James Freeman Sellers.

An elementary treatise on qualitative chemical analysis online

. (page 5 of 12)
Online LibraryJames Freeman SellersAn elementary treatise on qualitative chemical analysis → online text (page 5 of 12)
Font size
QR-code for this ebook


Special Method for Magnesium. Make a dilute solution
of alcana and record its absorption spectrum. Now add
a dilute neutral solution of magnesium chloride. The
alcana spectrum will be moved to the left in propor-
tion to the concentration of the magnesium-chloride
solution.

Special Method for Manganese. Boil the compound
with some lead dioxide and a little nitric acid and
test for the absorption spectrum of permanganic acid.
(See No. 11 on the table.)

Special Method for Cobalt (Wolff). Add ammonium sul-
phocyanate to the cobalt-chloride solution and shake
with alcohol (amyl preferable) and ether. This dis-
solves the cobaltous sulphocyanate, and the solution
gives a characteristic absorption spectrum.

Special Method for Iodine (Vogel). Iodine can be tested
for with the apparatus here shown : a is the gas flame
colored by the iodide ; b is a hard
glass tube held in position over
the smaller tube by a spiral cop-
per wire, G\ d is a hard glass
test-tube containing at its bot-
tom a mixture, e, of copper oxide
and an iodide ; / is a stream of
illuminating gas passing through
the apparatus and burning at a ;

g is the spectroscope ; h is a gas ^__

burner.

Bromine and chlorine can also be detected in the
same manner by using a bromide or a chloride instead of
an iodide. The copper iodide, or chloride, or bromide




64 CHEMICAL ANALYSIS

escapes with the gas and colors the flame green. The
spectroscope shows a number of bands, especially in the
green part of the spectrum, which are different for
iodine, chlorine, and bromine.



CHAPTER V

LIST AND PREPARATION OF REAGENTS

It is desirable that the student know not only the
chemical nature of reagents, but also their proper dilu-
tion. From a careful study of the principles of solu-
tion and of mass action, the reason for a knowledge of
dilution must be obvious.

In Exp. 4 an illustration is given of the different
effects of concentrated and dilute sulphuric acid on zinc.

Furthermore, it is desirable that reagents be so diluted
as to give uniform strengths, so that the volume of solu-
tion used will be an index of the quantity of reagent
employed. Most analysts use an arbitrary system of
dilutions that has no special significance, save that it
meets the empirical requirements of ordinary analysis.

Reddrop (1890) suggested that dilutions be based on
the equivalent weights of the reagents. The equiva-
lent weight of a substance is its molecular weight
divided by the number of its replaceable hydrogen
atoms, or those which are the equivalents of hydrogen.
For example, the equivalent weight of hydrochloric acid
is 36.5, obtained by dividing its molecular weight by its
number of replaceable hydrogen atoms :

36.5-1 = 36.5.

Likewise, the equivalent weights of sodium chloride,
sodium hydroxide, and silver nitrate are, respectively,

65



66 CHEMICAL ANALYSIS

58.5, 40, and 170. The equivalent weight of sul-
phuric acid is 49, obtained by dividing its molecular
weight by its number of replaceable hydrogen atoms :

98 -s- 2 = 49.
The following list gives some equivalent weights :

Hydrogen, H 1.

Oxygen, O 8.

Hydrochloric Acid, 1IC1 36.5.

Nitric Acid, HNO 3 63.

Sulphuric Acid, II 2 SO 4 . 49.

Acetic Acid, HC 2 H 3 O2 60.

Phosphoric Acid, H 3 PO 4 ....... 32.6.

Ammonium Chloride, NH 4 C1 53.7.

Barium Chloride, BaCl 2 103.8.

When equivalent weights are dissolved in equal
volumes of water, equal fractional parts of the solu-
tions will be equivalent. A normal solution of a sub-
stance is one which contains the equivalent weight of
that substance, in grams, dissolved in a liter of solution.
For example, a normal solution of sodium chloride is
its equivalent weight, 58.5 grams, dissolved in sufficient
water to produce a liter of solution.

