Systematic Analysis of Cations

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. 

  • 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. 

  1. Test tube 

  2. Test tube stand 

  3. Test tube holder 

  4. Corks 

  5. Boiling tubes 

  6. Delivery tube 

  7. Filter paper 

  8. Measuring cylinder 

  9. Reagents 


Procedures And Observation For Various Identification Tests – 

  1. 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 table 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. 


  1. 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 – 

  1. A charcoal cavity is taken for the experiment. 

  2. A little cavity is made on the charcoal bar using a borer. 

  3. Put a small amount of salt inside the charcoal cavity and mix it with  Sodium carbonate. If needed, pour some water. 

  4. 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 


  1. 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 –

  1. Take a platinum wire and twist it to make a small loop. 

  2. Take a Bunsen burner and heat it till the wire is red hot. 

  3. Put some Borax powder over a watch glass and dip the looped wire on it before heating it yet again. 

  4. The dipped Borax will fuse to give a transparent and colourless, glass-like bead. 

  5. 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 


  1. 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 – 

  1. Put some concentrated HCl on a watch glass. 

  2. Take the platinum wire and dip it in the Concentrated HCl solution and heat it by putting on the flame. 

  3. You will have to repeat this step unless the platinum wire shows any colour in the flame. 

  4. 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 Vedantu’s study material prepared by professional and skilled tutors. Download the Vedantu app or refer to the website. 

FAQ (Frequently Asked Questions)

1. How To Examine If Carbonate Ion Is Present In A Salt?

The insoluble carbonates can be precipitated from the salt mixture by adding Na2CO3 to the solution. Further, perform a charcoal cavity test to observe the presence of carbonate ions like Ba, Mg, Ca, etc.

2. What Is The Reason Iodine When Mixed With Starch Solution Imparts Blue Colour?

If an aqueous solution of the tri-iodide anion is added to the mixture, it forms a complex and turns the starch into a blue-black colour.

3. What Are Some Group 4 Cations?

A few group 4 cations formed after precipitation are Sr2+, Ba2+, Ca2+, etc. These can be precipitated in the form of carbonate or phosphate.

4. What Is The Reason HCl Is Used For The Flame Test?

In order to convert the elements to their metallic chloride form, the addition of concentrated HCl is integral. The reason is, chlorides are volatile and hence emits more energy which is released in the form of light energy in a flame test.