Cation – A cation is an ion carrying positive charge which is attracted to the cathode during the process of electrolysis.
A systematic analysis of cations is performed to separate and recognise cations which are commonly known from a mixture of salt. This experiment helps analyse the properties of cations and understand the concept of precipitation and formation of the complex at equilibrium. This qualitative analysis has been included in Class 12 practical syllabus of Chemistry to impart students with the knowledge of analysis of metallic elements and chemical research.
Aim of the Experiment
To recognise or identify cations from an inorganic salts mixture with the use of several tests and experiments.
Theory
The systematic analysis of salt mixture results in the removal of cations followed by precipitation reaction. You will see that the various experiments performed using different test reagents will have a varied set of reactions on cations. You will be able to determine the reasons for their separation from the salts.
Further, the qualitative analysis of cations is performed using a few preliminary tests. These tests are included in the following table.
SI. No. |
Name of Preliminary Test |
1 |
Physical examination |
2 |
Flame test |
3 |
Charcoal test |
4 |
Borax bead test |
It is to note that these don’t solidify evidence about the ions yet provide necessary insight regarding the cations involved in the salt mixture.
You can perform a systematic qualitative analysis of cations in three steps, as mentioned below.
Stage 1 – Segregation of Cations
Cations are segregated in 5 groups depending upon their solubility with the help of various precipitating reagents.
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Cations Group |
Solubility Property |
Process |
Ions Collected from the Precipitation |
Group 1 |
Insoluble chlorides |
The insoluble chlorides can be collected by adding 6M HCl to the salts causing Hg2Cl2, AgCl, PbCl2 to precipitate. The cations can be collected from the solution using the process of centrifugation or filtration. |
Hg22+, Ag+, Pb2+ |
Group 2 |
Acid-insoluble sulphides |
The solution’s pH is then adjusted to 0.5 before adding H2S to it. In this case, only metallic sulphide with a very low value of Ksp precipitates. The cations can be collected from the solution using the process of centrifugation or filtration. |
As3+, Cd2+, Bi3+, Cu2+, Hg2+, Sn2+and Sb3+. |
Group 3 |
Hydroxides or base-insoluble sulphides |
NaOH or Ammonia is mixed with the solution so that it turns into a basic solution. Then, (NH4)2S is added to the solution to get divalent metal ions as their sulphides and trivalent metal ions as their hydroxide. |
Fe2+, Co2+, Mn2+, Zn2+, and Ni2+ as their sulphide equivalent. Cr(OH)3 and Al(OH)3 precipitates in their hydroxide form. |
Group 4 |
Phosphate or insoluble carbonates |
You can add Na2CO3 to initiate precipitation of insoluble carbonates. You may as well add (NH4)2HPO4 for removal of same metal ions from the solution in the form of insoluble phosphates. |
Ba2+, Ca2+, Sr2+ |
Group 5 |
Alkali metals |
These alkali metals remain in the solution because none of the methods above can remove these from the solution. |
K+, Na+, Mg2+, NH4 |
Stage 2 – Here, the process of selective dissolution is used for separating the various cations precipitated in a group.
Stage 3 – Various test for cations are performed to identify and verify the cations present in them.
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Equipment Needed For The Experiment –
Since the qualitative analysis of cations and anions require you to perform various tests, you will need several instruments. Here are the instruments you will need.
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Test tube
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Test tube stand
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Test tube holder
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Corks
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Boiling tubes
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Delivery tube
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Filter paper
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Measuring cylinder
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Reagents
Procedures And Observation For Various Identification Tests –
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Preliminary Test To Identify Cations
Procedure –
This is the step of physical examination. You can look at the precipitation or salt’s colour and infer the ions it could possibly have. Look at the tabl
e mentioned below to understand this.
