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.
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.
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.
Test tube
Test tube stand
Test tube holder
Corks
Boiling tubes
Delivery tube
Filter paper
Measuring cylinder
Reagents
Procedures And Observation For Various Identification Tests –
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 –
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 –
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.
Observation –
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 –
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.
Observation –
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 –
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.
Observation –
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.
FAQs on Systematic Analysis of Cations
1. What is meant by the systematic analysis of cations in chemistry?
Systematic analysis of cations is a method used in qualitative analysis to identify the positively charged ions (cations) in an unknown salt. The process works by separating cations into different groups based on their selective precipitation using specific chemical reagents in a fixed order.
2. Why must the analysis of cations follow a strict sequence of groups?
The strict sequence is crucial because the group reagents are designed to precipitate only one specific group of cations at a time under certain conditions. For instance, the reagent for Group 3 would also precipitate cations from Group 4 if Group 3 ions were not removed first. Following the order ensures that tests for one group are not interfered with by cations from later groups, preventing false results.
3. Which cations belong to Group II, and what is the principle of their precipitation?
Group II cations include ions like Cu²⁺, Pb²⁺, and As³⁺. The principle for their precipitation is based on their very low solubility product (Ksp). The group reagent is H₂S gas passed through a solution made acidic with dilute HCl. The acid limits the concentration of sulphide ions, which is just enough to precipitate Group II sulphides but not the sulphides of cations in later groups.
4. How is the test for the ammonium ion (NH₄⁺) different from other cation tests?
The test for the ammonium ion is done separately on the original salt before starting the group analysis. This is because reagents used later, like ammonium hydroxide (NH₄OH), would add NH₄⁺ ions to the solution. To test for it, the salt is warmed with NaOH solution. If ammonium is present, it releases pungent ammonia gas, which can be identified by its smell or by its ability to turn moist red litmus paper blue.
5. What is the purpose of using concentrated HCl when performing a flame test?
Concentrated HCl is used to convert the salt into its metal chloride form. Metal chlorides are much more volatile than other salts like sulphates or carbonates. This means they turn into vapour more easily in the heat of the flame, allowing the metal atoms to get excited and emit their characteristic colour more intensely, making the test result clear and vibrant.
6. Which cations are included in Group IV, and what is their group reagent?
Group IV contains the cations Barium (Ba²⁺), Strontium (Sr²⁺), and Calcium (Ca²⁺). The group reagent used to precipitate them is a solution of (NH₄)₂CO₃ (Ammonium Carbonate), which is added in an alkaline medium (in the presence of NH₄Cl and NH₄OH). This causes the cations to precipitate as white carbonates.
7. What would happen if a student accidentally added the Group IV reagent before testing for Group II?
If the Group IV reagent, (NH₄)₂CO₃, were added before the Group II reagent (H₂S in acid), it would cause a major error. The cations from Group IV (Ba²⁺, Sr²⁺, Ca²⁺) would precipitate out of sequence. This would make the subsequent test for Group II invalid because the solution's composition would be wrong, and it would be impossible to correctly identify the remaining ions.
