George Samuel Newth.

A manual of chemical analysis, qualitative and quantitative online

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Group II. is only capable of separating the metals of this family
from those of III., IV., and V., but not from those of Group I.
If, therefore, the metals of Group I. are not first separated, they may
be precipitated along with the members of the second family by
the group-reagent for that family.

The various group-reagents (or general reagents) and the
particular compounds of the metals which are precipitated by them
are indicated in the following table :

GROUP I. General reagent, Hydrochloric acid, HC1,

precipitates

AgCl, Hg'sCl,, PbCl 2 (partially soluble).
1, H 2 WO 4 .)*



GROUP II. General reagent, Sulphuretted hydrogen, H 2 S,

precipitates in acid solution

PbS, Hg S, Bi S , CuS, CdS \ Insoluble in ammo-
(Ru 2 S 3( Rh 2 S 3 , PdS, OsS.) / nium sulphide.

Sb 2 S 3 , As 2 S 3 , Sn n S and Sn iv S 2 , AuS, PtS 2 ] Soluble in



TeS 2( Se, MoS,) e

GROUP III. General reagent, Ammonium sulphide, (NH ( ) S,

precipitates in presence of ammonium chloride and ammonia
(a) Hydrated ( AlaCHO),,, Cr 2 (HO) 6 .

compounds ( Be < H )" Zr ( HO )4- T h(HO) 4> Ce(HO) 3 , Sc(HO) 3 , Y(HO),,
LS I La(HO) 3 , Yb(HO) 3 , H 2 TiO 3 , H 3 TaO 4 , H 3 NbO 4 .)

* The compounds represented by the formulas in small type are those of
the so-called rare elements. They are included in the table in order to give, in
a bird's-eye view as it were, not only their position in the scheme of classifica-
tion, but also the composition of their compounds, which are precipitated by
the group-reagents.

C



1 8 Qualitative Analysis.

\ FeS, NiS, CoS, MnS, ZnS.

() Sulphides j (UO a S, InS, T1 2 S) (Vanadium

converted into soluble ammonium thiovanadate*).

GROUP IV. General reagent, Ammonium carbonate,
(NHj)^CO,, precipitates in presence of ammonium chloride
and ammonia

BaCO . SrCO , CaCO .

GROUP V. No general reagent. The group consists of
(NH 4 ), Na, X, Mg. (Li, Rb, Cs.)

* Vanadium belongs to the "arsenic and antimony" family in the natural
classification of the elements. The sulphide is, however, not precipitated by
H 2 S, V 2 O 5 being thereby reduced to V 2 O 4 , which gives a blue colour to the
liquid. " NH 4 C1, in presence of ammonia, precipitates white ammonium meta-
vanadate, NH 4 VO 3 , but ammonium sulphide converts this into the soluble
ammonium thiovanadate, which gives a brown colour to the solution. The
true group-reagent, therefore, does not actually precipitate this metal.



CHAPTER III.
REACTIONS OF THE METALS OF GROUP V.

THIS group contains the alkali metals (ammonium being regarded
as a metal), and also the element magnesium, which is more nearly
allied to the metals of the alkaline earths. The members of this
family are not precipitated by any group-reagent, but they are
^with the exception of ammonium) separately tested for in the
solution which is obtained after the metals of Groups I. to IV. have
been removed. By referring to the table on p. 17, it will be seen
that, in the course of separating the various groups, certain am-
monium compounds are employed, therefore it will be obvious that
it is necessary to test for this "metal " in the substance under ex-
amination before adding any ammoniacal compounds.

Ammonium, NH .

DRY REACTIONS. When heated alone in a glass tube, ammo-
nium salts undergo change.

(a) If the acid is readily volatile, the salt dissociates, but the
ammonia and the volatile acid, as they together pass away from
the heated area, immediately reunite, reproducing the original
compound, which then settles or condenses on the cool part of the
tube, forming a sublimate.

