Liquid-junction potential, increase

Gutbezahl and Grunwald considered liquid-junction potentials between a solution of aqueous potassium chloride and solutions of acids in ethanol-water mixtures both theoretically and experimentally. They concluded that for mixtures containing up to 33% ethanol the liquid-junction potential should be 6 mV or less. For solvents containing higher percentages of alcohol, the liquid-junction potential increases rapidly—25 mV for 50%, 44 mV for 65%, and 75 mV for 80% ethanol. These numerical values should not be interpreted too literally, particularly as the composition approaches 100% ethanol. Calculated liquid-junction potentials contain an indeterminate term that involves all quantities other than those arising from unequal transfer activity coefficients (such as dipole orientation effects). 

Changes in ionic strength will cause changes in the liquid junction potential of the reference electrode. If the concentration of positive ions increases, the liquid junction potential increases and shifts the isopotential point up and the pH downscale. If the concentration of negative ions increases, the liquid junction potential decreases or becomes more negative and shifts the isopotential point down and the pH upscale. 

In fact, some care is needed with regard to this type of concentration cell, since the assumption implicit in the derivation of A2.4.126 that the potential in the solution is constant between the two electrodes, caimot be entirely correct. At the phase boundary between the two solutions, which is here a semi-pemieable membrane pemiitting the passage of water molecules but not ions between the two solutions, there will be a potential jump. This so-called liquid-junction potential will increase or decrease the measured EMF of the cell depending on its sign. Potential jumps at liquid-liquid junctions are in general rather small compared to nomial cell voltages, and can be minimized fiirther by suitable experimental modifications to the cell. 

The results obtained with ISEs have been compared several times with those of other methods. When the determination of calcium using the Orion SS-20 analyser was tested, it was found that the results in heparinized whole blood and serum were sufficiently precise and subject to negligible interference from K and Mg ([82]), but that it is necessary to correct for the sodium error, as the ionic strength is adjusted with a sodium salt [82], and that a systematic error appears in the presence of colloids and cells due to complexa-tion and variations in the liquid-junction potential [76]. Determination of sodium and potassium with ISEs is comparable with flame photometric estimation [39, 113, 116] or is even more precise [165], but the values obtained with ISEs in serum are somewhat higher than those from flame photometry and most others methods [3, 25, 27, 113, 116]. This phenomenon is called pseudohyponatremia. It is caused by the fact that the samples are not diluted in ISE measurement, whereas in other methods dilution occurs before and during the measurement. On dilution, part of the water in serum is replaced by lipids and partially soluble serum proteins in samples with abnormally increased level of lipids and/or proteins. 

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 liquid-junction potentials. These effects are complex und highly dependent on the type of solvent or mixture used... 

The primary and the secondary buffers are separated by a liquid junction device, preferably a glass disk of fine porosity. Under these conditions, the contribution of the liquid junction potential to the cell voltage is very small. The increase in uncertainty is also very small. 

The half-wave potentials (corrected for changes in liquid-junction potential) for the one-electron reduction of aromatic hydrocarbons generally become more positive (the reduction is easier) as the dielectric constant of the solvent increases.44 This is in accord with the direction of the variation in solvation energy of the radical anions that is predicted by the simple Bom theory... 

The EMF of a cell with transference is increased or decreased, as compared with the EMF of a cell without transference, by the value of the liquid junction potential existing at the interface of both solutions according to the sign of this and to the sign of the electrode potentials. 

The influence of the relative values of the transference numbers, affecting the resultant value of the EMF of the concentration cell without transference, is clearly to be seen from the equation (VI-29) should t.. > <+ then eK is positive and in a concentration cell reversible with respect to cations the liquid junction potential is added to the sum of the electrode potentials should, however, < t+, then the liquid junction potential will lower the resultant EMF. In a concentration cell reversible with respect to anions (e. g. in a cell with chlorine electrodes) the EMF is decreased when ( >(+, and increased when t. < t+. 

Interface between two liquid solvents — Two liquid solvents can be miscible (e.g., water and ethanol) partially miscible (e.g., water and propylene carbonate), or immiscible (e.g., water and nitrobenzene). Mutual miscibility of the two solvents is connected with the energy of interaction between the solvent molecules, which also determines the width of the phase boundary where the composition varies (Figure) [i]. Molecular dynamic simulation [ii], neutron reflection [iii], vibrational sum frequency spectroscopy [iv], and synchrotron X-ray reflectivity [v] studies have demonstrated that the width of the boundary between two immiscible solvents comprises a contribution from thermally excited capillary waves and intrinsic interfacial structure. Computer calculations and experimental data support the view that the interface between two solvents of very low miscibility is molecularly sharp but with rough protrusions of one solvent into the other (capillary waves), while increasing solvent miscibility leads to the formation of a mixed solvent layer (Figure). In the presence of an electrolyte in both solvent phases, an electrical potential difference can be established at the interface. In the case of two electrolytes with different but constant composition and dissolved in the same solvent, a liquid junction potential is temporarily formed. Equilibrium partition of ions at the - interface between two immiscible electrolyte solutions gives rise to the ion transfer potential, or to the distribution potential, which can be described by the equivalent two-phase Nernst relationship. See also - ion transfer at liquid-liquid interfaces. 

