James Freeman Sellers.

An elementary treatise on qualitative chemical analysis online

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only the phosphates of Groups IV and V, and magnesium, not
those of Group III. But, though HC 2 H 3 O 2 is set free, its acid
effects are greatly weakened by the presence of an excess of a salt
having an ion in common with it ; and for this reason some free
HC 2 H 3 O 2 must be added with NaC 2 H 3 O 2 , though a large excess
of the acid should be avoided, as CrPO 4 is sparingly soluble in
it. (See Question 5, Theory of Solutions, p. 22.)



CHAPTER X

METALS OF GROUP IV: ZINC, MANGANESE, COBALT,
AND NICKEL

CHARACTERISTIC : Insolubility of the sulphides in alkaline solu-
tion.

GROUP REAGENT : Ammonium sulphide in presence of ammo-
nium hydroxide and ammonium chloride.

REACTIONS

Zinc (salt for study, zinc sulphate, ZnS0 4 ).

1. (NH 4 ) 2 S precipitates white zinc sulphide, ZnS,
soluble in strong acids ; insoluble in alkalies and
HC 2 H 3 O 2 . The precipitation is hastened in dilute
solutions by presence of NH 4 C1.

2. NH 4 OH and NaOH precipitate white zinc hydrox-
ide, Zn(OH) 2 , soluble in excess of reagent, forming
ammonium zincate, (NH 4 ) 2 ZnO 2 , 1 or sodium zincate,
Na 2 ZnO 2 . NH 4 C1 solutions dissolve Zn(OH) 2 .

3. Reducing flame on charcoal with Na 2 CO 3 gives a
yellow incrustation of zinc oxide, ZnO, turning white on
cooling. If this coating is moistened with cobalt nitrate
and again heated with the blowpipe, a green coloration
will appear, due to a double oxide of zinc and cobalt.

Manganese (salt for study, manganese sulphate, MnS0 4 ).

1. (NH 4 ) 2 S precipitates pink 2 manganous sulphide,
MnS, soluble in acids, including HC 2 H 3 O 2 ; insoluble
in alkalies. NH 4 C1 assists precipitation.

114



METALS OF GROUP IV 115

2. NH 4 OH and NaOH precipitate manganous hydrox-
ide, Mn(OH) 2 , which oxidizes to brown hydrated man-
ganic oxide, Mn 2 O 2 (OH) 2 . NH 4 C1 partly redissolves
Mn(OH) 2 if precipitated by NH 4 OH, but only partially
if precipitated by NaOH.

3. Fusion with Na 2 CO 3 and KNO 3 on a platinum foil
gives a soluble green mass of sodium manganate,
Na 2 MnO 4 , changing to red sodium permanganate,
Na 2 Mn 2 O 8 , on heating or acidifying. This reaction is
also produced by adding Na 2 O 2 to the solution and
warming till effervescence ceases.

4. Boiling with PbO 2 and H 2 SO 4 gives a deep-red
coloration of permanganic acid, H 2 Mn 2 O 8 .

5. Borax bead in the oxidizing flame gives an amethyst
coloration, sodium manganic metaborate, Na 6 Mn 2 (BO 2 ) 12 .
This color is destroyed by the reducing flame, due to the
formation of manganous metaborate.

6. Spectrum (see Special Method for Manganese, p. 63).

CHEMISTRY OF MANGANESE COMPOUNDS

There are six important oxides of manganese having the follow-
ing formulae : MnO, Mn 3 O 4 , Mn 2 O 3 , Mn() 2 , MnO 3 , Mn 2 O 7 . Of
these, the first, second, and third are basic, the first forming man-
ganous salts with acids, and the third, manganic salts. The fourth
oxide is both basic and acid, forming manganous salts with loss of
oxygen and also forming unstable manganites with more basic
oxides. The fifth and sixth are acid oxides and produce, respec-
tively, manganates and permanganates. The ready conversion of
the lower basic oxides to the higher acid oxides by oxidation is of
importance in qualitative analysis, on account of the solubility and
characteristic colors of the salts of the higher oxides. Reactions
3, 4, and 5 illustrate these effects.



