Qualitative Analysis of Cations and Anions

Some representative examples

A picture of crystalline solid, no water molecules

Do you know how to commonly tests for gases?

Test for cations

Please have revision of basic concepts• Homogeneous mixture (solution)• Precipitation• Solubility• Complex ions formation

Homogeneous…

• Homogeneous refers to a solution that is uniform throughout, clear in that it does not contain insoluble particles on standing or under the light. (eg. NaCl solution)

Precipitation...• Process in which an insoluble substance is

formed as a result of mixing two clear (may not be colourless) and homogeneous solutions together.

– (eg. NaCl(aq) + AgNO3 (aq) NaNO3(aq) + AgCl(s) )

Solubility of some hydroxide

Soluble Hydroxides Insoluble HydroxidesPotassium Hydroxide Most Hydroxides

including…Sodium Hydroxide Aluminium Hydroxide

Calcium Hydroxide ( sparingly soluble )

Zinc Hydroxide

Ammonia Solution Iron(II) Hydroxide

Iron(III) Hydroxide

Copper(II) Hydroxide

To illustrate the ions present in the following alkalis :

• Strong Alkalis– NaOH Na+ + OH-

– Ca(OH)2 Ca2+ + 2OH-

• Weak Alkali– NH3.H2O NH4

+ + OH-

• Notice that aqueous ammonia is a solution of ammonia gas in water. If this gas is dissolved in an organic solvent, it is not alkaline as it does not release OH- ions.

Complex Ions• Some of the insoluble

hydroxides can form complex ions with sodium hydroxide or ammonia solution which are soluble in water. This is the basis of test for cations.

Cation Soluble complex ionsAl3+ Al(OH)4

-, Aluminate ion

Zn2+ Zn(OH)4-, Zincate ion

Pb2+ PbO22- , Plumbite ion

Cu2+ Cu(NH3)42+,Complex ions

Investigation on zinc cation, Zn 2+

Zinc nitrate Zinc nitrate+ a few drops sodium hydroxide

Zinc nitrate + excess sodium hydroxide

Zinc nitrate+ a few drops ammonia solution

Zinc nitrate+ excess ammioniasolution

White ppt

Zn(NO 3)2 (aq)+ 2NaOH(aq) Zn (OH)2 (s)+ 2NaNO3(aq)

No ppt ??? white ppt dissolves forming a colourless solution

Zn(OH)2(s) + 2NaOH(aq) Na2Zn(OH)4(aq), complex ion formed

Investigation on aluminium cation, Al 3+

Aluminiumnitrate

Aluminium nitrate+ a few drops sodium hydroxide

Aluminium nitrate + excess sodium hydroxide

Aluminium nitrate+ a few drops ammonia solution

Aluminiumnitrate+ excess ammioniasolution

White ppt

Al(NO 3)3 (aq)+ 3NaOH(aq) Al (OH)3 (s)+ 3NaNO3(aq)

No ppt ??? white ppt dissolves forming a colourless solution

Al(OH)3(s) + NaOH(aq) NaAl(OH)4(aq), complex ion formed

Investigation on calcium cation, Ca 2+

Calcium chloride Calcium chloride+ a few drops sodium hydroxide

Calcium chloride + excess sodium hydroxide

Calcium chloride + any amount ammioniasolution (no reactionwith ammonia solution)

White ppt

CaCl2 (aq)+ 2NaOH(aq) Ca (OH)2 (s)+ 2NaCl(aq)

White pptthe white ppt is insoluble in excess NaOH

No complex ion formed. White ppt is Ca(OH)2

Investigation of Iron (II) Cation, Fe 2+ :

Adding a fewdrops of NaOH

Adding excess NaOH

The dirty green ppt in both cases is Fe(OH)2 , iron (II)hydroxide

Question :

Write the chemical equation that forms the dirty green pptof iron (II) hydroxide.

Investigation of Iron (II) Cation, Fe 2+ :

Adding a fewdrops of NH3.H2O

Adding excess NH3.H2O

The green ppt in both cases is Fe(OH)2 , iron (II)hydroxide

Investigation of Iron (III) Cation, Fe 3+ :

Investigation of Iron (III) Cation, Fe 3+ :

Adding a fewdrops of NaOH

Adding excess NaOH

The reddish brown ppt in both cases is Fe(OH)3 ,iron (III)hydroxide

Experiment 5B : Investigation of Iron (III)Cation, Fe 3+ :

Adding a fewdrops of NH3.H2O

Adding excess NH3.H2OThe reddish brown ppt in both cases is Fe(OH)3 , iron (III)

hydroxide

Question :

Write the chemical equation that forms the reddish brownppt of iron (III) hydroxide.

Compare & Contrast the different colours

of iron (II) & iron (III) compounds :

Pale green iron (II) sulphatesolution

Dirty green iron (II)hydroxide ppt

Dirty green ppt insoluble in excess NaOH

Orange yellowiron (III)sulphatesolution

Reddish brown iron (III)hydroxide ppt

Reddish brown ppt insoluble in excess NaOH

Pale blue Cu 2+ solution

Blue Cu(OH)2 ppt inNaOH

Blue ppt insoluble in excess NaOH

Blue ppt in NH3.H2O

Blue ppt soluble in excess NH3.H2Oforming a dark blue solutionA soluble complex ion is formed

Compare & Contrast the differences of adding NaOH & NH4OH to Copper (II) ion, Cu 2+ :

Test for Copper (II) Cation, Cu 2+ :

Test for ammonium cation, NH4+ :

NH4Cl(s) + NaOH(aq) NH3(g) + NaCl(aq) + H2O(l)

Test for puingent gas evolved with moist red litmus that turns blue

Cation Reaction with NaOH Reaction with NH3(aq)

Al3+ White precipitate, soluble in excess giving a colourless solution.

