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

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.

• You are provided with $\ce{Zn}$, not $\ce{Zn^2+}$, so you have to find a reaction between $\ce{Zn}$ and either $\ce{Fe^3+}$ or $\ce{Fe^2+}$. – orthocresol Jun 10 '16 at 12:33
• You are right, I'll change the equation. – daljit97 Jun 10 '16 at 14:06
• So you have a mixture of $\ce{Fe^2+}$ and $\ce{Fe^3+}$ ions, and you know that Zn only reacts with one of them, so maybe you can use that to determine the amount of $\ce{Fe^3+}$ ions? And then what can you use the dichromate to do? – orthocresol Jun 10 '16 at 14:54
• Well I can titrate the entire solution against the dichromate to find the amount of Fe2+ ions that reacted. – daljit97 Jun 10 '16 at 19:10

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: