Hydration, glass electrode

The concentration of the solution within the glass bulb is fixed, and hence on the inner side of the bulb an equilibrium condition leading to a constant potential is established. On the outside of the bulb, the potential developed will be dependent upon the hydrogen ion concentration of the solution in which the bulb is immersed. Within the layer of dry glass which exists between the inner and outer hydrated layers, the conductivity is due to the interstitial migration of sodium ions within the silicate lattice. For a detailed account of the theory of the glass electrode a textbook of electrochemistry should be consulted. 

As indicator electrodes glass and antimony electrodes are commonly used, but it must be noted that in benzene-methanol solutions, a glass-antimony electrode pair may be used in which the glass electrode functions as reference electrode. Glass electrodes should not be maintained in non-aqueous solvents for long periods, as the hydration layer of the glass bulb may be impaired and the electrode will then cease to function satisfactorily. 

A glass membrane in an electrolyte solution cannot be taken to be a homogeneous system in the direction perpendicular to the surface. When the membrane is in contact with the solution, water molecules can enter it and form a 5-100 nm thick hydrated layer [319]. The formation of this hydrated layer is actually a condition for good functioning of the glass electrode. The basic characteristics of the glass structure probably do not change in the hydrated layer, but the cation mobility increases considerably compared with the compact membrane interior... 

This electrode, also called the glass electrode, is specific to H+ ions. Glass in this case does not refer to the material of the electrode body but to the membrane that ensures contact with the solution. The membrane is a thin wall of glass that has a very high sodium content (25%). In the presence of water, hydration occurs and the membrane s surface becomes comparable to a gel while its interior corresponds to a solid electrolyte. 

Normally, glass electrodes must be soaked in water for a few minutes to a few hours for the electrode to develop a pH response. This allows hydration of the outer surface to take place with the formation of the hydrogen ion-selective sites. Similarly, in 90°C pH tests at Pennsylvania State University, it has been found necessary to allow the zirconia electrodes to soak for several hours before a pH response was observed. 

Before using the pH electrode, it should be calibrated using two (or more) buffers of known pH. Many standard buffers are commercially available, with an accuracy of 0.01 pH unit. Calibration must be performed at the same temperature at which the measurement will be made care must be taken to match the temperature of samples and standards. The exact procedure depends on the model of pH meter used. Modern pH meters, such as the one shown in Figure 5.8, are microcomputer-controlled, and allow double-point calibration, slope calculation, temperature adjustment, and accuracy to +0.001 pH unit, all with few basic steps. The electrode must be stored in an aqueous solution when not in use, so that the hydrated gel layer of the glass does not dry out. A highly stable response can thus be obtained over long time periods. As with other ion-selective electrodes, the operator should consult the manufacturer s instructions for proper use. Commercial glass electrodes are remarkably... 

The adsorption and desorption of metals onto the surface-exposed sorption sites of clay particles can approach equilibrium in seconds to minutes in well-mixed streamwaters. Similarly, the response of a pH electrode, which often stabilizes in a few seconds in stirred solutions, reflects the rapid rate of equilibration of aqueous protons with protons adsorbed on the hydrated surface of the glass electrode. 

The pH glass electrode functions as a result of ion exchange on the surface of a hydrated layer. The membrane of a pH glass electrode consists of chemically... 

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