Extracting Anthocyanin: a Natural PH Indicator

In this project I will be extracting a class of compounds called anthocyanins, which come from a broader class of molecules called flavonoids. Anthocyanins are special within themselves because they fall within a category of compounds which have halochromic properties.This means that they have a very special trait of changing color, depending on the pH of the solution it's mixed with.

This is due to the molecule losing a positive O+ charge and gaining a OH grouping. This seemingly minor change is what is responsible for the color change we see.

Things you will need:

- 1 Fresh Red Cabbage

- Distilled Water

-Filter

-Coffee Filter

-Beaker(s) (or stove pots)

-Variety of different pH solutions. (a list of common items and their pH is below)

- This project should be handled with care, like all projects.

The red cabbage should be chopped into fine pieces to allow the water to pull the anthocyanins from the vegatable. Ideally, the smaller the pieces the better, but not super fine so everything filters off later.

I have heard from many people that using a cheese shaver, that it makes it into perfectly size pieces.

Once this is done, we will need to add the red cabbage into our beaker (or stove pot) and add in distilled water until the cabbage is just covered. The less water the better. We will end up boiling off most of this, so the less water we start with, the less water we will have to boil off later.

This step is pretty simple. We will need to bring the cabbage water mixture to boil, followed by keeping it simmering for about 25 minutes. Mix well everything 10 minutes to break up chunks.

The purpose of this step is to let the anthocyanins really soak into the water. Eventually though, the cabbage will start degrading and come off in tiny pieces. This is fine, though can give you some troubles filtering for later.

In this step, we will filter off all of the red cabbage and just leave a solution of anthocyanins and water behind.

I used a filter and coffee filter, however any type of strainer will work. I have seen fabrics and metal strainers work just as well. Even a french press coffee maker will do the trick.

BE CAREFUL. The solution will be very hot.

The purpose of this step is to concentrate the anthocyanins to be used for the pH indicator.

This step is OPTIONAL, and the deluted version works pretty well. Though the stronger concentration can provide some additional color saturation which makes everything look nicer.

This is the final step.

You are now able to use the anthocyanin mixture to check the pH of solutions!

Here is a list of pH solutions to mix the anthocyanin solution with! (Clear solutions will give you the best color changes)

2 pH: Lemon Juice and Vinegar (acetic acid)

3 pH: Grapefruit and orange juice

4 pH: Apples, 7-UP Soda

5 pH: Pepto Bismol, Coffee

6 pH: Milk

7 pH: Water

8 pH: Baking Soda

9 pH: Hand Soap

10 pH: some laundry detergants

11 pH: ammonia

13 pH: Bleach

14 pH: Caustic soda ash

Since I feel comfortable with acids and bases, I used an acid with the pH of 0 to start, and continued to add a basic solution until I hit 14. This let me carefully increase pH and get a good range of colors.

This chemical function works because of a simple pH change, the compound goes through one major change. It simply loses a postive O+ and gains a -OH. This small change lets us see a variety of colors depending on what it is mixed with!