Glass electrodes acid error

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. °... 

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.)... 

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... 

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+...

A hydrolytic reaction that releases acid may be followed by titration with base. This is best done automatically by use of the pH-stat. A glass electrode registers the pH of the solution, which is kept constant by the automatic addition of base from a syringe controlled by an electronic circuit. Reaction volumes as low as 1 mL may be used, and the limit of detectability is about 50 nmol (5 to 10 /tL of base at 5 X 10-3 to 1 X 10-2 M). The usual source of error with this apparatus is the buffering effect of dissolved C02. 

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. 

Where a pH meter is standardized by use of an aqueous buffer and then used to measure the "pH" of a nonaqueous solution or suspension, the ionization constant of the acid or base, the dielectric constant of the medium, the liquid-junction potential (which may give rise to errors of approximately 1 pH unit), and the hydrogen ion response of the glass electrode are all changed. For these reasons, the values so obtained with solutions that are only partially aqueous in character can be regarded only as apparent pH values. 

R0.13.B. Error of glass electrode in hydrochloric acid solutions. (From L. Meites and L. C. Thomas, Advanced Analytical Chemistry. Copyright 1958, McGraw-Hill, New York. Used with permission of McGraw-Hill Company.)... 

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

FIGURE 23-7 Acid and alkaline error of selected glass electrodes at 25°C. (From R. G. Bates, Determination of pH, 2nd ed,. p. 365. New York Wiley, 1973. yVilh permission.)... 

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