116 CHEMICAL ANALYSIS

The higher oxides are also easily reduced to the lower, and
for this reason are extensively used in quantitative analysis.

The following equations illustrate a few of the many cases of
oxidation depending on the reduction of the higher oxides of
manganese :

(a) Oxidation of oxalic acid :

5H 2 C 2 4 + K 2 Mn 2 8 + 3H 2 SO 4 = 8H 2 O + 10CO 2 + 2MnSO 4
+ K 2 S0 4 ;

(5) Oxidation of hydrochloric acid :

K 2 Mn 2 O 8 + 16 HC1 = 2 KC1 + 2 MnCl 2 + 8 H 2 O + 10 Cl;
(c) Oxidation of ferrous to ferric salts :

10FeSO 4 + K 2 Mn 2 O 8 + 8 H 2 SO 4 = 5 Fe 2 (SO 4 ) 3 + K 2 SO 4
-f 2MnSO 4 + 8H 2 O.

In all these cases the reduction of K 2 Mn 2 O 8 is accompanied by
the destruction of the brilliant red color of the salt.



Cobalt (salt for study, cobalt nitrate, Co(N0 3 ) 2 ).

1. (NH 4 ) 2 S precipitates black cobaltous sulphide,
CoS, insoluble in cold dilute HC1 ; soluble in HNO 3
and in aqua regia. The presence of NH 4 C1 aids the
precipitation of CoS.

2. NH 4 OH precipitates blue cobaltous hydroxide,
Co(OH) 2 , soluble to a brown fluid in excess of re-
agent. NH 4 C1 hinders the precipitation.

3. NaOH precipitates blue cobaltous hydroxide,
Co(OH) 2 , not soluble in excess of reagent. NH 4 C1
prevents precipitation.

4. KCN precipitates light-brown cobaltous cyanide,
Co(CN) 2 , soluble in excess of reagent with formation
of potassium cobaltous cyanide, K 4 Co(CN) 6 . This
compound is decomposed by acids, in the absence of



METALS OF GROUP IV 117

an excess of KCN, with reprecipitation of Co(CN) 2 .
If K 4 Co(CN) 6 is oxidized with a mixture of NaOH and
bromine water, containing NaBrO, it changes to the
stable -ic salt, K 3 Co(CN) 6 , which will yield no precipi-
tate with acids.

5. KN0 2 added to the solution strongly acidified with
HC 2 H 3 O 2 gives a yellow precipitate, potassium cobaltic
nitrite, K 3 Co(NO 2 ) 6 . The precipitate will separate
after some hours in a warm place.

K 3 Co(NO 2 ) 6 is readily broken down by strong acids and alkalies,
but .is insoluble in HC 2 H 3 O 2 and in a solution of KNO 2 . Hence,
if strong acid is present, it is necessary to neutralize it with
Na 2 CO 3 and then to add an excess of HC 2 H 3 O 2 and KNO 2 .

6. Borax bead with both the oxidizing and reducing
flames gives a blue coloration of sodium cobaltous
metaborate, Na 2 Co(BO 2 ) 4 .

Nickel (salt for study, nickel chloride, NiCl 2 ).

1. (NH 4 ) 2 S precipitates black nickel sulphide, NiS,
sparingly soluble in excess of reagent ; soluble in hot
HNO 3 or aqua regia ; almost insoluble in dilute HC1.
The presence of NH 4 C1 aids the precipitation of NiS.

2. NH 4 OH precipitates greenish nickelous hydroxide,
Ni(OH) 2 , soluble in excess of reagent, forming a blue
fluid. NH 4 C1 prevents the precipitation.

3. NaOH precipitates Ni(OH) 2 , insoluble in excess of
reagent, but soluble in NH 4 C1.

4. KCN precipitates greenish nickelous cyanide,
Ni(CN) 2 , soluble in excess of reagent, forming potas-
sium nickelous cyanide, K 2 Ni(CN) 4 . NaBrO does not



118 CHEMICAL ANALYSIS

oxidize K 2 Ni(CN) 4 to the corresponding -ic salt, but to
black nickelic hydroxide, Ni(OH) 3 .

