GROUP II CATIONS
Group II Cations: Hg2+ , Pb2+ , Bi3+ , Cu2+ , Cd2+ , As3+ , Sb3+ , and Sn4+.
Precipitating Agent: S= (from thioacetamide in acidic solution)
PROCEDURE II - O
To 2 ml of Group II unknown (or decantant from Procedure I-1) add 6 MNH4OH to make the
solution just alkaline; then add 6 M HCl dropwise till the solution is just acidic; then add .5 ml of HCl in
addition. Add 2 ml of 3% H2O2 and heat; then add 1 drop of NH41. The hydrogen sulfide produced by the
hydrolysis of thioacetamide is a gas. It will tend to escape rapidly from the boiling solution. To get rapid
precipitation add 2-3 drops of thioacetamide every 30 seconds or so; stir and keep the solution in the hot
water bath between additions. Test for complete precipitation and centrifuge. Decantate: Contains Groups
III, IV, and V when working with general unknown; label and save.
ION
COLOR OF SOLUTION
OF ION
PRECIPITATED
FORM OF ION
COLOR OF THE
PRECIPITATE
Hg2+
Colorless
HgS
Black
Pb2+
Colorless
PbS
Black
Bi3+
Colorless
Bi2S3
Brown to Black
Cu2+
Blue
CuS
Black
Cd2+
Colorless
CdS
Yellow
As3+
Colorless
As2S3
Yellow
Sb3+
Colorless
Sb2S3
Orange
Sn4+
Colorless
SnS2
Yellow
PROCEDURE II-1
Wash the precipitate twice with 2 ml of 1% NH4NO3 to free it from acid. Centrifuge and discard wash
water. Now add 2 ml of 6 M NH4OH to the precipitate; then add 15 drops of thioacetamide. Heat and stir
in water bath for 5 minutes; then centrifuge.
Decantate: (NH4)3AsS3, (NH4)3SbS3, and (NH4)2SnS3, Section IIB.
Precipitate: HgS, PbS, Bi2S3, CuS, CdS, Section IIA.
SECTION IIA
PROCEDURE IIA-1
To the precipitate from procedure II-1 and 3 ml of 6 M HNO3, heat in water bath; stir and centrifuge.
Decantate: Pb(NO3)2, Bi(NO3)3, Cu(NO3)2, Cd(NO3)2 (Procedure IIA-3)
Precipitate: HgS (black) (Procedure IIA-2)
PROCEDURE IIA-2
Cover the residue (HgS) with 6M HCl and a few crystals of KClO3. Heat in an evaporating dish until
Cl2 is expelled. Apply the starch iodide paper test. (Wet starch iodide paper turns brown to purple in the
presence of Cl2. Dilute the solution with 1 ml of H2O and add a few drops of SnCl2. A white, black or gray
precipitate confirms the presence of Hg22+.
PROCEDURE IIA-3
To the decantate [Pb(NO3)2Bi(NO3)3, Cu(NO3)2, Cd(NO3)2] from Procedure IIA-1 add 1 ml of 6 M
H2SO4 and heat in the hood just until white fumes of H2SO4 appear. (Appearance of white fumes insures
the evaporation of HNO3; PbSO4 is soluble in HNO3.) Cool and dilute with H2O to a volume of 2 ml. A
white precipitate (PbSO4) confirms the presence of Pb2+.
Decantate: Bi2(SO4)3, CuSO4, CdSO4 (Procedure IIA-4)
Precipitate: PbSO4
PROCEDURE IIA-4
To the decantate [Bi2(SO4)3, CuSO4 and CdSO4] from Procedure IIA-3, add 15 M NH4OH until the
solution is strongly alkaline; then add 5 extra drops. Stir for a few minutes; Bi(OH) 3 is a white slowly
forming precipitate. (Procedure IIA-5)
Decantate: Cu(NH3)4SO4, Cd(NH3)4SO4 (Procedure IIA-6)
Precipitate: Bi(OH)3
PROCEDURE IIA-5
Wash precipitate twice with 1% NH4OH and discard wash water. To the precipitate, BI(OH) 3, add
freshly prepared sodium stannite, NaSn(OH)3 (SEE NOTE 7). The immediate blackening of the original
precipitate confirms the presence of Bi3+.