[Generally, however, a certain small amount of the dissociated
portions of the compound escapes recombination : e.g. heat a small
quantity of ammonium chloride in a dry test-tube ; notice that
white fumes are produced, which sublime up the tube. Now hold
a moistened red litmus paper in the mouth of the tube, and it will
be turned blue, showing that a portion of the ammonia escapes
from the tube before it meets the hydrochloric acid from which
it has been dissociated. For a moment discontinue heating, and
presently the blued paper will be reddened, for the molecules of
hydrochloric acid which have lost their partners (the escaped
ammonia) now make their way up the tube and act on the litmus
paper.]



2O Qualitative Analysis.

(V) If the acid is non-volatile, or volatile only at a high tempera-
ture, then the ammonium salt is decomposed, ammonia being
evolved, while the acid remains.

[E.g. Heat a little ammonium sulphate or phosphate in a test-
tube ; ammonia is rapidly evolved, and may be detected by its
characteristic smell.]

(<r) The ammonium salts of certain oxyacids which readily part
with oxygen (such as ammonium nitrate, nitrite, chromate) are also
decomposed by heat, the ammonia being oxidised to nitrogen or
oxides of nitrogen.

[E.g. Heat a few crystals of ammonium nitrate in a test-tube.
Examine the gas with a taper and a glowing splint of wood.]

NH 4 N0 3 = 2H 2 + N 2
NH 4 NO 2 = 2H 2 O + N 2
(NH 4 ) 2 Cr 2 7 = Cr 2 3 + 4H 2 O + N 2

Ammonium is separated from the other members of the group
by evaporating the solution to dryness, and strongly heating the
residue until the ammonia is completely expelled, which may generally
be regarded as accomplished when fumes are no longer given off.

WET REACTIONS. Ammonium salts are all soluble in water,
therefore it is only in concentrated solutions that any precipitations
with reagents can be formed. Use ammonium chloride.

Caustic alkalies (NaHO or KHO) and oxides or hydroxides
of metals of the alkaline earths (e.g. CaO, Ba(HO) 2 ), when heated
with an ammonium salt, cause the evolution of ammonia gas, NH 3 .

(NH 4 ) 2 SO 4 + 2NaHO = Na 2 SO 4 + 2H 2 O + 2NH 3
2NH 4 C1 + CaO = CaCl 2 + H 2 O + 2NH 3

In practice, sodium hydroxide solution is added either to the solid
salt or to its solution in water, and the mixture gently warmed.
The evolved ammonia may be recognized (i) by its characteristic
odour if present in sufficient quantities ; (2) by its power of restoring
the blue colour to moist reddened litmus paper, or of turning tur-
meric paper brown ; (3) by the formation of white fumes when a
glass rod moistened with strong hydrochloric acid is held in the
mouth of the test-tube.

[In special cases, as in the examination of natural waters, minute
traces of ammonia are detected by the use of Nessler's solution (a
solution of potassium mercuric iodide in potash), which gives either
a brown precipitate or a coloration, according to the amount of
ammonia present, 2(HgI 2 ,2KI) -I- sKHO + NH 3 = NHg 2 "I,H 2 O
+ 7KI +2H 2 O.]



Group V. 2 1

Platinum chloride, PtQ 4 , precipitates from concentrated
solutions a yellow crystalline compound, ammonium chloro-
'platinate, (NH 4 ) 2 PtCl fi (sometimes called ammonium platinic
chloride, 2NH 4 C1, PtCl 4 ), soluble in 170 parts of water at 10 ;
insoluble in alcohol and ether. This compound is distinguished
from the similar potassium salt in that, when strongly heated, it
leaves a residue of spongy platinum only.

Tartaric acid, H 2 (C 4 H 4 O, ; ), or hydrogen sodium tartrate,
HNa(C 4 H 4 O 6 ), produces in concentrated solutions a white pre-
cipitate of hydrogen ammonium tartrate, H(NH 4 )(C 4 H 4 O 6 ). Soluble
in water, in mineral acids, and in alkalies; insoluble in alcohol.
This compound is distinguished from the corresponding potassium
salt by the fact that when strongly heated, the carbonaceous residue
is without any alkaline reaction.

