Higher level 2009 Exam paper Q1
(b) why must potassium manganate be standardised? why is it necessary to standardise the potassium manganate solution immediately before use in titration? what reagent is used for this purpose?
(c)Describe how exactly 250cm of Fe............ Why was some sulfuric acid used in making up this solution?
(d) Explain why additional sulfuric acid must be added to the titration flask before each titration is carried out.
B) Potassium Manganate must be standardised as it is not a primary standard. It can not be made up accurately by simply weighing. It must be standardised before titration as it decomposes in sunlight. Ammonium Iron(II) sulfate is used as a source of Fe 2+ ions.
C) Usual volumetric analysis techniques, weigh accurately on electronic balance, place in 150cm deioinised water. Rinse the clock glass into beaker with deionised water. Stir. Pour into volumetric flask via funnel. Rinse beaker, stirrer and funnel into flask with deionised water. Make up to the graduation mark adding drop by drop at the neck and reading the bottom of the meniscus at eye level. Stopper and invert 20 times to ensure a homolegous solution.
Sulfuric acid is added to prevent hydrolysis and the Fe 2+ ions being oxidised to Fe 3+
D) More sulfuric acid is added to the titration to act as a supply of H+ ions so that the KMnO4 is reduced from the 7+ state to the 2+ state (purple to colourless) If it's not added the state of +4 is reached which is brown.
Hope this helps :)