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Measuring pH involve either the use of pH measuring electrodes or indicators whose colors are dependent on pH. A pH meter measures the difference in potential between a reference electrode insensitive to changes in pH and an electrode sensitive to such changes. pH indicators based on color changes are normally used in the form of pH papers. The paper is wetted with the solution being measured and the resulting color is compared with color standards to determine the pH. (reference)
A successful pH reading is dependent upon all components of the system being operational. Problems with any one of the three: electrode, meter or buffer will yield poor readings.
Electrodes: A pH electrode consists of two half-cells; an indicating electrode and a reference electrode. Most applications today use a combination electrode with both half cells in one body. Over 90% of pH measurement problems are related to the improper use, storage or selection of electrodes.
Meters: A pH meter is in reality a high precision and high impedance voltmeter capable of reading small millivolt changes from the pH electrode system. The meter is seldom the source of problems for pH measurements. Modern pH meters have temperature compensation (either automatic or manual) to correct for variations in slope caused by changes in temperature.
Buffers: These solutions of known pH value allow the user to adjust the system to read accurate measurements.
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