BEYOND - Jean-Marie Cavanihac's HOMEMADE pH TESTER FOR ACIDS AND BASES

ADULT SUPERVISION REQUIRED!
PLEASE READ THIS ENTIRE ARTICLE, INCLUDING ALL SAFETY PRECAUTIONS, BEFORE CONDUCTING ANY OF THE EXPERIMENTS DESCRIBED IN THIS ARTICLE.

The purpose of these experiments is to find out whether common materials around the home are acids and bases. Everybody knows that lemon has an acidic taste. Acids are chemicals which release hydrogen ions (a hydrogen atom which has lost its electron) when dissolved in water. They react with other elements like metals which give it back one of their electrons. In contrast, bases are alkaline substances. A base is a chemical which produces a SALT when mixed with an acid. For example:

Hydrochloric acid + Sodium hydroxide = Sodium chloride
HCl NaOH NaCl

Sodium chloride is the salt we use on the meat we eat. Note that HCl and NaOH are highly corrosive, whereas NaCl is inoffensive! In this reaction the base has neutralized the acid (or vice versa).

So how can we measure the strengths of acids or bases? The unit of measurement is called the pH which is related to the number of hydrogen ions (H⁺) in solution. The pH scale is logarithmic (i.e. a pH change of 1 means the concentration changes by 10 times). A pH of 1 is a very strong acid and 14 is a very strong base. In the middle is pH 7 which is neutral (e.g. water).

An easy (and funny) way to test the pH of liquids is to use the leaf properties of a vegetable: the red cabbage. Use the external leaves, which are much more colored than the one illustrated, and by using the process illustrated below you can make a colored solution with the leaf fragments. Don't use too much water to ensure a dark colored solution.

Note that this solution is quickly discolored if it's exposed to light (use a dark glass bottle). You can use the solution 'as is' to add to liquids under test or impregnate a filter paper (allow the paper to dry in a dark place before using; impregnate 2 or 3 times if the paper color is too light).

When the solution is made, it has a purple color which becomes red with acid and blue to green with bases.

To increase sensitivity, you can first impregnate the filter paper or blotting paper with the cabbage solution and then either 1) add a drop of vinegar to obtain pink paper (for a more sensitive to test for alkalis), or 2) add a drop of sodium bicarbonate to obtain blue paper (for a more sensitive test for acids). See the pictures below of the solution (upper series) and paper (lower series).

Test the following with the original solution: (Safety note: the materials in red should not be tested by youngsters, but should be tested by adults with the appropriate care taken in handling strong acids and alkalis).

baking soda (sodium bicarbonate)
lemon, tomatoes, orange juice, salt ,
white wine, vinegar, aspirin (if a youngster, drugs should not be tested without adult supervision)
lava soap, tonic water, DILUTED drain cleaner, oven cleaner, ammonium based detergents, lime scale remover .....

According to the color change, use pink or blue paper to increase sensitivity.

This project is also an opportunity, to learn about the hazards of the stronger acids or bases as they are corrosive to the skin and eyes. Never dilute strong acids by adding water to the acid: the reaction produces heat and can vaporize water, projecting drops of acid. Always add acid to the water. Sodium hydroxide in solution or pellets (drain cleaner), is used to unblock sink pipes, because it can dissolve organic matter!

Potassium hydroxide is much more corrosive. It's easy to identify hazard warning notices on packages or trucks which contain these chemicals: see picture right.

SAFETY NOTE

Youngsters should never handle chemicals such as strong acids or alkalis, or any container with a bright orange label like the one above. These labels warn that the contents can be dangerous, in this case corrosive, but other orange labels indicate that the contents can be harmful in other ways e.g. flammable, toxic. Adults should take great care as well, in handling, storage and disposal.

Another interesting experiment to try is as follows: Dampen a long strip of the purple paper with salt water, then place the terminals of a 9 volt battery on the wet strip for a few seconds (it should be a small i.e. low current capacity battery). The electrolytic reaction produces NaOH near the cathode and the paper turns green/yellow. In the past, this kind of paper was called 'polarity detector paper'.

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