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Qualitative Analysis of Group II Cations

Collect: 5 centrifuge tubes Labels Evaporating dish Latex gloves Two droppers Test tube holder and crucible tongsPrepare: Test tube rack, test tubes, and beaker Take out centrifuge*Conc. NH3(aq) and HCl: in hood

(2016/03/03 revised)

Test tube

Centrifuge tube

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Objective

To learn the techniques of separating and identifying some common cations

To understand the principles of precipitation and equilibrium of complex formation

Techniques Vortex mixer Precipitation Centrifuge Decantation Litmus and

universal indicator paperVortex Mixer Decantation

Cationic Solutions

(I) Insoluble chlorides: Hg22+, Ag+, Pb2+

(II) Insoluble sulfides in acidic medium: Hg2+, Pb2+, Cu2+, Bi3+, Cd2+, As3+, Sb3+, Sn4+ (metallic sulfide precipitates with smaller Ksp)

(III) Insoluble sulfide or hydroxides in alkaline medium: Al3+, Fe3+, Co2+, Ni2+, Cr3+, Zn2+, Mn2+ (metallic sulfide precipitates with greater Ksp)

(IV) Insoluble Carbonates: Mg2+, Ca2+, Sr2+, Ba2+

(V) Soluble cations: NH4+, Na+, K+

Introduction: Qualitative Analysis of Group I~V Cations

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Group 2 cations – Insoluble sulfides in acidic medium－HgS, PbS, CuS, Bi2S3, CdS, As2S3, Sb2S3, SnS2

Copper subgroup – Hg2+, Pb2+, Cu2+, Bi3+, Cd2+

The sulfides are insoluble in KOH solution, only soluble in nitric acid

Arsenic subgroup – As3+, Sb3+, Sn4+

The sulfides are thioamphoteric that are soluble in KOH(aq) and nitric acid

Mostly are toxic heavy metalsthus we only examine Cu2+, Bi3+, Sb3+, Sn4+

Group 2 Cations- Copper and Arsenic Subgroup

Cationic SolutionCu2+, Bi3+, Sb3+, Sn4+

(2, 2, 2, 8 drops)

Ppt 2-1Bi2S3, CuS, Sb2S3, SnS2

Soln 2-1

Ppt 2-2 (copper subgroup)Bi2S3, CuS

Soln 2-2 (arsenic subgroup)SbS3

3-, SbO33-, SnS3

2-, SnS2OH－, (KOH)

(pH 0.5)2 d 13%TA, Centrifuge and separate(Repeat 13%TA precipitation once)

Wash ppt with 1 d 6 M NH4Cl and 20 d of water, cfg.Add 10 d 0.5 M KOH to ppt and mix wellHeat in boiling water bathCfg. and separate the ppt and supernatant (Repeat this extraction with KOH once)

Flow Chart for Separating Copper and Arsenic Subgroups

CH3COH + NH4+ + H2S(aq)

OCH3CNH2 + H+ + 2H2O

△S

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Separate and Identify Cu2+ and Bi3+ Ppt 2-2 (Copper Subgroup)

Bi2S3, CuS

Soln 2-3Bi3+, Cu2+

Discard Residue(Contains S)

Ppt 2-4Bi(OH)3(s) (white)

Bi(s) (black)

Soln 2-4Cu(NH3)4

2+ (deep blue)

Add and mix with 15 M conc. NH3(aq) until basic (confirm with litmus test) Cfg. and separate ppt and supernatant

Wash ppt with NH4NO3/waterCfg. to get ppt.Add 5 d. 6 M HNO3/ 5 d. H2OHeat in boiling water bathCfg. and obtain soln

Add sodium stannite reagentSn(OH)3

- (freshly prepared)

2Bi(OH)3(s) + 3Sn(OH)3－ + 3OH－

→ 2Bi↓ + 3Sn(OH)62-

CuS, Bi2S3 + NO3- →

Cu2+, Bi3+ + S↓ + NO↑

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Soln 2-6SnCl62-, SbCl4－

Discard Residue

SbTest Sn test

Sb2S3(s) (orange)

Soln 2-2 (arsenic subgroup)SbS3

3-, SbO33-, SnS3

2-, SnS2OH－

Evaporate till approx. 4 d leftAdd 1 mL water and divide into 2 parts

15～20 d of conc. HClHeat in boiling water bath, till pptdissolvesCfg, pour supernatant into evaporating dish

