6 Analisis dan Kualitatif Ion

Qualitative Analysis of Cations

and Anions

Reaction can be detected by

•

_{Formation of gases}

•

Change the colour of the solution

•

Formation of precipitate

•

_{Change in temperature}

Test for cations

Please have revision of basic concepts

•

Homogeneous mixture (solution)

•

Precipitation

•

_Solubility

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

Precipitation...

•

_{Process in which an}

_{insoluble substance}

_{is formed as a}

result of

mixing two clear

(may not be colourless)

and

homogeneous solutions

together.

Solubility of some hydroxide

Soluble Hydroxides

Insoluble Hydroxides

Potassium Hydroxide

Most Hydroxides

including…

Sodium Hydroxide

Aluminium Hydroxide

Calcium Hydroxide

( sparingly soluble )

Zinc Hydroxide

To illustrate the ions present in the following alkalis :

•

Strong Alkalis

–

NaOH

_

Na

+

+ OH

-–

_Ca(OH)

2



Ca

2+

+ 2OH

-•

Weak Alkali

–

_NH

₃

_.H

₂

_O

_

_NH

₄+

_{+ OH}

-•

Notice that aqueous ammonia is a solution of ammonia gas in

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

Investigation on zinc cation, Zn

2+

Zinc nitrate Zinc nitrate + a few drops sodium

hydroxide

Zinc nitrate + excess

forming a colourless solution

Zn(OH)₂(s) + 2NaOH(aq)

Investigation on aluminium cation, Al

3+

forming a colourless solution

Al(OH)₃(s) + NaOH(aq)

Investigation on calcium cation, Ca

2+

Calcium chloride Calcium chloride + a few drops with ammonia solution)

White ppt

CaCl₂ (aq)+ 2NaOH(aq)

 Ca(OH)₂(s)+ 2NaCl(aq)

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

Investigation of Iron (II) Cation, Fe

2+

:

Adding a few

drops of NaOH

Adding excess

NaOH

The dirty green ppt in both cases is Fe(OH)

₂

, iron (II)hydroxide

Question :

Investigation of Iron (II) Cation, Fe

2+

:

Adding a few

drops of

NH

₃

.H

₂

O

Adding excess

NH

₃

.H

₂

O

Investigation of Iron (III) Cation, Fe

3+

:

Adding a

few

drops of

NaOH

Adding

excess

NaOH

Experiment 5B

:

Investigation of Iron (III)Cation, Fe

3+

:

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

Question :

Compare

&

Contrast

the different colours

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

Pale green

iron (II) sulphate solution

Dirty green iron (II)

hydroxide ppt

Dirty green ppt

insoluble in excess NaOH

Orange yellow iron (III)sulphate solution

Reddish brown

iron (III)hydroxide ppt

Pale blue Cu 2+ solution

Blue Cu(OH)₂ ppt in NaOH

Blue ppt insoluble in excess NaOH Blue ppt in NH₃.H₂O

Blue ppt soluble in

excess NH3.H2O

forming a dark blue solution

A soluble complex ion is formed

Compare

&

Contrast

the differences

Test for ammonium cation, NH

₄+

:

NH

₄

Cl

_(s)

+ NaOH

_(aq)



NH

_3(g)

+ NaCl

_(aq)

+ H

₂

O

_(l)

Cation Reaction with NaOH Reaction with NH_3(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

Test for Anion

Test for sulphate anion, SO

₄

2-Sulphate solution

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

White ppt

formed

Precaution : Colour of white ppt such as barium sulphate

may 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

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 ppt

formed

Compare & Contrast the differences in the

colours of the silver precipitates

AgCl

AgBr

AgI

Test for iodide anion, l

-

:

iodide solution

Acidify silver nitrate with nitric acid

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

Yellow ppt

formed

Alternative Test for Cl

-

& I

-

anions :

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

Explanation:

Bright yellow PbI₂ lead(II)iodide

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

A

B

KNO

_3(s)

+ NaOH

_(aq)

+ Devarda’s alloy



NH

_3(g)

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

Test for nitrate anion, NO

₃-

:

Test for carbonate anion, CO

₃- :

CaCO

_3(s)

+ 2HCl

_(aq)



CO

_2(g)

+ CaCl

_2(aq)

+ H

₂

O

_(l)

Summary

Anion

Test

Result

CO

₃2-

Add dilute acid

Effervescence,

carbon dioxide gas

produced

Cl

-

_{Acidify with HNO}

3(aq)

, followed

by AgNO

_3(aq)

White precipitation

I

-

Acidify with HNO

3(aq)

, followed

by Pb(NO

₃

)

_2(aq)

Yellow precipitation

NO

₃-

Add NaOH

(aq)

followed by Al

foil; warm carefully

Ammonia gas

produced

SO

₄2-

Acidify with HNO

3(aq)

, followed

by Ba(NO

₃

)

_2(aq)

Summary

CATIONS (+ IONS)

ANIONS (- IONS)

1)Preliminary

Observation:

Appearance(Colour)

1)Preliminary

Observation:Solubility in

Water

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

Test for Cations

•

Using NaOH:

•

_{Using NH}

₃

_.H

₂

_{O (aqueous ammonia):}

•

_{For example:}

Zn

2+

(aq) + OH

-

(aq)



Zn(OH)

2

(s)

Solubility of Hydroxides

Soluble hydroxides

Insoluble hydroxides

Potassium hydroxide,

Sodium hydroxide,

Calcium hydroxide

(sparingly soluble),

Ammonia solution

Most hydroxides

including

Aluminium hydroxide,

Zinc 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