Adventures in Ethics and Science

Yesterday, we had an urge to do some experimentation and I had a red cabbage that had overstayed its welcome in the refrigerator crisper drawer.

So of course, we made cabbage-water indicator.

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An indicator is a substance that produces a color change that gives you information about whether the stuff you’re testing with it is acidic, basic, or neutral. Some indicators have just two states. For example, phenolphthalein, the indicator beloved by chemistry students (in part because of the fun of spelling it), is frequently used to find the equivalence point in acid-base titrations. In acidic and neutral solution, phenolphthalein is colorless. When you are one drop of base past a neutral solution (in which acid and base are perfectly balanced), phenolphthalein gives a pale pink color. The more basic the solution, the pinker the phenolphthalein will make it.

Other indicators — including the one we made — give a range of colors.

How to make the indicator:
Shred some red cabbage. (We used 3/4 of a very small head. If you don’t have a sharp knife, tearing up the cabbage leaves will work, too.)

Put the cabbage pieces in a clean container. (We used a Pyrex 4-cup measure, but you can use a bowl or even a plastic zipper bag.)

Cover the cabbage with very hot water. Let it steep until that water has cooled (to somewhere between lukewarm and room temperature).

The purple liquid you’ve made is your indicator. Pour it into a container and compost the cabbage. (If some flecks of cabbage remain in your cabbage water, it’s not a big deal.)

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Next, cast your eyes around the kitchen for substances that might be acids or bases.

Liquids are good, as are fruits that can be made to give up juice without too much urging.

Solids that you have around for baking are also good. (Think salt, baking soda, sugar, cream of tartar, …)

We stuck to edibles in our experiment. You could check out non-edibles that you have around for cleaning, but some of them may introduce more danger (e.g., from caustic ingredients) than you want to deal with.

There is, after all, something appealing about being able to do chemistry without having to locate safety goggles and nitrile gloves.

Besides your cabbage-water indicator and the substances you will be testing with that indicator, you will also need small containers for mixing. We used tea cups because they were small, shallow, and white inside — which makes it easier to see and compare color changes. If you use clear glass containers (or test tubes, if you have them), you will probably want a piece of white paper to put under (or behind) them. Otherwise, you might lose the color-changes in your table top.

We found it handy to keep a teacup of cabbage-water indicator as a reference for color comparisons.

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So, we poured more indicator into the rest of the tea cups and started testing our kitchen reagents. (Actually, we had to stop to wash out tea cups part way through because we had more things we wanted to test than we had tea cups. Thanks goodness for digital cameras!)

Here’s what we found:

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Fresh lemon juice turns the cabbage-water indicator a lovely pink.

This was the first substance we tested, and we were actually surprised at how dramatic the color change was.

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Fresh orange juice turns the cabbage-water indicator an orangish-pink. (Orange juice is itself orange.) There’s definitely a color change from purple to pink, but the pink is not as dramatic as what we got from the lemon juice.

Next time, we might try using full-strength lemon juice and comparing it to diluted lemon juice in terms of the color when added to the indicator. (For that experiment, we’d actually want to measure the amounts of cabbage water and lemon juice being used. After all, the water in the indicator solution also dilutes the lemon juice.)

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Rice wine vinegar turned the indicator pink. It also made us think of sushi.

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Apple cider vinegar also turned the indicator pink.

Potentially, you could test as many different kinds of vinegar as you have in your pantry, but after a couple vinegars both give the same color, you have a pretty good hunch that other vinegars will do the same thing. (Some of your other vinegar options — like balsamic vinegar — may be dark enough to make the color change hard to notice if you add significant volumes of the vinegar to your indicator. Luckily, you only need to add a wee glug of vinegar to see something happen.)

If you hadn’t drawn the conclusion before you get to the vinegar, the vinegar gives you good reason to infer that cabbage-water indicator turns pink in the presence of an acid. (The vinegar bottle labels usually say something about the vinegar being diluted to achieve a particular % acidity.)

Having worked out what to expect from indicator + acid, we added a known base to the cabbage-water.

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A bit of baking soda added to the indicator gave a distinct blue color. Next time, we might try adding different amounts of baking soda to identical volumes of cabbage-water to see if the results display a range of blues.

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Seeing an unguarded mug of tea on the counter, we added a glug of that to the indicator. It got darker, but was still purplish rather than pinkish or bluish. We concluded that the tea must be neutral.