5. KN0 2 produces no precipitate with nickel salts.

6. Borax bead in the oxidizing flame gives yellow
Na 2 Ni(BO 2 ) 4 ; in the reducing flame it gives gray
metallic nickel.

Why concentrated IIC1 does not dissolve CoS and NiS has not
been satisfactorily explained. It would seem reasonable that as
II 2 S does not precipitate the sulphides of cobalt and nickel from
a solution of their chlorides, their sulphides should be soluble in
HC1. It has been surmised that immediately after precipitation
the sulphides undergo polymerization, i.e., a locking together of
several of their molecules, to form very insoluble compounds,
(CoS) x and (NiS) x .

PROCESS OF SEPARATION

The separation of the members of this group is based
upon the facts that Zn(OH) 2 is soluble in excess of
NaOH ; that MnS is soluble in HC 2 H 3 O 2 ; and that
the borax bead, KNO 2 , and KCN with NaBrO give dis-
tinctive reactions with cobalt and nickel salts. The
process of separation is a,s follows :

Boil off excess of NH 4 OH, add NH 4 C1, and then
(NH 4 ) 2 S in moderate excess. Filter and wash 1 thor-
oughly, rejecting the washings. The residue (a) may
consist of the sulphides of the group. The nitrate (a)
may contain members of Groups V and VI, and pos-
sibly Mn, which separates slowly from dilute solutions.
MnS oxidizes readily to brown Mn 2 O 2 (OH) 2 , espe-
cially if in dilute solutions, or if the residue is exposed
to the air for a short while, and this may appear as






METALS OF GROUP IV 119

a brown precipitate l in the filtrate from the sulphides
of Group IV. Therefore set this filtrate aside, and
if such a precipitate appears, filter and preserve the
filtrate to be tested for Groups V and VI.

Dissolve the residue (a) in boiling dilute HC1, to
which a small crystal of KC1O 3 is added. Continue
the boiling till all free chlorine is expelled ; then add
while stirring an excess of NaOH. After cooling, filter.
The filtrate (b) may contain Na 2 ZnO 2 , and the residue (b)
may consist of Mn(OH) 2 , Co(OH) 2 , and Ni(OH) 2 . Pass
H 2 S through the filtrate (b). A white precipitate con-
firms the presence of zinc.

The washed residue (b) is dissolved in a small
quantity of hot HC1. After nearly neutralizing with
NH 4 OH, some NH 4 C 2 H 3 O 2 is added and II 2 S is passed
till the precipitation is completed. Filter. The resi-
due (c) may consist of CoS and NiS. The filtrate (<?)
may contain Mn(C 2 H 3 O 2 ) 2 , and is to be concentrated
to a small bulk and tested in either of the following
ways :

First: Add NH 4 OH, NH 4 C1, and (NH 4 ) 2 S. A yel-
low or green precipitate, appearing after some time,
confirms MnS.

Second: Add a solution of Na 2 CO 3 , dissolve the
white precipitate in HC1, and add NH 4 OH, NH 4 C1,
and (NH 4 ) 2 S. In case of doubt, test the solution for
manganese with the spectroscope.

Test the residue (c) with the borax bead for cobalt. .

Dissolve the residue (c) in a very little aqua regia,
evaporate off chlorine and excess of acids, and divide
into two parts.



120 CHEMICAL ANALYSIS

To one portion add Na 2 CO 3 till alkaline, then
HC 2 H 3 O 2 in excess, and finally add some solid KNO 2 .
After standing twenty-four hours in . a warm place, a
yellow precipitate confirms K 3 Co(NO 2 ) 6 . Filter and
add excess of NaOH, which will precipitate Ni(OH) 2
if nickel is present.

Make the second part neutral by adding NaOH, test-
ing with litmus paper ; then add KCN until the yellow
precipitate redissolves. Digest with constant stirring
for 10 minutes, or till the dark color disappears. Filter
into a large test-tube, and add to the filtrate an equal
bulk of NaOH and sufficient bromine water to produce
a permanent red color. A black precipitate on gently
warming confirms nickel.