Izutsu et al. studied the compiexing of Na" in acetonitrile solution with various protic and aprotic solvents using an ion-sensitive glass electrode. Parker s assumption of negligible liquid junction potential with an tetraethylammonium picrate salt bridge was adopted and found to be valid, even when water was added. The formation constants increased in the order methanol < H2O < DMF < NJ -dimethylace-tamide DMSO < HMPA. 

III. Free Diffusion Junction.—The free diffusion type of boundary is the simplest of all ir. practice, but it has not yet been possible to carry out an exact integration of equation (41) for such a junction. In setting up a free diffusion boundary, an initially sharp junction is formed between the two solutions in a narrow tube and unconstrained diffusion is allowed to take place. The thickness of the transition layer increases steadily, but it appears that the liquid junction potential should be independent of time, within limits, provided that the cylindrical symmetry at the junction is maintained. The so-called static junction, formed at the tip of a relatively narrow tube immersed in a wdder vessel (cf. p. 212), forms a free diffusion type of boundary, but it cannot retain its cylindrical symmetry for any appreciable time. Unless the two solutions contain the same electrolyte, therefore, the static type of junction gives a variable potential. If the free diffusion junction is formed carefully within a tube, however, it can be made to give reproducible results. ... 

Liquid junction potentials may arise from the transfer of ionic species through the transition region. The liquid junction potential makes a contribution to the emf of the cell it increases with increasing difference between the two solutions that form a single Junction. The liquid junction potential can often be kept small by using a concentrated salt bridge. 

An increasing interest in nonaqueous media, which began in the seventies of the XXth century, also resulted in the extended studies on the properties of the relevant Hg[solvent interface. Most of the papers discussing the dependence of Epzc of Hg on the solvent used were published in the eighties [2]. The pzc values for selected solvents, including water for comparison, are collected in Table 1. They are expressed versus standard hydrogen electrode (SHE) and, in order to eliminate the unknown liquid junction potential, also versus the bis(biphenyl)chromium(l)/(0) standard potential, that was assumed to be solvent independent. 

The increase in HCl concentration results in an increase in the liquid junction potential. 

As the concentration of the (dissimilar) electrolyte on the other side of the boundary (in the test solution) increases, or as the ions are made differenf the liquid-junction potential will get larger. Very rarely cm the liquid-junction potential be considered to be negligible>The liquid-junction potential with neutral salts... 

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 values cited neglect the liquid-junction potential at the KCl boundary, which in the case of strong acids, for example, increases the absolute value an average of several millivolts. 

Measurements in low-conductivity waters There has been a recent increase in interest in the measurement of pH in low ionic strength waters from upland areas because of the need to assess acidification of these waters. In low ionic strength waters, i.e., those that have a conductivity of less than lOOpScm, problems in pH measurement can arise that are associated either with the low-conductivity solution or variation in the liquid junction potential. The variation in liquid junction potential is caused by blockage of the junction with precipitates and this is more of a problem for the silver/silver chloride reference electrode than the saturated calomel electrode. The problem of measurement in... 

The intercept voltage, Vq, is principally the aggregate of the anode and cathode half-cell reversible potentials, the anode and cathode overpotentials, and the liquid-junction potential at the membrane (usually small). For a typical new cell with activated cathodes, Vq is about 2.4 V. With uncoated cathodes, this increases to about 2.7 V. 

The extent of sample contamination by the Hlling solution, of course, also depends on the length of time the reference electrode is in contact with the sample and the volume of the sample. One method of reducing contamination is by use of more dilute filling solutions. This may increase liquid junction potential. It may also... 

There are four common sources of error that can occur when making pH measurements on difficult samples. The first is high sample resistance which can result in slow response and increased noise pickup. The second is lack of compatibility of the reference filling solution with the sample or poor performance from a particular type of junction in the sample. This lack of compatibility results in a large unstable liquid junction potential which can cause slow response, instability, and/or significant error. The third common source of error is contamination of the sample. This may be... 

The low ionic strength and low conductivity of some nonaqueous solvents (see Table A.4) may result in severe noise pickup and large liquid junction potentials. These effects can be minimized by increasing the ionic strength of the solvent with a neutral electrolyte such as a quaternary ammonium salt. The addition of a neutral salt to the solvent increases its ionic strength, however, and consequently affects the hydrogen ion activity. Normally this effect is insignificant when compared with the potential error without the salt. 

Usually, the more hydrophobic an IL, the higher the solubility of water in the IL is. The solubility of water in moderately hydrophobic ILs suitable to an ILSB is typically 0.5-1 weight percent, which corresponds to the mole fraction of water being ca. 0.2 at room temperature, and increases with temperature [44-46]. A high content of water in an ILSB is unwelcome. When the ambient temperature varies during use or storage of ILSB, tiny water droplets may appear as suspensions in the ILSB or may absorb more water from a sample solution than that at a normal operating temperature. Such variability of water content in ILSB can be deleterious for the reproducible liquid junction potential, and minimization of water dissolution is preferable. However, the use of more hydrophobic ILs than those exemplified in... 

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