the left and right, any potentials generated at the interfaces cancel each other (in principle), resulting in the system voltage being dependent only on the interaction of the glass membrane and the test solution. The measuring part of the electrode, the glass bulb on the bottom, is coated both inside and out with a ~10 nm layer of a hydrated gel. These two layers are separated by a layer of dry glass. The silica glass structure (that is, the conformation of its atomic structure) is shaped so that it allows Na+ ions some mobility. The metal cations (Na+) in the hydrated gel diffuse out of the glass and into solution while H+ from solution can diffuse into the hydrated gel. It is the hydrated gel which makes the pH electrode an ion-selective electrode. H+ does not cross through the glass membrane of the pH electrode, it is the Na+ which crosses and leads to a change in free energy. When an ion diffuses from a region of activity to another region of activity, there is a free energy change and this is what the pH meter actually measures. The hydrated gel membrane is connected by Na+ transport and thus the concentration of H+ on the outside of the membrane is 'relayed' to the inside of the membrane by Na+. All glass pH electrodes have extremely high electric resistance from 50 to 500 MΩ. Therefore, the glass electrode can be used only with a high input-impedance measuring device like a pH meter, or, more generically, a high input-impedance voltmeter which is called an electrometer. Limitations The glass electrode has some inherent limitations due to the nature of its construction. Acid and alkaline errors are discussed above. An important limitation results from the existence of asymmetry potentials that are present at glass/liquid interfaces. The existence of these phenomena means that glass electrodes must always be calibrated before use; a common method of calibration involves the use of standard buffer solutions. Also, there is a slow deterioration due to diffusion into and out of the internal solution. These effects are masked when the electrode is calibrated against buffer solutions but deviations from ideal response are easily observed by means of a Gran plot. Typically, the slope of the electrode response decreases over a period of months. Storage Between measurements any glass and membrane electrodes should be kept in a solution of its own