PH is an indicator of the acidity or alkalinity of a substance. Our bodies generally stay at pH 7, which means we are neutral---neither acidic nor alkaline. Drinking water and aquarium water should be close to pH 7. Testing pH with homemade pH strips is a good way to check your home water supply, and it can also be a fun way to teach kids about pH.
Make pH Strips
To make your own pH strips, chop three or four red cabbage leaves. You should take leaves from the outside of the cabbage that have the most color. Put the chopped leaves in a pot with just enough water to cover them, and boil it for 15 minutes. Strain the leaves from the water, and simmer the water until it is reduced to about half a cup. This produces a very intense purple liquid full of anthocyanins, a natural type of dye that is reactive to changes in pH.
Cool the purple water and put it in a shallow pan. Place several paper towels or coffee filters into the pan and stir them gently to evenly distribute and soak up the dye. If there is any excess dye, pour it down the drain carefully to avoid staining the sink. Lay the paper out to air-dry overnight, and then cut it into strips.
To test the pH of a liquid, dip the strips into a sample of the liquid, wait a few seconds, and look at the color of the paper. If the color doesn't change at all, it's pH 7, or neutral. The more acidic a substance is, the redder the paper will turn, and bases will turn the paper blue, green or yellow, depending on how alkaline they are. Be aware that anything that turns the paper bright red or bright yellow is caustic and should not come into contact with your skin.
Water and pH
The drinking water in your house should be pH neutral, or slightly alkaline. The alkalinity may result from the presence minerals like calcium and magnesium, which, in small amounts, are not harmful and actually make the water taste better. However, if water is acidic, it could contain dangerous chemicals like iron or lead that make water taste bad and are harmful to drink.
If you keep an aquarium, you should find out the ideal pH for your fish and make sure only fish with similar requirements share a tank. Most fish prefer pH-neutral water, though some have more extreme requirements. Additionally, a healthy aquarium should have a constant, unchanging pH. If your fish have ongoing health problems, try testing the water once a day for a week to see if there are any fluctuations in pH that are negatively affecting your fish.