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Systematic Analysis of Anions

Chemistry

Systematic Analysis of Anions

Last updated date: 17th Apr 2024

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Overview of Systematic Analysis of Anions

Anion is basically having more electrons than a cation or a proton thus providing it with a negative charge. For forming an anion the atom needs to gain one or more electrons. The number of electrons that will be gained tells about the charge on the anion. Suppose if an anion is formed by gaining one electron it will have one negative charge, if it is forked by gaining two electrons then 2 negative charges and so on. These electrons are basically pulled away from other atoms that have less affinity for them. For finding out about the anion in a particular sample anion analysis is done.

Analysis of anions may indicate what kind of anions are present in a sample and also the amount of anions that are present in the given sample provided to you. To identify and quantify the anion composition in the sample traditional chemistry uses methods known as colorimetric methods. In modern chemistry, there is a method known as anion exchange chromatography or ion chromatography. In this method, you can not only separate the ions present in the given samples but also quantify the individual anion that is present in the given sample. It takes nearly 10 to 30 minutes in order to find multiple anions in a single run.

This article provided to you by Vedantu will help you to know more about the systematic analysis of anions. This is going to give you a brief view of how to do the analysis of an anion in the given sample provided to you. You are provided with experiments that will help you to understand it well along with this you are provided with the information about solubility product and common ion effect and further information related to this anion analysis. Besides this, Vedantu provides you with a set of frequently asked questions that answer the queries of the students around there. Many times children have few questions related to this topic and these frequently asked questions help them to clear most of their doubts.

Inorganic salts are obtained with a base through either partial or complete neutralization of acid. The part that the acid contributes is known as an anion in the salt formation and the part that the base contributes is known as a cation. The preliminary examination results are important clues about the presence of some cations or anions. The systematic analysis of anions is an integral part of salt analysis (or the qualitative inorganic analysis).

Experiment

Aim: To identify the anionic radicals that are present in an inorganic mixture of salts by performing different tests.

Theory: Qualitative analysis involves the identification and detection of acidic and basic radicals, which are present in inorganic salts. Inorganic salts are produced by the reaction of both acids and bases or the acidic oxides with either a base or basic oxides.

Some of the examples of the reaction of both acids and bases or the acidic oxides with a base or basic oxides are chemically represented below:

CO₂ + 2NaOH → Na₂CO₃ + H₂O

NaOH + HCl → NaCl + H₂O

2NaOH + H₂SO₄ → Na₂SO₄+ 2H₂O

KOH + HNO₃ → KNO₃ + H₂O

Many organic compounds are crystalline solids because they have pre-defined geometrical shapes. In general, they contain the oppositely charged ions or particles, which are called radicals.

In the salt analysis, two fundamental principles are of great use of:

Solubility Product
Common Ion Effect

Solubility Product

Solubility products can be defined as a product of ion concentrations, which are elevated to a power equal to the occurrences count of ions in an equation by representing the electrolyte dissociation at a given specific temperature when the solution is saturated. Under all the conditions, the solubility product is not given as the ionic product, but only if the solution gets saturated. Talking about solubility equilibrium then it is a type of dynamic equilibrium. It occurs when chemical compounds in the solid-state are in chemical equilibrium with the solution of that compound.

Common Ion Effect

The phenomenon which suppresses the degree of dissociation of any of the weak electrolytes by adding some amount of strong electrolyte which contains a common ion is known as the common ion effect. For example, by adding a strong electrolyte sodium acetate which contains a common acetate ion, ionization of weak electrolyte acetic acid can be suppressed. Common Ion Effect refers to the decrease in solubility of an ionic precipitate by addition to the solution of soluble compounds having an ion in common with the precipitate that is formed. This effect is based on the fact that both the original salt and the other added chemical have one ion in common between them.

Materials Required:

Boiling tubes
Test tubes
Test tube holder
Corks
Test tube stand
Delivery tube
Filter paper
Reagents

Apparatus Setup

(Image will be updated soon)

Procedure

S. No	Experiment	Observation	Inference
1.	Preliminary Reactions	salt is the colourless one	Fe²⁺, Ni²⁺, Fe³⁺, Co²⁺ ions are absent.
	Appearance	If the salt is green	Maybe, the Ni²⁺, Fe²⁺, Cu²⁺
		If the salt is brown	Maybe, the Fe²⁺
		If the salt is pink	Maybe, the Mn²⁺, Co²⁺.
		If the salt is blue	Cu²⁺
2	Action of Heat: Take a less quantity of salt in a dry test tube, and heat it gently.	A colourless gas with pungent odour which turns moist red litmus blue paper.	Maybe, the NH₄⁺ salt.
		Reddish-brown vapours can be obtained, which turn acidified ferrous sulfate brown paper.	Maybe, the NO₃^-
		In the cold, the substance is white and hot, yellow.	Maybe, the (Zn)²⁺
3	Flame Test: Add one drop of Concentrated HCl to a less amount of salt in a watch glass and keep it in a paste. Now, add the paste into the non-luminous bunsen burner base using a glass rod.	Crimson red colour flame. Crimson red colour flame. Bluish green flame Brick red colour flame.	Maybe, the Sr²⁺ Maybe, the Ba²⁺ Maybe, the Ca²⁺ Maybe, the Cu²⁺