Equal volumes of normal solutions whose solutes
react with one another should neutralize each other per-
fectly, leaving no excess of either reagent, as in the fol-
lowing equation : AgNO 3 + NaCl = AgCl + NaNO 8 .

Experiment 19

Arrange two burettes. Fill the one with a normal solution of
sulphuric acid, and the other with a normal solution of sodium
hydroxide. Into a beaker or flask measure out 20 c.c. of the



LIST AND PREPARATION OF REAGENTS 67

alkali and add about 1 c.c. litmus solution. Now add the acid

from the burette, drop by drop, stirring or shaking constantly,

till the blue color changes to red. Compare the volumes of the
two solutions used.

NOTE. The normal solutions in this experiment should be prepared
by the instructor.

Theoretically, solutions of all reagents should be of
equivalent strengths ; but it has not been found practi-
cal to give all of them these values because of their
lack of uniform solubilities. It is convenient to adopt
the normal solution as a standard and to express all
dilutions as multiples or fractions of normal.

The letter N is used to denote a normal solution, and
all variations from normal are expressed in terms of N.
For example, the best dilution for sulphuric acid is
about five times its normal strength, expressed thus:
5NH 2 SO 4 , or 5N solution; and the best dilution for

silver nitrate is about one-fifth its normal strength,

w w

expressed thus : AgNO 3 , or solution.

o 5

It is sufficient for the purposes of qualitative analysis
if the strength of the reagents be approximately known,
and it will be understood that the strengths given below
are only approximate.

LIST OF REAGENTS

Solutions 1. Acetic Acid, HC 2 H 3 2 -f- Aq. 1 volume

80% acid to 2 volumes water = 5 N solution.

2. Concentrated Hydrochloric Acid, HC1. Sp. gr. 1.20 =
UN solution.

3. Dilute Hydrochloric Acid, HC1 -f- Aq. 1 volume con-
centrated acid to 3 volumes water = 5 N solution.



68 CHEMICAL ANALYSIS

4. Hydrosulphuric Acid Gas, H 2 S. For generating the
gas Kipp's apparatus is generally used. 1 The generator
should be kept in a hood with a good draught. The mate-
rials used in the generator are lumps of ferrous sulphide,
FeS, and dilute hydrochloric or sulphuric acid. Often
when the acid seems exhausted it will renew its activity if
it is removed from the generator, and the lumps of ferrous
sulphide thoroughly washed with water.

5. Hydrosulphuric Acid Solution, H 2 S + Aq. The gas is
passed into cold water to saturation. The solution of the
gas should be kept in the dark or in bottles of deeply
colored glass, as sunlight decomposes the acid with sepa-
ration of sulphur.

6. Concentrated Nitric Acid, HISTOg. Sp. gr. 1.42 = 16 N
solution.

7. Dilute Nitric Acid, HN0 3 + Aq. 5 volumes concen-
trated acid to 11 volumes water = 5 N solution.

8. Concentrated Sulphuric Acid, H 2 S0 4 . Sp. gr. 1.84
= 36 N solution.

9. Dilute Sulphuric Acid, H 2 S0 4 ' 2 H 2 + Aq. 1 volume
concentrated acid to 6 volumes water = 5 N solution. In
diluting the concentrated acid, it should be added to the
water very slowly, in a large porcelain dish, and allowed to
cool before using.

10. Tartaric Acid, HaC^^e + Aq. 1 part crystals to
13 parts water = N solution. The acid decomposes in
solution and should be prepared fresh each time.

11. Aqua Regia, HC1 + HN0 3 . 1 volume concentrated
nitric to 3 volumes concentrated hydrochloric acid. This
proportion is sometimes varied for specific purposes. As a
solvent, just enough of the reagent should be used to dis-
solve the substance completely. A large excess must be
avoided, since if allowed to remain it decomposes other
reagents, while if evaporated certain volatile chlorides,



LIST AND PREPARATION OF REAGENTS 69

e.g., arsenic and mercuric chlorides, are liable to be lost.
Prepare aqua regia fresh each time it is needed.

