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 Hg2

2+.

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)4

2+]. 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 H2S.) Centrifuge and save decantate.

Decantate: SnCl62-, SbCl6

3-, 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 (NH4)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 [SnCl6

2-, 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 SnCl4

2-, and HgCl4

2- according to the following equations:

HgS + HCl + KClO3 (HgCl4)2- + S + H2O

SnCl42- + Hg2Cl2

2Hg

black + SnCl6

2-

SnCl42- + 2HgCl4

2- Hg Cl

white2 2 + SnCl6

2-

+ 4Cl=

7. 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 (White Precipitate)

Sn(OH)2 + NaOH NaSN(OH)3 (Clear)

8. 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= CuSCd2+ + 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 + aqua

regia

HgCl

colorless4

2 +

SnCl2 Hg Cl

white2 2 +

Hg

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

black +

Na2SN(OH)6

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

CdFe(CN)6 Whitecadium

hexacyanoferrate II or cadium

ferrocyanide

Cu2+ + CN- Cu(CN3)2-

complex ion does not interfere with the test for Cd2+

Cd2+ + CN- Cu(CN)32-

Cd2+ + S= CdS Yellow

PROCEDURE Section IIB

As2S3 + HCl As2S3 Yellow Precipitate

Sb2S3 + HCl [SbCl4]-

SnS2 + HCl [SnCl6]2-

PROCEDURE IIB-8

(SbCl4)- + Sn Sb

black + SnCl4

=

SbCl63- + Mg

Sb

black + Mg2+ +

Cl-

SnCl62- + Mg SnCl4

= + Mg2+ + Cl-

SnCl42- + HgCl2

Hg Cl

white2 2 +

SnCl6=

Hg2Cl2 + SnCl4=

Hg

black + SnCl6

=

TechniquesIn this lab, you will perform simple test tube experiments and make careful observations. To separate ions of each group, you will form an insoluble precipitate and centrifuge to remove solid from the ions that remain in solution. You will need to carefully wash the precipitate and decant the solution to remove all soluble ions. You will use pH paper when you acidify and basify your samples. Finally, you will heat in a water bath to perform some reactions. Be sure to use clean glassware to avoid contamination of your sample. Wash glassware with soap and rinse with distilled water.

2nd analytical group of cationsThe 2nd analytical group of cations consists of ions that forms acid-insoluble sulfides. Cations in the 2nd group include: Cd2+, Bi3+, Cu2+, As3+, As5+, Sb3+, Sb5+, Sn2+, Sn4+ and Hg2+. Pb2+ is usually also included here in addition to the first group.The reagent can be any substance that gives S2− ions in such solutions; most commonly used are H2S (at 0.2-0.3 M), AKT (at 0.3-0.6 M). The test with the sulfide ion must be conducted in the presence of dilute HCl. Its purpose is to keep the sulfide ion concentration at a required minimum, so as to allow the precipitation of 2nd group cations alone. If dilute acid is not used, the early precipitation of 4th group cations (if present in solution) may occur, thus leading to misleading results. Acids beside HCl are rarely used. Sulfuric acid may lead to the precipitation of the 4th group cations, while nitric acid directly reacts with the sulfide ion in the reagent, forming colloidal sulfur.The precipitates of these cations are almost indistinguishable, except for CdS which is yellow. All the precipitates, except for HgS, are soluble in dilute nitric acids. HgS is soluble only in aqua regia, which can be used to separate it from the rest. The action of ammonia is also useful in differentiating the cations. CuS dissolves in ammonia forming an intense blue solution, while CdS dissolves forming a colourless solution. The sulfides of As3+, As5+, Sb3+, Sb5+, Sn2+, Sn4+ are soluble in yellow ammonium sulfide, where they form polysulfide complexes.This group is determined by adding the salt in water and then adding dilute hydrochloric acid followed by hydrogen sulfide. Usually it is done by passing

hydrogen sulfide over the test tube for detection of 1st group cations. If it forms a reddish brown or black precipitate then Bi3+, Cu2+, Hg2+ or Pb2+ is present. Otherwise, if it forms a yellow precipitate, then Cd2+ or Sn4+ is present; or if it forms a brown precipitate, then Sn2+ must be present; or if a red orange precipitate is formed, then Sb3+ is present.To distinguish between ions in the black or reddish brown precipitate, it is first boiled in diluted HNO3. If it is insoluble, then Hg2+ is present. If it is soluble, then Cu2+, Hg2+ or Pb2+ may be present; sulfuric acid is then added to the resulting solution. If a white precipitate forms, then Pb2+ may be present; if no precipitate is formed, then a new solution is made by adding an excess of ammonium hydroxide in the original salt solution. A resulting blue color indicates the presence of Cu2+, and a white precipitate indicates bismuth. Otherwise, if the precipitate is insoluble then Hg2+ is present.To distinguish between ions in the yellow precipitate, an excess of NaOH is added to the original salt solution to form a white precipitate. The test tube is then shaken, and if the white precipitate dissolves, then Sn4+ is present; otherwise, Cd2+ is present.Confirmation test for lead:Pb2+ + 2 KI → PbI2 + 2 K+Pb2+ + K2CrO4 → PbCrO4 + 2 K+Confirmation test for copper:2 Cu2+ + K4[Fe(CN)6] + CH3COOH → Cu2[Fe(CN)6] + 4 K+Cu2+ + 2 NaOH → Cu(OH)2 + 2 Na+Cu(OH)2 → CuO + H2O (endothermic)Confirmation test for bismuth:Bi3+ + 3 KI (in excess) → BiI3 + 3 K+BiI3 + KI → K[BiI4]Bi3+ + H2O (in excess) → BiO+ + 2 H+Confirmation test for mercury:Hg2+ + 2 KI (in excess) → HgI2 + 2 K+HgI2 + 2 KI → K2[HgI4] (red precipitate dissolves)2 Hg2+ + SnCl2 → 2 Hg + SnCl4 (white precipitate turns gray)