Systematic Analysis of Anions

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Introduction

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 the crystalline solids because they have the 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:

  1. Solubility Product

  2. Common Ion Effect


1. 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.


2. 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.


Materials Required

  • Boiling tubes

  • Test tubes

  • Test tube holder

  • Corks

  • Test tube stand

  • Delivery tube

  • Filter paper

  • Reagents

Apparatus Setup

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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.

  1. Crimson red colour flame.

  2. Crimson red colour flame.

  3. Bluish green flame

  4. 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 CO32-

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



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.

There is no characteristic gas that is released from ammonium.

Ammonium is absent.


Sodium Carbonate Extract

Confirmatory Tests for S2, CO32-, SO32-, 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 CO32– 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 SO42-

A HCl soluble acid white precipitate.

The anion is SO32-


No characteristics are precipitated.

Absence of SO32- and SO42-.

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 SO42- 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 H2SO4 solution without interfering.

At liquid junction, a brown ring can be obtained.

Nitrate anion - NO3– is present.

No brown ring is formed

Nitrate anion - NO3– 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

CO32‒ (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

SO32‒ (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

SO42‒ (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

PO43‒ (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

C2O42‒ (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 ________ (CO32‒, SO32‒, S2‒, NO2‒, SO42‒, Cl, NO3‒, I, Br, C2O42‒, PO43‒, CH3COO) anion.


Precautions

  • Read the label carefully on a bottle before using chemicals or reagents. Never use reagent which is unlabeled.

  • In smelling vapours or chemicals, be careful. Always fan the vapours 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.

FAQ (Frequently Asked Questions)

Q1. How to Test the Presence of Sulphide Ions?

Answer: H₂S rotten egg smell is observed when adding dilute sulphuric acid to salt. The confirmatory test is the Ag₂S black spot, which is observed when adding S₂-ion to the silver foil.

Q2. What is the Term Common Ion Effect?

Answer: 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.

Q3. How to Test the Carbonate Ion’s Presence?

Answer: A dilute acid can be used to detect the carbonate ions, CO₃²⁻ 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.

Q4. Why Does Iodine Result in a Blue Colour with a Starch Solution?

The iodine test helps to test the presence of starch. And starch becomes the intense “blue-black” colour because of the production of an intermolecular load-transfer complex by adding the aqueous solutions of triiodide anion.