12. Ammonium Chloride, NH 4 C1 + Aq. Use 1 part crys-
tals to 4 parts water, and allow it to rise to the natural tem-
perature ; then dilute with 1 part water = 5 N solution.

13. Ammonium Carbonate, (NH 4 ) 2 C0 3 + Aq + NH 4 OH.

4 parts solid ammonium carbonate dissolved in 7 parts

5 N NH 4 OH ; then dilute with 14 parts water = 5 N
solution.

14. Ammonium "Sesqui" Carbonate. Dissolve 1 part
solid (NH 4 ) 2 C0 3 in 9 parts water and add for each 10 c.c.
of the liquid 5 drops of strong ammonia (Hager). Pre-
pare fresh each time.

15. Concentrated Ammonium Hydroxide, NH 4 OH -f- Aq.
Sp. gr. 0.90 = 9N solution.

16. Dilute Ammonium Hydroxide, NH 4 OH -f Aq. 2 vol-
umes concentrated ammonia water (sp. gr. 0.90) to 5 volumes
water = 5 N solution. Both concentrated and dilute ammonia
attack glass vessels ; and if white flakes appear in the clear
solutions, they should be filtered out before use.

17. Ammonium Oxalate, (NH 4 ) 2 C 2 O 4 -H 2 O -f- Aq. 1 part

W

solid crystals to 25 parts water = solution.

18. Ammonium Sulphide, (NH 4 ) 2 S + Aq + NH 4 OH.
Saturate 3 parts 5N ammonia with hydrogen sulphide
gas, and then add 2 parts 5N ammonia solution. This
reagent should be made frequently, as it decomposes on
standing.

19. Yellow Ammonium Sulphide, (NH 4 ) 2 Sx -f- Aq + ]STH 4
OH. Digest a solution of (NH 4 ) 2 S with a little powdered
roll sulphur. An excess of sulphur must be avoided, as it
produces the red solution containing higher sulphides.

20. Barium Chloride, BaCl 2 - 2 H 2 + Aq. 1 part solid
crystals dissolved in 10. parts water = N solution.



70 CHEMICAL ANALYSIS

21. Bromine, Br. Should be kept in a dark, glass-
stoppered bottle.

22. Dilute Bromine, Br -f Aq. Make a saturated so-
lution by shaking an excess of bromine in water =

L

solution. Stronger solutions of bromine can be made by
adding potassium bromide to the water solution.

23. Calcium Hydroxide (lime water), Ca(OH) 2 -f Aq.
Saturate freshly boiled distilled water by shaking an
excess of freshly slaked lime in it, and allowing the

TVT

excess of lime to settle. Filtrate = solution.

2(j

24. Chlorine Water, Cl + Aq. Saturate cold water

with chlorine gas = solution. Should be kept in the
o

dark and in brown bottle.

25. Cobalt Nitrate, Co(N0 3 ) 2 -6H 2 O + Aq. 1 part solid
crystals to 7 parts water = N solution.

26. Ferric Chloride, FeCl 3 + Aq. 1 part solid salt to
20 parts water = N solution.

27. Ferrous Sulphate, FeS0 4 -7H 2 + Aq. 1 part solid
crystals to 7 parts water = N solution. It is best to pre-
pare this solution fresh, when needed.

28. Lead Acetate, Pb(C 2 H 3 2 ) 2 -3H 2 + Aq. 4 parts
solid to 21 parts water = N solution.

29. Magnesium Sulphate, MgS0 4 - 7 H 2 + Aq. 1 part
crystals to 8 parts water = N solution.

30. Magnesia Mixture, MgCl 2 + NH 4 C1 + NH 4 OH + Aq.
Dissolve 6 grams magnesium chloride crystals and 165
grams ammonium chloride in 300 c.c. water ; then add

N
300 c.c. 5 N ammonia solution ; and dilute to 1 liter =

solution.

31.- Mercuric Chloride, HgCl 2 + Aq. 1 part solid salt

to 37 parts water = - solution.



