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.
To recognise or identify cations from an inorganic salts mixture with the use of several tests and experiments.
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.
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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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.
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.
Test tube stand
Test tube holder
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.
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.
A charcoal cavity is taken for the experiment.
A little cavity is made on the charcoal bar using a borer.
Put a small amount of salt inside the charcoal cavity and mix it with Sodium carbonate. If needed, pour some water.
With the use of a reducing flame and a mouth blowpipe, heat the mixture present in the charcoal cavity and observe the changes.
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.
Take a platinum wire and twist it to make a small loop.
Take a Bunsen burner and heat it till the wire is red hot.
Put some Borax powder over a watch glass and dip the looped wire on it before heating it yet again.
The dipped Borax will fuse to give a transparent and colourless, glass-like bead.
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.
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.
Put some concentrated HCl on a watch glass.
Take the platinum wire and dip it in the Concentrated HCl solution and heat it by putting on the flame.
You will have to repeat this step unless the platinum wire shows any colour in the flame.
Subsequently, dip it in the solution of concentrated HCl followed by dipping in salt. Observe the colour it imparts in the flame.
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.
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.