[When tartaric acid is used, the acid previously in combination
with the ammonia is liberated by the double decomposition, thus

NH 4 C1 + H 2 (C 4 H 4 6 ) = H(NH 4 )(C 4 H 4 6 ) + HC1

And as the precipitate is soluble in mineral acids, the delicacy of the

reaction is increased by employing hydrogen sodium tartrate as the

reagent, in which case no free acid is formed in the reaction, thus

NH 4 C1 + HNa(C 4 H 4 G ) = H(NH 4 )(C 4 H 4 O 6 ) + NaCl]

Sodium, Na.

DRY REACTION. Sodium compounds, when heated upon a
platinum wire in a Bunsen flame, undergo volatilisation, and
impart to the flame a brilliant golden yellow colour. This flame-
reaction is the most characteristic and delicate test for this metal.*
WET REACTIONS. All sodium salts are soluble ; sodium platino-
chloride is soluble in water, in alcohol, and in ether. Hydrogen
sodium tartrate also is freely soluble in water. Sodium pyroanti-
monate,t however, is less soluble in water than the corresponding

* When the light emitted by heating a sodium salt in the Bunsen flame is
examined by the spectroscope (see p. 27), it is found to be monochromatic, i.e.
to consist of one colour only, namely, pure yellow light. Any coloured object
whose colour contains no yellow in its composition, when illuminated only by
the light emitted by the sodium flame, appears black. The red colouring matter
in blood, for example, contains no yellow ; hence blood, when seen only in the
light of the sodium flame, looks black: for this reason, the hands and face of a
person illuminated only by the sodium flame appear black and dirty in propor-
tion as the individual has a high colour or not. Indigo, and many other
common coloured materials, similarly absorb the yellow light emitted by sodium,
hence if the sodium flame be viewed through a thin stratum of such a coloured
solution, the yellow light is entirely intercepted. For the use that is made of



this property, see potassium (p. 22).
f Formerly misnamed sodiu



iium metantimonate. For a comparison of phos-
phates, arsenates, and antimonates, see Newth's " Inorganic Chemistry," p. 458.









22



Qualitative Analysis.



potassium salt, and is therefore precipitated by the addition of a
strong solution of potassium pyroantimonate to a strong solution of
a sodium salt, such as sodium chloride, thus

H 2 K 2 Sb 2 O 7 + 2NaCl = H 2 Na 2 Sb 2 O, + 2KC1

Potassium, K.

DRY REACTION. When potassium compounds are heated upon
a platinum wire in a Bunsen flame, they impart to the flame a pale
violet or lilac colour. This delicate tint, however, is completely
masked by the intense yellow colour which the presence of even
minute quantities of sodium compounds impart to the flame.

Introduce a fragment of potassium nitrate into the Bunsen
flame upon a loop of clean platinum wire ; * notice the lilac colour
imparted to the flame. Now look at the flame through a potassio-
scopefi ,and observe that it appears a brilliant crimson- red colour.
Upon another wire introduce a particle of sodium chloride into the
flame, and notice that when this is examined through the potassio-
scope, the intense golden yellow light is absolutely cut off, and is
invisible. Now touch the wire containing the nitre with a fragment
of sodium chloride, and again bring it into the flame. The yellow
of the sodium completely overpowers and masks the violet of the
potassium when viewed direct, but if looked at through the potassio-
scope, the red colour due to the potassium shines up as brilliantly as
before, while the yellow is completely intercepted \ (see also p. 33).

* By merely touching the wire with the fingers, it contracts sufficient sodium
compounds to give the yellow flame. To clean it, it should be dipped in hydro-
chloric acid and heated until it ceases to
impart any colour to the flame.

f The potassioscope is a little instru-
ment consisting of a flat circular glass cell
filled with a solution of one of the rose-
aniline blue colouring matters. It possesses
the advantage over the older indigo prism,
in that it not only makes it possible to de-
tect potassium in the presence of sodium
(which a solution of indigo does equally
well), but, owing to the fact that it entirely
intercepts the red light emitted by lithium,
barium, strontium, and calcium, it ren-
ders it possible to detect potassium with
certainty, even when any of these other
elements are present.

