Skip to main content

Water Science for Schools

Water Basics • Earth's Water • Water Cycle • Special Topics • Water Use • Activity Center • Water Q&A • Galleries • Search this site • Help • Water glossary • Site map • Contact us • Back • Home

Water Science Pictures
pH meter measures acidity

Measuring pH

Picture of an electronic pH meter used in a lab. 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.

Taking pH at home or school

Picture of litmus paper, which can be used for a quick estimation of pH in water. One of the most popular school science projects is to take the pH of water from different sources. Chances are your school (and certainly not you) does not have an electronic pH meter lying around, but you can still get an estimate of pH by using litmus paper. Litmus paper, which can be found at pet-supply stores (to check the pH of aquariums) is simply a strip of paper that, when a sample of water is dropped onto it, turns a certain color, giving a rough estimate of pH.

Pictures: The water around us Measuring water Human's influence Water use
Picture gallery Water Science home page

Accessibility FOIA Privacy Policies and Notices

Take Pride in America home page. USA.gov U.S. Department of the Interior | U.S. Geological Survey
URL: http://ga.water.usgs.gov/edu/phmeter.html
Page Contact Information: Howard Perlman
Page Last Modified: Friday, 07-Nov-2008 15:45:06 EST