Glass electrodes potential difference

The origin of the glass electrode potential is not discussed here, but it may be helpful to mention that the glass membrane functions as a cation exchanger and that a Nemst potential is observed if such a membrane separates two solutions at two different concentrations ... 

As a result of a variable liquid-junction potential, the measured pH may be expected to differ seriously from the determined from cells without a liquid junction in solutions of high acidity or high alkalinity. Merely to affirm the proper functioning of the glass electrode at the extreme ends of the pH scale, two secondary standards are included in Table 8.14. In addition, values for a 0.1 m solution of HCl are given to extend the pH scale up to 275°C [see R. S. Greeley, Anal. Chem. 32 1717 (I960)] ... 

When first developed, potentiometry was restricted to redox equilibria at metallic electrodes, limiting its application to a few ions. In 1906, Cremer discovered that a potential difference exists between the two sides of a thin glass membrane when opposite sides of the membrane are in contact with solutions containing different concentrations of H3O+. This discovery led to the development of the glass pH electrode in 1909. Other types of membranes also yield useful potentials. Kolthoff and Sanders, for example, showed in 1937 that pellets made from AgCl could be used to determine the concentration of Ag+. Electrodes based on membrane potentials are called ion-selective electrodes, and their continued development has extended potentiometry to a diverse array of analytes. 

Two methods are used to measure pH electrometric and chemical indicator (1 7). The most common is electrometric and uses the commercial pH meter with a glass electrode. This procedure is based on the measurement of the difference between the pH of an unknown or test solution and that of a standard solution. The instmment measures the emf developed between the glass electrode and a reference electrode of constant potential. The difference in emf when the electrodes are removed from the standard solution and placed in the test solution is converted to a difference in pH. Electrodes based on metal—metal oxides, eg, antimony—antimony oxide (see Antimony AND ANTIMONY ALLOYS Antimony COMPOUNDS), have also found use as pH sensors (8), especially for industrial appHcations where superior mechanical stabiUty is needed (see Sensors). However, because of the presence of the metallic element, these electrodes suffer from interferences by oxidation—reduction systems in the test solution. 

This method involves very simple and inexpensive equipment that could be set up m any laboratory [9, 10] The equipment consists of a 250-mL beaker (used as an external half-cell), two platinum foil electrodes, a glass tube with asbestos fiber sealed m the bottom (used as an internal half-cell), a microburet, a stirrer, and a portable potentiometer The asbestos fiber may be substituted with a membrane This method has been used to determine the fluoride ion concentration in many binary and complex fluondes and has been applied to unbuffered solutions from Willard-Winter distillation, to lon-exchange eluant, and to pyrohydrolysis distil lates obtained from oxygen-flask or tube combustions The solution concentrations range from 0 1 to 5 X 10 M This method is based on complexing by fluonde ions of one of the oxidation states of the redox couple, and the potential difference measured is that between the two half-cells Initially, each cell contains the same ratio of cerium(IV) and cerium(tll) ions... 

Consider the apparatus shown in Figure 14-3. A glass tube is fitted with electrodes so that a potential difference of 10,000 volts can be applied across a space filled with a desired gas at various pressures. Suppose neon, for example, is placed in the tube. With the voltage applied, the gas will begin to conduct electricity when its pressure is reduced to about 0.01 atmosphere. The tube then glows with the familiar color of a neon sign. If a different gas is used, the color is different, but otherwise, the behavior is about the same. If the pressure is reduced still further to about 10 atmosphere, the glow from the gas... 

Logarithmic scale for expressing acidity or alkalinity of water (7.0 to 0 indicates increasing acidity 7.0 to 14 indicates increasing alkalinity). Measured by means of a glass electrode/reference electrode pair immersed in the water sample under test. The potential difference depends upon the pH which is then displayed on a pH meter (high input impedance, millivoltmeter). 

The principle of pH electrode sensing mechanisms which are based on glass or polymer membranes is well investigated and understood. Common to all potentiometric ion selective sensors, a pH sensitive membrane is the key component for a sensing mechanism. When the pH sensitive membrane separates the internal standard solution with a constant pH from the test solution, the potential difference E across the membrane is determined by the Nemst equation ... 

Selective ion electrodes (SIE). Selective ion electrodes are essentially variants of the well-known pH meter. They are membrane indicator types of electrodes in which a potential is developed across a membrane in the presence of the ion the size of the potential is related to the concentration and hence can be used to quantitatively detect and measure the species. However, instead of a glass membrane, as in the pH meter, the membranes consist of organics that are immersible in water. For example, anion-sensitive electrodes use a solution of an anion exchange resin in an organic solvent the liquid can be held in the form of a gel, for example, in polyvinyl chloride. The ion reacts with the organic membrane, setting up an equilibrium between the free ion in solution and the ion bound to the membrane, generating a potential difference, which is measured. 

When the concentration of H+ is different on either side of the membrane, a potential difference is generated, which is related to the activity of H+ ions in solution, i.e. pH. The latter is determined using an electronic millivoltmeter, the pH meter, which monitors the potential difference between the glass electrode and an internal reference electrode of Ag/AgCl (currently preferred to the mercurous chloride (Hg) electrode for environmental purposes). After calibration, the instrument will directly yield the pH of a solution. 

Figure 18.2—Measurement of pH. The concentration of H+ ions can be determined from the potential difference between the reference electrode and the glass electrode. Details of the membrane, which is permeable to the H1 ion, are shown. When an H+ ion forms a silanol bond, a sodium ion moves into the solution to preserve electroneutrality. A cross-section of the membrane showing this exchange reaction is presented (IUPAC conventions are not followed to improve clarity in the diagram). Prior to its use, the pH meter is calibrated with a buffer solution of known pH.

A difference of 59.16 mV (at 25°C) builds up across a glass pH electrode for every factor-of-10 change in activity of H+ in the analyte solution. Because a factor-of-10 difference in activity of H+ is 1 pH unit, a difference of, say, 4.00 pH units would lead to a potential difference of 4.00 X 59.16 = 237 mV. The charge of a calcium ion is n = 2, so a potential difference of 59.16/2 = 29.58 mV is expected for every factor-of-10 change in activity of Ca2+ in the analyte measured with a calcium ion-selective electrode. 

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