Identification of Anions from the Volatile Products

S. No	Experiment	Observation	Inference
4	Dilute H2SO4 action: Add either 1 or 2ccs of dilute H₂SO₄ to a less portion of salt in a test tube and then warm it gently.	Brisk effervescence can be obtained from odourless, colourless gas which turns lime water.	Anion is the Carbonate CO₃^2-
		colourless gas with rotten egg odor can be obtained, by turning lead acetate black paper.	Here, Sulphide anion exists.
		Colourless gas with a burning sulfur odour which turns acidified dichromate green.	Sulfate anion exists.
		Reddish brown gas obtains with a fishy odor, and turns acidified brown ferrous sulphate..	Nitrate anion is present.
		colourless vinegar-flavored gas is obtained.	Maybe, the acetate anion.
		No characteristic observation.	Absence of above mentioned anions.
5	Action of Con. H₂SO₄: Add 2-3 ccs of Con. H₂SO₄ to a small amount of salt taken in a test tube and heat it gently.	Reddish-brown vapours that turn moist red paper fluorescent.	Maybe, the bromide anion.
		colourless gas with a pungent smell that provides dense white fumes with a dipped glass rod in NH₄OH solution.	Maybe, the chloride anion.
		Violet colored vapours turns either blue or violet starch paper.	Maybe, the iodide anion.
		Reddish-brown vapours turns brown ferrous sulfate paper into the acidified one.	Maybe, the nitrate anion.
		Reddish-brown vapours turns the brown ferrous sulfate paper as the acidified one.	Maybe, the nitrate anion.
		Reddish-brown vapours turn brown ferrous sulfate paper into acidified.	Maybe, the nitrate anion.
		No characteristic observation.	Absence of all the anions, mentioned above.
6	Concentrated H₂SO₄ with Cu turnings action: Mix with some Cu bits and a small amount of salt taken in the test tube, add 2 to 3 ccs of H₂SO₄ and then heat it.	It can be observed that the reddish brown gas evolution turns acidified ferrous sulfate paper into brown.	Nitrate anion exists
6		No reddish brown vapours occur.	Nitrate anion is absent.
7	Concentrated H₂SO₄ with MnO₂ Action: Add the equal amount of MnO₂ to a less amount of salt in the test tube and then, add some ccs of Concentrated H₂SO₄ and heat gently.	A greenish yellow gas turns blue or starch iodide paper.	Maybe, the chloride anion.
		Reddish brown vapours can be obtained that turn the moist fluorescent red paper.	Maybe, the bromide anion.
		Violet vapours can be obtained that turn starch paper violet or blue.	Maybe, the iodide anion.
		No characteristic coloured vapours can be obtained.	Absence of the anions mentioned above.
8	NaOH action: Add some ccs of 10% NaOH solution to a pinch of salt taken in a test tube and warm it gently.	It is obtained as colourless gas with a smell of pungent type which produces dense white fumes with glass rod dipped in the HCl acid.	Maybe, the ammonium ion.
8		There is no characteristic gas that is released from ammonium.	Ammonium is absent.

Sodium Carbonate Extract

Confirmatory Tests for S₂–, CO₃^2-, SO₃^2-, CH₃COO– and NO₂– anions:

When the salt is water-soluble, confirmatory anion testing can be carried out using the water extract and when the salt becomes water-insoluble, by using sodium carbonate extract. Confirmation of CO₃^2– can be done either with the help of aqueous salt solution or with a solid salt as such, as the carbonate ions are composed in the extract of sodium carbonate. Water extraction is formed by dissolving salt in water.

Sodium Carbonate Extract Preparation

Take 1 gram of salt in a porcelain dish or a boiling tube. Now, mix 3 g of solid sodium carbonate approximately with 15 mL of distilled water. And, remove the contents and cook for up to 10 minutes. Filter, cool, and collect the filtrate in the test tube and label it as a sodium carbonate extract.

