Selected Reactions of Some Cations

A. Reaction with NaOHA1. Formation of Insoluble OH- precipitates with strong baseReagent: 3.00M NaOHAnalytes: all cationsResultsChemical Reaction ObservationAg+ + OH- AgOH Brown ppt.Cu2+ + 2OH- Cu(OH)2 Blue ppt.Co2+ + 2OH- Co(OH)2 Wine Red-pink ppt.Ni2+ + 2OH- Ni(OH)2 Green ppt.Cr3+ + 3OH- Cr(OH)3 Grey-green ppt.Al3+ + 3OH- Al(OH)3 Gelatinous white ppt.Zn2+ + 2OH- Zn(OH)2 White ppt.Fe3+ + 3OH- Fe(OH)3 Red-orange ppt.Ba2+ + 2OH- Ba(OH)2 White ppt.Ca2+ + 2OH- Ca(OH)2 White ppt.Mg2+ + 2OH- Mg(OH)2 White ppt.NH4

+ + OH- NH3 + H2O Pungent, colorless gas due to evolution of NH3

*Few drops ~ 2-3 drops (addition of excess NaOH may lead to incorrect conclusion)A2. Solubility of the hydroxide precipitates with excess NaOHReagent: 3.00M NaOHAnalytes: Hydroxide precipitates in A1Results:Chemical Reaction ObservationCr(OH)3 + OH- [Cr(OH)4]- Gray-green ppt. Green solutionAl(OH)3 + OH- [Al(OH)4]- Gel white ppt. Colorless solutionZn(OH)2 + 2OH- [Zn(OH)4]2- White ppt Colorless solution

A3. Reprecipitation of those that dissolved in A2Reagent: 3.00M HNO3Analyte: Tetrahydro-complexes in A2Results:Chemical Reaction Observation[Cr(OH)4]- + H+ Cr(OH)3 + H2O Green solution Gray green ppt.[Al(OH)4]- + H+ Al(OH)3 + H2O Colorless solution Gel white ppt.[Zn(OH)4]- + 2H+ Zn(OH)2 + 2H2O Colorless solution White ppt

Excessive addition of the acid may further dissolve the hydroxide precipitate such that: [Cr(OH)4]- (green solution) + H+ Cr(OH)3 (grey-green ppt) + 3H+ Cr3+ + 3H2O (green solution)

------------------------------------------------------------------------------------------------------------------------------------------B. Reaction with NH3

B1. Formation of Insoluble OH- precipitates with weak baseReagent: 6.00M NH3

Analytes: all cationsResults

NH3 + H2O ↔ NH4+ + OH-Chemical Reaction ObservationAg+ + OH- AgOH Brown ppt.Cu2+ + 2OH- Cu(OH)2 Blue ppt.Co2+ + 2OH- Co(OH)2 Wine Red-pink ppt.Ni2+ + 2OH- Ni(OH)2 Green ppt.Cr3+ + 3OH- Cr(OH)3 Grey-green ppt.Al3+ + 3OH- Al(OH)3 Gelatinous white ppt.Zn2+ + 2OH- Zn(OH)2 White ppt.Fe3+ + 3OH- Fe(OH)3 Red-orange ppt.Ba2+ + 2OH- NVR NVR

Ca2+ + 2OH- Ca(OH)2 White ppt.Mg2+ + 2OH- Mg(OH)2 White ppt.NH4

+ NVRB2. Solubility of the hydroxide precipitate with excess NH3

Reagent: 6.00M NH3

Analytes: Hydroxide precipitates in B1Results:Chemical Reaction ObservationCu(OH)2 + 4NH3 [Cu(NH3)4]2+ + 2OH- Blue ppt Deep blue solutionCo(OH)2 + 4NH3 [Co(NH3)4]2+ + 2OH- Wine red-pink ppt Amber solutionNi(OH)2 + 4NH3 [Ni(NH3)4]2+ + 2OH- Green ppt. Blue solutionZn(OH)2 + 4NH3 [Zn(NH3)4]2+ + 2OH- White ppt Colorless solution

