Test for Anions

What is an anion?

We will look at 1.The Chloride Ion Cl - 2.The sulfate ion SO The sulfite ion SO The carbonate ion CO The hydrogen carbonate ion HCO The nitrate ion NO The phosphate ion PO 4 3-

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 2cm 3 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 AgNO 3 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 2cm 3 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? Ba 2+ + SO 4 2-  BaSO 4 ( Barium sulfate) Ba 2+ + SO 3 2-  BaSO 3 (Barium sulfite)

How do we distinguish between the 2? 3. Add about 2cm 3 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 BaCl 2 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: Ba 2+ + SO 4 2-  BaSO 4 ( Barium sulfate) Ba 2+ + SO 3 2-  BaSO 3 (Barium sulfite) BaSO 4 + HCl  No reaction SO H +  SO 2 + H 2 O

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 2cm 3 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 : Carbonate No precipitate : Hydrogen Carbonate (unless boiled

Carbonate and Hydrogen Carbonate Ions Test: Add dilute HCl to the solids. Distinguish the two by adding MgSO 4 to a fresh solution. Observation: A gas is given off that turn limewater milky. Precipitate forms: Carbonate No Precipitate: Hydrogencarbonate (precipitate forms on boiling

Formulas CO H +  CO 2 + H 2 O HCO H +  CO 2 + H 2 O Reacting with limewater Ca(OH) 2 +CO 2  CaCO 3 + H 2 O

Formulas Reacting with Magnesium Sulfate Mg 2+ + CO 3 2-  MgCO 3 Mg 2+ + HCO 3 -  Mg(HCO 3 ) 2 With boiling Mg(HCO 3 ) 2  MgCO 3 + CO 2 + H 2 O

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) sulfate 3.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 FeSO 4 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 1cm 3 of water and shake to dissolve 2.Pour in ammonium molybdate until the test tube is half full 3.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