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Harpic Power Plus Colour Change Chemistry

Introduction

I was cleaning manganese dioxide-stained glassware in the lab with Harpic Power Plus - it’s a dilute solution of hydrochloric acid, and cleans brown manganese stains very nicely. I must have had a small amount of potassium permanganate still in one of the beakers, as I noticed the cleaning solution change colour from bright blue to clear green, then to yellow, then red, and finally back to a pale yellow clear solution over the course of 10 minutes or so.

After a few false starts, I’ve replicated it. This works with the mid-2025 formulation of Harpic Power Plus. Formulations from other years may behave differently.

Materials

You need:

  • PPE - gloves at least.
  • 300ml water (tap water is fine)
  • 20ml Harpic Power Plus
  • A few tiny grains of potassium permanganate

Method

Start by making a weak solution of Harpic in water. I found 20ml of Harpic to 300ml of water worked well. Tap water was fine; there’s nothing too critical in this experiment except the quantity of permanganate.

Get 300ml of water, stirring. Heat isn’t needed, but may speed up the reaction.
Get 300ml of water, stirring. Heat isn’t needed, but may speed up the reaction.
A bottle of 2025 vintage Harpic Power Plus.
A bottle of 2025 vintage Harpic Power Plus.
20ml of Harpic in 300ml of water makes a nice blue solution
20ml of Harpic in 300ml of water makes a nice blue solution

You only need a small quantity of permanganate. A few large (sub-mm) crystals work well. You can always add more if needed.

A very old bottle of permanganate
A very old bottle of permanganate
This is actually slightly too much to add.
This is actually slightly too much to add.
Immediately after the addition, not much is seen to happen.
Immediately after the addition, not much is seen to happen.
After about 30 seconds, the colour starts to fade and turn green-ish.
After about 30 seconds, the colour starts to fade and turn green-ish.
The colour will fade from deep blue to a clear, almost greenish colour over the next minute or so. I used water at ~12 °C; warmer or colder will change the reaction rate.
40 seconds after addition.
40 seconds after addition.
60 seconds after addition.
60 seconds after addition.
Eventually, the colour will start to shift to yellow. If you don’t get this, add another crystal of permanganate and keep stirring.
1min 10 starting to turn yellow.
1min 10 starting to turn yellow.
1min 30 definite yellow.
1min 30 definite yellow.

I was called away, so I don’t know the exact timings for the following colour changes, but it was on the order of a few minutes.

Yellow!
Yellow!

As long as you have a small excess of permanganate in the beaker, the colour will start to shift to red-purple. The trick is not to let the purple permanganate overwhelm the colour. Less permanganate is more in this reaction.

Going Red.
Going Red.
A dirty red/orange solution.
A dirty red/orange solution.

If you want to speed up the final clearing step, now is the time to start gentle heating. The colour in the solution is coming from manganese dioxide particles in suspension, and heat will speed up their dissolution by the hydrochloric acid. Without heating, it took about 20-25 minutes to become crystal clear.

Starting to clear.
Starting to clear.
Crystal clear.
Crystal clear.

Proposed reaction scheme

The blue dye in the Harpic formulation seems to bleach easily. I should probably take a sample and see if I can identify it. It doesn’t seem to stain fabric.

  1. The permanganate reacts with the acid in Harpic and liberates a small amount of chlorine; this is probably what bleaches the blue colour. This produces manganese dioxide in fine suspension, which can appear a dirty yellow colour.
  2. The yellow combined with residual blue causes the shift to green.
  3. Eventually, all the blue is bleached, and the manganese dioxide concentration is high enough to make the solution yellow.
  4. Excess permanganate dissolves, causing the solution to change colour towards orange/red/purple, depending on the permanganate concentration.
  5. The hydrochloric acid in the solution dissolves the manganese dioxide to form manganese chloride, which is colourless in such low concentrations. This shifts the colour back to dirty yellow.
  6. After a long enough time, all the permanganate has dissolved and reacted to become manganese dioxide, and this has dissolved. The solution becomes clear.

It’s really a competition reaction where various reactions occur simultaneously, and the colour of the solution tells you which is currently dominant.