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UCL Home  /  Geography  /  Resources  /  Laboratory  /  Laboratory Methods  /  Water Analysis  /  pH Measurement

pH Measurement

The measurement of pH is performed electrometrically using a pH meter with either a general purpose combination electrode or a low ionic combination electrode, depending upon the ionic concentration of the sample. In low ionic strength solutions a standard pH electrode will give a sluggish response and poor reproducibility. However a low ionic electrode must not be used in solutions of high ionic strength (above 50uS) as this will result in damage to the electrode.

The electrode should never be allowed to dry out and therefore must be returned to a beaker or bottle of either pH4 buffer or standard electrode filling solution between measurements. In the absence of either of these solutions, distilled water can be used, but the electrode should not be stored for any great length of time in this.

The principal of the probe requires the glass electrode to adsorb a layer of the sample onto its surface; the resultant potential difference being a function of the hydrogen ion (H+) concentration in the sample and the electrolyte contained within the electrode (Wetzel and Likens, 1991).

Prior to measurement the pH meter is calibrated using a freshly made buffer solution (pH7) and the slope of the electrode adjusted against a pH4 buffer. Temperature compensation is adjusted manually according to the ambient sample temperature. The electrode is thoroughly rinsed with distilled water before each measurement.

Water samples should be collected in a clean glass beaker, well flushed with the sample, and the electrode allowed to stand for several minutes without agitation before the pH value is determined.