Observation –
SI. No. |
Colour of Salt |
Conclusions |
1 |
Light green |
Possibility of Ferrous salts |
2 |
Bluish green or blue |
Nickel or copper salts |
3 |
Dark green |
Possibly Chromium salts |
4 |
Flesh colour or light pink |
Salts of Manganese |
5 |
Dark brown |
Might be Ferric salts |
6 |
White |
It depicts Fe, Cu, Mn, Ni, Co are absent. |
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Charcoal Cavity Test
The cation is converted to metal carbonate first and then heated so that it decomposes to produce metal oxide. You can detect the cation present in the salt by observing the colour of bead or the residue in charcoal cavity.
Procedure –
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A charcoal cavity is taken for the experiment.
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A little cavity is made on the charcoal bar using a borer.
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Put a small amount of salt inside the charcoal cavity and mix it with Sodium carbonate. If needed, pour some water.
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With the use of a reducing flame and a mouth blowpipe, heat the mixture present in the charcoal cavity and observe the changes.
Observation –
Observation |
Conclusion |
A yellow residue or incrustation while hot and white when cold. |
Zn2+ |
Brown residue & incrustation. |
Cd2+ |
A brittle bead with an incrustation of colour brown or yellow. |
Bi3+ |
A metallic shining bead with incrustation soft yellow and marks paper. |
Pb2+ |
Garlic odour, white fumes, and white incrustation. |
As3+ |
No incrustation with red residue. |
Cu2+ |
No incrustation with black residue. |
Mn2+, Fe3+, Ni3+ |
White residue. |
Might be Mg2+, Ca2+, Al3+, Ba2+ |
Smoke, white liquid globule with no bead. |
Hg |
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Borax Bead Test
Borax bead test is performed to observe manganese, nickel, copper, or iron ions in the salt mixture by heating it in oxidised as well as reduced flame and observing the change in its colour.
Procedure –
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Take a platinum wire and twist it to make a small loop.
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Take a Bunsen burner and heat it till the wire is red hot.
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Put some Borax powder over a watch glass and dip the looped wire on it before heating it yet again.
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The dipped Borax will fuse to give a transparent and colourless, glass-like bead.
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Make sure to touch the bead (hot) with Hydrochloric acid (HCl) and immerse it in salt. Then, heat the bead in oxidising and reducing flames and observe the change in bead colour.
Observation –
Colour Of Bead (Oxidising Flame) |
Colour Of Bead (Reducing Flame) |
Conclusion |
||
Hot |
Cold |
Hot |
Cold |
|
Yellow |
Yellow |
Green |
Green |
Iron |
Green |
Blue |
Colourless or reddish |
Colourless or reddish |
Copper |
Pink |
Pink |
Colourless |
Colourless |
Manganese |
Brown |
Brown |
Black / Grey |
Black / grey |
Nickel |
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Flame Test
It is an important test for systematic analysis of cations as the 5th group cations show characteristic colour when exposed to flame in this experiment. These ions in their chloride form can impart heat energy which is released in the form of light energy when exposed to non-luminous flame. The reason various ions exhibit different colours is due to the reason that every metal ion has a different level of light energy.
Procedure –
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Put some concentrated HCl on a watch glass.
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Take the platinum wire and dip it in the Concentrated HCl solution and heat it by putting on the flame.
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You will have to repeat this step unless the platinum wire shows any colour in the flame.
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Subsequently, dip it in the
solution of concentrated HCl followed by dipping in salt. Observe the colour it imparts in the flame.
Observation –
Flame Colour |
Conclusion |
|
Through naked eye |
With the help of blue glass |
|
Golden yellow |
No colour |
Na+ |
Brick red colour |
Light yellow |
Ca2+ |
Crimson red colour |
Crimson or purple |
Sr2+ |
Violet |
Pink |
K+ |
Green flashes |
No characteristic colour observed |
Mn2+ or Zn2+ |
Blue or bluish green |
Blue or bluish green |
Cu2+ |
Light green colour |
Bluish green |
Ba2+ |
Now that you are aware of the systematic analysis of cations, you will be able to perform the experiments quickly and observe the results. These will prepare you for the viva questions and practical examination in a better way. To improve your knowledge and understanding, you can refer to ’s study material prepared by professional and skilled tutors. Download the app or refer to the website.