Glass electrode constant

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. 

Ionic Equilibria.. The ion product constant of D2O (see Table 3) is an order of magnitude less than the value for H2O (24,31,32). The relationship pD = pH + 0.41 (molar scale 0.45 molal scale) for pD ia the range 2—9 as measured by a glass electrode standardized ia H2O has been established (33). For many phenomena strongly dependent on hydrogen ion activity, as is the case ia many biological contexts, the difference between pH and pD may have a large effect on the iaterpretation of experiments. 

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. 

This technique with two indicator electrodes, proposed in 1956 by Dubois and Walisch146, is an intermediate between bipotentiometry and biamperometry, because neither the current nor the potential across the electrodes of the same metal are kept strictly constant. The authors opinion, e.g., in the titration of iodide with bromate in hydrochloric acid, that the AE value does not matter much, has been contradicted by Kies (ref. 141a, p. 18). As a method of alkalimetry or acidimetry it cannot be preferred, like any other dead-stop technique to the usual glass electrode methods147. Nevertheless, the fact that the apparatus permits a choice of adjustment towards constant current or constant potential can be useful, but then the method approaches either bipotentiometry or biamperometry. 

Oumada et al. [148] described a new chromatographic method for determining the aqueous pKa of dmg compounds that are sparingly soluble in water. The method uses a rigorous intersolvent pH scale in a mobile phase consisting of a mixture of aqueous buffer and methanol. A glass electrode, previously standardized with common aqueous buffers, was used to measure pH online. The apparent ionization constants were corrected to a zero-cosolvent pH scale. Six sparingly soluble nonsteroidal antiinflammatory weak acids (diclofenac, flurbiprofen, naproxen, ibu-profen, butibufen, fenbufen) were used successfully to illustrate the new technique. 

For current practice, the described method of pH measurement is too tedious. Moreover, not hydrogen but glass electrodes are used for routine pH measurements (see Section 6.3). Then the expression for the EMF of the cell consisting of the glass and reference electrodes contains a constant term from Eq. (6.3.10), in addition to the terms present in Eq. (3.3.3) this term must be obtained by calibration. Further, a term describing the liquid junction potential between the reference electrode and the measured solution must also be included. 

The membrane of the glass electrode is blown on the end of a glass tube. This tube is filled with a solution with a constant pH (acetate buffer, hydrochloric acid) and a reference electrode is placed in this solution (silver chloride or calomel electrodes). During the measurement, this whole system is immersed with another reference electrode into the test solution. The membrane potential of the glass electrode, when the internal and analysed... 

To some extent, the constant K is a function of the area of glass in contact with the acid analyte. For this reason, no two glass electrodes will have the same value of K... 

SO Electrode. A gas-sensing SO2 electrode marketed by Ionics, Inc. was used to provide additional VLE data at 25°C as a function of composition. Aqueous SO2 equilibrates across a polymeric membrane with a filling solution containing about 0.1 M NaHSO-j. Ionic species do not diffuse across the membrane. A small combination glass electrode measures the pH of the filling solution. The SO2 activity (Pso ) is proportional to the activity of H+ (10"PH), because the bisulfite activity is constant ... 

The electromotive force (emf) of liquid membrane electrodes depends on the activity of the ions in solution and their performance is similar in principle to that of the glass electrode. To characterize the behavior of liquid membrane electrodes, the linearity of the emf measurements vs. concentration of a certain ion in solution is checked. Additional performance data are the Nernstian slope of the linear range and the pH range over which the potential of the electrode is constant. 

This technique uses both direct and back titrations of weak acids and bases. Values of are obtained directly. In purely aqueous media, over the pH range 2-10, the titration of dilute (0.005 to 0.05 M) solutions of weak monovalent acids and bases with a glass electrode can lead to reliable thermodynamic pKs. Over this pH interval, the activity coefficients of the ionic species can be calculated by means of the Debye-Hiickel equation. Also, the activity coefficients of the neutral species remain essentially constant and... 

The sensor for ion solvation was a silver wire for Ag+ and an univalent cation-sensitive glass electrode for Li+ and Na+. In detennining AG (M+, R -> S) from Eq. (6.13), they assumed that the LJPs at PC/(PC + DMSO) and PC/DMSO were negligible (Ej = 0). At the same time, they detennined in PC the step-wise formation constants (/ ) for the complexation of Ag+, Li+ and Na+ with DMSO (see Section 6.3.3), and used them to calculate the Gibbs energies of transfer of those ions from PC to (PC + DMSO) and to neat DMSO. Equations (6.14) and (6.15) were used in the calculation ... 

A potentiometric method for determination of ionization constants for weak acids and bases in mixed solvents and for determination of solubility product constants in mixed solvents is described. The method utilizes glass electrodes, is rapid and convenient, and gives results in agreement with corresponding values from the literature. After describing the experimental details of the method, we present results of its application to three types of ionization equilibria. These results include a study of the thermodynamics of ionization of acetic acid, benzoic acid, phenol, water, and silver chloride in aqueous mixtures of acetone, tetrahydrofuran, and ethanol. The solvent compositions in these studies were varied from 0 to ca. 70 mass % nonaqueous component, and measurements were made at several temperatures between 10° and 40°C. 

We have recently devised a rapid and convenient method for determination of the ionization constant for water in mixed aqueous organic solvents (11-16). The method utilizes glass electrodes and gives results in satisfactory agreement with earlier work. 

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