¼ small spatula H2C2O4(s)2 d 13%TAWarm in water bath

Hg2Cl2(s) (white)Hg (black)

Small Al strip /6 M HClHeat in boiling water bath to dissolveCfg. and obtain solnAdd 0.1 M HgCl2 to solution

SbS33- + H+ → Sb2S3(s)

Sb2S3(s) + H+ + Cl- → 2SbCl4－(aq)

SnCl62- + 3H2C2O4 →Sn(C2O4)3

2- (stable)SbCl4- + H2S → Sb2S3 (orange)

Sn2+ + 2HgCl2 → Hg2Cl2(s) + Sn4+ + 2Cl-

Separate and Identify Sn4+ and Sb3+

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Separate Copper and Arsenic Subgroups

• Add 13%TA • Heat in warm water• Produce sulfide ppt

• Separate ppt and supernatant by decantation

1) 1 d 6 M NH4Cl/20 d H2O to wash ppt, then cfg.

2) Add 10 d 0.5 M KOH to ppt

3) Heat in boiling water bath4) Cfg. and separate5) Repeat KOH extraction once Sulfide ppt

Ppt 2-2Copper subgroup

CuS, Bi2S3

Soln 2-2Arsenic subgroup

Contains Sn4+, Sb3+

• Centrifuge • Balance diagonally

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Copper Subgroup: Use HNO3to Dissolve CuS, Bi2S3

3(CuS, Bi2S3)(s) + 2NO3-(aq) + 8H+

(aq) → 3(Cu2+、Bi3+)(aq) + 2NO(g)↑ + 3S↓ + 4H2O

The water bath should be keep boiling Mix the HNO3 and ppt thoroughly Heat to the black sulfides disappear, and the solution appears to be

off-white for elemental sulfur forms

• Heat till reaction starts (starts to bubble)

• Ppt is dissolved and appears white and milky

• Add 5 d water and 5 d 6 M HNO3

• Heat in boiling water bath

Ppt 2-2Copper

subgroupCuS, Bi2S3

• 1 d 0.2 M NH4NO3/20 d H2O to wash ppt,

• Cfg. to get ppt.

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Separate and Identify Cu2+

and Bi3+

CentrifugeCfg. and take supernatant

ContainsCu2+, Bi3+

Do not add more NaOH when ppt dissolved

3(aq)NaOH

2(s)NaOH

2(aq) Sn(OH)Sn(OH)SnCl

:freshly reagent stannite sodium Prepare*(aq)(aq)

Add conc.NH3 to basic

Cu(NH3)42+

Bi

Add sodium stannite reagent*

(confirm with litmus test)

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Separate and Identify Sn4+

and Sb3+

SbS33- + SbO3

3- + 6H+ → Sb2S3(s)(orange) + 3H2OSnS2OH- + H+ → SnS2(s)(yellow) + H2O

Soln 2-2Contains

Sn4+, Sb3+

Add conc. HCl slowly

Evaporate to 4 d

Dissolve when heatedTransfer to evaporating dish

Excess conc. HCl(ca. 15-20 d)

• 1/4small spatula H2C2O4(s)• 2 d 13%TA• Heat in water bath

• Small Al strips / HCl• Heat till dissolve, • Cfg. to get soln• Add HgCl2 to liquid portion

Sb

Sn • Add 1 mL H2O• Separate into 2

tubes

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Cu2+ Bi3+ Sb3+ Sn4+

Expected Results

• Keep the resulting solutions for TA to check

• Record all the observations, i.e. color, ppt, rate etc.

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Use centrifuge tubes in centrifuge, do not use test tubes Centrifuge tubes should be placed in opposite sides to

keep balanced The lid should be closed during use; the centrifuge should

be started from slow to check if there are unusual sounds, then the speed can be increased

If there are unusual sounds or movement in the centrifuge, the power should be turned off first to fix

There must be at least one person watching the centrifuge when in use

Centrifuge for approx. 1~2 min, open lid when the centrifuge has completely stopped

Manipulate the Centrifuge

Wear gloves Always use test tube holders, test

tube racks, and crucible tongs Add reagents on top of the

centrifuge tube to avoid contamination of chemicals

Do not take excess chemicals to avoid pollution

Solutions must be mixed thoroughly after adding reagents

Dip solution on testing paper to determine acidity

All heating should be done in the hood

Conc. NH3(aq) and HCl(aq) are in hood

Recycle heavy metal waste 14

Notice