Redox and redox equilibria
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I am confused with the method of using colorimetry to determine the amount of iron(II) present in iron tablets. I will carry out a titration with potassium manganate(VII). however, I do not know what to do with the solution once it turns pink. Do I keep it for colorimetry or not? Also, how do I convert iron(II) into iron(III)? What would be a suitable filter to add for colorimetry? With what solution e.g. sodium thiocyanate? Please can you also help me what would be suitable quantities of whatever solution to add and the concentration required if any?
Corrie writes ...
In the titration of a solution containing Fe(II) ions with K-manganate(VII) under acid conditions, the appearance of a pink colour indicates the end-point of the titration. All the Fe(II) ions have been oxidised to Fe(III) ions and you can do the necessary calculations to link the amount of manganate (VII) used to the amount of Fe(II) present.
For colorimetry involving the red complex of Fe(III) ions with thiocyanate ions, you will have to use a filter somewhere in the green to blue region of the spectrum. The complex, being red, absorbs blue-green light.
To convert all the Fe(II) present to Fe(III) you could use drops of K-manganate(VII) solution, as in the titration, with plenty of acid present. In fact you could start by using the solution at the end of the titration - which is possibly what you were thinking about in your question - providing it was only just a very feint pink colour.
The sodium (or potassium) thiocyanate should be in large excess - so make up a solution of it once you have found the Fe(II) concentration from your titration. I would mix equal amounts of the two solutions to start with and test the absorbance. You may have to dilute the Fe(III) solution if the absorbance is too high.
You will also need a calibration curve for your colorimeter, using solutions with known concentrations of the red complex made up using a standard solution of a stable iron salt, such as iron(III) ammonium sulphate, and an excess of thiocyanate, made up in dilute sulphuric acid.
Keep the thiocyanate concentration the same in all the samples tested.
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updated: 05 February 2007