LIST AND PREPARATION OF REAGENTS 71

32. HydrocMorplatinic Acid, H 2 PtCl 6 -6H 2 + Aq. 1 part
solid salt to 12 parts water = N solution. The solution
can also be prepared by dissolving 0.30 gr. platinum foil
in aqua regia, evaporating to dryness, and redissolving in
10 c.c. 5 N HC1.

33. Potassium Chromate, K 2 O0 4 -f Aq. 1 part solid
salt to 10 parts water = N solution.

34. Potassium Cyanide, KCN -f- Aq. 1 part solid salt
to 15 parts water = N solution. Prepare fresh for each
experiment.

35. Potassium Ferricyanide, K 3 Fe(CN) 6 -3H 2 -f Aq.
1 part solid salts to 9 parts water = N solution.

36. Potassium Ferrocyanide, K 4 Fe(CN) 6 -3H 2 + Aq.
1 part solid salt to 10 parts water = N solution.

37. Potassium Hydroxide, KOH -f- Aq. 1 part solid
caustic potash to 3.5 parts water = 5 N solution.

38. Potassium Sulphocyanate, KCNS + Aq. 1 part solid
salt to 10 parts water = N solution.

39. Silver Nitrate, AgN0 8 -f- Aq. 1 part solid salt to

M-

30 parts water = solution.

40. Sodium Acetate, NaC 2 H 3 2 -3H 2 + Aq. 1 part solid
salt to 8 parts water = N solution.

41. Sodium Carbonate, Na 2 C0 3 - 1 OH 2 + Aq. 1 part
solid crystals to 7 parts water = N solution.

42. Sodium Hydroxide, NaOH -f- Aq. 1 part solid to 5
parts water 5 N solution. First dissolve the solid base
in a little water, allow to cool, and then dilute to the
required volume.

43. Sodium Phosphate, HNa 2 P0 4 -12H 2 + Aq. 1 part
solid crystals to 8 parts water = N solution.

44. Stannous Chloride, SnCl 2 -2H 2 + Aq. Dissolve 3
parts solid salt in 3 parts 5 N hydrochloric-acid solution,
and dilute with 20 parts water = N solution. Pieces of



72 CHEMICAL ANALYSIS

granulated tin should be kept in the solution. An excel-
lent quality of solid stannous chloride can be made by
heating granulated tin with repeated small quantities of
concentrated HC1, added at intervals whenever ebullition
ceases. When all the tin is dissolved, evaporate to dry-
ness on the water bath.

Solvents. 45. Alcohol, C 2 H 6 OH, 95 per cent.

46. Carbon Disulphide, CS 2 .

47. Ether, (C 2 H 5 ) 2 0, commercial.

48. Ether-Alcohol, 1 volume absolute ether to 1 volume
absolute alcohol.

49. Petroleum Ether.

50. Water, distilled.

Dry Reagents. 51. Ammonium Chloride, NH 4 C1.

52. Ammonium Carbonate, (NH 4 ) 2 C0 3 .

53. Cobalt Nitrate, Co(N0 3 ) 2 -6H 2 0.

54. Lead Peroxide, Pb0 2 .

55. Manganese Peroxide, Mn0 2 .

56. Microcosmic Salt, HNa(NH 4 )P0 4 -8H 2 0.

57. Potassium Carbonate, K 2 C0 3 , anhydrous.

58. Potassium Cyanide, KCN.

59. Potassium Disulphate, HKS0 4 .

60. Potassium Nitrate, KlSTOg.

61. Potassium Nitrite, KN0 2 .

62. Sodium Carbonate, Na 2 C0 3 , anhydrous.

63. Sodium Tetraborate (borax), Na 2 B 4 7 10 H 2 0.

64. Sodium Peroxide, Na 2 2 .



CHAPTER VI

SYSTEMS OF ANALYTICAL EXAMINATION

Analysis by the Dry Way. The chief operations in
analysis by this system are the observations of (a) ,
Oxidation and Reduction, and (b) Flame Coloration.