CHAPTER XI

METALS OF GROUP V : BARIUM, STRONTIUM, AND CALCIUM

CHARACTERISTIC : Insolubility of the carbonates in alkaline
solution.

GROUP REAGENT : Ammonium carbonate in presence of ammo-
nium hydroxide and ammonium chloride.

REACTIONS

Barium (salt for study, barium nitrate, Ba(N0 3 ) 2 ).

1. (NH 4 ) 2 C0 3 precipitates white barium carbonate,
BaCO 3 , soluble in acids (except H 2 SO 4 ) and in acid
ammonium carbonate, H(NH 4 )CO 3 . As (NH 4 ) 2 CO 3
easily dissociates into H(NH 4 )CO 3 and NH 3 , it is
necessary to add NH 4 OH before (NH 4 ) 2 CO 3 . NH 4 C1
solutions dissolve BaCO 3 slightly, especially, while
boiling, giving off NH 3 and CO 2 .

2. H2S0 4 (also CaSO 4 and SrSO 4 ) precipitates white
barium sulphate, BaSO 4 , insoluble in dilute acids and
alkalies; somewhat soluble in hot concentrated acids.

3. (NH 4 ) 2 C 2 4 precipitates white barium oxalate,
BaC 2 O 4 , soluble in acids, including HC 2 H 3 O 2 .

4. I^CrC^ precipitates yellow barium chromate,
BaCrO 4 , soluble in HC1 and HNO 3 ; somewhat solu-
ble in HC 2 H 3 O 2 and in NH 4 C1; insoluble in potas-
sium dichromate, K 2 Cr 2 O 7 , which latter salt is formed
by the action of acids on K 2 CrO 4 :

2K 2 Cr0 4 + 2HC1 = K 2 Cr 2 O 7 + 2 KC1 + H 2 O.

121



122 CHEMICAL ANALYSIS

Hence, in acid solutions of barium salts, K 2 CrO 4
should be added in excess to neutralize the acids.

5. Ether-alcohol does not dissolve barium nitrate,
Ba(NO 3 ) 2 . As Ba(NO 3 ) 2 is soluble in water, it is
necessary to conduct the experiment with a perfectly
dry salt.

6. Heated on a platinum wire in the flame of the
Bunsen lamp, barium salts give a yellow-green color,
which, seen through a cobalt glass, appears blue-green.

7. Spectrum (see Table VII, p. 58).

Strontium (salt for study, strontium nitrate, Sr(N0 3 ) 2 ).

1. (NH 4 ) 2 C0 3 precipitates white strontium carbonate,
SrCOg, which for the most part behaves towards rea-
gents like BaCO 3 . It is less soluble in NH 4 C1 than
BaCOg.

2. ILjSC^ (also CaSO 4 ) precipitates white strontium
sulphate, SrSO 4 , sparingly soluble in water, but more
soluble in HC1 and HNO 3 ; insoluble in (NH 4 ) 2 SO 4 .

3. (NH 4 ) 2 C 2 4 gives a reaction similar to 3, under
barium, except that SrC 2 O 4 is almost insoluble in
HC 2 H 3 2 .

4. K^CrO^ does not precipitate yellow strontium chro-
mate, SrCrO 4 , except on long-standing and in concen-
trated neutral solutions. SrCrO 4 is quite soluble in



5. Ether-alcohol does not dissolve strontium nitrate,
Sr(NO 3 ) 2 , but as it is very soluble in water, it is essen-
tial to conduct the experiment with a freshly heated
anhydrous salt.



METALS OF GROUP V 123

6. Heated in a non-luminous flame on a platinum
wire, strontium salts give a deep red color, which, seen
through a blue glass, appears blue.

7. Spectrum (see Table VII, p. 58).

Calcium (salt for study, calcium nitrate, Ca(N0 3 ) 2 ).

1. (NH 4 ) 2 C0 3 gives a reaction similar to 1, under
barium, except that CaCO 3 is more soluble than BaCO 3
in NH 4 C1 and in H 2 S0 4 .

2. I^SO^ precipitates white calcium sulphate, CaSO 4 ,
sparingly soluble in water and acids ; insoluble in
alcohol. In this latter experiment it is necessary
that the calcium salt be in a concentrated solution.

