Practical investigations
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I am doing an A2 practical planning to find the % mass of iron in iron cast by redox titration. How do I go about it? Do I react the iron cast with conc. H2SO4? How can I make sure all the iron are in the form of Fe2+ before I titrate it with potassium manganate? To titrate with MnO4, one of the solution has to be acidified, so do I add H2SO4 again? Should it be concentrated or dilute? Thank you!
Ulex says
To answer your questions as you have asked them. Yes, you react the powdered sample of cast iron with sulphuric acid. 1 mole dm-3 should be fine, no need to use concentrated. So long as the acid is in excess, the iron will go into solution as Fe2+(aq). Air will oxidise it only very slowly. Yes, you should add even more 1M sulphuric acid when titrating. The equation for the reaction demands 8H+(aq) for every MnO4-.
Always carry out a risk assessment and check with your teacher before starting any practical work.
To anticipate your next question: how much of everything should be used? I reproduce a question and answer which another student received.
Question
I need to find the percentage by mass of iron in a sample of cast iron, with the method involving a redox titration. I’m going to use KMNO4 and will dissolve the cast iron in the acid so that it converts into Fe2+ ions. I also understand how to do the calculations to work out the percentage by mass. However, I’m not sure of how to work out the inital concs and initial mass - do I just randomly pick a number or is there actually a way of working out how much you should use? thanks a lot!
Ulex says ...
A certain amount of guesswork is involved. A reasonable amount of cast iron to use might be just over 0.5 g – you’ll see why in a moment. Potassium manganate(VII) will be used to titrate with. It has a very intense colour and it isn’t very soluble so specify a concentration which isn’t too great. Guess at, say, 0.05M – you can check whether this is appropriate and change it if you need to. Try the following steps.
1 Work out the number of moles of iron in 0.5 g (if you specify 0.56 g this works out as 0.01 mol !) assuming for the moment that the cast iron sample is pure iron.
2 The minimum number of moles of sulphuric acid required to dissolve the iron will be the same as this; aim to use 1 mol dm-32SO4 and provide a generous excess
3 Put your solution into a 100 cm3 standard flask and make up to the mark with 1 mol dm-3
4 Pipette out 10.0 cm3 portions and titrate with 0.05 mol dm-34 ; Work out from the equation what volume of solution should be required. Is this a reasonable volume, not too much or too little? Now remember that your sample isn’t pure iron so the actual titration value will be smaller than you have worked out. Is this still reasonable ? If your titration volume works out as more than 50 cm3 (say) your KMnO4 is too dilute, if it works out as less than 10 cm3 it is too concentrated – adjust accordingly.
Always carry out a risk assessment and check with your teacher before starting any practical work.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 09 May 2006