PROCEDURE IIA-6
Use the decantate from Procedure IIA-4 which may contain Cu(NH3)4so4, Cd(NH3)4SO4 [A blue color
at this point indicates the presence of Cu(NH3)42+]. Divide the decantate into two parts (a) and (b). To
portion (a) just acidify by adding 6 M HOAc; then add 2 drops in excess; now add potassium
hexocyanoferrate K4Fe(CN)6 (also known as potassium ferrocyanide). Pink to purple red precipitate
indicates the presence of Cu2+.
If Cu2+ is present, [WARNING: Be sure that the 2nd portion of sample (b) is basic; if acidic, HCN
is evolved when NaCN is added, HCN is very poisonous gas.] Add a few drops of NaCN till blue color
disappears [it might be necessary to add solid NaCN] to form dicyanocopper I complex which will not
interfere with the Cd2+ test. At this time the blue color should disappear, indicating the complexing of all
the Cu. Now add a few drops of IM thioacetamide and heat. A yellow precipitate confirms the presence of
Cd2+.
If Cu2+ is absent, then test for Cd2+ as described above but do not add the NaCN.
SECTION IIB
PROCEDURE IIB-7
The decantate from Procedure II-1 contains (NH4)3AsS3, (NH4)3SbS3, (NH4)2SnS3. Add 6 M HCl till
the solution is just acidic; stir, centrifuge and discard decantate.
PRECIPITATE: As2S3, Sb2S3, SnS2
Transfer precipitate As2S3, Sb2S3 and SnS2 to an evaporating dish and heat for a few minutes (do not
boil) with 3 ml of 6 MHCl until H2S is completely expelled. Apply lead acetate paper test. (Wet lead
acetate paper turns brown to black in the presence of H 2S.) Centrifuge and save decantate.
Decantate: SnCl62-, SbCl63-, colorless (Procedure IIB-9)
Precipitate: As2S3, yellow (Procedure IIB-8)
PROCEDURE IIB-8
Wash residue with 10 drops of H2O added to 3 drops of 3 M HCl, centrifuge and discard wash water.
Dissolve the residue (As2S3) in 1 ml of concentrated HNO3 and warm till brown fumes fail to appear [all
solid should dissolve; any precipitate might be discarded]. Add 2 ml of (NH 4)2MoO4 and heat; yellow
precipitate confirms the presence of As3+. (A white precipitate in the absence of As3+ is due to the
decomposition of (NH4)2MoO4 and is not to be confused with the test for the As3+).
PROCEDURE IIB-9
Use the decantate from Procedure IIB-7 [SnCl62-, SbCl63-]. Divide the solution into two parts (a) and
(b). Pour solution (a) onto a piece of tin on a clean silver coin (if the coin is not clean, wash with 3 M
HNO3 for a few seconds and wash with distilled water). A fine black deposit on the coin confirms the
presence of Sb. This precipitate is insoluble in NaOCl solution. Any arsenic which might have precipitated
is soluble in NaOCl.
To part (b) add an equal volume of HCl and a piece of Mg ribbon 3 inches long. Allow all the Mg to
dissolve completely; remove any Mg which did not dissolve from the test tube at this point. Then add two
drops of HgCl2. A white, black or gray precipitate which develops after several minutes confirms the
presence of Sn2+.
NOTES
1.
The sulfides HgS, CuS, Bi2S3, PbS, CdS exhibit basic properties while AS2S3,, SnS3, Sb2S3 exhibit
acidic properties; thus treating the sulfides of Group II cations with a base dissolve the solids that
exhibit acidic properties.
2.
The addition of NH4OH and HCl to the unknown of Group II cations is to control the pH, the S =
varies inversely with the square root of H+. If a large excess of HCl is added, some cations of
Group II will not precipitate (the greater the H+, the smaller the S=). The H+ concentration needed
to regulate the [S=] is .2 - .3 M.
3.
