Liquid-junction potential temperature effect

The list of error sources continues, just to mention a few the ionic strength of the sample, the liquid-junction and residual liquid-junction potentials, temperature effects, instabilities in the galvanic cell, carryover effects, improper use of available corrections (e.g., for pH-adjusted ionized calcium or magnesium). An error analysis goes beyond the limited scope of this paper more details are presented elsewhere [10]. 

The e.m.f. of a thermogalvanic cell is the result of four main effects (a) electrode temperature, (b) thermal liquid junction potential, (c) metallic thermocouple and (d) thermal diffusion gradient or Soret. 

The most widely used reference electrode, due to its ease of preparation and constancy of potential, is the calomel electrode. A calomel half-cell is one in which mercury and calomel [mercury(I) chloride] are covered with potassium chloride solution of definite concentration this may be 0.1 M, 1M, or saturated. These electrodes are referred to as the decimolar, the molar and the saturated calomel electrode (S.C.E.) and have the potentials, relative to the standard hydrogen electrode at 25 °C, of 0.3358,0.2824 and 0.2444 volt. Of these electrodes the S.C.E. is most commonly used, largely because of the suppressive effect of saturated potassium chloride solution on liquid junction potentials. However, this electrode suffers from the drawback that its potential varies rapidly with alteration in temperature owing to changes in the solubility of potassium chloride, and restoration of a stable potential may be slow owing to the disturbance of the calomel-potassium chloride equilibrium. The potentials of the decimolar and molar electrodes are less affected by change in temperature and are to be preferred in cases where accurate values of electrode potentials are required. The electrode reaction is... 

By far the biggest problems with the stability and the magnitude of the liquid junction potentials arise in applications where the osmotic or hydrostatic pressure, temperature, and/or solvents are different on either side of the junction. For this reason, the use of an aqueous reference electrode in nonaqueous samples should be avoided at all cost because the gradient of the chemical potential of the solvent has a very strong effect on the activity coefficient gradients of the ions. In order to circumvent these problems one should always use a junction containing the same solvent as the sample and the reference electrode compartment. 

In practice, the value of k is never obtained as such, because the meter is adjusted so that the standard reads the correct value for its pX, the scale being Nernstian. As k contains in addition to the reference electrode potentials, a liquid-junction potential and an asymmetry potential, frequent standardization of the system is necessary. The uncertainty in the value of the junction potential, even when a salt bridge is used, is of the order of 0.5 mV. Consequently the absolute uncertainty in the measurement of pX is always at least 0.001/(0.059// ) or 0.02 if n = I, i.e. a relative precision of about 2% at best. For the most precise work a standard addition technique (p. 32) and close temperature control are desirable. All measurements should be made at constant ionic strength because of its effect on activities. Likewise,... 

Kinetic methods call for no measurements of absolute values of the parameter typically used to monitor reactions (absorbance, fluorescence intensity, potential), but rather for their temporal variations as a result, kinetic measurements are free from the effects of factors that introduce errors in absolute values (e.g., turbidity, the liquid-liquid junction potential, and the presence of other absorbing or fluorescent substances provided they do not take part in the reaction of interest or alter the parameter response). However, strict timing and temperature control (to within 0.01-0.1°C) are essential to kinetic methods, which thus require modern, powerful instrumentation. 

The Soret effect is effectively a liquid junction potential produced by a temperature gradient in a homogeneous electrolyte. The activity coefiicients of ions are generally not identical consequently, a temperature gradient produces a driving force, which will be opposed by potential. The implications of the Soret effect in an ILIT perturbation were discussed by Smalley et al. [5], who showed that there is a linear relationship between the Soret potential, A Fsoret, and the temperature difference between the electrode surface and the bulk solution, AT, i.e. ... 

Of these three issues, the first two are the most serious, with the first severely limiting the systems that can be studied to those that are stable in the presence of hydrogen, and the second hmiting the upper temperature. The third constraint is not a major issue in high subcritical systems, because the transference numbers of the ions of most, if not all, binary electrolytes tend toward 0.5 with increasing temperature however, at temperatures above the critical temperature the solubility of a salt is severely restricted and it may not be possible to attain a sufficiently high concentration to suppress the liquid junction potential. Note that the isothermal liquid junction is most effectively suppressed if the transference numbers of the cation and the anion of the background electrolyte are equal, a condition that is fulfilled by KCl at ambient temperature (and hence the reason for the choice of KCl in ambient temperature studies). 

The standardization parameters are determined by a nonhnear least squares procedure. The a term mainly corresponds to the negative log of the activity coefficient of the [ff] at the working temperature and ionic strength. 5 refers to the Nemst slope. The /h term corrects pH readings for the nonlinear pH response due to liquid junction and asymmetry potentials in the acidic region (1.5-2.5), while /oh corrects for high-pH (11-12.2) nonlinear effects. = [ff] [OH ] is ionic product of water. AT varies as a... 

Wild found that thermoelectric currents are produced at the contacts of solutions in the tubes at different temperatures, and the thermoelectric forces are stronger than the liquid contact potentials. He used the same solution (e.g. CUSO4) in the lower parts of both tubes, and the same solution (e.g. ZnS04) above. He found a thermoelectric force between two solutions of the same salt of different concentrations. The thermoelectric forces obeyed Volta s law. E. du Bois-Reymond and Wild did not detect a Peltier effect by passing a current through the junctions, but the effect was later observed by Schultz-Sellack. ... 

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