The Water Science Picture Gallery

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. The 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. So, how does this contraption work? The water sample is placed in the little cup and the glass probe at the end of the retractable arm is placed in the water. The back of the probe is connected to the main box by electrical wires, and at the tip of the probe there is a thin glass bulb. Inside 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. In other words, the voltage of the second electrode responds to the amount of free hydrogen ions (the pH) in the 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 pH, such as pure water with a neutral pH of 7.0. The knobs on the box are used to adjust the displayed pH value to the known pH of the solution, thus calibrating the meter.

Picture gallery Water Science home page USGS Water Resources The URL for this page is http://ga.water.usgs.gov/edu/phmeter.html Comments? Contact Howard Perlman Last Modified: Wednesday, 28-May-2003 14:18:59 EDT