The USGS Water Science School
The U.S. Geological Survey analyzes hundreds of thousands of water samples every year. Many measurements are made right at the field site, and many more are made on water samples back at the lab. pH is an important water measurement, which is often measured both at the sampling site and in the lab. There are large and small models of pH meters. Portable models are available to take out in the field and larger models, such as this one, are used in the lab.
To use this pH meter, the water sample is placed in the cup and the glass probe at the end of the retractable arm is placed in the water. Inside the thin glass bulb at the end of the probe there are two electrodes that measure voltage. One electrode is contained in a liquid that has a fixed acidity, or pH. The other electrode responds to the acidity of the water sample. A voltmeter in the probe measures the difference between the voltages of the two electrodes. The meter then translates the voltage difference into pH and displays it on the little screen on the main box.
Before taking a pH measurement, the meter must be "calibrated." The probe is immersed in a solution that has a known and stable pH (a "buffer solution"). The knobs on the box are used to adjust the displayed pH value to the known pH of the solution, thus calibrating the meter.
One of the most popular school science projects is to take the pH of water from different sources. Your school might have electronic pH meters for school projects, as these meters have come down in prices a lot. The most basic way of testing if a solution is acidic or basic is to use litmus paper, which uses a color change to give you this information. There are also paper pH test strips available that can give you a better estimate of pH of solutions. With these, the strips show a certain color to indicate certain pH values. These strips can be used to in school science classes and fairs, to test the water in an aquarium, or to test one's urine, even.