When studying flame reactions, it is
often of the greatest convenience to use a
stand on which to support the platinum
wire, so that the hands may be free ; a
simple stand is readily constructed as
shown in Fig. 8. A piece of glass tube
or glass rod is inserted in a large cork
(rubber, being heavier, makes a steadier
loot), and a piece of galvanized iron wire is twisted two or three times round




FIG. 8.



Group V. 23

WET REACTIONS. Most potassium salts are soluble in water.
Use a solution of potassium chloride.

Platinum chloride, PtCl 4 , produces, with concentrated solu-
tions of potassium salts, a yellow crystalline precipitate of potassium
chloro-platinate (or potassium platinic chloride), K 2 PtCl 6 , soluble
in 1 10 parts of water at 10 (therefore more soluble than the
corresponding ammonium compound). Soluble in alkalies (there-
fore the solutions used should be acid) ; nearly insoluble in alcohol ;
quite insoluble in a mixture of alcohol and ether (therefore the .
precipitation of this compound is promoted by the addition of
alcohol).

Hydrogen sodium tartrate, HNa(C 4 H 4 O 6 ), gives, with solu-
tions of potassium salts, a white crystalline precipitate of hydrogen
potassium tartrate, HK(C 4 H 4 O 6 ) ; soluble in much water, and also
in acids and alkalies (therefore the solution should be both concen-
trated and neutral). The precipitate is insoluble in alcohol.

Hydrofluosilicic acid (or silico-flnoric acid), H 2 SiF 6 ,
throws down a white precipitate of gelatinous appearance, consisting
of potassium silicofluoride, ICjSiFg, sparingly soluble in water.



Magnesium, Mg.

In the " natural classification " of the elements, magnesium is
associated with the metals of the alkaline earths (Be, Ba, Sr, Ca)
on the one hand, and with zinc and cadmium on the other. Its
position along with the alkalies in Group V. of the analytical
classification, is simply because it differs from the other members of
its own natural family in that the presence of ammonium chloride
prevents the precipitation of magnesium hydroxide by ammonia
in Group III., and also of magnesium carbonate by the group
reagent of Group IV.

DRY REACTION. When magnesium salts are strongly heated
in the outer blowpipe, a white infusible residue of the oxide is left.
If, after cooling, the residue be moistened with a drop or two of
cobalt nitrate solution and again strongly heated in the outer blow-
pipe flame, the mass acquires a pink colour. This reaction is
reliable only in the absence of other metallic oxides.

WET REACTIONS. Of the common salts of magnesium, the
sulphate, chromate, nitrate, and halogen salts are soluble in water.
One prominent characteristic of magnesium compounds is the

the rod with one end projecting at right angles to the upright. The little
glass tube into which the platinum wire is fused, is then slipped over the project-
ing iron wire. This arrangement admits of the wire being raised or lowered as
desired, while at the same time it readily remains in any position.



24 Qualitative Analysis.

readiness with which they form " double " salts, many of which are
soluble in water. Use magnesium sulphate.

Alkaline hydroxides (NH 4 HO, KHO, NaHO, Ca(HO) 2 , or
Ba(HO) 2 ) precipitate from solutions of magnesium sulphate or
chloride, white magnesium hydroxide, Mg(HO) 2 . Almost insoluble
in water ; soluble in acids, soluble in ammonium chloride

Mg(HO) 2 + 4NH 4 C1 - 2NH 4 HO + MgCl 2 ,2NH 4 Cl (soluble
double salt)

.Owing to the solubility of magnesium hydroxide in ammonium
chloride, only -half the magnesium is precipitated from magnesium
chloride by means of ammonia, thus

2MgCl 2 + 2NH 4 HO = Mg(HO) 2 + MgCl 2 ,2NH 4 Cl
If ammonium chloride is previously present in sufficient quantity to
combine with the whole of the magnesium hydroxide, the alkaline
hydroxides give no precipitate.