(a) Oxidation and reduction have been explained
under ignition operations. These include fusion in
crucibles, closed tube reductions, oxidation and reduc-
tion with fluxes on a platinum wire, and reduction
on charcoal with and without fluxes.

(b) Flame colorations have been explained under
simple flame colorations and spectroscopy.

Though these operations are indispensable to the
analyst, they do not constitute an independent system,
but are only used for preliminary and confirmatory
observations.

Analysis by the Wet Way. Solution is the basis of
this system of analysis. Advantage is taken of the
following facts :

(a) The metallic ions of most compounds behave
alike towards certain reagents, regardless of the acid
radicals which may be present. For example, all solu-
ble silver salts will give insoluble silver chloride with
all soluble chlorides.

(b) Differences of solubility of similar compounds of
different metals may be utilized in separating them into

73



74 CHEMICAL ANALYSIS

groups. For example, silver, lead, and copper sulphides
are insoluble in acidified solutions, while some other
sulphides, e.g., those of zinc and barium, are soluble
under like conditions. Silver and lead chlorides are
insoluble, and copper chloride is soluble in acidified
solutions. Such differences of solubility afford an easy
means of separating and detecting these metals.

(<?) Physical characteristics, color, odor, etc., are used
for detecting individual substances. For example, the
soluble salts of both cadmium and copper are precipi-
tated by hydrogen sulphide ; but as the one sulphide
is a bright yellow and the other black, the two can be
distinguished.

(d) The principles of the periodic law of elements
are in part regarded in analytical chemistry. The
periodic groups sodium, potassium, lithium ; barium,
strontium, calcium ; chlorine, bromine, and iodine -
are also utilized as analytical groups.

An ideal natural system of classification would have
analytical groups to coincide with periodic groups ; but
in this respect analytical classification is somewhat arti-
ficial, and depends more upon differences in degree of
the physical property of solubility than on chemical
properties. For example, magnesium, zinc, and cad-
mium belong to the same periodic group, but, by reason
of the differences in the solubilities of their sulphides,
the three metals are placed in three separate analytical
groups.

By (a) all salts require two analyses : first, for
metals ; and, second, for the acid radicals. By (b) and
(c) the metals are divided into groups depending on



SYSTEMS OF ANALYTICAL EXAMINATION 75

the insolubility of their chlorides, sulphides, hydroxides,
and carbonates.

There are six groups of metals :
Group I, whose chlorides are insoluble in aqueous

solution ;
Group II, whose sulphides are insoluble in acidified

(HC1) solution;
Group III, whose hydroxides are insoluble in alkaline

(NH 4 OH) solution ;
Group IV, whose sulphides are insoluble in alkaline

(NH 4 OH) solution ;
Group V, whose carbonates are insoluble in alkaline

(NH 4 OH) solution ;
Group VI, which has no group characteristic.

The group reagent would be for :
Group I, a soluble chloride (HC1) ;
Group II, a soluble acidified sulphide (H 2 S with HC1) ;
Group III, a soluble alkali (NH 4 OH with NH 4 C1);
Group IV, a soluble alkaline sulphide [(NH 4 ) 2 S with

NH 4 OH and NH 4 C1] ;
Group V, a soluble alkaline carbonate [(NH 4 ) 2 CO 3 with

NH 4 OH and NH 4 C1] ;
Group VI, no group reagent.

By (b) and (d) the acids are also divided into groups
depending on the insolubility of their barium and silver
salts, hence the three groups : -
Group I, whose barium salts are insoluble in aqueous

solution ;
Group II, whose silver salts are insoluble in dilute

acid solution ;
Group III, which has no group reagent.



76 CHEMICAL ANALYSIS

By (b) and (c) the individual metals and acid radicals
included in the groups are separated and distinguished.
For example, silver, lead,- and mercury (mercurous)
chlorides are thrown down by hydrochloric acid as
white precipitates.

