxide, MnO, from Manganous sulphate. — To a solution
of manganous sulphate a solution of potassium oxalate in requisite quantity
is added. A white precipitate falls. Allow to stand for a short time, filter,
wash and dry in the steam oven.

MnS04 + K2C2O4 = K2SO4 + MnC204.

The perfectly dry manganous oxalate is transferred to a hard glass tube
drawn out near each end, and a current of dry carbon dioxide is passed through
it. When all air is displaced, the tube is heated.

MnC204 = MnO-HCO-hC02.

When the white substance has turned into an olive green powder (MnO)
the tube is sealed whilst carbon dioxide is still passing through.



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U TEXT-BOOK OF PRACTICAL CHEMISTRY

On exposure to air, even at the ordinary temperature, MnO absorbs
oxygen and forms higher oxides of manganese.

Ferrous oxide, FeO, is prepared from ferrous oxalate in a precisely similar
way. It is a black powder.

Stannous oxide, SnO, may also be obtained thus, but stannous hydroxide,
being far more stable than the oidde, may be used instead of the oxalate.
It is prepared from stannous chloride in the usual way. SnO is stable at the
ordinary temperature as thus prepared. It is a black powder.

17. Chromic oxide, Cr203. — For use as a pigment this oxide is not prepared
according to No. 11, as other methods give a more brightly coloured product.
Several special methods are in use, which are merely mentioned here.

(1) Heat carefully in a basin (preferably metal) a little anmionium dichro-
mate ; it decomposes violently :

(^K^)^Cifi, = Cr^Os + Ng + 4H,0.

(2) An intimate mixture of potassium dichromate and ammonium chloride
in the required proportions may be used instead of ammonium dichromate ;
in this case it is necessary to heat the mixture fairly strongly in a crucible.
Boil the product with water, filter, wash, dry and ignite. Use it for preparing
metallic chromium (No. 32).

(3) Mix intimately equal weights of potassium dichromate and boric acid
with enough water to make a stiff paste. Transfer to a clay crucible, heat
gently till dry and then fairly strongly. When cold extract with water and
boil well. Filter, wash and dry. The product is a partly hydrated oxide,
Ouignet's §reen, Cr20(0H)4.

18. Antimony trioxide, 8b203. — The first portions of the distillate in the
preparation of antimony trichloride (No. 118) will contain a considerable
amount of this substance, also much hydrochloric acid. On diluting it largely,
partial hydrolysis takes place :

SbCla + H,0 = SbOCl ; + 2H01.

Hence dilute the liquor with much cold water and allow to settle. Wash
several times by decantation with cold water, and then suspend in much water
and boil for some time. Filter, wash and dry at 200° C. A sample is boiled
with sodium carbonate solution, filtered and the filtrate acidified with nitric
acid and tested for a chloride. If any is present, the hydrolysis was not
complete.

19. Antimony pentoxide, Sb^Os. — This is prepared similarly to arsenic
pentoxide (q.v.) by oxidising the trioxide with strong nitric acid. When
strongly heated it forms stable Sb204. Hence dry the above product at
275*^ C.

20. Bismnth pentoxide, BigOs (so-called '' bismuthic acid ").

(a) Bismuth trioxide (No. 11) is suspended in an excess of hot 30% potash
solution, and chlorine is passed in, using a funnel as delivery tube (Fig. 20).

(&) Of, Some potassium chloride is added to the above suspension, and it
is electrolysed in a platinum basin (made the anode).

In either case a dark-coloured insoluble deposit is produced, said to be
potassium bismuthate, KBiOg. Dilute the liquid considerably, decant, wash
with water, and boil the powder with strong nitric acid for a short time.



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METALLIC OXIDES 15

Bismuthic acid, HBiOj, is produced, a scarlet powder ; dilute the liquid,
decant, filter, wash and dry at 100° C. Oxygen is evolved if the heating is
continued beyond the time necessary to drive off the " hydrate " water.
Heated very strongly, it passes into Bi204 (?).

B. PEROXIDES.

A metallic peroxide is one which

(a) When heated gives off oxygen and leaves a lower oxide.

(b) When treated with hot hydrochloric acid gives off either chlorine or
oxygen.

(c) When heated with strong sulphuric acid gives off oxygen.
There are two general methods of preparation :

(i) The dry method, in which the lower oxide is heated in air or with a
suitable oxidising agent and thus made to take up oxygen.

(ii) The wet method, in which the hydroxide of the lower oxide is precipi-
tated by an alkali and then oxidised by a solution of an oxidising agent.

Examples of the Dry Method.

21. Barimn peroxide, BaOs* — Barium monoxide, BaO, from No. 5, is
placed in a hard glass tube and heated to dull redness in a slow stream of
oxygen which is dried by strong sulphuric acid and passed over soda-lime to
absorb any carbon dioxide. The process is continued till the gain in weight
is approximately that demanded by the equation

2BaO + 02C^ 2BaOj.

Air may be used, but the process will take considerably longer. *

The product may be used for the preparation of hydrogen peroxide

(No. 27).

When BaOj is heated to a bright red heat the above reaction goes from

right to left.

The method forms the basis of Brin's process for obtaining oxygen from

the air.

