6 Analisis dan Kualitatif Ion

Qualitative Analysis of Cations
and Anions
Some representative examples

Reaction can be detected by
•
•
•
•
•

Formation of gases
Change the colour of the solution
Formation of precipitate
Change in temperature
Etc..

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
Potassium Hydroxide

Insoluble Hydroxides
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 ions

Al3+

Al(OH)4-, Aluminate ion

Zn2+

Zn(OH)42-, 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
White ppt

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

Zinc nitrate
+ excess
sodium
hydroxide

Zinc nitrate
+ a few drops
ammonia
solution

Zinc nitrate
+ excess
ammionia
solution

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+

Aluminium
nitrate

Aluminium
nitrate
+ a few drops
sodium
hydroxide
White ppt

Al(NO3)3 (aq)+ 3NaOH(aq)
 Al (OH)3 (s)+ 3NaNO3(aq)

Aluminium
nitrate
+ excess
sodium
hydroxide

Aluminium
nitrate
+ a few drops
ammonia

solution

Aluminium
nitrate
+ excess
ammionia
solution

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
ammonia
solution
(no reaction
with ammonia
solution)

White ppt
White pptthe white ppt is insoluble in excess NaOH

CaCl2 (aq)+ 2NaOH(aq)

 Ca(OH)2(s)+ 2NaCl(aq)

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

Investigation of Iron (II) Cation, Fe

2+

:

Question :
Write the chemical equation
that forms the dirty green ppt
of iron (II) hydroxide.
Adding a few
drops of NaOH

Adding excess
NaOH

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

Investigation of Iron (II) Cation, Fe

Adding a few
drops of
NH3.H2O

2+

:

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

Adding a
few
drops of
NaOH

3+

:

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+

:

Question :
Write the chemical equation
that forms the reddish brown
ppt of iron (III) hydroxide.
Adding a
few
drops of
NH3.H2O

Adding
excess
NH3.H2O

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

Compare & Contrast the different colours
of iron (II) & iron (III) compounds :

Pale green
iron (II) sulphate
solution

Dirty green
iron (II)
hydroxide ppt

Orange yellow
iron (III)sulphate
solution

Dirty green ppt
insoluble in excess NaOH

Reddish brown
ppt insoluble in
excess NaOH

Reddish brown
iron (III)hydroxide ppt

Compare & Contrast the differences

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

Blue ppt
soluble in
Blue ppt in NH3.H2O excess NH3.H2O
Pale blue Cu 2+ solution
forming a dark
blue solution
Blue Cu(OH)2 ppt in
A soluble complex
NaOH
ion is formed
Blue ppt insoluble in excess NaOH

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

Reaction of Cations
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.

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-

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

White
ppt
formed

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

Sulphate
solution

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

White ppt(BaSO4
fromFeSO4)
observed only on
standing

White ppt of BaSO4
from ZnSO4

White ppt (BaSO4 from CuSO4)
observed only on standing

Test for chloride anion, Cl - :
Acidify silver nitrate with nitric acid i.e. nitric acid
+silver nitrate in a separate test-tube

White
ppt
formed
AgCl(s) +

chloride
solution
NaNO3(aq) 

acidified AgNO3(aq) +

NaCl(aq)

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

AgCl
White ppt

AgBr
Cream ppt

AgI
Yellow ppt

Test for iodide anion, l - :
Acidify silver nitrate with nitric acid
i.e. nitric acid +silver nitrate in a separate test-tube

Yellow
ppt
formed
AgI(s) +

iodide
solution
NaNO3(aq) 

acidified AgNO3(aq) +

NaI(aq)

Alternative Test for Cl- & I- anions :

A

Bright yellow PbI2
lead(II)iodide

B

White PbCl2
lead(II) chloride

Explanation:
Test-tube A : Lead(II)nitrate + potassium iodide  lead (II) iodide + potassium nitrate
Test-tube B :Lead(II)nitrate + potassium chloride lead (II) chloride+ potassium nitrate

Test for nitrate anion, NO3- :

KNO3(s) + NaOH(aq) + Devarda’s alloy NH3(g)

(just Al powder or Al + Zn powder)

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

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
CO32-

Test
Add dilute acid

Cl-

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

I-

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

Yellow precipitation

NO3-

Add NaOH(aq) followed by Al
foil; warm carefully
Acidify with HNO3(aq), followed
by Ba(NO3)2(aq)

Ammonia gas
produced

SO42-

Result
Effervescence,
carbon dioxide gas
produced
White precipitation

White precipitation

Summary
CATIONS (+ IONS)
1)Preliminary
Observation:
Appearance(Colour)

ANIONS (- IONS)
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+
Ca 2+
Zn 2+
Pb 2+
Al 3+
Cu 2+
Fe 2+
Fe 3+

Ammonium ion
Calcium ion
Zinc ion
Lead (II) ion
Aluminium ion
Copper (II) ion
Iron (II) ion
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),

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

Ammonia solution

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
3+
Al(OH)
, aluminate ion
4
Al
Zn

2+

Zn(OH)4 - , zincate ion

Pb

2+

PbO22- ,

Cu

2+

Cu(NH3)4 2+ ,complex

plumbite ion

ions