The separation is accomplished by observing facts
like these : Lead chloride is soluble in hot water ; sil-
ver and mercurous chlorides are not. Silver chloride
is soluble in ammonia ; white mercurous chloride is not
soluble, but is blackened by that reagent. Thus, both
the differences of solubility and color are used for sepa-
rating and detecting metals.



PART II REACTIONS AND SEPARATIONS



CHAPTER VII

METALS OF GROUP I : SILVER, MERCURY (MERCUROUS
SALTS, HgO, AND LEAD

CHARACTERISTIC : Insolubility of the chlorides in cold water or
dilute HC1.

GROUP REAGENT: Dilute hydrochloric acid.

REACTIONS

Silver (salt for study, silver nitrate, AgN0 3 ).

1. HC1 precipitates white silver chloride, AgCl, dark-
ening in the sunlight; insoluble in HNO 3 ; 2 soluble in
NH 4 OH.

The soluble compound formed by adding NH 4 OH to AgCl
is variously regarded as 2AgCl-3NH 3 , AgCl-2NH 3 , and
AgCl 3 NH 3 , though the preponderance of authority favors
2AgCl-3NH 8 . NH 4 OH also unites with many other salts,
especially those of mercury, copper, and cobalt (which see).

1 The student is urged again to be methodical and thoughtful in his
laboratory exercises, especially in the execution of the reactions and
separations in this part of the book.

For convenience and economy of space, the reagents hereafter referred
to are generally expressed by molecular formulas, not by names; but
the student should not on this account contract the bad habit of calling
chemical substances by their formulas. For illustration, " hydrogen sul-
phide " not " H 2 S " should be the spoken name for the compound.

77



78 CHEMICAL ANALYSIS

Some seem to be derivatives pf ammonia, and others of the
quasi-metal, ammonium. (See Remsen's Inorganic Chemistry,
p. 274.)

The theory that these substances are chemical compounds
not " molecular additions " is in harmony with their conduct.
In all of them there has been a shifting of the atoms of the
metallic salts and ammonia to form more complex ions, thus pro-
ducing radical changes in their solubilities and chemical conduct.

2. B^S precipitates black silver sulphide, Ag 2 S, sol-
uble in boiling HNO 3 .

3. NH^OH 1 precipitates brown silver oxide, Ag 2 O,
soluble in excess of reagent.

4. NaOH gives similar results to NH 4 OH, except
that the precipitate is not soluble in excess of reagent.

5. K 2 CrO 4 precipitates dark-red silver chromate,
Ag a CrO 4 , soluble in hot HNO 3 and in NH 4 OH.

6. Metallic copper deposits metallic silver on its surface.

7. Reducing flame on charcoal with Na 2 CO 3 , or, better,
with the fusion mixture of Na 2 CO 3 and K 2 CO 3 , gives
a metallic silver bead.

Na 2 CO 3 acts partly as a flux, thus making possible the ioniza-
tion of AgNO 3 and Na 2 CO 3 , and partly as an active chemical
agent. The following reactions occur here :

2 AgN0 3 + Na 2 C0 3 - Ag 2 CO 3 + 2 NaNO,;
Ag 2 CO 3 + heat = Ag 2 O + CO 2 ;



Mercurous Mercury, Hg' (salt for study, mercurous
nitrate, Hg 2 (N0 3 ) 2 ).

1. HC1 precipitates white mercurous chloride or calo-
mel, Hg 2 Cl 2 , soluble in hot HNO 3 ; insoluble in
NH 4 OH, forming a black substance.



METALS OF GROUP I 79

- The black insoluble substance is amido-mercurous chloride
(" black precipitate "), NH 2 Hg / 2 Cl, or possibly a mixture of
NH 2 Hg"Cl (" white precipitate ") and metallic mercury :

Hg 2 Cl 2 + 2 NH 4 OH = NH 2 HgCl + Hg + NH 4 C1 + 2 H 2 O.