B3. Reprecipitation of those that dissolved in B2Reagent: 3.00M CH3COOHAnalyte: Tetraammine-complexes in B2Results:Chemical Reaction Observation[Cu(NH3)4]2+ + 2H+ + 2H2O Cu(OH)2+ 4NH4

+ Deep blue solution Blue ppt[Co(NH3)4]2+ + 2H+ + 2H2O Co(OH)2+ 4NH4

+ Amber solution Wine red-pink ppt[Ni(NH3)4]2+ + 2H+ + 2H2O Ni(OH)2 + 4NH4

+ Blue solution Green ppt.[Zn(NH3)4]2+ + 2H+ + 2H2O Zn(OH)2+ 4NH4

+ Colorless solution White pptExcessive addition of the acid may further dissolve the hydroxide precipitate such that: [Cu(NH3)4]2+ (deep blue complex) + 2H+ Cu(OH)2 (blue ppt.) + 2H+ Cu2+ + 2H2O(blue solution)

------------------------------------------------------------------------------------------------------------------------------------------C. Reaction with HCl

C1. Formation of Insoluble Cl-saltReagent: 6.00M HClAnalytes: all cationsResultsChemical Reaction ObservationAg+ + Cl- AgCl White ppt.

AgCl (s) + hot H2O NVRAgCl + NH3 Ag(NH3)2 + + Cl-

------------------------------------------------------------------------------------------------------------------------------------------D. Reaction with (NH4)2S

D1. Formation of Insoluble sulfide-saltReagent: (NH4)2SAnalytes: all cationsResultsChemical Reaction Observation2Ag+ + S2- Ag2S Brown ppt.Cu2+ + S2- CuS Black ppt.Co2+ + S2- CoS Black ppt.Ni2+ + S2- NiS Black ppt.2Cr3+ + 3S2- + 6H2O 2Cr(OH)3 + H2S Grey-green ppt.Al3+ + 3S2- + 6H2O 2Al(OH)3 + H2S White ppt.Zn2+ + S2- ZnS White ppt.Fe3+ + S2- S + Fe2+

and/orFe2+ + S2- FeS

Yellow ppt. and/or black ppt

D2. Solubility of precipitate from E1Reagent: 1M HClAnalytes: precipitates from E1ResultsChemical Reaction ObservationCr(OH)3 + 3H+ Cr3+ + 3H2O Grey-green solutionAl(OH)3 + 3H+ Al3+ + 3H2O Colourless solution

ZnS + 2H+ Zn2+ + H2S Colourless solutionFeS + 2H+ Fe2+ + H2S Yellow solution

D3. Solubility of the remaining undissolved sulfides from D2Reagent: 6M NH3

Analytes: unreacted precipitates from D2ResultsChemical Reaction ObservationAg2S + 4H+ + 2NO3

- 2Ag+ + 2NO + 2H2O + S Yellow ppt.3CuS + 8H+ + 2NO3

- 3Cu2+ + 2NO + 4H2O + 3S Yellow ppt. and blue solution3CoS + 8H+ + 2NO3

- 3Co2+ + 2NO + 4H2O + 3S Yellow ppt. and wine red solution3NiS + 8H+ + 2NO3

- 3Ni2+ + 2NO + 4H2O + 3S Yellow ppt. and green solution*Results are sometimes not observedin the laboratory

------------------------------------------------------------------------------------------------------------------------------------------E. Reaction with (NH4)2CO3

E1. Formation of carbonate-precipitatesReagents: 3.00M NH4NO3, 6M NH3 and 1.50M (NH4)2CO3

