Identifying anions and cations

8/17/2019 Identifying anions and cations

http://slidepdf.com/reader/full/identifying-anions-and-cations 1/10

CHM-CAPE-QA-01

Qualitative Analysis: Identification of substances based on their reactions and physical characteristics.Copyright © 2014 Mark December

Cation Analysis

To about 1 cm3/ 0.001 dm3 portions of the solution of the unknown substance, add the reagents shown in the table below, drop wise until

they are present in excess.

Cations Addition of NaOH Addition of NH3 (aq) Confirmatory Test Ammonium - NH4

+ No precipitate formed. Warm the solutiongently, ammonia gas will evolve.Smell the product to detect the gas or placedamp red litmus in the gas. Damp red litmusturns blue. Or dip a glass rod into the solutionand place in conc. HCl or any other hydrogenhalide. Ammonia gas forms white fumes withthe conc. HCl.

NH4+(aq) + OH-(aq) → NH3 (g) + H2O (l)

Solutions mix and no precipitate forms. Warm the solution with dil. AqueousNaOH . Ammonia gas evolves.

NH4Cl(aq) + NaOH(aq) → NH3(g) + NaCl(aq) + H2O(I)

Sodium – Na+ No precipitate No precipitate Do a flame test. Sodium gives a yellow flame.

Potassium – K+ No precipitate No precipitate Do a flame test. Potassium gives a lilac flame.

Copper (II) – Cu2+ Pale blue precipitate, turns black on heating.Insoluble in excess alkali.

Cu2+(aq) + 2OH-

(aq) → Cu(OH)2(s)

Blue precipitate. Soluble in excess aqueousammonia forming a deep blue solution

containing the tetraamine copper (II) ion,[Cu(NH3)4]2+.

Cu2+(aq) + 2OH-

(aq) → Cu(OH)2 (s)

Cu2+(s) + 4NH3 (aq) → [Cu(NH3)4]2+

Tetraamine copper (II) ion

Add potassium iodide, KI .Brown solution, White precipitate.

2Cu2+(aq) + 4I- (aq) → 2CuI (s) + I2 (aq)

White Brown



CHM-CAPE-QA-01

Cations Addition of NaOH Addition of NH3 (aq) Confirmatory TestIron (II) / Ferrous – Fe2+ Dirty green precipitate, turns brown on

exposure to air. Insoluble in excess alkali.

Fe2+(aq) + 2OH-

(aq) → Fe(OH)2 (s)

Dirty green precipitate, turns brown in air dueto atmospheric oxidation of the ion to Fe3+ ions.Insoluble in excess aqueous ammonia.

Fe2+(aq) + 2OH-

(aq) → Fe(OH)3 (s)

Fe2+(aq) → Fe3+(aq) + e-

Add potassium hexacyanoferrate (III)solution, K 3Fe(CN)6 . Dark blue precipitate formed.

Fe2+(aq)

+ [Fe(CN)6]3-(aq)

→Fe(III)4[Fe(II)(CN)6]3 (s)

Iron (III) /Ferric – Fe3+ Red-brown precipitate. Insoluble in excessalkali.

Fe3+(aq) + 3OH-

(aq) → Fe(OH)3 (s)

Red-brown precipitate. Insoluble in excessalkali.

Fe3+(aq) + 3OH-

(aq) → Fe(OH)3 (s)

Add potassium hexacyanoferrate (II)solution, K 4Fe(CN)6 (aq). Dark blueprecipitate formed.

Fe3+(aq)

+ [Fe(CN)6]4-(aq)

→Fe(III)4[Fe(II)(CN)6]3 (s)

OR

Add ammonium thiocyanate solution,NH 4SCN (aq) or potassium thiocyanatesolution, KSCN (aq). Blood red solutionformed.

With SCN-(aq):

Fe3+(aq) + SCN-

(aq) + 5H2O(l) →

[Fe(H2O)5SCN]2+ (aq)

Chromium (III) –

Cr 3+

Grey-green precipitate. Soluble in excessalkali.