White precipitate, soluble in excess

NH4+ Ammonia gas produced on

warming.-

Ca2+ White precipitate, insoluble in excess.

No precipitate, or very slightly white precipitate.

Cu2+ Light blue precipitate, insoluble in excess.

Light blue precipitate, soluble in excess giving a dark blue solution.

Fe2+ Green precipitate, insoluble in excess.

Fe3+ Reddish brown precipitate, insoluble in excess.

Zn2+ White precipitate, soluble in excess, giving a colourless solution.

Reaction of Cations

NOTE : Na+ and K+ have no ppt in any amount of sodium hydroxide and ammonia solution.

Test for Anion

( - ve ION )

Test for sulphate anion, SO4

2-

Sulphate solution

Acidify barium nitrate with nitric acidie. nitric acid + barium nitrate in a separate test-tube

White pptformed

acidified Ba(NO3)2(aq) + Na2SO4(aq) BaSO4(s) + 2NaNO3(aq)

Precaution : Colour of white ppt such as barium sulphatemay be obscured by the colour of the cation.Some egs below :

White ppt of BaSO4 from ZnSO4

White ppt(BaSO4 fromFeSO4)observed only on standing

White ppt (BaSO4 from CuSO4)observed only on standing

Test for chloride anion, Cl - :

chloride solution

Acidify silver nitrate with nitric acid i.e. nitric acid +silver nitrate in a separate test-tube

White pptformedAgCl(s) + NaNO3(aq) acidified AgNO3(aq) + NaCl(aq)

Compare & Contrast the differences in the colours of the silver precipitates

AgCl AgBr AgI

White ppt Cream ppt Yellow ppt

Test for iodide anion, l - :

iodide solution

Acidify silver nitrate with nitric acidi.e. nitric acid +silver nitrate in a separate test-tube

Yellow pptformedAgI(s) + NaNO3(aq) acidified AgNO3(aq) + NaI(aq)

Alternative Test for Cl- & I- anions :

Test-tube A : Lead(II)nitrate + potassium iodide lead (II) iodide + potassium nitrateExplanation:

Bright yellow PbI2lead(II)iodide

Test-tube B :Lead(II)nitrate + potassium chloride lead (II) chloride+ potassium nitrate

A B

White PbCl2lead(II) chloride

KNO3(s) + NaOH(aq) + Devardro’s alloy NH3(g)

Test for pungent gas evolved with moist red litmus that turns blue

Test for nitrate anion, NO3- :

(just Al powder or Al + Zn powder)

Test for carbonate anion, CO3- :

CaCO3(s) + 2HCl(aq) CO2(g) + CaCl2(aq) + H2O(l)

Test for gas evolved with limewater that turns chalky

Summary

Anion Test ResultCO3

2- Add dilute acid Effervescence, carbon dioxide gas produced

Cl- Acidify with HNO3(aq), followed by AgNO3(aq)

White precipitation

I- Acidify with HNO3(aq), followed by Pb(NO3)2(aq)

Yellow precipitation

NO3- Add NaOH(aq) followed by Al

foil; warm carefullyAmmonia gas produced

SO42- Acidify with HNO3(aq), followed

by Ba(NO3)2(aq)

White precipitation

Summary

CATIONS (+ IONS) ANIONS (- IONS)1)Preliminary Observation: Appearance(Colour)

1)Preliminary Observation:Solubility in Water

2)Thermal Decomposition: Residue (Colour)

2)Thermal Decomposition:Gas evolved

3)Using NaOH and NH3.H2O

List of Cations

• NH 4+ Ammonium ion• Ca 2+ Calcium ion• Zn 2+ Zinc ion• Pb 2+ Lead (II) ion• Al 3+ Aluminium ion• Cu 2+ Copper (II) ion• Fe 2+ Iron (II) ion• Fe 3+ Iron (III) ion

Test for Cations

• Using NaOH:• Using NH3.H2O (aqueous ammonia):

• For example:Zn 2+(aq) + OH-(aq) Zn(OH)2(s)

Note: Cation from alkali metal hydroxide, insoluble

Solubility of Hydroxides

Soluble hydroxides Insoluble hydroxides

Potassium hydroxide, Sodium hydroxide, Calcium hydroxide (sparingly soluble), Ammonia solution

Most hydroxides including Aluminium hydroxide, Zinc hydroxide, Iron (II) hydroxide, Iron (III) hydroxide, Copper (II) hydroxide.

Solubility of Complex ions

• Some of the insoluble hydroxides can form complex ions with sodium hydroxide or ammonia solution which are soluble in water. This is the basis of test for cations.

Cation Soluble Complex ions

Al 3+ Zn 2+

Pb 2+

Cu 2+

Al(OH)4 - , aluminate ion

Zn(OH)4

- , zincate ion PbO2

2- , plumbite ion

Cu(NH3)4

2+ ,complex ions