Experimental procedures of cation QA. Coloured pictures fully available.

1. IDENTIFYING CATIONS:
TECHNIQUES & OBSERVATIONS
Tan Wan Jun
Please view in ‘Slide Show’ mode.

2. Using NaOH(aq)
Pour unknown salt solution into test tube
until a height of about 1cm (~1cm3).
Add 3 drops of NaOH(aq) to solution.
Shake the mixture.
Record observation(s):
Any ppt? Colour of ppt formed?
If there is ppt, add excess NaOH(aq) to about
half test tube full. Shake the mixture.
Record observation:
Is ppt soluble in excess NaOH(aq)?
How to Perform Qualitative Cation Analysis
Note:
1. If unknown salt is given in solid form, dissolve a
small amount in a test tube with deionised water.
2. Separate the resulting solution into 2 equal
portions (~1cm3 each) for testing using NaOH(aq)
and NH3(aq) respectively.
Repeat analysis using NH3(aq)
b
dc
a
unknown

3. Cation NaOH(aq) NH3(aq)
Al3+ White ppt Soluble in
excess to
give
colourless
Solution
White ppt Insoluble
in excess
Pb2+ White ppt Soluble in
excess to
give
colourless
solution
White ppt Insoluble
in excess
Zn2+ White ppt Soluble in
excess to
give
colourless
solution
White ppt Soluble in
excess to
give
colourless
solution
Ca2+ White ppt Insoluble
in excess
No ppt (solution remained colourless)
Note: How do you distinguish between Pb2+ ions and Al3+ ions?

4. Cation NaOH(aq) NH3(aq)
Cu2+ Light blue
ppt
Insoluble
in excess
Light blue
ppt
Soluble in
excess to
give
dark blue
solution
Fe2+ Dirty-green
ppt
Insoluble
in excess
Dirty-green
ppt
Insoluble
in excess
*Fe3+ Reddish-
brown ppt
Insoluble
in excess
Reddish-
brown ppt
Insoluble
in excess
Note: On standing in air (oxygen), the dirty-green ppt of Fe(OH)2 turns reddish-brown due to
oxidation of Fe2+ ions.
*Colour of solution containing
Fe3+ may range from colourless
to pale yellow depending on
concentration of Fe3+.
Excess reagent:
NaOH(aq)/NH3(aq)
Dirty green ppt with
reddish-brown tinge
Stand in air

5. Using Sodium Hydroxide Solution, NaOH(aq)
Salt
Solution
No ppt
Colourless, pungent
gas (NH3) evolved on
heating. Turns moist
red litmus blue.
NH4
+
White ppt
ppt soluble in
excess NaOH(aq)
to form a
colourless
solution
Zn2+
Al3+
Pb2+
ppt insoluble in
excess NaOH(aq)
Ca2+
Coloured ppt
Light blue ppt
insoluble in excess
NaOH(aq)
Cu2+
Dirty-green ppt
insoluble in excess
NaOH(aq)
Fe2+
Reddish-brown ppt
insoluble in excess
NaOH(aq)
Fe3+
Add NaOH(aq)
If you cannot decide whether the ppt is soluble in excess reagent or not, pour away some of the ppt, leaving just a little
bit at the bottom of the test tube, before adding more reagent to about half the test tube. Hold the test tube against
the light. If you can see particles of the ppt, the ppt is not soluble in excess reagent.

6. Salt
Solution
No ppt
Ca2+, NH4
+
White ppt
ppt soluble in
excess NH3(aq) to
form a colourless
solution
Zn2+
ppt insoluble in
excess NH3(aq)
Al3+, Pb2+
Coloured ppt
Light blue ppt
soluble in excess
NH3(aq) to form a
dark blue solution
Cu2+
Dirty-green ppt
insoluble in excess
NH3(aq)
Fe2+
Reddish-brown ppt
insoluble in excess
NH3(aq)
Fe3+
Using Aqueous Ammonia, NH3(aq)
Add NH3(aq)
If you cannot decide whether the ppt is soluble in excess reagent or not, pour away some of the ppt, leaving just a little
bit at the bottom of the test tube, before adding more reagent to about half the test tube. Hold the test tube against
the light. If you can see particles of the ppt, the ppt is not soluble in excess reagent.

7.  The precipitate in each of the reactions with NaOH(aq)/NH3(aq) is the
insoluble hydroxide of the metal ion.
 E.g. Copper(II) ion + hydroxide ion  copper(II) hydroxide; Cu2+(aq) + 2OH-(aq)  Cu(OH)2(s)
 Some of the precipitates dissolve in excess NaOH(aq)/NH3(aq) due to the
formation of compounds that are soluble in water (complexes).
 Pb2+ ions can be distinguished from Al3+ ions by adding hydrochloric acid, HCl,
to the unknown solution.
 If a white ppt forms, Pb2+ is present – due to the insolubility of lead(II) chloride, PbCl2.
 If NH4
+ ion is present in the unknown, addition of NaOH(aq) followed by
warming will produce ammonia gas, NH3(g).
 Refer to the video on how testing for NH3(g) is done.
 When a gas is evolved, look out for the following: Any effervescence? Colour
and odour of gas? Action of the gas on moist litmus paper?
Review

8. Test for Ammonium Ion
Things to note:
1)Remember to wet litmus
paper before use. Can you
recall the reason?
[A moist
litmus paper allows the gas
to dissolve in water, forming
H+(aq) or OH-(aq) ions.]
2)Observe how warming is
done: Remove test tube
from Bunsen flame
periodically to prevent
contents from spurting.

9.  Tan, Y. T., Chen, L. K., Sadler, J. & Clare, E. (2007). Salts. In GCE ‘O’ Level
Chemistry Matters (pp.194-211). Singapore: Marshall Cavendish International
Pte. Ltd.
 Video “Test for ammonium ion” obtained and modified from:
Yue-Chang, T. H. (n.d.). Ammonium ion test. Retrieved November 26, 2008,
from http://www.youtube.com/watch?v=GkWAamtZnAc
References & Acknowledgements