Most solutions prepared in the biological laboratory must have a carefully controlled pH. Buffers are prepared by adjustment to a specific pH with strong acid and base solutions, using a meter to monitor the pH. A pH meter is a volt meter that measures the electrical potential between two electrodes. One electrode is in contact with your solution, and the other is in contact with a reference solution. Usually both of these electrodes are combined in a single pH probe that you place in your solution. These meters can read to the nearest 0.1 pH unit, but require frequent calibration with reference buffers of known pH.