3. (NH 4 ) 2 C 2 4 precipitates white calcium oxalate, sol-
uble in strong acid; insoluble in HC 2 H 3 O 2 .

4. K 2 Cr0 4 gives no precipitate if the solution is acidi-
fied with HC 2 H 3 O 2 .

5. Ether-alcohol dissolves calcium nitrate, Ca(NO 3 ) 2 .

6. Heated in a non-luminous flame on a platinum wire,
calcium salts give a pale red color, which appears green-
gray if seen through a blue glass.

7. Spectrum (see Table VII, p. 58).

PROCESS OF SEPARATION

The separation of the members of this group may be
made by three methods, which are based on the follow-
ing phenomena :

Method 1. (a) Insolubility of Ba(NO 3 ) 2 and Sr(NO 3 ) 2

in ether-alcohol;

(b) Insolubility of BaCrO 4 in K 2 Cr 3 O 7 solu-
tion.



124 CHEMICAL ANALYSIS

Method 2. (a) Insolubility of BaCrO 4 in K 2 Cr 2 O 7 solu-
tion ;

(b) Insolubility of SrSO 4 in (NH 4 ) 2 SO 4 solu-
tion.

Method 3. Differences between the spectra of barium,
strontium, and calcium salts.

The solution is first made alkaline with NH 4 OH,
NH 4 C1 is added in moderate quantity, and finally
(NH 4 ) 2 CO 3 solution is added till the precipitation is
completed. The mixture should be warmed (not boiled),
filtered, and washed with ammoniated water. The
filtrate (a) may contain members of Group VI. The
residue (a) may consist of BaCO 3 , SrCO 3 , and CaCO 3 .

Method 1. Dissolve the residue (a) in an evaporat-
ing dish with a minimum of dilute HNO 3 , and evapo-
rate carefully to dryness. Remove to an iron plate (a
small clean sand bath may be used) and heat to dull
redness, till all traces of moisture are expelled, tested
by holding a cold dry watch-glass or beaker over the
dish. After cooling, quickly triturate the mass in a
dry mortar with about 10 c.c. ether-alcohol. Transfer
to a small dry flask and shake the corked flask at inter-
vals. After about an hour filter through a dry paper
(filtrate (b)) and wash the residue (b) with a little ether-
alcohol till the drippings show no cloudiness with dilute
H 2 SO 4 . Dissolve the residue (5), possibly consisting
of Ba(NO 3 ) 2 and Sr(NO 3 ) 2 , in warm water acidified
with a few drops of HC 2 H 3 O 2 , and while boiling add
K 2 CrO 4 till the solution ceases to smell of HC 2 H 3 O 2 .
A yellow precipitate confirms BaCrO 4 . Filter, and to
the filtrate (<?) add NH 4 OH and (NH 4 ) 2 CO 3 . A white
precipitate confirms SrCO 3 . To the filtrate (b) add



METALS OF GROUP V 125

some dilute H 2 SO 4 . A white precipitate confirms
CaS0 4 .

In the separation of the nitrates of the group with ether-alco-
hol, not only the external moisture but also the water of crystal-
lization of the nitrates must be expelled. The temperature may
reach 180 and not injure the salts, but above that degree the
nitrates are dissociated into the insoluble oxides.

Method 2. Dissolve the residue (a) in a small amount
of dilute HC 2 H 3 O 2 and add K 2 CrO 4 till the solution
ceases to smell of HC 2 H 3 O 2 . A yellow precipitate,
residue (5), confirms BaCrO 4 . Filter. Add to the
filtrate (b) a concentrated solution of (NH 4 ) 2 SO 4 , and
boil. Filter. The residue (c) indicates SrSO 4 , which can
be confirmed with the flame. Add to the filtrate (c) some
(NH 4 ) 2 C 2 O 4 . A white precipitate confirms CaC 2 O 4 .

Method 3. Dissolve the residue (a) in the least pos-
sible amount of HC1 and evaporate to a thin paste.
Test with the spectroscope for barium, strontium, and
calcium.

