GROUP IV CATIONS
QUALITATIVE ANALYSIS
A.
INTRODUCTION
Ions in the Group: Ca2+, Sr2+, Ba2+.
Precipitating Agent: (NH4)2 CO3 in ammonical buffer.
Inspection of the periodic chart will show that the ions of this group all fall in Group II A of the
chart. Due to the principle of periodicity of chemical properties, it is reasonable to expect that all three of
these ions will form similar chemical compounds. The separation of the ions is accomplished by making
use of the relatively small differences in the solubilities of their salts.
+1-
0CaCrO4
-1SrCrO4
CaSO4
-2-
SrSO4
-3-
-4-
CaCO3
SrC2O4
BaC2O4
SrCO3
BaCO3
BaCrO4
CaC2O4
-5BaSO4
Comparison of the solubilities of the chromate, sulfate, carbonate, and oxylate of Ca 2+, Sr2+,
and Ba2+
Use of flame test in group IV:
Each of the ions in group IV gives characteristic color in a flame.
Ion
Color
Ca2+
Orange-red
Sr2+
Carmine
Ba2+
Yellow-green
The flame test should be performed on the undiluted sample acidified with concentrated HCl.
There are several problems encountered in interpreting the results of flame test. If cooper is present, its
color in the flame may be mistaken for barium. Sodium can obscure the barium flame entirely. (Small
quantities of sodium ion will be present in almost any sample.) If any two of the group IV ions are present
in relatively large amounts, their flame colors may obscure the color from the ions present in the smaller
amount. For these and other reasons, flame test results should never be taken as conclusive evidence for the
presence or absence of an ion.
The sulfide ion remaining in solution after separation of group III must be removed. This is
accomplished by acidifying the solution thus forming H2S, which is expelled by boiling. Any solid present
at this point will be sulfides that were not completely removed at the proper time or elemental sulfur.
Centrifuge and discard any solids.
B. GROUP IV PROCEDURES
PROCEDURE IV-1 Removal of S2- ions and precipitation of Group IV
Acidify decantate from procedure III-O with conc.HCl till just acidic and boil
down to a volume of 1.5 ml. (Do this in an evaporating dish.) Centrifuge and
discard any solid that may be present. Apply flame test to undiluted part of this
solution. (If you are using unknown containing Group IV cations, take 2.5 ml of
that solution.) If the sample is a Group IV unknown, the above acidification is
not necessary. Add about 30 drops NH4 Cl (unless it was added in Group III).
Then add 6 M N4H OH dropwise with stirring until the solution is alkaline.
Heat the buffered solution in a water bath and add about 1 ml of 2 M
(NH4)2CO3. Test for complete precipitation. Centrifuge, label, and save the
decantant for Group V analysis. Wash the precipitate with 1 ml hot water;
combine the water from the washing with the Group V decantate.
Decantate:
Group V
Precipitate:
BaCO3, SrCO3, CaCO3 White
PROCEDURE IV-1
Dissolution of the carbonates and separation of Ba2+.
Add about 1 ml of 6 M acetic acid to the above precipitate. Heat for several
minutes until al of the solid has dissolved. Add more acetic acid if necessary to
give complete dissolution. Any solid at this point may be discarded. Add about
0.25g ammonium acetate; add 1 M K2Cr2O7 . A yellow precipitate confirms the
presence of Ba2+. Test for complete precipitation. Avoid a large excess of
Cr2O7= which might precipitate Sr2+ as SrCrO4 which is also yellow. The
addition of Cr2O7= to the super-natant liquid should be done drop at a time.
Decantate:
Sr2+, Ca2+, CrO42- ion
Precipitate:
BaCr4 Yellow
PROCEDURE IV-2
Removal of excess CrO42- ion
Add concentrated NH4OH to the decantate from Procedure IV-1 until the
solution is basic. (Stir throughly to insure that the enitre sample is basic.) Heat
in a water bath and add (NH4)2CO3. Test complete precipitation; centrifuge and
discard the supernatant liquid. Wash the precipitate with water, 1 ml at a time,
until the precipitate is white and free of any yellow color.
Precipitate:
SrCO3, CaCO3 White
PROCEDURE IV-3
Dissolve the precipitate from Procedure IV-2 in 1 ml of 6 M acetic acid. If the
solution is not completely clear, filter it. Divide the solution into two equal
portions (A) and (B).
PROCEDURE IV-4
Identification of Sr
To portion (A) of the solution from Procedure IV-3add 1 ml of saturated CaSO4
solution. (Be careful that only the solution is added and that absolutely none of
the solid CaSO4 is introduced into the sample). The appearance of a white
precipitate (SrSO? ) confirms the presences of Sr2+. The precipitate may be
slow in forming. If Sr2+ is present, it must be removed else it will interfere with
the test for Ca2+ .
PROCEDURE IV-5
Removal of Sr2+ from solution
If Sr2+ is found to be present in Procedure IV-4, it must be removed. To do this,
add 2 ml (NH4)2SO4 to portion (B) from Procedure IV-3, and centrifuge to
remove SrSO4. Test for complete precipitation. If Sr2+ is not found in Procedure
IV-4, this step is not necessary.
Decantate:
Ca2+
Precipitate:
SrSO4 White
PROCEDURE IV-6
Identification of Ca
To the decantate from Procedure IV-5 or the other half of the solution from
Procedure IV-4, add 2 ml (NH4)2C2O4 (Ammonium oxalate); Mix completely;
allow to stand 10-15 minutes. The appearance of a white precipitate confirms
the presence of Ca.
C. CHEMICAL REACTIONS FOR THE GROUP IV ANALYSIS
PROCEDURE IV-0
Ba2+ + CO32
Sr2+ + CO32Ca2+ + Co32-
PROCEDURE IV-1
BaCO3 + H+
BACO3 White
SrCO3 White
CaCO3 White
Ba2+ + H2O + CO2
SrCO3 + H+
Sr2+ + H2O + CO2
CaCO3 + H+
Ca2+ + H2O + CO2
Ba2+ + CrO42-
BaCrO4 Yellow
Sr2+ + CO32-
SrCO3 White
Ca2+ + CO32-
CaCO3 White
PROCEDURE IV-3
SrCO3 + H+
Sr2+ + H2O + CO2
PROCEDURE IV-4
Sr2+ + SO42-
SrSO4 White
PROCEDURE IV-5
Sr2+ + SO42-
SrSO4 White
PROCEDURE IV-6
Ca2+ + C2O42-
PROCEDURE IV-2
CaC2O4 White