Thursday, January 15, 2009

pH Meter Calibration and Use



A pH meter is an electronic instrument used to measure the pH (acidity or alkalinity) of a liquid (though special probes are sometimes used to measure the pH of semi-solid substances). A typical pH meter consists of a special measuring probe (a glass electrode) connected to an electronic meter that measures and displays the pH reading.

Before use pH meter must be calibrated. Each pH electrode used for measurements is slightly different and its characteristic changes with aging. For very precise work the pH meter should be calibrated before and after each measurement. For normal use calibration should be performed at the beginning of each day. The reason for this is that the glass electrode does not give a reproducible e.m.f. over longer periods of time.

Calibration should be performed with at least two standard buffer solutions that span the range of pH values to be measured. For general purposes buffers at pH 4 and pH 10 are acceptable. The pH meter has one control (calibrate) to set the meter reading equal to the value of the first standard buffer and a second control (slope) which is used to adjust the meter reading to the value of the second buffer. A third control allows the temperature to be set. Standard buffer sachets, which can be obtained from a variety of suppliers, usually state how the buffer value changes with temperature.

pH meter calibration is more often referred to as pH electrode calibration, as parameters set are not device dependent, but electrode dependent. Correct procedure of the pH meter calibration is usually described in the pH meter manual. General outline of the procedure is presented in the pH electrode calibration section.

The calibration process correlates the voltage produced by the probe (approximately 0.06 volts per pH unit) with the pH scale. After each single measurement, the probe is rinsed with distilled water or deionized water to remove any traces of the solution being measured, blotted with a clean tissue to absorb any remaining water which could dilute the sample and thus alter the reading, and then quickly immersed in another solution. When not in use, the probe tip must be kept wet at all times. It is typically kept immersed in a an acidic solution of around pH 3.0. In an emergency, acidified tap water can be used, but distilled or deionised water must never be used for longer-term probe storage as the relatively ionless water 'sucks' ions out of the probe through diffusion, which degrades it.

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