Alkali errors

Figure 23.8 shows the readings of a glass electrode [the measured values of of a cell of the type (23.5)] as a function of solution pH. In the range from acidic to neutral solutions, this curve perfectly obeys Eq. (23.7) (i.e., the potential varies linearly by 0.06 V per unit of pH). However, in alkaline solutions the curve departs from this function ( alkali error of the glass electrode ) in strongly alkaline solutions the readings of the electrode are practically independent of solution pH. This is due to violation of the selectivity conditions. At a pH value of 10 and a sodium ion... 

Addition of an alkali metal oxide as a "network modifier to the "network former causes pH sensitivity, i.e., small amounts of alkali metal induce superficial gel layer formation as a merely local chemical attack and so with limited alkali error larger amounts will result in more pronounced dissolving properties of the glass up to complete dissolution, e.g., water-glass with large amounts of sodium oxide. Simultaneous addition of an alkaline earth metal oxide, however, diminishes the dissolution rate. Substitution of lithium for sodium in pH-sensitive glass markedly reduces the alkali error. 

Alkali error occurs when cations (typically Na+) replace H+in the gel layer and thus pH values lower than the actual are measured. 

If the concentration of hydrogen ions is equal to or larger than that of the hydroxide ions, this effect can be ignored. At pH values of over 10, however, the concentration of hydrogen ions is so small that the pH values measured are too low, as the glass membrane only responds to the hydroxide ions. This effect is called the alkali error. Alkali-proof glass electrodes do exist in which the alkali error is low. (For measurement of pH see also... 

The investigations of Eisenman et al. [16] concerning the alkali error of pH glass electrodes led to the development of glasses of the aluminum silicate series, which in addition to a sensitivity for hydrogen ions show a high specificity for sodium ions. Typical sodium-selective glass compositions are, for example ... 

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