# Determine percentage of iron(II) and iron(III) in a solution

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Hi I am stuck on the following question.
Outline a plan of an experiment to determine the percentage of iron present as iron(III) in a
solution containing Fe3+(aq) and Fe2+(aq) ions. You are provided with zinc, a standard
solution of potassium dichromate(VI) and dilute sulphuric acid. Zinc can reduce Fe3+(aq) to
Fe2+(aq).
Write equations for all the reactions that occur. Explain how you would use the zinc and
how you would calculate the final answer.

I know the following equations are involved but don't know the method.
2Fe3+ + Zn --> 2Fe2+ + Zn2+
6 Fe2+ + Cr2O7 2- + 14 H+ → 6 Fe3++ 2 Cr3+ + 7 H2O​

Last edited:

Borek
Mentor
Reducing Fe(II) to Fe with zinc is not easy, as they both happily react with water. Reducing Fe(III) to Fe(II) on the other hand, is quite easy.

Reducing Fe(II) to Fe with zinc is not easy, as they both happily react with water. Reducing Fe(III) to Fe(II) on the other hand, is quite easy.
Yes, sorry that was a mistake. I changed the equation.

Borek
Mentor
Assuming you titrate the original sample - what do you determine?

Assuming you titrate the original sample - what do you determine?
Titrating the original solution against Cr2O72-, I will find the amount of Cr2O72- I need to convert all Fe2+ ions into Fe3+ ions. Also titrating against zinc will give the amount of Fe3+ ions in the solution.

Borek
Mentor
Titrating the original solution against Cr2O72-, I will find the amount of Cr2O72- I need to convert all Fe2+ ions into Fe3+ ions.

Sure, finding the amount of dichromate used is a part of the calculations, but that's not what I was asking about. Amount of which ions you will find in the end of the calculations?

Sure, finding the amount of dichromate used is a part of the calculations, but that's not what I was asking about. Amount of which ions you will find in the end of the calculations?
Well, since I know the amount of dichromate I can find the amount of Fe2+ ions that reacted (dichromate x 6), so the titrated solution will only contain Fe3+ ions.

Borek
Mentor
OK, so let's assume now you know how much Fe(II) there were in the solution. Can you think of a way of determining the total amount of iron?

OK, so let's assume now you know how much Fe(II) there were in the solution. Can you think of a way of determining the total amount of iron?
Alright, so I know the amount of Fe2+ and can also determine the amount of fe3+ using zinc. So I can do (amount of Fe2+) / (amount of Fe2+ + amount of Fe3+) *100. Would that be a correct procedure?

Borek
Mentor
Alright, so I know the amount of Fe2+ and can also determine the amount of fe3+ using zinc.

Beware: what you wrote (while correct in general) may mean you are actually wrong in your thinking. Amount of Fe(III) can't be determined directly.

So I can do (amount of Fe2+) / (amount of Fe2+ + amount of Fe3+) *100. Would that be a correct procedure?

That's the definition of what you are expected to do, so yes, that's the correct approach (just see above).

Beware: what you wrote (while correct in general) may mean you are actually wrong in your thinking. Amount of Fe(III) can't be determined directly.

That's the definition of what you are expected to do, so yes, that's the correct approach (just see above).
Hmm why is not possible to determine the Fe3+ ion directly, isn't the reaction with zinc suppose to help with that?

Borek
Mentor
Hmm why is not possible to determine the Fe3+ ion directly, isn't the reaction with zinc suppose to help with that?

Please describe step by step how you are going to determine amount of Fe(III) in the sample.

Please describe step by step how you are going to determine amount of Fe(III) in the sample.
Well I was thinking just to look at the amount of Fe3+ ions reduced by the zinc (assuming that only Fe3+ ions will be reduced to Fe2+).

Borek
Mentor
Well I was thinking just to look at the amount of Fe3+ ions reduced by the zinc (assuming that only Fe3+ ions will be reduced to Fe2+).

How? You have no easy control over how much zinc reacted. Thing you can easily measure is the amount of dichromate used for Fe(II) oxidation.