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hi how can I control relative of Fe2+ andFe3+ when we adding raw iron to acid solution To get the ratio 2:1

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closed as off-topic by Mithoron, Tyberius, Todd Minehardt, airhuff, bon Feb 6 '18 at 8:16

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Most acids aren't strong oxidisers and really only yield $\ce{Fe^2+}$ when dissolving iron. The most common oxidising acid is nitric acid, $\ce{HNO3}$, which does fully oxidise the iron to $\ce{Fe^3+}$.

If you don't have access to nitric acid (now a restricted material in most European countries), you can dissolve the iron in hydrochloric acid, $\ce{HCl}(aq)$, then oxidise the $\ce{Fe^2+}$ subsequently with hydrogen peroxide $\ce{H2O2}(aq)$ solution. Then boil that solution for a bit to get rid of any excess peroxide.

You can also buy ferric chloride solution, $\ce{FeCl3}(aq)$, as etching solution.

Now how do you create a solution with a known $\frac{\ce{Fe^2+}}{\ce{Fe^3+}}$ ratio?

Add a calculated amount of steel wool to a known quantity of the ferric solution. The following reaction takes place:

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

Control the concentration and quantity of ferric solution and the amount of steel wool used and you can obtain any $\frac{\ce{Fe^2+}}{\ce{Fe^3+}}$ ratio. For example, if you set aside precisely half of your ferric solution, then add an excess of steel wool to the other part, filter of the unreacted iron (leave it to react overnight, e.g.) and mix both parts then your ratio will be $\frac21$.

This how many instructables for ferrofluid are written. I've done this myself and it works well.

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