We recently ordered some potassium ferrocyanide from our friendly chemical company.

It turned up and this is what it looks like.

Yes, you guessed it.

It's not potassium ferrocyanide, but potassium ferricyanide.

The former contains iron in the plus two oxidation state and the latter in the plus three oxidation state.

The subtle but important difference.

It's obviously an easy mistake to make if a professional supplier can get it wrong.

But we figured we'd keep it because it's an interesting oxidizing agent.

Given though that we wanted to order the ferrocyanide,

the first thing we're going to do is show you how to convert it back to the ferrocyanide in a nice clean way.

So here's 30 grams.

We've got a hot plate stirrer and a nice big beaker set up on this equipped with a magnetic stirrer to do the reaction in.

Use a large beaker because the reaction is going to foam quite a bit and you don't want it to overflow.

First of all, we add 100 mils of water to the beaker.

And now with stirring, here's our 30 grams of ferrocyanide crystals.

After a few minutes, we should go into solution.

The crystals are bright orange, but the solution is a yellow color, but retains its intensity even when diluted quite a lot.

Our product contains an extra atom of potassium.

So in order to provide this,

we need the equivalent of potassium hydroxide.

This is 5.2 grams.

Let's add this to the solution.

Unlike most iron salts, the addition of potassium hydroxide doesn't form a precipitate.

But ferrocyanide is a complex with the cyanide strongly bonded to the iron atom at the center.

You can't think of it as iron-3 in the same way as a simple iron salt.

It doesn't behave in the same way chemically.

Okay now, we need a reducing agent.

Something that can donate an electron to the iron and change its oxidation state 2 plus 2 from plus 3.

We're going to use 8 mils of 30% hydrogen ferroxide solution.

You're probably thinking, hang on a minute.

Hydrogen ferroxide isn't a reducing agent.

It's an oxidizing agent, right?

Well, let's see what happens.

We add the ferroxide solution slowly.

There's a definite reaction with lots of gas given off.

You can also see the color changing slightly.

It's less orange now and more of a yellow-brown-green color.

Let's keep the addition going.

The color is now white-green.

The color of iron in its plus-2 oxidation state.

And the color of potassium ferrocyanide.

What is happening is that the hydrogen peroxide is being oxidized by the ferrocyanide into oxygen.

It actually acting as a reducing agent.

What's really neat is that in conjunction with potassium hydroxide, the only other byproduct of the reaction is water.

Leaving us with pure ferrocyanide in solution.

It rapidly decomposed by the iron salts in the solution.

If we check the pH.

You can see that the solution is neutral.

No more hydroxide present.

So we place the resulting solution in a pilot's baking dish.

And evaporate it down.

Soon we get some nice yellow crystals of potassium ferrocyanide forming in the mixture.

And as the water evaporates the whole mixture turns into crystals.

And then finally these dry out and become anhydrous.

The anhydrous product is a paler yellow color.

And we broke the lumps into a fine powder to make sure it was completely dry.

Here's what we ended up with.

Around 34 grams of anhydrous potassium ferrocyanide as a pale yellow solid.

This is a quantitative yield from the starting material.

One final note.

You may end up with a product with a brighter yellow or even slightly orange color like this.

If this is the case then what happened was one of two possibilities.

Firstly, if you used below a stoichiometric amount of potassium hydroxide accidentally.

Then you might not have converted all of the starting material.

Secondly though, it's possible for the potassium hydroxide to get used up locally in the reaction.

Leading to hydrogen peroxide then oxidizing the ferrocyanide back to the starting material again in the neutral pH conditions.

So use a slight excess of potassium hydroxide.

And add a little more if your solution at the end is still slightly orange.

We find potassium ferrocyanide very useful as a source of alkali cyanides via the reaction we've covered in a previous video.

Check it out for more details.

You might ask why we don't do the reaction directly using ferricyanide and sodium metal.

Firstly it requires more sodium metal.

But more importantly the sodium metal is a reducing agent.

Whilst the ferrocyanide is an oxidizing agent.

The reaction is much more vigorous.

The sodium usually burns with a lot of flame.

It's potentially dangerous.

And the alkali cyanide product is quite impure and contains a lot of sodium hydroxide.

Thanks for watching.

And if you are going on to make cyanides.

Then please be very careful.

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