Systematic Analysis of Anions -CoolGyan

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

Tabel of Contents

Aim Theory Materials Required Apparatus Setup Procedure Results Precautions

Aim:

To identify the anionic radicals present in an inorganic mixture of salts by performing various tests.

Theory:

Qualitative analysis involves the detection and identification of acidic and basic radicals present in inorganic salts. Inorganic salts are formed by the reaction of acids and bases or acidic oxides with a base or basic oxides.

Some of the examples of the reaction of acids and bases or acidic oxides with a base or basic oxides are

NaOH + HCl → NaCl + H₂O

CO₂ + 2NaOH → Na₂CO₃ + H₂O

KOH + HNO₃ → KNO₃ + H₂O

2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

Most of the organic compounds are crystalline solids as they have defined geometrical shapes. Usually, they consist of oppositely charged particles or ions called radicals.

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

Solubility Product
Common Ion Effect

1. Solubility Product:

Solubility product is defined as a product of ion concentrations elevated to a power equal to the number of occurrences of ions in an equation representing electrolyte dissociation at a given temperature when the solution is saturated. Under all conditions, the solubility product is not the ionic product, but only if the solution is saturated.

2. Common Ion Effect:

The phenomenon that suppresses the degree of dissociation of any weak electrolyte by adding a small amount of strong electrolyte that contains a common ion is called the common ion effect. For example, by adding a strong electrolyte sodium acetate containing common acetate ion, ionization of weak electrolyte acetic acid is suppressed.

Materials Required:

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

Apparatus Setup:

Systematic Analysis of Anions

Procedure:

S.No	Experiment	Observation	Inference
1	Preliminary reactions	Given salt is colourless	Fe²⁺, Fe³⁺, Ni²⁺ and Co²⁺ ions are absent.
	Appearance	If the given salt is green	May be Fe²⁺, Ni²⁺, Cu²⁺
		If the given salt is brown	May be Fe²⁺
		If the given salt is pink	May be Co²⁺. Mn²⁺
		If the given salt is blue	Cu²⁺
2	Action of Heat: Take a small quantity of the salt in a dry test tube, heat it gently.	A colorless gas with a pungent odor that turns moist red litmus blue paper.	May be NH₄⁺ salt.
		Reddish brown vapors are obtained that turn acidified ferrous sulfate brown paper.	May be NO₃^–
		In cold, the substance is white and in hot, yellow.	May be (Zn)²⁺
3	Flame Test: Add a drop of Con.HCl to a small amount of salt in a watch glass and put it in a paste. Put the paste into the base of the non-luminous bunsen burner with the help of a glass rod	(a) Apple green colour flame. (b) Crimson red colour flame. (c) Brick red colour flame. (d) Bluish green flame	May be Ba²⁺ May be Sr²⁺ May be Ca²⁺ May be Cu²⁺

Identification of Anions from Volatile Products:

S.No	Experiment	Observation	Inference
4	Action of dilute.H₂SO₄: Add 1 or 2ccs of dilute H₂SO₄ to a small portion of the salt in a test tube and warm it gently.	Brisk effervescence is obtained from colorless, odorless gas that turns lime water.	Anion is Carbonate CO₃^2-
		Colorless gas with an odor of rotten eggs is obtained, turning lead acetate black paper.	Sulphide anion is present.
		Colorless gas with the odor of burning sulfur that turns acidified dichromate green.	Sulfate anion is present.
		Reddish brown gas is obtained with a fishy odor that turns acidified brown ferrous sulphate.	Nitrate anion is present.
		Colorless vinegar-flavored gas is obtained.	May be 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 vapors that turn moist red paper fluorescent.	May be bromide anion.
		Colorless gas with a pungent smell that provides dense white fumes with a dipped glass rod in NH ₄ OH solution.	May be chloride anion.
		Violet colored vapors that turn blue or violet starch paper.	May be iodide anion.
		Reddish-brown vapors turning brown ferrous sulfate paper into acidified.	May be nitrate anion.
		No characteristic observation.	Absence of all above-mentioned anions.
6	Action of Con. H₂SO₄ with Cu turnings: Mix with a few Cu bits a small amount of the salt taken in a test tube, add 2 – 3 ccs of H₂SO₄ and heat it.	It is observed that copy evolution of reddish brown gas turns acidified ferrous sulfate paper brown.	Nitrate anion is present
6		No reddish brown vapours.	Absence of nitrate anion.
7	Action of Con. H₂SO₄ with MnO₂: Add an equal amount of MnO₂ to a small amount of salt in a test tube and add a few ccs of Con. H₂SO₄ and heat gently.	A greenish yellow gas turns violet (or) blue starch iodide paper.	May be chloride anion.
		Reddish brown vapors are obtained that turn moist fluorescent red paper.	May be bromide anion.
		Violet vapors are obtained that turn starch paper blue (or) violet.	May be iodide anion.
		No characteristic coloured vapours are obtained.	Absence of all above mentioned anions.
8	Action of NaOH: Add a few ccs of 10% NaOH solution to a pinch of salt taken in a test tube and warm it gently.	It is obtained a colorless gas with a pungent smell that produces dense white fumes with a glass rod dipped in HCl.	May be ammonium ion.
8		There is no characteristic gas released from ammonium.	Absence of ammonium.

Sodium Carbonate Extract:

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

Confirmatory anion testing is carried out using water extract when salt is water-soluble and using sodium carbonate extract when salt is water-insoluble. Confirmation of CO₃^2– is done using aqueous salt solution or using solid salt as such, as carbonate ions are contained in the sodium carbonate extract. Extract of water is produced by dissolving salt in water.