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Tonic water turned the indicator pink. Tonic water must be acidic. (We think this is most likely due to the carbonation. However, we didn’t test any seltzer water to follow up on this hunch because we didn’t want to open a fresh bottle just for this experiment. Tight funding is a problem all the way around!)

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We added a bit of milk (nonfat) to some indicator. The result was a bit more opaque and milky-looking, but still purple. We concluded that milk is neutral.

Then, we got a little fancy and separated an egg.

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We poured the eggwhite into a tea cup of cabbage-water indicator. The awesome thing about the eggwhite is that it doesn’t immediately go into solution with the indicator like the other substances we tested.

In other words, the eggwhite and indicator don’t give a visible reaction until you stir them together.

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After some stirring, the indicator with eggwhite turned blue. We concluded that eggwhite is basic.

This is a very easy experiment to set up, and the color changes are surprisingly vivid. I’m now inclined to keep red cabbage on hand at all times. (And, we still have about 12 oz. of cabbage-water in a jar in the fridge. Obviously, we’re going to have to test it in a day or two to see if the indicator quality persists or degrades.)

Comments

  1. #1 Susan B.
    September 26, 2008

    Another interesting experiment would be to take a couple of known acids and known bases and combine acid and base pairs in various proportions, then see what proportion of, say, vinegar to baking soda is required to keep the indicator purple. If you took several different acids and added a range of baking soda amounts to each, you’d be able to rank the acidity by how much base is required to make them neutral. This would be a nice introduction to pH values.

  2. #2 SimonG
    September 26, 2008

    I might have to try this with my nephews. Good to encourage an interest in science, and the eldest is already interested in natural history.

    I recall when I was about 8 my dad brought home a reel of Universal Indicator paper, “borrowed” from the lab at work. I had a lot of fun with that, especially since at school we just had litmus paper. Odds and ends he brought back from work were often more interesting than “real” presents. I had a large collection of magnets, (some quite large), ball bearings and old electrical gear, and a lot of miscellaneous office supplies.

  3. #3 Ross
    September 26, 2008

    If you wanted to keep an indicator around the house all the time, turmeric is a bit easier to keep (and it’s cheap!). It turns from golden yellow in acidic and neutral solutions to quite a deep red in alkaline. (Its midpoint is very close to the pH ranges possible with dissolved baking soda).

    The sprogs might like experimenting to find out how cabbage juice’s range of blues match up with the turmeric’s range of colours. Just make sure they don’t get any on their clothes!

  4. #4 Becca
    September 26, 2008

    Awesome!
    I am now going to find an opportunity to write “a wee glug” in my lab notebook.

  5. #5 Missy Ph.D.
    September 26, 2008

    Wow. I don’t remember doing experiments like this when I was in school! Definitely something that I’d like to experiment with my kids (though I’m not yet married now) :D

  6. #6 Academic
    September 26, 2008

    All of these really cool kitchen experiments are making me miss my childhood! (Although, I confess mine were less cool than yours.)

  7. #7 Richard Simons
    September 27, 2008

    I once cooked some red cabbage and made gravy using the water. The gravy went pale blue, a most unappetizing colour. That is when I realized I should probably have not used self-raising flour.

  8. #8 Killinchy
    September 27, 2008

    Fifty years ago when I was the chem lab tech at my school in England (the school paid me to prep stuff for the chem teacher who had a demonstration for just about every lesson), I had to make some Hopner’s indicator.

    This weird mixture of dyes and indicators was cherry red in an acid solution, emerald in a basic solution, and in a neutral solution, had a sort of metallic colour. Metallic? Well, the solution wasn’t colourless, but it didn’t seem to have any particular colour.

    I have no idea what the recipe was (Shades of McArthur Park)

  9. #9 Lyle G
    September 28, 2008

    I have made indicator by exptacting red cabbage with alcohol. I expect that it keeps better than a water extraction. It takes on a brownish tinge after a while.

  10. #10 Lab Cat
    September 29, 2008

    A great kitchen lab experiment. Thank you.

    If you want to test to see if carbonation is responsible for the acidic reaction of tonic water, try boiling some tonic water – this removes the carbonation.

  11. #11 Carrie
    September 29, 2008

    Mahalo nui Janet for this post. We are definitely going to try this at home!

  12. #12 kojiu
    August 24, 2009

    seriously, try camelias or blueberries.
    remember: a universal indicator chart wont aply to this other stuff used. its totaly diferent.

  13. #13 nazeea
    May 2, 2010

    this is so cool.i might use it in my teaching.thanks

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