Cr 3+(aq) + 3OH-

(aq) → Cr(OH)3 (s)

Cr(OH)3 (s) + 3NaOH(aq) →Na3Cr(OH)6 (aq)

Sodium chromate (III)

Grey-green precipitate. Soluble in excessalkali.

Cr 3+(aq) + 3OH-

(aq) → Cr(OH)3 (s)

Cr 3+(s) + 6NH3 (aq) → [Cr(NH3)6]3+

(aq)

Tetraamine zinc (II) ion



CHM-CAPE-QA-01

Cations Addition of NaOH Addition of NH3 (aq) Confirmatory TestManganese (II) – Mn2+ Cream precipitate. Insoluble in excess alkali.

Mn2+(aq) + 2OH-

(aq) → Mn(OH)2 (s)

Cream precipitate. Insoluble in excess alkali.

Mn2+(aq) + 2OH-

(aq) → Mn(OH)2 (s)

Nickel (II) – Ni2+ Green precipitate. Insoluble in excess alkali.

Ni2+(aq) + 2OH-(aq) → Ni(OH)2 (s)

Green precipitate. Soluble in excess alkaliproducing a violet-purple colour as the

tetraamine nickel (II) ion forms.

Ni2+(aq) + 2OH-

(aq) → Ni(OH)2 (s)

Ni2+(s) + 4NH3 (aq) → [Ni(NH3)4]2+

Tetraamine nickel (II) ion

Cobalt (II) – Co2+ Blue-green precipitate. Insoluble in excess.

Co2+(aq) + 2OH-

(aq) → Co(OH)2 (s)

Blue-green precipitate. Soluble in excess.

Co2+(aq) + 2OH-

(aq) → Co(OH)2 (s)

Co3+(s) + 6NH3 (aq) → [Co(NH3)6]3+

Hexaamine cobalt (III)

Lead (II) – Pb2+ White precipitate. Soluble in excess alkali.

Pb2+(aq) + 2OH- → Pb(OH)2 (s) --- White

Pb(OH)2 (s) + 2NaOH(aq) → Na2Pb(OH)4 (aq)

Sodium plumbate (II)

White precipitate. Insoluble in excessaqueous ammonia.

Pb2+(aq) + 2OH- → Pb(OH)2 (s) --- White

Add potassium iodide solution, KI (aq). Yellow precipitate formed.

Pb2+(aq) + 2I-(aq) → PbI2 (s)

Add potassium chromate solution,K 2 CrO 4 (aq). Yellow precipitate formed.

Pb2+(aq) + CrO4

2- (aq) → PbCrO4 (s)

Most lead salts are insoluble- SO42-,

CO32-, Cl-. The chloride is soluble in

hot solution.



CHM-CAPE-QA-01

Cations Addition of NaOH Addition of NH3 (aq) Confirmatory TestZinc – Zn2+ White precipitate. Soluble in excess alkali.

Zn2+(aq) + 2OH- → Zn(OH)2 (s) --- White

Zn(OH)2 (s) + 2NaOH(aq) → Na2Zn(OH)4 (aq)

Sodium zincate

White precipitate. Soluble in excess aqueousammonia forming tetraamine zinc (II) ion,[Zn(NH3)4]2+.

Zn2+(aq) + 2OH- → Zn(OH)2 (s) --- White

Zn2+(s) + 4NH3 (aq) → [Zn(NH3)4]2+ (aq)

Tetraamine zinc (II) ion

Aluminium – Al3+ White precipitate. Soluble in excess alkali.

Al3+(aq) + 3OH- → Al(OH)3 (s) --- White

Al(OH)3 (s) + NaOH(aq) → NaAl(OH)4 (aq) Sodium aluminate

White precipitate. Insoluble in excessaqueous ammonia.

Al3+(aq) + 3OH- → Al(OH)3 (s) --- White

Solutions are acidic hence pH < 7, blue litmus goes red /pink and effervesceswith CO3

2- / HCO3-.

