Hquid junction

The measurement of pH using the operational ceU assumes that no residual Hquid-junction potential is present when a standard buffer is compared to a solution of unknown pH. Although this may never be stricdy tme, especially for complex matrices, the residual Hquid-junction potential can be minimised by the appropriate choice of a salt-bridge solution and caHbration buffer solutions. 

Other problems occur in the measurement of pH in unbuffered, low ionic strength media such as wet deposition (acid rain) and natural freshwaters (see Airpollution Groundwatermonitoring) (13). In these cases, studies have demonstrated that the principal sources of the measurement errors are associated with the performance of the reference electrode Hquid junction, changes in the sample pH during storage, and the nature of the standards used in caHbration. Considerable care must be exercised in all aspects of the measurement process to assure the quaHty of the pH values on these types of samples. 

Reference Electrodes and Liquid Junctions. The electrical cincuit of the pH ceU is completed through a salt bridge that usually consists of a concentrated solution of potassium chloride [7447-40-7]. The solution makes contact at one end with the test solution and at the other with a reference electrode of constant potential. The Hquid junction is formed at the area of contact between the salt bridge and the test solution. The mercury—mercurous chloride electrode, the calomel electrode, provides a highly reproducible potential in the potassium chloride bridge solution and is the most widely used reference electrode. However, mercurous chloride is converted readily into mercuric ion and mercury when in contact with concentrated potassium chloride solutions above 80°C. This disproportionation reaction causes an unstable potential with calomel electrodes. Therefore, the silver—silver chloride electrode and the thallium amalgam—thallous chloride electrode often are preferred for measurements above 80°C. However, because silver chloride is relatively soluble in concentrated solutions of potassium chloride, the solution in the electrode chamber must be saturated with silver chloride. 

Samples that contain suspended matter are among the most difficult types from which to obtain accurate pH readings because of the so-called suspension effect, ie, the suspended particles produce abnormal Hquid-junction potentials at the reference electrode (16). This effect is especially noticeable with soil slurries, pastes, and other types of colloidal suspensions. In the case of a slurry that separates into two layers, pH differences of several units may result, depending on the placement of the electrodes in the layers. Internal consistency is achieved by pH measurement using carefully prescribed measurement protocols, as has been used in the determination of soil pH (17). 

Other difficulties of measuring pH in nonaqueous solvents are the complications that result from dehydration of the glass pH membrane, increased sample resistance, and large Hquid-junction potentials. These effects are complex and highly dependent on the type of solvent or mixture used (1,5). 

Gronet C, Lewis NS (1983) Design of a 13% efficient n-GaAsi xPx semiconductor-hquid junction solar cell. Nature 300 733-735... 

The same reference electrode can be used to characterize electrodes in contact with different electrolytes therefore, the cell used to determine the electrode potential often includes a liquid junction (electrolyte-electrolyte interface). In this case the electrode potential is understood as being the corrected OCV value, which is the value for this cell after elimination of the hquid-junction potential. For instance, for the zinc electrode (2. b), the expression for the electrode potential can be written as... 

A simple system in which transport processes occur that are also characteristic of membrane processes is the Hquid junction formed between two electrolyte solutions in the same solvent. The region in which one electrolyte passes into the other is frequently a porous diaphragm of various construction (fig. 2.2). A second type of Hquid junction is the free diffusion region (fig. 2.3). 

The only process occurring in a Hquid junction is the diffusion of various components of the two solutions in contact with it. The various mobilities of the ions present in the Hquid junction lead to the formation of an electric potential gradient, termed the diffusion potential gradient. A potential difference, termed the liquid-junction potential, A0x,. is formed between two solutions whose composition is assumed to be constant outside the Hquid junction. 

Bard A. J., Bocarsly A. B., Fan F. F., Walton E. G. and Wrighton M. S. (1980), The concept of Fermi level pinning at semiconductor/hquid junctions. Consequences for energy conversion efficiency and selection of useful redox couples in solar devices , J. Am. Chem. Soc. 102, 3671-3677. 

Prokopuk N. and Lewis N. S. (2004), Energetics and kinetics of interfacial electron-transfer processes at chemically modified InP/hquid junctions , J. Phys. Chem. B 108, 4449-4456. 

Parkinson B. A., Heller A. and Miller B. (1979), Effects of cations on the performance of the photoanode in the -GaAs/K2Se-K2Se2-KOH/C semiconductor hquid junction solar cell , J. Electrochem. Soc. 126, 954-960. 

The cell Zn(s) ZnCl2(aq) CuCl2(aq) Cu(s) has been used to illustrate the hquid junction using a narrow tube. It has a positive emf. Explain the direction of migration of the ions... 

The residual liquid-junction potential, combined with the uncertainty in the standard buffers, limits the absolute accuracy of measurement of pH of an unknown solution to about 0.02 pH unit. It may be possible, however, to discriminate between the pH of two similar solutions with differences as small as 0.004 or even 0.002 pH units, although their accuracy is no better than 0.02 pH units. Such discrimination is possible because the hquid-junction potentials of the two solutions will be virtually identical in terms of true a. For example, if the pH values of two blood solutions are close, we can measure the difference between them accurately to 0.004 pH. If the pH difference is fairly large, however, then the residual hquid-junction potential will increase and the difference cannot be measured as accurately. For discrimination of 0.02 pH unit, large changes in the ionic strength may not be serious, but they are important for smaller changes than this. 

The chemical reaction of Eq. (13-145) is an oxidation which involves direct transfer of the released electrons to the metal electrode. If the hquid-junction potential is neglected, the electromotive force of the cell of Eq. (13-144) may be written in the form... 

Quasi-elastic Laser Scattering (QELS) provides a versatile tool for monitoring in situ the frequency of thermally induced capillary waves at the Uquid/hquid junction [26, 27]. Unlike the previous techniques based on an absorption/fluores-cence signal arising from molecular probes at the interface, QELS is not sensitive to specific molecules but to the interfacial tension of the molecular junc-... 

A practical limitation of a different kind arises in measurements when the pH is less than 3 or more than 11. The hquid junction potential of the common pH cell shows a great variability under these conditions, preventing, for example, the use of potassium tetroxalate (pH = 1.675 at 20°C) as a primary pH standard. 

As mentioned above, electrons are injected into the p-doped area and holes into the n-doped area of the semiconductor if a p-n junction is under forward bias (Figure 2.9). In the case of some semiconductors, for example, GaAs or GaP, the subsequent recombination of the minority carriers occurs with light emission. This effect is used in so-called light-emitting diodes. Emission effects have also been observed with semiconductor-hquid junctions and corresponding recombination effects in semiconductors are treated in Section 7.5. 

In the proposed standardization procedure, we assume that the contribution fiom the isothermal hquid junction potential (ITLJP) of the reference electrode to the cell potential, due to interfacing the internal reference electrode solution with the external solution, is negligible or has been calculated with sufficient accuracy The concentration of hydrogen ion in the HCl solution niH+), for example, may be estimated by considering the HCl dissociation reaction, ... 

As noted above, an external pressure-balanced electrode suffers from two irreversible thermodynamic potentials the isothermal liquid junction potential and the thermal hquid junction potential (thermal diffusion potential). The external pressure balanced reference electrode (EPBRE) is amenable to formal thermodynamic analysis in terms of Thermocell I ... 

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