Practical investigations
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I have recently done a practical experiment and had quite a large error margin with my results. I dissolved iron(II) sulphate crystals in 50 ml of sulphuric acid, all mixed with distilled water to get a final volume of 250 ml. 25 ml of this solution was then added to 20 ml of sulphuric acid, and titrated using a burette, with potassium manganate(VII) (permanganate). Can you help me with finding where the errors for the potassium permanganate titration volume errors might have come from?
Igloo writes
You haven’t given me enough information to answer this question properly, but here are some ideas. If your answer is way out, then you have probably made some mistakes with your calculation.
Normally this titration works extremely well, so in theory you should achieve excellent results. The end-point of the titration is very sharp and easy to detect.
Here are some possible sources of error:
[1] Iron(II) sulphate is difficult to obtain in a very pure state since it is susceptible to oxidation. It is better to use ammonium iron(II) sulphate as a standard.
[2] Did you measure its mass accurately enough, and did you transfer all of it to the volumetric flask?
[3] Was the sulphuric acid used sufficiently concentrated, e.g. about 1 molar? If not, your titration might have given a dirty brown colour at the end-point rather than a colourless mixture.
[3] Did you remember to mix the iron(II) sulphate solution thoroughly by upending the solution in the volumetric flask several times? This error gives rise to titration readings which vary wildly.
[4] If you are using the potassium manganate(VII) solution to find out information about the iron(II) sulphate, you will be relying on the purity of the potassium manganate(VII). If so, did you standardise the potassium permanganate before use? If not made up freshly, solutions of this oxidant do not have a reliable and fixed concentration.
Always carry out a risk assessment before starting any practical work, and check with your teacher.
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: 12 March 2007