S. No	Experiment	Observation	Inference
9	Silver nitrate test: Add the dilute HNO₃ to sodium carbonate extract portion until effervescence gets stopped. Now, add some drops of AgNO₃ solution in excess, 2 to 3 numbers.	NH₄OH of curdy white soluble precipitate.	Maybe, the chloride anion.
		A precipitate of pPale yellow NH₄OH, which is sparingly soluble.	Maybe, the bromide anion.
		Insoluble yellow coloured precipitation in the NH₄OH.	Maybe, the iodide anion.
		None of the characteristics precipitate.	All the above mentioned anions are absent.
10	Barium Chloride test: Add the solution of BaCl₂ to about 1-2 ccs of the extract (after the acetic acid neutralization and boiling of CO₂). Now, add the dilute hydrochloric acid to the ppt portion, mentioned above.	An insoluble white precipitate in the HCl acid.	The anion is SO₄^2-
		A HCl soluble acid white precipitate.	The anion is SO₃^2-
		No characteristics are precipitated.	Absence of SO₃^2- and SO₄^2-.
11	Lead acetate test: Add the lead acetate solution to up to 1 or 2 ccs of the extract (after the acidification with acetic acid and CO₂ boiling off, cooling).	White ppt, which is soluble in excess ammonium acetate solution.	Presence of SO₄^2- can be confirmed.
12	Ferrous Sulphate Test: (Brown ring test) Add the dilute H₂SO₄ in drops to about one or two cc of extract until the effervescence gets stopped. And, add some drops in excess, add 2 to 3 solution drops of freshly prepared FeSO₄. Keep it in a slanting position on the test tube, add Concentrated H₂SO₄solution without interfering.	At liquid junction, a brown ring can be obtained.	Nitrate anion - NO^3-is present.
12		No brown ring is formed	Nitrate anion - NO^3- is absent.
13	Ferric chloride test: Take extract in the test tube for up to one or two ccs and add neutral FeCl3 solution. If required, split and filter the solution or filtrate into 2 parts:	Deep red colouring produced.	Acetate anion is confirmed. (CH₃COO^-)
	1. Add dilute HCl	Red colouring has disappeared.	CH₃COO^- can be confirmed.
	2. Add water and boil to the 2nd part	Reddish brown precipitate.	CH₃COO^- can be confirmed.
14	Calcium Chloride Test: In the test tube, add diluted to the extract of sodium carbonate portion. Boil CO₂ with acetic acid and add some solution drops of calcium chloride.	A white calcium oxalate precipitate can be obtained.	Confirms the oxalate anion presence.
	Incorporate the diluted HNO₃ to the white and hot ppt	Precipitate dissolves.
15	Ethyl Acetate Test: Add some drops of ethanol to the pinch of salt, which is taken in a test tube, followed by either 1 or 2ccs of H₂SO₄. Now, gently heat it and cool it down. Na₂CO₃ version.	There is an odour which is pleasant and fruity.	It can be confirmed that anion acetate is present.

Observations and Inference

S. No	Anions	Observation
1	CO₃^2- (the Carbonate anion)	CO₂ gas evolves with brisk effervescence with the dilute sulphuric acid, that turns the lime water into milky.
2	S₂^- (the Sulphide anion)	Add a solution drop for the sodium nitroprusside. It appears as violet or purple.
3	SO₃^2- (the Sulfite anion)	A white precipitate can be formed with a barium chloride solution, which dissolves in sulfur dioxide gas and dilute hydrochloric acid also develops.
4	SO₄^2- (the Sulfate anion)	Take 1 mL salt water extract in sodium carbonate or water and add BaCl₂ solution after the acidification with dilute hydrochloric acid. Insoluble white precipitate in the concentrated HCl or HCl. It turns out to be HNO₃.
5	NO₂^- (the Nitrite anion)	Add some drops of iodide potassium solution and some drops of starch solution, acidified with acetic acid. Then, blue colour appears.
6	NO₃^- (the Nitrate anion)	Take 1 mL of salt solution in a test tube in water. Add a concentration of 2 mL. Mix thoroughly with H₂SO₄ solution. Now, cool the mixture under tap. Add freshly prepared ferrous sulfate without shaking on sides of the test tube. At the two solution’s junctions, a dark brown ring can be formed.
7	Cl^- (the Chloride anion)	Take 0.1 g of salt in a test tube, add a pinch of manganese dioxide and 3-4 drops of concentrated acid with sulfuric acid. Heat the reaction mixture. A greenish yellow chlorine gas, which is detected by its bleaching action and strong odor
8	Br^- (the Bromide anion)	Take 0.1 gm of salt in the test tube with a MnO₂ pinch. Add concentrated sulphuric acid and heat it with 3-4 drops. There exists an evolution of intense brown fumes.
9	I^- (the Iodide anion)	Take 1 mL of the salt solution in water in the test tube. Add a concentration of 2 mL. Mix thoroughly with H₂SO₄ solution. Cool mixture under the tap. And, add freshly prepared ferrous sulfate without shaking the sides of the test tube. At the two solution junction, a dark brown ring can be formed.
10	PO₄^3- (the Phosphate anion)	Acidify the salt solution or sodium carbonate extract in water and concentrated HNO₃ and add the ammonium molybdate solution and heat to boil. A precipitate of canary yellow can be formed.
11	C₂O₄^2- (the Oxalate anion)	Take 1 mL of sodium carbonate extract or acetate acidified water extract and add calcium chloride solution. An insoluble white precipitate can be formed in the ammonium oxalate solution and oxalic acid, but it is soluble in dilute nitric acid and dilute hydrochloric acid.
12	CH₃COO^- (the Acetate anion)	Add 1 mL and 0.2 mL concentration of ethanol. Heat the H₂SO₄ solution. The presence of acetate ion can be confirmed by fruity odor.

