Glass alkaline error

The user must be alert to some shortcomings of the glass pH electrode. For example, in solutions of pH 11 or more, the electrode shows a so-called alkaline error in which it responds also to changes in the level of alkali metal ions (particularly sodium) ... 

Because soda-glass membranes contain a high proportion of sodium ions, they exhibit a marked response to sodium ions in solution. The effect becomes increasingly significant as the hydrogen ion activity decreases, i.e. at high pH, and it is sometimes referred to as the alkaline error. At pH 12, the error is about 0.3 of a pH unit if the solution is 0.1 M with respect to sodium ions, and 0.7 of a pH unit if the solution is 1 M in sodium ions. Other monovalent cations such as lithium and potassium have a similar but smaller effect. By replacing the sodium in... 

Figure 3.12 Demonstration of the extent of the alkaline error, showing plots of the deviations in pH measurement under aqueous alkaline conditions as determined with a standard glass electrode (the Coming 015 ). The figures in parentheses represent the concentrations of the metal hydroxide salt in mol dm". From Christian, G. D., Analytical Chemistry, 5th Edn, Wiley, 1994. Reprinted by permission of John Wiley Sons, Inc.

Figure 15-J5 Acid and alkaline errors of some glass electrodes. A Coming 015, H2S04. B. Corning 015, HCI. C Corning 015,1 M Na+...

Thus, online measurements of composition are usually limited to some overall property. A typical example is pH, defined as the absolute value of the logarithm of the molar concentration (or, more exactly, activity) of hydrogen ion pH can be measured by exploiting the electric potential established between two proper electrodes immersed in the sample fluid, usually a glass membrane electrode and a reference electrode [15], Notwithstanding the temperature dependence and the alkaline error (at high pH, a marked sensitivity to the effect of Na+ and of other monovalent... 

The alkaline error , often found in pH electrodes, arises because in very alkaline solution [Na+] or [K+] is normally very high, making a significant extra contribution to the potential as expressed through the Henderson equation (13.10). Minimization of this error is done by using glass of special composition and with very low selectivity coefficients for Na+ and K+. 

The alkaline error can be satisfactorily explained by assuming an exchange equilibrium between the hydrogen ions on the glass surface and the cations in solution. This process is simply the reverse of that shown in Equation 21-6 ... 

The alkaline error. The ordinary glass electrode becomes somewhat sensitive to alkali metal ions and gives low readings at pH values greater than 9. 

The alkaline error arises when a glass electrode is employed to measure the pH of solutions having pH values in the 10 to 12 range or greater. In the presence of alkali ions, the glass surface becomes responsive to not only hydrogen ions but also alkali metal ions. Measured pH values are low as a result. 

Alkaline error The tendency of many glass electrodes to provide an anomalously low pH response in highly alkaline environments. 

What is the alkaline error and the acid error of a glass membrane pH electrode ... 

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