Alkaline carbonates (K 2 CO 3 , Na 2 CO 3 , (NH 4 ) 2 CO 3 ) pro-
duce in solutions of magnesium salts, in the absence of ammonium
salts, precipitates of basic carbonates of magnesium, the composition
of which varies with conditions of temperature and concentration.
The precipitate with (NK 4 ) 2 CO 3 only separates out after a short
time. In the presence of ammonium chloride tJiese reagents give no
precipitate.

Hydrogen disodium phosphate, HNa 2 PO 4 , precipitates
hydrogen magnesium phosphate, HMgPO 4 , and tri-magnesium
phosphate, M*g 3 (PO 4 ) 2 . In the presence of ammonium chloride,
however, the double ammonium magnesium phosphate is thrown
down as a white crystalline precipitate, NH 4 MgPO 4 . It is
appreciably soluble in water, but insoluble in ammonia; hence
ammonia must be previously added.

In very dilute solutions the precipitation only takes place on
long standing. It is accelerated by stirring with a glass rod, the
deposition first appearing where the rod has rubbed the glass
vessel. The precipitate is soluble in acids, even acetic acid, but
reprecipitated by ammonia.

SYSTEMATIC PLAN OF ANALYSIS FOR THE METALS OF

GROUP V.

After having carefully gone through the various reactions for
the metals of Group V., the student should proceed to the examina-
tion of a few solutions containing mixtures of two or more salts of
these metals.



Detection of the Metals of Group V. 25

The various special tests by which the individual members of
this group are recognised are not, for the most part, interfered with
by the presence of the rest of the group. Therefore a complete
separation of all the metals is not necessary. Thus, the test for
ammonium (evolution of ammonia by heating with sodium
hydroxide) can be made in the presence of Mg, K, and Na com-
pounds ; and obviously must be made in a separate portion of the
solution under examination from that in which sodium is to be
tested for, as it involves the addition of a sodium compound.

The test for magnesium, likewise (precipitation of NH 4 MgPO 4 ),
may be made in the presence of all the other members ; and
clearly must be made also in a separate portion of the solution, as
it involves the addition of both ammonium and sodium compounds.
Similarly, the flame tests for potassium and sodium are not inter-
fered with by the presence of ammonium or magnesium. The
flame reaction for potassium, however, unless examined by the aid
of the potassioscope, must always be corroborated by the forma-
tion of potassium chloroplatinate. But before this test can be
applied, ammonium salts must first be removed.

Solutions may be examined for the metals of Group V.,
NH 4 , Na, K, Mg, by the following system :

DETECTION OF THE METALS OF GROUP V.

Operation i. To a portion of the solution add NaHO, and heat
in a test-tube. The evolution of ammonia (detected by its odour,
and its action on test-papers) proves the presence of NH 4 .

Operation 2. To a second portion add NH 4 C1, NH 4 HO, and
HNa 2 PO 4 . A white crystalline precipitate of NH 4 MgPO 4 proves
the presence of Mg.

Operation 3. Evaporate another (and larger) portion to dryness
in a porcelain dish. If ammonium salts are present * (already
ascertained in Operation i), they must be removed. For this pur-
pose, scrape the residue out of the dish and strongly heat it on the
lid of a platinum crucible (or a piece of platinum foil), until, on
momentarily withdrawing it from the flame, fumes are no longer
visible.

Dissolve the residue in a small quantity of water, and add one
drop of HC1. Dip a clean platinum loop into the solution, and heat

* In a complete analysis, ammonium salts are always present here, as they will
have been introduced in the process of separating the other groups. Under
these circumstances, therefore, the operation of removing ammonium com-
pounds is always necessary. The substance under analysis is tested for
ammonium before the ammoniacal reagents are introduced.



26 Qualitative Analysis.

it in a Bunsen flame. An intense * yellow coloration proves the
presence of Na. A lilac colour indicates the presence of K.

In either case examine the flame through the potassioscope ; a
crimson flame indicates K.

Add to the solution of the residue a few drops of PtCl 4 , and stir
with a glass rod. A yellow precipitate of K 2 PtCl 6 confirms K.



APPENDIX TO CHAPTER III.

