pH testing strips—also called litmus strips—are thin, small sheets of paper, each about 2 inches (5.1 cm) long by 1⁄2 inch (1.3 cm) wide. They are coated with a chemical solution which changes color when exposed to basic or acidic substances. pH strips can be purchased at most health-food stores or at any science supply store. You can also purchase pH strips at major online retailers, or at most pet-supply stores. Place about 1 inch (2.5 cm) of the pH strip into water. Dip either side; the entire strip is pH sensitive. Be sure to keep a firm hold on one end of the strip, though, so that it doesn’t slip into the water you’re planning to measure.  For example, draw a glass of water from your tap if you’re trying to ascertain the pH of the drinking water. Or, if you’re trying to adjust the pH level in a fish tank, dip a strip into the water at the top of the tank. Alternately, you can use a water dropper to place 1 or 2 drops of water directly onto the center of the litmus strip. Once the pH strip has been submerged for at least 10 seconds, pull it out and look at the end. The pH strip will have changed color to indicate the presence of base or acid in the water. If the color of the strip doesn’t change immediately, wait a few seconds. Your pH testing kit should have come with a color chart. The chart indicates the color that litmus paper will turn when exposed to a basic or acidic substance. Match the color of the pH strip to a color on the chart, and you’ll have a good idea of the water’s pH. In broad terms, a red litmus strip indicates highly acidic water, yellow indicates mildly acidic, green indicates mildly basic, and purple indicates highly acidic. Litmus strips are a quick and effective gauge of pH levels, but are not the most precise method of measurement. A digital probe reader (often called a “pH meter”) is a much more accurate device, which will determine the pH of a liquid to 2 decimal places. To use the pH meter, submerge the tip of the instrument in water until the display shows the pH reading.  pH meters are commonly used in science labs and can be purchased or rented through science-supply stores. The pH meter may need to be re-calibrated between each usage. Follow the printed directions included with the meter for best results.
Purchase pH testing strips. Dip 1 pH strip in a sample of water for 10 seconds. Observe what color the pH strip turns. Compare the pH strip to a pH color indicator. Gauge pH levels using a digital probe reader for more accurate data.