Potassium Dichromate Preparation

Chromium 3-Oxide

Potassium Hydroxide

3.2 Molar Ratio

Sodium Hydroxide

Potassium Nitrate

1.5 Molar Ratio

ratio to the chromic oxide used.

We put all three starting ingredients into a blender and converted them into a well-mixed

powder as you can see here.

There's still a few lumps but nothing serious.

Now you'll need to work quickly because the potassium hydroxide will rapidly absorb moisture.

Here's our trusty stainless steel food shaker.

Not exactly stainless though after what we've used it for.

Let's get the mixture in.

Here we go.

The container is about half full.

So we'll balance this on the gas burner and heat it strongly.

It doesn't take long before the mixture inside starts to melt.

Okay.

Pretty soon it's all melted and starts to bubble away.

And soon after the mixture appears to thicken again, then bubbling begins to eject material

out of the container.

So we put a lid on the container to prevent this, and continue to heat.

Soon the mixture in.

And inside the container becomes a white solid paste.

And you can see a hint of orange color around the outside of the mixture.

We put the lid on the container and then applied full heat for 15 minutes non-stop.

At the end of this time the mixture inside was quite a yellow color as you can see.

Using some tongs to hold the hot water in.

We used a spatula to break up the solid inside and give it a thorough mix.

Then we put the lid on again and gave it another 15 minutes constant heating on full power.

Then we slowly allowed the container to cool to room temperature.

Strangely the contents of the container appeared almost red in color, but when take out, either on cooling or exposure to air, they turn green again like this.

We scraped out the contents of the steel container into a large beaker.

It's changed color to a light green.

We added about 50 ml of boiling water to the steel container and the rest of the container.

The remaining solids dissolved and suspended very easily.

Then we poured this into the beaker containing the rest of the solids.

We added about another 25 ml of hot water, making up to a total volume of about 100 ml.

And then placed on a hot plate in order to keep the liquid hot, and gave a good stir to break up any lumps of material.

Now time to filter the hot mixture.

The filter, as you can see is a beautiful intense yellow.

We then transferred the green filtered solid back into the beaker and added a further 10 ml of hot water.

40 ml of boiling water.

Then after heating and stirring again, we filtered.

More intense yellow filtered.

Still a few yellow crystals visible in the filter though.

So we added a little more boiling water to get all of our product dissolved out.

As you can see, there wasn't much unreacted chromic oxide left, perhaps just a few grams.

Here's our filtrate, which contains our product, potassium chromate.

We put this on a hot plate with a small stirrer and heated in order to evaporate down a little.

We also did a little test to check the pH of this solution.

Strongly alkaline still.

Chromate is the chromium 6 species which is favored in alkali solutions.

However in acid conditions chromate changes to dichromate.

This has reduced down a little bit, and we're starting to see some crystals forming on the

surface of the hot liquid.

Here we've got some concentrated hydrochloric acid.

We're now going to slowly add this to the solution in order to take the pH down until

it's strongly acid.

Careful doing this because the neutralization produces some heat, and you don't want the

toxic chromate jumping out of the beaker.

As you can see the solution turns the dark color.

There's also an orange tint forming, which is our dichromate.

We've added about 50 ml of hydrochloric acid now.

So let's test the pH again.

It's difficult to know if the red is the color of the solution or the reaction of the indicator.

But looking at the edges where the liquid is diffusing through the paper, it's strongly

acid.

So we covered up the mixture and chilled it.

And now we're going to test it again.

in the fridge for about an hour. Dichromate is not very soluble in cold water, but the

other salts present in solution should stay in solution. And sure enough after an hour,

we've got lots of brilliant orange crystals formed in the liquid. We filter these off.

Most of the dark color is staying in the portrait, leaving the orange crystals behind.

We then wash the crystals twice using 20 ml each of ice cold water. This got rid of the

residual dark color. Then we got them as dry as we could on the pump.

Dichromate is not very soluble in cold water, but the other salts present in solution should

The filtrate does still contain some dichromate, but we'll wait and see what our yield is

lie before we decide whether to process this further.

We couldn't get the solid completely dry, but we got it to this stage and then we placed

into an oven at 120 degrees C for a few hours in order to dry completely.

Here's the product.

18 grams of potassium dichromate as a dry fine powder with a beautiful bright orange

color.

This represents a 74% yield on the starting chromium oxide, which is great.

We decided not to further work up the dark colored filtrate, although you could evaporate

it further and try to produce more crystals if you wanted to.

Well this is good.

But because the powder is very fine and not nice.

The color is slightly more yellow.

Let's make some of that beautiful orange crystalline product like you would obtain commercially.

To do this we simply recrystallize the potassium dichromate from boiling hot water using about

35 to 40 mils in this case.

In the boiling water the solid dissolves readily forming a dark red colored solution.

Here's the result.

Now switch off the heat and allow the beaker to cool as slowly as possible.

The slower the cooling the larger the individual crystals.

Pretty soon orange crystals will start to form in the solution.

Once the beaker is at room temperature, chill it down to get as much solid out as possible.

Then filter.

Wash with a little ice cold water.

And dry thoroughly on the pump.

Re-crystallization is quite efficient.

You can see there's some dichromate in the filtrate, but not that much.

Here's our final commercial quality product.

Beautiful orange crystals of potassium dichromate.

We use dichromate in some of our other videos as an oxidizer.

agent for organic reactions to check these out to get some interesting ideas

on what you can use it for thanks for watching