22. Red lead, Pb304. — This may.be obtained by heating litharge to
350^-400° C. in air or oxygen, either in a similar manner to that of No. 17,
or by spreading the litharge on a clean sand-tray and heating with a rose-
burner. In either case the operation is lengthy, and the regulation of tem-
perature difficult. A small quantity may be conveniently prepared by
weighing out litharge and potassium chlorate in the proportion 3PbO : KCIO3.
The chlorate is dissolved in the minimum of water, and the litharge made into
a paste with the solution. It is then dried carefully and heated to a tempera-
ture of 400°-450° {no higher). When the action is complete allow to cool,
grind up the mass, extract with boiling water, and finally filter, wash and dry
at 100°.

23. Lead peroxide, PbOg (Kassner, Chemische Industrie, 18, 104, 120).—
When a mixture of litharge and calcium carbonate are heated in air, the
reaction

PbO + 2CaC03 + O2 = CajjPbO^ + 2CO2



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16 TEXT-BOOK OF PRACTICAL CHEMISTRY

takes place, calcium plumbate being formed. This is decomposed by nitric
acid thus : Ca^PbO^ + 4HNO3 = 2Ca(N08)2 + PbO^.

Hence mix intimately litharge and precipitated calcium carbonate in the
proportions demanded by the equation, put into a clay crucible and ignite
strongly in a furnace, allowing free access of air, and stirring frequently with
a stout iron wire. The ignition is continued until a small sample of the
product gives no test for carbonate. Allow to cool, and slowly add the
powdered mass to a slight excess of hot dilute nitric acid kept hot in a large
basin. Decant and treat the residue with more boiling dilute nitric acid.
Finally filter, wash and dry the dark-brown oxide, and dry at 120*^ C. in an
air-oven.

Examples op the Wet Method.

24. Lead peroxide. — If lead hydroxide be precipitated from a solution of a
lead salt, and treated with an oxidising agent such as hypochlorite, hydrogen
peroxide, chlorine, etc., it is oxidised to lead dioxide.

The most suitable oxidising agent is bleaching powder, which contains the
necessary alkali as well as the oxidising agent.

To a solution of 50 grams of lead acetate kept hot in a large basin, add
gradually a filtered solution of bleaching powder, until a specimen of the clear
liquid, treated with bleaching solution and heated, gives no more precipitate.

PbCCaHaOji)^ + Ca(0H)2 = Pb(OH)j + Ca(C2H302)2,
Pb(0H)2 -H CaOClg = CaClg -f- PbOjj + HjO.
Then allow to settle, decant, wash twice by decantation with hot water, then
once with hot water containing a little nitric acid. Filter, wash and dry at
120® C. in air oven.

25. Lead sesqai-ozide, Pb203. — This oxide is obtained by the oxidation
of a strongly alkaline lead solution.

To a solution of 50 grams of lead acetate in 250 c.cs. of water in a large
basin add 10% caustic soda solution, gently warming meanwhile, till the
precipitate formed at first redissolves. Then add a solution of bleaching
powder, freshly made by grinding bleaching powder in a large mortar with
cold water and filtering. The liquid is heated meanwhile, and the hypochlorite
solution slowly added until no more precipitate forms. Allow to settle,
decant at once, wash with boiling water by decantation, filter, wash and dry
at lOO*'. An orange powder.

N.B. — Unless the washing from excess of bleaching solution is done at
once, atmospheric carbon dioxide will soon precipitate calcium carbonate,
from which the PbgOg cannot be freed. It is therefore better, though not so
economical, to use sodium hypochlorite solution.

The action of dilute nitric acid on Pb^Og and Pb304 is as follows :

Pb203 + 2HNO3 = Pb(N03)2 4- PbOg 4- H2O.
Pb304 4- 4HNO3 = 2Pb(N08)2 + Pb02 4- 2H2O,
thus indicating that they are both salts of plumbic acid.

PbjOj is lead meta-plumbate (i.e. from HgPbOs).
Pb304 is lead ortho-plumbate (i.e. from H4Pb04).



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METALLIC OXIDES 17

Acetic acid acts similarly to nitric.

26. Barium peroxide (hydrated), BaOs^SHsO.— 50 grams of the crude
barium peroxide from No. 21 are ground with water to a thin cream, cooled
to 0® C, and added slowly to the calculated amount of ice-cold dilute hydro-
chloric acid (about 2 N) until present in slight excess. Filter (or decant) and
pour into it a saturated barium hydroxide solution at 0° C. imtil no more
precipitate forms. Filter the crystalline mass on the pump, wash with
several smaU quantities of ice-water, and dry at 130^ C, at which temperature
the water of crystallisation is lost.

Reactions :

BaOg + 2Ha = BaOj + H^O^,
Ba(0H)2 + HaOjj = BaOg + 2H,0.
The BaOj then crystallises with SHjO, forming most probably

Ba(0H)4 . 6H2O.

27. Hydrogen peroxide, H2O2. — Crude BaOs from No. 21 is- added a little
at a time to 5% phosphoric acid solution cooled with ice and contained in a
mortar. Grind thoroughly and continue the addition and grinding until the
reaction to litmus is neutral. Allow to settle and decant the liquid as far as
possible from the insoluble barium phosphate. Extract the residue once or
twice with a little ice cold water and add the extractions to the main portion.
Finally filter the decanted liquids. The strength may be found by titration
with permanganate (Quantitative Analysis, p. 249), or by measuring the
oxygen evolved when the peroxide is catalytically decomposed by MnOj
(p. 276).

The solution may be concentrated by evaporation at 75° C. in a perfectly



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