H2O2 is added to oxidize Sn2+ to Sn+4.
4.
NH4I is added to reduce As5+ to As3+.
5.
The unknown might turn cloudy when diluted or when NH4OH is added. This cloudiness is due to
the formation of BiOCl or SbOCl. The solution should clear up when HCl is added. Regardless of
whether the solution clears or not, the cations of Group II will precipitate as sulfides when
thioacetamide is added.
6.
KClO3 is added to oxidize Sn2+ to Sn+4 and the HCl is added to form SnCl42-, and HgCl42according to the following equations:
HgS + HCl + KClO3
7.
SnCl42- + Hg2Cl2
2Hg
+ SnCl62black
SnCl42- + 2HgCl42-
Hg 2 Cl 2
+ SnCl62- + 4Cl=
white
To prepare a solution NaSN(OH)3, add NaOH to SnCl2 till a white precipitate forms, then add
NaOH dropwise while stirring till the precipitate dissolves.
NaOH + SnCl2
Sn(OH)2 + NaOH
8.
(HgCl4)2- + S + H2O
Sn(OH)2
(White Precipitate)
NaSN(OH)3 (Clear)
Section IIB
H2S is expelled so that it will not react with the cations of Group IIB.
9.
Mg is added to reduce antimony ion to metallic antimony and Sn4+ to Sn2+.
Precipitation of Group II Cations
Hg2+ + S=
HgS
Pb2+ + S=
PbS
Bi3+ + S=
Bi2S3
Cu2+ + S=
CuS
Cd2+ + S=
CdS
As3+ + S=
As2S3
Sb3+ + S=
Sb2S3
Sn4+ + S=
SnS2
PROCEDURE II-1
As2S3 + NH4OH + H2S
(NH4)3AsS3) Ammonium thioarsenite
Sb2S3 + NH4OH + H2S
(NH4)3SbS3 Ammonium thioantimonite
SnS2 + NH4OH + H2S
(NH4)2SnS3 Ammonium thiostannate
PROCEDURE IIA-1
3PbS + 2NO3- + H+
3Pb2+ + 3S + 2NO + 4H2O
Bi2S3 + 2NO3- + H+
2Bi3+ + 3S + 2NO + 4H2O
3CuS + 2NO3- + H+
3Cu2+ + 3S + 2NO + 4H2O
3CdS + 2NO3- + H+
3Cd2+ + 3S + 2NO + 4H2O
PROCEDURE IIA-2
HgS +
HgCl 4 2
colorless
aqua
regia
+ SnCl2
Hg 2 Cl 2
Hg
+
white
black
PROCEDURE IIA-3
Bi2(OH)3 + NH4OH
Bi(OH)3
CuSO4 + NH4OH
Cu(NH3)4SO4
CdSO4 + NH4OH
Cd(NH3)4SO4
Bi2(OH)3 + NaSn(OH)3
Bi
+ Na2SN(OH)6
black
PROCEDURE II-A
Cu(NH3)42(+) + K4Fe(CN)6
Cu2Fe(CN)6
Pink
Blue
cupric ferrocyanide
or copper hexacyanoferrate II
Cd(NH3)42+ + K4Fe(CN)6
Cu2+ + CN-
Cu(CN3)2-
CdFe(CN)6
White
cadium hexacyanoferrate II
or cadium ferrocyanide
complex ion does not interfere with the test for Cd2+
Cd2+ + CNCd2+ + S=
Cu(CN)32CdS Yellow
PROCEDURE Section IIB
As2S3 + HCl
As2S3 Yellow Precipitate
Sb2S3 + HCl
[SbCl4]-
SnS2 + HCl
[SnCl6]2-
PROCEDURE IIB-8
(SbCl4)- + Sn
Sb
+ SnCl4=
black
SbCl63- + Mg
Sb
+ Mg2+ + Clblack
SnCl62- + Mg
SnCl4= + Mg2+ + Cl-
SnCl42- + HgCl2
Hg 2 Cl 2
+ SnCl6=
white
Hg2Cl2 + SnCl4=
Hg
+ SnCl6=
black