Test for Anions

What is an anion?

An ion that carries a negative charge

In the next 2 weeks we will look detecting the presence of some of these anions

We will look at

1. The Chloride Ion Cl-

2. The sulfate ion SO42-

3. The sulfite ion SO32-

4. The carbonate ion CO32-

5. The hydrogen carbonate ion HCO3-

6. The nitrate ion NO3-

7. The phosphate ion PO43-

1. To detect the presence of chloride ions

Apparatus: 1.Test Tube, 2.Spatula, 3.a chloride salt, 4.silver nitrate, 5.dilute ammonia and 6.deionised water

Procedure

1. Place 2cm3 of water in test tube adding a small amount of the salt, shaking to dissolve

2. Add a few drops of silver nitrate. What happens??

3. Add about the same amount of ammonia as water. What happens?

Chloride ions

Test: Add a few drops of AgNO3 to a solution of the solid

Observation: A white precipitate is formed which is soluble in dilute ammonia

Formula: Ag+ + Cl- AgCl

The down pointing arrow means that a precipitate has formed

A precipitate is the name given to an insoluble material that settles out of a solution

2+3: To detect the presence Sulfite and Sulfate ions

Apparatus: 1.2 test tubes, 2. deionised water, 3. sulfate salt, 4. sulfite salt, 5. dilute HCl,6. Barium Chloride

Procedure:1. Add about 2cm3 of water to both test tubes

and make up a solution of sulfite salt in one and sulfate in the other, make sure you label your test tubes!

2. Add a few drops barium chloride to each test tube to each, note what happens?

Ba2+ + SO42- BaSO4 ( Barium sulfate)

Ba2+ + SO32- BaSO3 (Barium sulfite)

How do we distinguish between the 2?

3. Add about 2cm3 of dilute hydrochloric acid to each test tube

What happens? Barium sulfate is insoluble in HCl

Barium sulfite is soluble

Sulfate and Sulfite ions

Test: Add BaCl2 solution to the test tubes, distinguish the two by adding dilute HCl

Observation: A white precipitate is formed, when HCl is added to this precipitate, if the precipitate remains : sulfate if it dissolves sulfite

Formulas:

Ba2+ + SO42- BaSO4 ( Barium sulfate)

Ba2+ + SO32- BaSO3 (Barium sulfite)

BaSO4 + HCl No reaction

SO3- + 2H+ SO2 + H2O

4+5: Carbonate and Hydrogen Carbonate ions

Apparatus: 2 test tubes, carbonate and hydrogencarbonate salt, dilute HCl, magnesium sulfate and deionised water

Procedure:1. Place aprox 1cm of carbonate salt in a test

tube, do the same and add about 2cm3 of acid into the test tubes. What happens?

Distinguishing between carbonate and hydrogencarbonate

2. Add Magnesium Sulfate to a fresh solution of the salt.

Precipitate forms : CarbonateNo precipitate : Hydrogen Carbonate (unless boiled

Carbonate and Hydrogen Carbonate Ions

Test: Add dilute HCl to the solids. Distinguish the two by adding MgSO4 to a fresh solution.

Observation: A gas is given off that turn limewater milky.Precipitate forms: CarbonateNo Precipitate: Hydrogencarbonate (precipitate forms on boiling

Formulas

CO32- + 2H+ CO2 + H2O

HCO3- + 2H+ CO2 + H2O

Reacting with limewaterCa(OH)2 +CO2 CaCO3 + H2O

Formulas

Reacting with Magnesium SulfateMg2+ + CO3

2- MgCO3

Mg2+ + HCO3- Mg(HCO3)2

With boilingMg(HCO3)2 MgCO3 + CO2 + H2O

6. To detect the presence of Nitrate ions

Apparatus: Test Tube, Nitrate Salt, iron(ii) sulfate, Concentrated sulfuric acid.

Procedure: 1. Place a small amount of nitrate salt in a test tube and

dissolve with some deionized water.2. Add the same quantity of freshly prepared iron(ii)

sulfate3. Carefully add some concentrated sulfuric acid, VERY

DANGEROUS! Note what happens?

Nitrate Ions

Test: Brown Ring Test, To a solution of the solid, add a freshly prepared FeSO4 solution, add concentrated sulfuric acid

Observation: A brown ring is formed at the junction of the 2 liquids

Brown ring is due to the presence of nitrate ion

7. To detect the presence of a phosphate ion

Apparatus: Test tube, beaker, hot plate, soluble phosphate salt, ammonium molybdate solution, concentrated nitric acid, deionized water

Procedure: 1. Add a few crystals of phosphate salt to about 1cm3 of

water and shake to dissolve2. Pour in ammonium molybdate until the test tube is half

full3. Add 5 drops of concentrated nitric acid VERY

DANGEROUS! What happens?

7. Phosphate ions

Test: Ammonium molybdate is added to a solution of the solid. Concentrated nitric acid is added. Solution id warmed if needed

Observation: Yellow precipitate is formed

Phosphate ions are present