Unit A23 5.3 Volumetric analysis

Question 1

5.3.1 - Titrating iodine with sodium thiosulfate solution using starch and hence estimate oxidising agents by their reaction with excess acidified potassium iodide

Answer 1

• Rinse a pipette with potassium iodate(V) solution and using a pipette filler, pipette 25.0 cm3 of standard potassium iodate(V) solution into a conical flask.
• Add approximately 1.5 g of potassium iodide (excess) and excess of sulfuric acid (by measuring cylinder).
• Rinse the burette with the standard sodium thiosulfate, and then fill the burette with sodium thiosulfate, making sure the tap is filled.
• Add standard sodium thiosulfate solution from the burette until the solution is straw/yellow in colour and add 3 drops of starch indicator solution.
• Titrate until the indicator changes colour
• Repeat until results are concordant (within 0.1 cm3)

If hydrogen peroxide is used as an oxidising agent, instead of the first step, pipette 25.0 cm3 of hydrogen peroxide into a conical flask; add a few drops of ammonium molybdate catalyst and proceed as above.

Question 2

5.3.2 - Titrate acidified potassium manganate(VII) solution with reducing agents

Answer 2

• Rinse the pipette with a solution of a reducing agent and use the pipette and safety filler to pipette 25.0 cm3 into a conical flask.
• Rinse the burette with a standard solution of potassium manganate(VII) and then fill the burette ensuring the tap is filled.
• Add excess sulfuric acid using a measuring cylinder, to the conical flask
• Add potassium manganate(VII) solution from the burette into the conical flask and swirl until the solution turns from colourless to pink
• Repeat until results are concordant (within 0.1 cm3)

Question 3

5.3.4 - Determine the purity of a group II metal oxide or carbonate by back titration

Answer 3

• Weigh out a mass of the solid (Group II metal, Group II oxide or Group II carbonate)
• Add an excess of hydrochloric acid
• Add the solution to a 250 cm3 volumetric flask and make up with deionised water until the bottom of the meniscus is on the mark
• Rinse a pipette with the solution from the volumetric flask, and use the pipette and safety pipette filler to pipette 25.0 cm3 into a conical flask.
• Add 3 drops of phenolphthalein indicator
• Rinse a burette with standard alkali solution, and fill the burette
• Add alkali from the burette, to the conical flask until the indicator changes colour
• Repeat until results are concordant (within 0.1 cm3)