Analytes: all cationsResults:Chemical Reaction ObservationBa2+ + CO3

2- BaCO3 White ppt.Ca2+ + CO3

2- CaCO3 White ppt.Mg2+ + CO3

2- MgCO3 White ppt.E2. Flame TestReagent: 12.0M HCl (to dissolve CO32- precipitates)Analytes: Precipitates from E1 dissolved in 12M HClCation Flame ColorBa2+ Yellow-greenCa2+ Brick redMg2+ None

------------------------------------------------------------------------------------------------------------------------------------------ATQ1.

a. Four Main Groups of CationsCations: Ag+, Cu2+, Co2+, Ni2+, Cr3+, Al3+, Zn2+, Fe3+, Ba2+, Ca2+, Mg2+, NH4

+

Group 1: insoluble Cl- salt (Ag+)Group 2: insoluble S2- salts (Cu2+, Co2+, Ni2+, Cr3+, Al3+, Zn2+, Fe3+)Group 3: insoluble CO3

2- salts (Ba2+, Ca2+, Mg2+)Group 4: NH4

+

b1. Separation of CationsN/A, since there is only one cation that gives ppt with Cl- which is Ag+

b2. Separation of Group 2

Cations(procedure C)

Group 1ppt formed

Supernate + (NH4)2S(procedure D1)

Group 2ppt formed

Supernate + (NH4)2CO3

(procedure E)

Group 3ppt formed

Group 4supernate

c. Separation for a mixture of Ag+, Cu2+, Zn2+, Fe3+, Mg2+

Ions + HCl(Ag+, Cu2+, Fe3+, Zn2+, Mg2+)

Ppt formed(AgCl)

Supernate + (NH4)2CO3(Cu2+, Fe3+, Zn2+, Mg2+)

Ppt formed + Excess HCl(Fe(OH)3, MgCO3)

Ions + (NH4)2S(Mg2+ and Fe3+)

Red Orange ppt

(Fe(OH)3)

Supernate(Mg2+)

Supernate + excess HCl(complexes of Cu2+, Zn2+)

Ions + Excess NaOH

Blue ppt(Cu(OH)2)

Supernate[Zn(OH)4]2+

Ions + HCl(Ag+, Cu2+, Fe3+, Zn2+, Mg2+)

Ppt formed(AgCl)

Supernate + (NH4)2CO3(Cu2+, Fe3+, Zn2+, Mg2+)

Ppt formed + Excess HCl(Fe(OH)3, MgCO3)

Ions + (NH4)2S(Mg2+ and Fe3+)

Red Orange ppt

(Fe(OH)3)

Supernate(Mg2+)

Supernate + excess HCl(complexes of Cu2+, Zn2+)

Ions + Excess NaOH

Blue ppt(Cu(OH)2)

Supernate[Zn(OH)4]2+

2.a. Fe3+ and Al3+ add (NH4)2CO3 Fe(OH)2 (red orange ppt) + supernate (Al3+ ion)b. Al3+ and Zn2+ add excess NH3 Al(OH)3 (gelatinous white ppt) + supernate [Zn(NH3)4]2+

c. Cr3+ + Al2+ add few drops of NaOH Cr(OH)3 (gray-green ppt) + Al(OH)3 (gelatinous white ppt)

Group 2 + Excess NaOH(Cu2+, Co2+, Ni2+, Cr3+, Al3+, Zn2+, Fe3+)

Ppt formed + HCl(Insoluble OH- ppt of Cu2+, Co2+, Ni2+, Fe3+)

Supernate + excess HCl(tetra-hydroxocomplexes of Cr3+, Al3+, Zn2+)

Ions + Excess NH3(dissolved ions of Cu2+, Co2+, Ni2+, Fe3+)

Ions + Excess NH3(dissolved ions of Cr3+, Al3+, Zn2+)

Ppt(insoluble OH- ppt

of Fe3+)

Supernate(tetraammine

complexof Co3+, Ni2+)

Ppt(insoluble OH- ppt

of Cr3+, Al3+)

Supernate(tetraammine

complex of Zn2+)