Calcium – Ca2+ White precipitate. Insoluble in excess alkali.

Ca2+(aq) + 2OH-

(aq) → Ca(OH)2 (s) --- White

No precipitate. Add sodium carbonate, Na2 CO 3. Whiteprecipitate formed.

Ca2+(aq)

+ CO32-

(aq) → CaCO3 (s) --- White

OR

Do a flame test. Calcium gives a brick-red flame.

Magnesium – Ma2+ White precipitate. Insoluble in excess alkali.

Mg2+(aq) + 2OH-

(aq) → Mg(OH)2 (s) --- White

No precipitate



CHM-CAPE-QA-01

Cations Addition of NaOH Addition of NH3 (aq) Confirmatory TestStrontium – Sr 2+ White precipitate. Insoluble in excess alkali.

Sr 2+(aq) + 2OH-

(aq) → Sr(OH)2 (s) --- White

No precipitate Do a flame test. Strontium gives acrimson flame.

Barium – Ba2+ White precipitate. Insoluble in excess alkali.

Ba2+(aq) + 2OH-(aq) → Ba(OH)2 (s) --- White

No precipitate Do a flame test. Barium gives a yellow-green (apple green) flame.



CHM-CAPE-QA-01

Anion Analysis

Anions in the solid state can be tested by the action of heat, dilute nitric or hydrochloric acid or concentrated sulphuric acid.In solutions, anions can be tested as shown in the table below.To test with Silver nitrate add excess nitric acid to solution to decompose any sulphite or carbonate and then add the silver nitrate followedby aqueous ammonia. To test with Barium chloride add aqueous barium nitrate or chloride and then add excess nitric acid to any precipitate.

Solid Aqueous Solutions

Anions Heat Dilute HNO3 /Dilute HCl

Conc. H2SO4,heated.

Barium Chloride,(BaCl2) + HNO3

Silver Nitrate,(AgNO3) +

Ammonia solution,(NH3 (aq))

Lead Nitrate,Pb(NO3)2

Hydrogencarbonate/bicarbonate - HCO3

-

Carbon dioxide,CO2 and H2Oevolved.

2HCO3-

→ CO32-

(s) +H2O(l) + CO2 (g)

Carbon dioxideforms a whiteprecipitate with limewater.

CaOH2 (aq) + CO2(g) → CaCO3(s) + H2O(l)

Carbon dioxide, CO2 evolved.

H+(aq) + HCO3

2-(aq) →

H2O(I) + CO2 (g)

Carbon dioxide formsa white precipitatewith lime water.

CaOH2 (aq) + CO2(g) →CaCO3(s) + H2O(l)

Carbon dioxide,CO2 evolved.

H+(aq) + HCO3

2-(aq)

→ H2O(I) + CO2 (g)

Carbon dioxideforms a whiteprecipitate with limewater.

CaOH2 (aq) + CO2(g) → CaCO3(s) + H2O(l)

Carbonate – CO32- Those of metalsbelow Calcium in theseries decompose togive oxide andCarbon dioxide,CO2 (g)

Carbon dioxide evolved.

2H+(aq) + CO3

2-(s) →

H2O (l) + CO2 (g)

Carbon dioxide evolved.

2H+(aq) + CO3

2-(s) →

H2O (l) + CO2 (g)

White precipitate.Soluble in excessHNO3. CO2 (g) evolved.

Ba2+(aq) + CO3

2-(aq) →

BaCO3 (s)

White precipitate.Soluble in nitric acid.

Pb2+(aq) + CO3

2-(aq) →

PbCO3 (s)



CHM-CAPE-QA-01



Conc. H2SO4,heated.





Carbonate – CO32- CO3

2- → O2-(s) +

CO2(g)

Test CO2 withlimewater.

Test CO2 with

limewater.

Test CO2 with

limewater.

Chloride – Cl- Hydrogen chloride,HCl(g) evolved.Some Cl2 (g) if astrong enoughoxidising agent ispresent.

