|Glass Electrode for pH Measurement|
pH-glass electrodes respond to hydrogen ions, i.e. they are ion selective electrodes for H+. The selective measuring element is a glass membrane of special composition. Before the electrode is ready for measuring, the glass membrane is watered and thus forms a thin, gel-type swelling layer. The swelling process is completed in 24-28 h at room temperature. The formation of this swelling layer is accelerated at higher temperatures. It can be damaged in abrasive media, organic solvents and F--containing solutions. A combined glass electrode is illustrated in Fig.1-2.
Fig. 2 Glass electrode and various potentials in a connecting chain; Ea is a H+ activity dependent potential.
A few important terms which always appear in connection with pH-measurements are explained in the following:
If the glass electrode is immersed into a measuring solution which is identical to the inner buffer, then theoretically the measured voltage should be zero; the condition is that the measuring chain is symmetrical, i.e. its outer reference electrode and its inner working electrode are identical. The deviation that occurs of a few mV is called asymmetric voltage and depends on the construction of the measuring chain. The inner buffer mostly has a pH-value of 7; at pH =7 theoretically 0 mV should be measured, i.e. the cell zero point is at pH 7.
In strong alkali solutions measuring errors can arise because alkali ions contribute to the voltage besides the hydrogen ions. A pH-value which is too low is then simulated.
The state of the pH-measuring chain is subject to temporary fluctuations. Therefore, depending on the accuracy required, the pH should be periodically calibrated, e.g. daily. In this procedure, the actual characteristic line of the electrode in the form of the asymmetric voltage and the Nernst slope are made to cover the characteristic line of the pH-meter (Fig. 3).
Fig. 3 Electrode calibration: 1) asymmetric and 2) slope calibration.
Adjusting the asymmetric voltage (given as Uas or pHas): Measuring chain first immersed in buffer pH7 adjust with potentiometer Uas.
Adjustment of the Nernst slope: (is often given as relative slope, e.g. rel. slope = 0.98 or 98%)
Measuring chain immersed in the second buffer (pH = 7). The pH-value of this second buffer should be chosen in the region of the samples be measured. Adjust pH-value of the buffer potentiometer slope.With microprocessor controlled pH-meters, the order of the buffer solutions is not important as a calibration line is calculated from the measured data.
One uses for the pH-calibration buffer, solutions with known pH-values. The pH-value of these buffer solutions is of course dependent on the temperature. For exact measurements, the calibration temperature should be the same as the measuring temperature.