2. E^S precipitates a black mixture of mercuric sul-
phide 1 and mercury, HgS + Hg, soluble in aqua regia.

The action of aqua regia depends on the following :

3 HC1 + HN0 3 - C1 2 + NOC1 + 2 H 2 O ;
2 HgS + C1 2 + 2 NOC1 = 2 HgCl 2 + 2 NO + 2 S.

3. NH 4 OH produces a black precipitate 2 of unknown
composition, insoluble in excess of reagent.

4. NaOH produces a black precipitate of mercurous
oxide, Hg 2 O, insoluble in excess of reagent.

5. SnC^ precipitates gray metallic mercury, distinct
when boiled with HC1.

The action of SnCl 2 on mercurous salts depends on the tend-
ency of stannous salts to oxidize to the stannic condition, on
which account they act as energetic reducing reagents. Mercurous
salts are reduced to metallic mercury, and mercuric salts are first
reduced to mercurous and then to metallic mercury :

Hg 2 Cl 2 + SnCl 2 = 2 Hg + SnCl 4 ;
2HgCl 2 + SnCl 2 = Hg 2 Cl 2 + SnCl 4 ;
Hg 2 Cl 2 + SnCl 2 = 2 Hg + SnCl 4 .

6. KI precipitates green mercurous iodide, Hg 2 I 2 ,
soluble in acids.

7. Metallic copper deposits metallic mercury, distinct
when polished.

8. Heating in a closed tube, with fusion mixture,
deposits globules of metallic mercury on the sides of
the tube.



80 CHEMICAL ANALYSIS

Lead (salt for study, lead nitrate, Pb (N0 3 ) 2 ).

1. HC1 precipitates white lead chloride, PbCl 2 , solu-
ble in boiling water, and in concentrated HC1.

2. HgS precipitates black lead sulphide, PbS, soluble
in hot HNO 3 .

3. NH 4 OH and NaOH precipitate white lead hydroxide,
Pb(OH) 2 , soluble in excess of NaOH, forming the
sodium salt Na 2 PbO 2 . If NH 4 OH is used for pre-
cipitation, and NaOH for redissolving the precipitate,
the latter should be filtered and washed before add-
ing the NaOH. This is necessary in order to elimi-
nate ammonium salts which prevent the formation of
Na 2 Pb0 2 .

4. 1^804 precipitates white lead sulphate, PbSO 4 ,
soluble in hot NaOH and in NH 4 C 2 H 3 O 2 .

5. I^CrC^ precipitates yellow lead chromate, PbCrO 4 ,
soluble in NaOH, forming Na 2 PbO 2 .

6. KI precipitates yellow lead iodide, Pblg, 1 soluble
in hot water and in excess of KI.

7. Metallic zinc deposits metallic lead.

8. Reducing flame on charcoal, with fusion mixture,
precipitates metallic lead.

PROCESS OF SEPARATION

The separation of the members of this group is based
upon the fact that PbCl 2 is soluble in hot water,
and that AgCl is soluble in NH 4 OH. The process
of separation is as follows :

Add cold dilute HC1 so long as the white precipi-
tate continues to be formed; then add about ten drops



METALS OF GROUP I 81

in excess. Filter and wash 1 with cold water 2 (ice water
is preferable). Filtrate (a) may contain members of
all the subsequent groups ; residue (a) may consist of
PbCl 2 , AgCl, and Hg 2 Gl 2 . Pour boiling water on the
residue in the filter 3 and test the filtrate (b) with K 2 CrO 4
for lead, and, if present, continue the washing with hot
water till all traces of it disappear. Add warm NH 4 OH
to the residue (b) on the filter and test the filtrate (c)
for silver 4 by neutralizing with HNO 3 . Precipitation of
AgCl confirms the presence of silver. If a black residue
(c) is left after adding NH 4 OH, mercury is probably
present. Confirm by dissolving in a little aqua regia,
evaporating excess of chlorine and acids, and adding
SnCl 2 .

A certain amount of care must be observed on adding HC1,


1 2 3 5 7 8 9 10 11 12

Online LibraryJames Freeman SellersAn elementary treatise on qualitative chemical analysis → online text (page 5 of 12)