14. Redox (REDOX TITRATIONS)
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Give the general steps of a redox titration calculation
Construct and balance the overall redox equation by combining the two half equations Find moles of given substance Find moles of required substance using molar ratio from overall redox equation Convert moles to concentration
Describe the colour changes involved in the titration of Fe2+ and MnO4-
Colourless/ pale green —> pale pink Fe2+ solution will be colourless in conical flask Add MnO4- (which is dark purple) from brunette until Fe2+ is oxidised to Fe3+. You then add 1 drop of excess MnO4- and see a pale pink colour MnO4- is self indicating
Describe the colour changes involved in the titration between I2 and S2O32-
Blue-black —> colourless Iodine is brown in conical flask Add sodium thiosulfate (which is colourless )from a burette until conical flask contents are pale straw yellow. Then add a couple of drops of starch indicator, where now the contents of the conical flask goes blue black Continue to add S2O32- until blue black goes colourless
Why do we preform a redox titration with Fe2+ and MnO4-? (give general method)
To find the mass of iron in iron tablets: 1. Grind up iron tablets with pestle and mortar. Transfer to weighing boat and measure mass. Transfer crushed tablets to small beaker. Reweigh the weighing boat (weighing by difference) 2. Add sulfuric acid to the beaker and dissolve the tablets. (we need the solution to be acidified as potassium manganate works in acidic conditions)Transfer the solution to a 250cm3 volumetric flask. Add washings. Make up to the line with distilled water. Invert to shake to ensure uniform concentration 3. Pipette the 25cm3 of the Iron (II) solution into a conical flask 4. Fill the burette with potassium manganate solution, K2MnO4 5. Titrate Fe2+ solution until the mixture has turned from colourless/ very pale green to just tuned pink. MnO4- is self indicating. 6. Repeat until concordant - 0.01dm3
Give 5 procedural errors in redox titration, and ways to overcome them
Stirring may not be sufficient to ensure all iron dissolved - warm it may help instead Transfer of the solution means you may loose some mass of iron - so ensure the beaker/ all equipment is rinsed The solution may not be mixed - invert volumetric flask to ensure uniform concentration of iron solution Glassware may not be read accurately - parallax error (not read at eye level) - to fix, read glassware from the bottom of the meniscus End point may not be clear/ obvious - use a white tile