How to determine the percentage of iron(II) and iron(III) in a solution?

Question

Outline a plan of an experiment to determine the percentage of iron present as iron(III) in a solution containing $\ce{Fe^3+(aq)}$ and $\ce{Fe^2+(aq)}$ ions. You are provided with zinc, a standard solution of potassium dichromate(VI) and dilute sulfuric acid. How you would use the zinc and how you would calculate the final answer.

So I know that the following equations will be involved

• $\ce{2Fe^3+ + Zn -> 2Fe^2+ + Zn^2+}$

• $\ce{6 Fe^2+ + Cr2O7^2- + 14 H+ → 6 Fe^3+ + 2 Cr^3+ + 7 H2O​}$

I am very unsure on the method to use, I know that I can titrate a sample of the solution against $\ce{K2Cr2O7}$ to eliminate $\ce{Fe2+}$ ions, but I am clueless on how to proceed after.

Answer 1

You’re already very close. You have realised that:

1. You can titrate the $\ce{Fe^{+II}}$ content with a potassium dichromate standard solution; and

2. You can turn $\ce{Fe^{+III}}$ into $\ce{Fe^{+II}}$ by adding metallic zinc.

Note that in the first bullet point of mine, your second reaction, you are creating $\ce{Fe^{+III}}$, hence iron(III) that was present before you started won’t be modified. Similarly for the other reaction and iron(II). That should lead us to following methodology, hidden in a spoiler tag:

1. Titrate the iron(II) content in an aliquot of your solution. That gives you the content of iron(II) and only iron(II). 2. Take another aliquot and reduce all iron(III) to iron(II). That will give you the total iron content. 3. Substract the value from 1. from the value from 2. to get the content of iron(III).