Results: The salt contains ________ (CO₃^2‒, SO₃^2‒, S^2‒, NO^2‒, SO₄^2‒, Cl^‒, NO₃^‒, I^‒, Br^‒, C₂O₄^2‒, PO₄^3‒, CH₃COO^‒) anion.

Precautions: Read the label carefully on a bottle before using chemicals or reagents. Never use reagents which are unlabeled.

In smelling vapors or chemicals, be careful. Always fan the vapors to your nose gently

Always use hand gloves and an apron as an eye protector in the chemical laboratory.

Unnecessarily, do not mix chemicals with reagents and do not taste the chemicals at all.

Never throw or add sodium metal into the dustbin or sink.

Always pour acid into water for dilution. Never add acid to the water.

Be careful when heating the test tube. When adding a reagent or heating, the test tube must never point to anyone.

Keep cleaning your working environment. Never ever throw in the sink any papers and also glass. Always use a dustbin for this purpose.

Always pour acid into water for dilution.

Be careful with the flammable substances, explosive compounds, electrical appliances, toxic gasses, glass products, hot substances, and flames.

Always wash hands after the laboratory work has been over.

Always use less quantity of reagents. The excessive use of reagents not only leads to chemicals being wasted, but they also cause environmental damage.

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FAQs on Systematic Analysis of Anions

1. How to Test the Presence of Sulfide Ions?

When we add sulphuric acid in its dilute form to any salt then there is a production of H₂S and thus hydrogen sulfide gas smells of the rotten egg. This hydrogen sulfide contains hydrogen ions and sulfide ions. The confirmation test of the sulfide ion you can get when there is the formation of a black spot of arsenic sulfide, that is Ag₂S. This arsenic sulfide black spot is observed only when sulfide ions, that are 9s, S₂ ions, are added to the silver foil and a reaction between the sulfide ion and silver foil takes place.

2. What is the Term Common Ion Effect?

The common ion effect defines the effect on balance that takes place when adding to the solution, a common ion that is already present in the solution. In general, the common ion effect decreases the solubility of the solvent. Mainly the common ion effect refers to the decrease in the solubility of precipitates of ions when there is an addition of soluble compounds with an ion that is common in both the original salt that is added and the chemical we are adding afterwards into it.

3. How to Test the Carbonate Ion’s Presence?

Carbonate ions can be detecTed whether in their solid compound or in the solution also. To test the compound an acid is added. This acid is dilute acid, let us say the dilute hydrochloric acid can be added to test the compound containing the carbonate ions.A dilute acid can be used to detect the carbonate ions, CO₃^2- bubbles release when the acid can be added to the test compound, generally diluted hydrochloric acid. And, the carbon dioxide causes bubbles using Limewater and that gas is carbon dioxide. These carbon dioxide bubbles are released in the presence of these carbonate salts in dilute acids.

4. Why Does Iodine Result in a Blue Color with a Starch Solution?

Whenever we add iodine into the starch solution that is in the presence of starch there is a change in the color of iodine. Amylase is the starch that is basically responsible for the change in color of iodine to blue-black in the presence of it. The iodine molecules basically slip inside the coil of amylase molecules which results in the formation of a linear tri iodide complex which is soluble and slips into starch molecules that turns blue-black in color. Thus, you can say that this change in the blue-black color of iodine is due to the production of intermolecular load transfer complexes by adding the aqueous solutions of triiodide ions.

5. Why is an anionic analysis done before the cations analysis?

You will observe that anionic analysis is often done before the cationic analysis. There are certain anions such as oxalate, phosphate and fluoride that interfere with the detection of the cations of group 3 such as chromium and iron and aluminum. Therefore it is necessary that you have the formation of these anions before going for the analysis of the cations. So that we can proceed further only after the removal of such ions from the solutions.