Magnesium, in many respects, behaves like the three other
alkali-earth metals barium, strontium, and calcium. In one
respect, however, magnesium differs very materially from the
other kindred metals. Its salts readily dissolve in ammonium
salts, but only to a very limited degree do ammonium salts affect
the solubility of the salts of barium, strontium, and calcium.
Hence NH 4 C1 must be added with (NH 4 ) 2 CO 3 to prevent the
precipitation of MgCO 3 , but an excess of the reagent must be
avoided lest it also dissolve the other carbonates. Heating is
usually conducive to precipitation ; but in this case great heat
dissociates (NH 4 ) 2 CO 3 into NH 3 and H(NH 4 ) 2 CO 3 , which latter
acid salt, being poorly ionized, does not precipitate the carbon-
ates of the group. Hence the solution must be warmed, but not
boiled.



CHAPTER XII

METALS OF GROUP VI: MAGNESIUM, AMMONIUM,
POTASSIUM, AND SODIUM

No group characteristic. No group reagent.
REACTIONS

Magnesium (salt for study, magnesium chloride, MgCl 2 ).



1. HN^PC^ precipitates white crystalline acid magne-
sium phosphate, HMgPO 4 , very soluble in acids ; some-
what soluble in water. If NH 4 C1 and NH 4 OH are
added before HNa 2 PO 4 , the more insoluble (NH 4 )MgPO 4
will be precipitated. It is soluble in acids, even
HC 2 H 3 O 2 . Precipitation is hastened by rubbing the
side of the vessel with a glass rod.

2. NH 4 OH and NaOH precipitate white magnesium
hydroxide, Mg(OH) 2 , soluble in acids and in NH 4 C1.

3. Spectrum (see Table VII, p. 58).

Ammonium (salt for study, ammonium chloride, NH 4 C1).

1. H^PtClg 1 in neutral or acid solutions precipitates
yellow crystalline ammonium platinic chloride,
(NH 4 ) 2 PtCl 6 , insoluble in alcohol; sparingly soluble
in water and dilute acids. As the salt is somewhat
soluble in water, it is best to add H 2 PtCl 6 , evaporate
nearly to dryness, and add a few drops of alcohol.

2. H 2 C 4 H 4 6 in neutral concentrated solutions precipi-
tates white acid ammonium tartrate, H(NH 4 )C 4 H 4 O 6 ,

126



METALS OF GROUP VI 127

insoluble in alcohol; somewhat soluble in water; sol-
uble in acids. The reagent should be added like
H 2 PtCl 6 .

3. NaOH sets free NH 3 , detected by the odor, with red
litmus paper, or with a glass rod moistened with con-
centrated HC1.

4. Heated on a platinum foil, all ammonium salts are
completely volatilized.

Potassium (salt for study, potassium chloride, KC1).

1. H^PtClg gives a reaction very similar to 1, under
ammonium.

2. H^E^Og gives a reaction very similar to 2, under
ammonium.

3. Heated in a non-luminous flame, potassium salts
color the flame violet, which appears purple when seen
through a blue glass.

4. Spectrum (see Table VII, p. 58).

Sodium (salt for study, sodium chloride, NaCl).

1. H2K 2 Sb 2 7 in concentrated neutral solutions precipi-
tates sodium pyro-antimoniate, H 2 Na 2 Sb 2 O 7 .

2. Heated in a non-luminous flame, sodium salts color
the flame yellow, which color cannot be seen through a
blue glass.

3. Spectrum (see Table VII, p. 58).

PROCESS OF SEPARATION

As the group is composed of metals not bound by a
common group reagent, and as salts of some of the



128 CHEMICAL ANALYSIS

metals may have been used as reagents in previous
groups, the detection of the individuals cannot be made
entirely by separations, but in part by separations and
in part by individual tests in the original solutions.

The detection of the members of this group is based
upon the facts that NH 4 MgPO 4 is insoluble in presence
of NH 4 OH and NH 4 C1; that K 2 PtCl 6 and NH 4 PtCl 6
are insoluble in alcohol solutions ; and that ammonium
salts are dissociated by strong alkalies, and completely
volatilized by strong heat.

Concentrate the solution by evaporation. Divide the
solution into two unequal parts.

