Student name: ...........................................................................
Group: .......................................................................................
Laboratory Nr. 4
Qualitative Analysis of the Mixture of Cations and Anions
Purpose: to study specific tests for detecting in an unknown sample the presence of common cations
(metal ions) and anions.
Ions are tested for by using the typical method (qualitative reaction) of forming a characteristic
indication (insoluble precipitates, intensive colour, odour, gas formation)
Theory: Analysis of an unknown sample (sometimes called X) to discover what elements it contains –
is called qualitative analysis.
There are many different schemes of qualitative analysis, but all have features in common. One such
feature is the formation of a precipitate (white or of various colours) when certain reagents are added
to a solution of the sample, often after it has been made acidic or basic by the addition of acid or base.
Other features can be: formation of soluble compounds with intensive colour (mainly complexes),
bubbles of gases, specific smell of gases, specific forms of crystalline precipitation, etc.
Experiments:
Identification of cations - NH4+, Na+, K+, Ca2+, Cu2+, Fe2+, Fe3+
1. NH4+ can be found by the reaction with alkalis. This ion is identified by an intensive smell of
the ammonia:
NH4Cl + KOH  KCl + NH3 (g) + H2O
Experiment: In a test tube containing small amount of given unknown X solution (about 0.5 cm3) a
piece of solid KOH is added and strongly shaken. NH4+ ion is identified by the strong specific smell of
NH3.
2. Na+ can be found by the reaction with potassium decavanadate - K6[V10O28]. Presence of this
ion is confirmed by the formation of yellow shiny sparkling precipitate. Divalent ions of other metals
should be bound before Na+ identification into citrate complexes:
2NaCl + K6[V10O28]  K4Na2[V10O28] (s) + 2KCl
Experiment: In a test tube small amount of given X solution (about 0.5 cm3) is mixed with an equal
amount of potassium citrate, then several droplets of reagent K6[V10O28] are added. Na+ is identified
by the formation of yellow shining crystalline precipitate.
3. K+ can be found by the reaction with perchloric acid - HClO4 and identified by the formation
of white precipitate of potassium perchlorate:
KCl + HClO4  KClO4 (s) + HCl
Experiment: In a test tube containing small amount of given X solution (about 0.5 cm3) several
droplets of reagent HClO4 are added and K+ is identified by the formation of white milky precipitate.
4. Ca2+ can be found by the reaction with potassium oxalate – K2C2O4 and identified by the
formation of white precipitate of calcium oxalate. Divalent ions of other metals should be bound
before Ca2+ identification into citrate complexes:
CaCl2 + K2C2O4  CaC2O4 (s) + 2KCl
Experiment: In a test tube small amount of given X solution (about 0.5 cm3) is mixed with an equal
amount of potassium citrate, then several droplets of reagent K2C2O4 are added. Ca2+ is identified by
the formation of white milky precipitate.
5. Cu2+ can be found by the reaction with ammonium (NH3) aqua solution and identified by the
formation of soluble blue copper complex:
CuSO4 + 4NH3  [Cu(NH3)4]SO4
Experiment: Simply add to your X sample several droplets of the specific reagent and observe for the
above indicated features.
6. Fe2+ can be found by the reaction with potassium hexacianoferrate (K3[Fe(CN)6]) and
identified by the formation of dark blue precipitate:
3FeSO4 + 2K3[Fe(CN)6]  Fe3[Fe(CN)6]2 (s) + 3K2SO4
Experiment: Simply add to your X sample several droplets of the specific reagent and observe for the
above indicated features.
7. Fe3+ can be found by the reaction with ammonium rodanide (NH4SCN) and identified by the
formation of soluble red coloured complex compound:
Fe(NO3)3 + 6NH4SCN  (NH4)3[Fe(SCN)6] + 3NH4NO3
Experiment: Simply add to your X sample several droplets of the specific reagent and observe for the
above indicated features.
Identification of anions - CO32-, Cl-, SO42-, NO3-, PO431. CO32- can be found by the reaction with strong acids and identified by the effervescence of
carbon dioxide:
Na2CO3 + 2HNO3  2NaNO3 + CO2 (g) + H2O
2. Cl- can be found by the reaction with silver nitrate and identified by the formation of white
milky precipitate of silver chloride:
CaCl2 + 2AgNO3  2AgCl (s) + Ca(NO3)2
3. SO42- can be found by the reaction with barium chloride and identified by the formation of
white precipitate of barium sulphate:
K2SO4 + BaCl2  BaSO4 (s) + 2KCl
Experiment (for identification of CO32-, Cl-, SO42- ions): In a test tube small amount of given X
solution (about 1 cm3) is mixed by the equal amount of 6M HNO3. If effervescence of carbon dioxide
is observed this confirms the presence of CO32- ion. Then the acidified X solution is divided into 2 test
tubes and one is checked for Cl- ion, another for SO42- ion by simply adding the above mentioned
specific reagents. Both ions are identified by formation of white milky precipitates.
5. NO3- can be found by the reaction with diphenylamine and identified by the formation of dark
blue soluble compound:
Experiment: In a test tube containing solution of diphenylamine (diphenylamine is dissolved in the
concentrated sulphuric acid) several droplets of a given X solution are added. If solution becomes dark
blue, this confirms about the presence of NO3- ion. Be careful as concentrated strong acid is used in
this experiment!
6. PO43- can be found by the reaction with ammonium molybdate – (NH4)2MoO4 and identified
by the formation of yellow precipitate of (NH4)3[PMo12O40]:
Experiment: In a test tube containing solution of (NH4)2MoO4 several droplets of a 6M HNO3 are
added. The content of the beaker is shaken until all white precipitate dissolves. Then 2-3 droplets of a
given X solution are added and PO43- ions are identified by the formation of yellow precipitate.
CONCLUSIONS
Nr
NAME
Example
25
Daniel ….
CATIONS found in X solution
ANIONS found in X solution
K+, Cu2+, ….
SO42-, …