Systematic Analysis of Anions

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. Preliminary examination gives quite important clues about the presence of some anions or cations.

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 they have defined geometrical shapes. Usually, they consist of oppositely charged particles or ions called radicals.

In the salt analysis, two fundamental principles of great use

  1. Solubility Product
  2. 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:

  1. Test tubes
  2. Boiling tubes
  3. Test tube holder
  4. Test tube stand
  5. Corks
  6. Filter paper
  7. Delivery tube
  8. Reagents

Apparatus Setup:

Systematic Analysis of Anions

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 CO32-
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 NH4OH 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
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.
There is no characteristic gas released from ammonium. Absence of ammonium.

Sodium Carbonate Extract:

Confirmatory Tests for CO32-, S2–, SO32-, NO2 and CH3COOanions:

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 CO32– 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 SO42-
A HCl soluble white precipitate. The anion is SO32-
No characteristics precipitate. Absence of SO42- and SO32-.
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 SO42- 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.
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 coloring produced. Acetate anion is confirmed. (CH3COO)
1. Add dil. HCl Red coloring is gone. CH3COOis confirmed.
2. Add water and boil to the second part Reddish brown precipitate. CH3COOis 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.
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 CO32‒

(Carbonate anion)

CO2 gas is evolved with brisk effervescence with dilute sulphuric acid, which turns lime water milky.
2 S2‒ (Sulphide anion) Add a drop of solution for sodium nitroprusside. Appears purple or violet coloring.
3 SO32‒ (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 SO42‒ (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 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.
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 PO43‒ (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 C2O42‒ (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:

  1. The given salt contains ________ (CO32‒, S2‒, SO32‒, SO42‒, NO2, NO3, Cl, Br, I, PO43‒, C2O42‒, CH3COO) anion.

Precautions:

  1. Read the label carefully on the bottle before using any reagent or chemicals. Never use a reagent that is unlabeled.
  2. Always use an apron, an eye protector and hand gloves in the chemical laboratory.
  3. In smelling chemicals or vapors, be careful. Always gently fan the vapors to your nose
  4. Do not unnecessarily mix chemicals with reagents. Do not taste chemicals at all.
  5. For dilution, always pour acid into water. Never add acid to water.
  6. Never add or throw sodium metal into the sink or dustbin.
  7. 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.
  8. For dilution, always pour acid into water. Never add acid to water.
  9. Keep clean your working environment. Never throw in the sink any papers and glass. For this purpose, always use dustbin.
  10. Be careful with explosive compounds, flammable substances, toxic gasses, electrical appliances, glass products, flames and hot substances.
  11. Always use the minimum quantity of reagents. The use of excessive reagents not only leads to chemicals being wasted, but also causes environmental damage.
  12. Always wash your hands after the laboratory work has been completed.

Viva Questions:

1. How will you test the presence of carbonate ion?

Ans: A dilute acid is used to detect carbonate ions, CO32- 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.

2. What do you understand by the term common ion effect?

Ans: 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.

3. Why does iodine give a blue colour with starch solution?

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

4. How do you test the presence of sulphide ion?

Ans: Rotten egg smell of H2S is observed when adding dilute sulphuric acid to the salt. Confirmatory test is black spot of Ag2S is observed when adding S2-ion to silver foil.

5. Describe the layer test for bromide and iodide ions.

Ans: 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.