Glass acid error

FIGURE 5-7 The alkaline and acid errors of several glass pH electrodes. A, Corning 015/H2SO4 B, Corning 015/HC1 C, Coming 015/1 M Na+ D, Beckman-GP/1 M Na+ E, L N BlackDot/lM Na+ F, Beckman E/1M Na+ G, Ross electrode. (Reproduced with permission from reference 16.)... 

Acid error. In strong acid, the measured pH is higher than the actual pH, perhaps because the glass is saturated with H+ and cannot be further protonated (Figure 15-15). 

The acid error. Values registered by the glass electrode tend to be somewhat high when the pH is less than about 0.5. 

Acid error The tendency of a glass electrode to register anomalously high pH response in highly acidic media. 

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

Glass electrodes also show an error in extremely acidic solutions (pH < 0.5). The acid error is in the opposite direction to the aUcaline error the measured pH values are too high. The cause of the acid error is not understood. 

For electromotive force measurements there are many suitable electrodes, and only the most novel features can be summarised here. The glass electrode is generally usable in these solvents. It has been found that in solutions of hydrochloric acid in anhydrous acetic acid, the glass electrode is subject to acid errors of as much as 70 mV relative to the chloranil electrode, owing to the incorporation of chloride ions in the surface gel layer of the glass. ° Since ethylenediamine reacts with calomel but not with mercury (II) chloride, a suitable reference electrode for this solvent can be constructed of a mercury pool in contact with ethylenediamine saturated with respect to both HgClg and LiCl. °... 

For loiu pH streams, the concern is the acid error and dehydration of the glass surface from an acidic environment. While high temperatures do not greatly increase the acid error, extremely hot acidic streams, dramatically decreases the glass s life expectancy. 

In addition to these challenges, pH membrane electrodes may exhibit deviations from Nemstian behavior in alkaline and acid media. This depends on the glass composition and is established by the manufacturer. Alkaline error arises from ion-exchange with other (alkaline) cations, while acid error originates from acid extraction into the glass gel layer. These processes are analogous to ionophore-based membranes, which are treated in some detail below. 

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