Preparation of Sodium Carbonate Extract:

Take 1 g of salt in a boiling tube or porcelain dish. Mix approximately 3 g of solid sodium carbonate with 15 mL of distilled water. Remove the contents and cook for about 10 minutes. Cool, filter, collect the filtrate in a test tube and label it as an extract of sodium carbonate.

S.No	Experiment	Observation	Inference
9	Silver nitrate test: Add dilute HNO₃ to a portion of the sodium carbonate extract until the effervescence stops. Add a few drops of AgNO₃ solution in excess, 2 – 3nos.	Curdy white NH₄OH soluble precipitate.	May be chloride anion.
		Pale yellow precipitate in NH₄OH that is sparingly soluble.	May be bromide anion.
		Insoluble yellow precipitation in NH₄OH.	May be iodide anion.
		No characteristics precipitate.	Absence of all above mentioned anions.
10	Barium Chloride test: Add BaCl₂ solution to about one or two ccs of the extract (after acetic acid neutralization and CO₂ boiling). Add dilute hydrochloric acid to a portion of the ppt above.	An insoluble white precipitate in HCl.	The anion is SO₄^2-
		A HCl soluble white precipitate.	The anion is SO₃^2-
		No characteristics precipitate.	Absence of SO₄^2- and SO₃^2-.
11	Lead acetate test: Add lead acetate solution to about one or two ccs of the extract (after acidification with acetic acid, CO₂ boiling off and cooling).	White ppt, soluble in excess of the solution of ammonium acetate.	Presence of SO₄^2-is confirmed.
12	Ferrous Sulphate Test: (Also called Brown ring test) Add dilute H₂SO₄ in drops to about 1 or 2cc of extract until the effervescence stops. And a few drops in excess add two to three drops of freshly prepared FeSO₄ solution. Keep in a slanting position the test tube, add Con. H₂SO₄ without interfering with the solution	At the liquid junction, a brown ring is obtained.	Nitrate anion (NO₃^–) is present.
12		No brown ring formed	Nitrate anion (NO₃^–) is absent.
13	Ferric chloride test: Take the extract in a test tube for about 1 or 2ccs and add a neutral FeCl₃ solution. If necessary, filter and split the solution or filtrate into two parts:	Deep red colouring produced.	Acetate anion is confirmed. (CH₃COO^–)
	1. Add dil. HCl	Red colouring is gone.	CH₃COO^–is confirmed.
	2. Add water and boil to the second part	Reddish brown precipitate.	CH₃COO^–is confirmed.
14	Calcium Chloride Test: In a test tube, add dil to a portion of the extract of sodium carbonate. Boil off CO₂ with acetic acid. Add a few drops of a solution of calcium chloride.	A white calcium oxalate precipitate is obtained.	Confirms the presence of oxalate anion.
14	Incorporate dil. HNO₃ to the hot and white ppt	Precipitate dissolves.
15	Ethyl Acetate Test: Add a few drops of ethanol to a pinch of salt taken in a test tube, followed by 1 or 2ccs of H₂SO₄. Heat it gently and cool it down. Version Na₂CO₃.	There is a pleasant fruity odor.	It is confirmed that anion acetate is present.

Observations and Inference:

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

Results:

The given salt contains ________ (CO₃^2‒, S^2‒, SO₃^2‒, SO₄^2‒, NO₂^‒, NO₃^‒, Cl^‒, Br^‒, I^‒, PO₄^3‒, C₂O₄^2‒, CH₃COO^‒) anion.

Precautions:

Read the label carefully on the bottle before using any reagent or chemicals. Never use a reagent that is unlabeled.
Always use an apron, an eye protector and hand gloves in the chemical laboratory.
In smelling chemicals or vapors, be careful. Always gently fan the vapors to your nose
Do not unnecessarily mix chemicals with reagents. Do not taste chemicals at all.
For dilution, always pour acid into water. Never add acid to water.
Never add or throw sodium metal into the sink or dustbin.
Be careful as the test tube is heated. When heating or adding a reagent, the test tube should never point to yourself or your neighbors.
For dilution, always pour acid into water. Never add acid to water.
Keep clean your working environment. Never throw in the sink any papers and glass. For this purpose, always use dustbin.
Be careful with explosive compounds, flammable substances, toxic gasses, electrical appliances, glass products, flames and hot substances.
Always use the minimum quantity of reagents. The use of excessive reagents not only leads to chemicals being wasted, but also causes environmental damage.
Always wash your hands after the laboratory work has been completed.

Frequently Asked Questions on

How will you test the presence of carbonate ion?

A dilute acid is used to detect carbonate ions, CO₃^2- Bubbles are released when an acid is added to the test compound, usually diluted hydrochloric acid. Carbon dioxide causes the bubbles using Limewater and the gas is carbon dioxide.

What do you understand by the term common ion effect?

The common ion effect describes the effect on balance that occurs when adding to a solution a common ion an ion that is already contained in the solution. Generally, the common ion effect decreases a solvent’s solubility.

Why does iodine give a blue colour with starch solution?

The iodine test is used to test for starch presence. Starch becomes an intense “blue-black” colour due to the formation of an intermolecular load-transfer complex by adding aqueous solutions of the triiodide anion.

How do you test the presence of sulphide ion?

Rotten egg smell of H₂S is observed when adding dilute sulphuric acid to the salt. The confirmatory test is the black spot of Ag₂S is observed when adding S^2-ion to silver foil.

Describe the layer test for bromide and iodide ions.

In the presence of diluted hydrochloric acid, the layer test is performed by adding Carbon Disulphide to the solution. It will give an orange layer in the case of bromine, whereas in the case of iodine it will give a violet layer.