LITHIUM, RUBIDIUM, AND CAESIUM.

These three elements are usually placed in the category of rare
metals. It must be remembered, however, that there are degrees 01
rarity ; and while the compounds of rubidium and caesium are
certainly among the very rare substances with which the chemist
comes into contact, those of lithium, on the other hand, are very
widely distributed and are much more frequently met with.f

Lithium, Iii.

DRY REACTION. Lithium salts impart to the flame a brilliant
carmine-red colour.

WET REACTIONS. All the common salts are readily soluble in
water, except the carbonate, phosphate, and oxide, which are
soluble with difficulty. The chloride and nitrate are soluble in a
mixture of alcohol and ether (distinction from Na and K, the
chlorides and nitrates of ivhich are not soluble}.

Na 2 CO 3 and K 2 CO 3 precipitate Li 2 CO 3 from cold moderately
concentrated solutions (i part dissolves in 100 parts of water).

HNa 2 PO 4 gives a white precipitate, on boiling, of Li 3 PO 4 . The
precipitation is complete in the presence of NaHO.

PtCl 4 gives no precipitate (distinction from NH 4 , K, Rb, Cs).

HNa(C 4 H 4 O 6 ), hydrogen sodium tartrate, gives no precipitate.

Rubidium, Kb, and Caesium, Cs.

The compounds of these metals present the very closest
resemblance to those of potassium, and there are scarcely any

* More or less of a yellow flame is usually obtained, owing to the presence
of traces of sodium compounds as impurities in the reagents previously used in
separating the groups in the course of a complete analysis.

f Perhaps a rough idea of the relative rarity of the compounds of these
metals might be gained by a comparison of their cost. Lithium salts can be
obtained for about 12^. per pound, while rubidium and caesium salts cost
about 5-y. per drachm, i.e. at the rate of ^64 per pound.



The Rare Metals of Group V. 27

chemical reactions by which they can be distinguished. The
separation of these metals is based on the different degrees of
solubility of their chloro-platinates. In this respect, as well as in
their other properties,* rubidium stands intermediate between
potassium and caesium.

When heated in a Bunsen flame, rubidium and caesium salts
impart to it a lilac colour, which to the unaided eye is absolutely
indistinguishable from that produced by potassium compounds ;
and when compounds of these metals, as well as those of lithium,
are mixed with comparatively minute quantities of sodium salts, the
colours they give to the flame are completely overpowered and
masked by the yellow of the sodium. By means of the spectroscope,
however, not only are the apparently identical colours given by
potassium, rubidium, and caesium proved to consist of light of
different quality or composition, but the presence of any or all of
them is easily and certainly detected even when admixed with
sodium salts.

The spectroscope is an instrument by means of which the light
emitted by strongly heated substances can be examined after it has
been made to pass through a glass prism. Its use depends upon
the fact that different coloured lights possess different degrees of
refrangibility ; that is to say, different coloured rays of light are
bent out of their straight course, by passage through a prism, at
different angles. Ordinary white light is composed of rays of all
degrees of refrangibility, hence, when such light passes through a
prism, the various coloured rays are separated, and spread out in the
order of their refrangibility, the least refrangible red at one
extreme, to the deep violet at the other. This familiar " rainbow "
coloured band of light is called the continuous spectrum.

In the spectroscope the light is passed through a narrow slit at
one end of a small telescope, and an image of the slit is received
upon a glass prism. This bends the light out of its straight course,
and spreads it out into the various colours of which it is composed.
If white light be admitted, then the continuous spectrum is seen,
which is an infinite number of images of the slit arranged side by
side ; if such a monochromatic light as that given by heating sodium
salts in the flame be used, then one image of the slit is seen in that
part of the spectrum which corresponds to the particular degree of
refrangibility of the light. In this case we say that the spectrum
of sodium consists of one line (that is, one image of the slit) in the
yellow, or one yellow iine.\ The light which passes out of the prism
is usually examined by a second telescope, which can be revolved



Online LibraryGeorge Samuel NewthA manual of chemical analysis, qualitative and quantitative → online text (page 3 of 45)