First Part (smaller). Add NH 4 OH, NH 4 C1, and
HNa 2 PO 4 . A white crystalline precipitate appearing
immediately or after some time confirms NH 4 MgPO 4 .

The precipitate should be crystalline, and if the sides of the
vessel are scratched with a glass rod the crystals will adhere to
the vessel in clusters in the path of the scratching. As the crys-
talline structure of the precipitate is distinctly characteristic of
NH 4 MgPO 4 , it is often desirable to confirm this with the micro-
scope. A simple method is to allow the crystals to grow for sev-
eral hours. Then carefully decant off the liquid, and examine
the sides of the glass vessel with a reading glass or simple micro-
scope. If the precipitate appears flocculent and crystals cannot be
detected, examine it for*Al(OH) 3 with the spectroscope. A1(OH) 3
can possibly appear at this stage of the separation because it is
soluble in excess of NH 4 OH, and may have been brought over
from Group III.

Second Part (larger). Evaporate to dry ness and
expel ammonium salts by removing the mass from
the dish, and heating on a platinum foil till no white
fumes appear immediately after removal from the flame.



METALS OF GROUP VI 129

Moisten the residue with HC1 and test for potassium
and sodium with the simple flame and with the spectro-
scope. Confirm the test for potassium by adding to
the moistened residue some H 2 PtCl 6 and alcohol.

Both ammonium and potassium salts give very similar precipi-
tates with H 2 PtCl 6 ; hence it is necessary to remove all the ammo-
nium salts by sublimation. Care should be taken not to heat the
substance too much, as potassium chloride is also somewhat vola-
tile and might be lost.

As ammonium salts are used as reagents in several
groups, it is obviously necessary to test for ammonia in
the original solution.

Concentrate some of the original solution and test
with NaOH for ammonium salts.



CHAPTER XIII

ACIDS OF GROUP I: CHROMIC, CARBONIC, SILICIC, SUL-
PHUROUS, SULPHURIC, PHOSPHORIC, BORIC, OXALIC,
TARTARIC, AND HYDROFLUORIC ACIDS

CHARACTERISTIC : Insolubility of their barium salts in neutral
solution.

GROUP REAGENT : Barium chloride.

KEACTIONS

Chromic Acid, H 2 Cr0 4 (salt for study, potassium chro-
mate, K 2 Cr0 4 ).

1. BaClg precipitates yellow barium chromate, BaCrO 4 ,
soluble in dilute acids, except H 2 SO 4 .

2. H 2 S reduces CrO 3 to O 2 O 3 ; so that Cr will be
detected in the analysis for the metals, even though it
originally were present in its acid state of oxidation.

3. Pb(C 2 H 3 2 ) 2 precipitates yellow lead chromate,
chrome yellow, insoluble in HC 2 H 3 O 2 .

(For other reactions, see Chromium as a metal.)

Carbonic Acid, H 2 C0 3 (salt for study, sodium carbonate,
Na 2 C0 3 ).

1. BaC^ precipitates white barium carbonate, BaCO 3 ,
insoluble in water; soluble in acids, except H 2 SO 4 .
This reaction occurs only with salts of H 2 CO 3 , not with
the free acid,

130



ACIDS OF GROUP I 131

2. HC1 and other acids, excepting H 2 S and HCN,
decompose carbonates with evolution of CO 2 . This
gas is readily soluble in water ; and in dilute solutions
of the carbonates, it may not be formed in sufficient
quantity to oversaturate the solution and escape. From
concentrated or hot solutions, it escapes with efferves-
cence. Being heavier than air, it may be detected by
decantation into a test-tube containing lime water, its
presence being shown by the appearance of a milky
precipitate :

Ca(OH) 2 + CO 2 = CaCO 3 + H 2 O.

An excess of CO 2 will dissolve the precipitate first
formed. Sulphur dioxide, SO 2 , will also produce a
white precipitate with lime water, but can usually be
detected by its odor.

Silicic Acid, H 4 Si0 4 (salt for study, sodium silicate,
Na 4 Si0 4 ).

1. BaC^ precipitates white barium silicate, Ba 2 SiO 4 ,


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