H+(aq) + Cl-(s) →

HCl (g)

2Cl-(aq) → Cl2 (g) +2e-

White precipitate.Soluble in dilute andconc. NH3 (aq) formingthe diamine silver (I)ion, [Ag(NH3)2]+ (aq)

Ag+(aq) + Cl-(aq) →

AgCl(s) , white

AgCl(s) + 2NH3 (aq) ↔[Ag(NH3)2]+

(aq) + Cl-(aq)

White precipitate.Soluble on heating,re-precipitates oncooling.

Pb2+(aq) + 2Cl-(aq) →

PbCl2 (s) , white

Bromide – Br - Br 2 (l) and HBr (g)

evolved.

H+(aq) + Br -(s) ↔

HBr (g)

2Br -(aq) ↔ Br 2 (g) +2e-

Cream precipitate.Soluble in conc. NH3(aq)

but insoluble in dil.NH3(aq).

Ag+(aq) + Br -(aq) →

AgBr (s) , cream

White precipitate.Soluble on heating,re-precipitates oncooling.

Pb2+(aq) + 2Br -(aq) →

PbBr 2 (s) , white



CHM-CAPE-QA-01



Conc. H2SO4,heated.





Iodide – I- HI (g), SO2 (g) , I2 (g) andH2S (g) formed.

2I-(aq) → I2 (aq) + 2e

Yellow precipitate.Insoluble in dilute andconc. NH3 (aq)

Ag+(aq) + I-(aq) → AgI(s) ,

Bright Yellow precipitate.

Pb2+(aq) + I-(aq) →

PbI2(s)

Sulphate – SO42- Add excess dilute HNO3

(aq) making the solutionto be tested acidic. Thisremoves anycontaminating

carbonates present.Then add the BaCl(aq) orBa(NO3)2 (aq).

White precipitate.Insoluble in acid.

Ba2+(aq) + SO4

2-(aq) →

BaSO4 (s)

White precipitate.Insoluble in diluteHNO3 (aq).

Pb2+(aq) + SO4

2-(aq) →

PbSO4 (s)

Sulphite – SO32-

Sulphites are morethermally unstablethan the sulphatesand can be testedusing heat.

SO2 (g) evolved.

2H+(aq) + SO3

2- →H2O(l) + SO2 (g)

SO2 (g) evolved.

SO2 (g) is colourless.It turns acidified

potassiumdichromate (VI),Cr 2 O 7

2- /H + fromorange to green.

White precipitate.Soluble in acid (SO2 evolved)

Ba2+(aq) + SO3

2-(aq) →

BaSO3 (s)

White precipitate.Soluble in diluteHNO3.

Pb2+(aq) + SO3

2-(aq) →

PbSO3(s)



CHM-CAPE-QA-01



Conc. H2SO4,heated.





Chromate – CrO42- Add dil. H 2 SO 4 tothe solution.

Colour changesfrom yellow toorange due to theformation of thedichromate ion,CrO7

2-(aq).

CrO42-

(aq) + 2H+(aq)

→ CrO72-(aq) + H2O(l)

Nitrate – NO3- Nitrates high in the

reactivity seriesdecomposes to givethe nitrite andoxygen gas.

Eg: NaNO3

NaNO3 (s) →NaNO2(s) + O2(g)

Nitrates low in thereactivity seriesdecomposes to givethe oxide, nitrogen

Warm with conc.H 2 SO 4. NO 2(g) andHNO 3(aq) produced.

HNO3(aq) + NO3-(s)

→ HNO3 (aq) +HSO4

-(aq)

Add a small piece ofCu. Solution turnsblue and brown fumes of NO2(g) evolves.



CHM-CAPE-QA-01

dioxide and oxygengas.

Eg: Cu(NO3)2

Cu(NO3)2 (s) →CuO(s) + O2(g) +NO2(g)

4H+(aq) + 2NO3

- (aq) +

Cu(s) → Cu2+(aq) +

4NO2(g) + 2H2O(l)

Documents

Identifying anions and cations