Conductance, liquid junction potentials

An element of uncertainty is introduced into the e.m.f. measurement by the liquid junction potential which is established at the interface between the two solutions, one pertaining to the reference electrode and the other to the indicator electrode. This liquid junction potential can be largely eliminated, however, if one solution contains a high concentration of potassium chloride or of ammonium nitrate, electrolytes in which the ionic conductivities of the cation and the anion have very similar values. 

For a uni-univalent electrolyte, the multiplier term in (6.24) cancels out and the liquid junction potential can be calculated from the equivalent ionic conductivities Xi of the two solutions, from the Sargent equation. 

The combination of the attraction of the electric field and the retarding effects leads to a maximum velocity for each ion. Measurement of these velocities gives information about the structure of the solution. Different cation and anion velocities give rise to a potential difference this is the liquid junction potential. It is interesting to know the magnitude of this potential, as it affects the measured potential of the whole electrochemical cell in other words, ion conductivities need to be measured. 

Ah already stated the liquid junction potential results from the different mobility of ions. Consequently no diffusion potential can result at the junction of the electrolyte solution the ions of which migrate with the same velocity. It is just this principle on which the salt bridge, filled by solutions of those salts the ions of which have approximately the same mobilities, is based (the equivalent conductivities of ions Kf and Cl- at infinite dilution at 25 °C are 73.5 and 70.3 respectively and the conductivities of ions NH+ and NOg are 73.4 and 71.4 respectively). Because ions of these salts have approximately the same tendency to transfer their charge to the more diluted solution during diffusion, practically no electric double layer is formed and thus no diffusion potential either. The effect of the salt bridge on t he suppression of the diffusion potential will be better, the more concentrated the salt solution is with which it is filled because the ions of the salt are considerably in excess at the solution boundary and carry, therefore, almost exclusively the eleotric current across this boundary. 

Electrolyte junction — A liquid junction is the region of contact of two different -> electrolyte solutions kept apart by a porous -> diaphragm, such as sintered glass or ceramic. At the contact a -> Galvani potential difference appears, which is called -> liquid junction potential (Ej). In the case of two solutions of the same electrolyte, but with different concentrations (c(a) and c(/S)), the potential Ej is defined by the equation Ej = (t+-t-) ln ry, where t+ and t are - transport numbers of the cation and anion, respectively. If the concentration of one of the ions is the same in both solutions, but the other ion differs (e.g., NaCl and KC1), the potential Ej is given by the Henderson equation, which is reduced to the Lewis-Sargent relation for a 1 1 electrolyte Ej = ln, where A (/3) and A (a) are molar conductivities of the electrolytes in the com-... 

A two-compartment electrochemical cell contains NaCl in one compartment and KCl in the other. The compartments are separated by a porous partition. Concentrations of both the electrolytes are equal. If /l,3ciand are the equivalent conductivities of the two solutions, show that the liquid junction potential is given by... 

The theoretical basis of the use of a bridge containing a concentrated salt solution to eliminate liquid junction potentials is that the ions of this salt are present in large excess at the junction, and they consequently carry almost the whole of the current across the boundary. The conditions will be somewhat similar to those existing when the electrolyte is the same on both sides of the junction. When the two ions have approximately equal conductances, i.e., when their transference numbers are both about 0.5 in the given solution, the liquid junction potential will then be small [cf. equation (36a)]. The equivalent conductances at infinite dilution of the potassium and chloride ions are 73.5 and 76.3 ohins cm. at 25, and those of the ammonium and nitrate ions are 73.4 and 71.4 ohms cm. respectively the approximate equality of the values for the cation and anion in each case accounts for the efficacy of potassium chloride and of ammonium nitrate in reducing liquid junction potentials. 

When the current does not flow through battery the measurable diflerence in electric potential between the terminals of the two electrodes is the result of all the equilibrium potential differences at the interphase between the conducting phases in contact. In the example of the Daniell cell, with both electrodes having copper terminals, there are three interfacial potential differences (apart from the small liquid junction potential difference at the contact between the two electrolyte phases) one potential difference at the contact between the zinc rod and the copper terminal (Zn/Cu) and two potential differences at the metal-solution interphases (Zn/Zn + and Cu/Cu +), which are mainly due to the charge transfer processes. 

To establish a pH scale, Sorensen chose a dilute hydrochloric acid solution for a standard. He took the concentration of hydrogen ions in such a solution to be given by aC, where C is the concentration of hydrochloric acid and a is a degree of dissociation determined from conductance measurements. His procedure had drawbacks first, there is evidence that the extrapolation procedure does not actually reduce the liquid-junction potential of Cell (3-6) to zero second, the hydrochloric acid is completely dissociated (dissociation constant about 1.6 x 10 ), and therefore the concentration of H is C rather than a somewhat smaller quantity. 

Use the Henderson equation to estimate the liquid junction potentials for the following systems assume that the limiting molar conductivities given in table 6.2 can be used to calculate the ionic mobility. 

Potentiometric titration experiments of Zr chloride, nitrate, sulphate and perchlorate solutions were conducted at (25.00 + 0.05)°C until the onset of precipitation. Initial solutions (0.038, 0.019, 0.0095 and 0.0047 M in Zr) contained < 0.4% Hf and had an excess of 2 M of the acid of the anion studied. Titrations were performed with carbonate free 0.101 N NaOH. Glass electrodes where calibrated regularly but no correction for differences between liquid junction potential of reference and measured solutions was performed. The pH convention used was not reported and it is assumed that a NBS type convention was used. The pH at the onset of precipitation and coagulation of an uncharacterised and presumably amorphous solid were determined optically. The pH of coagulation was the pH at whieh the precipitate coagulated and the supernatant solution was clear. Reproducibility of these characteristic pH values was within 0.05 to 0.07 pH units. 

The imprecision of 0.02 in the definition of pH is made up of approximately equal contributions from two quite different sources. One source of imcertainty is the variability in the liquid jimction potential [45, 59, 61] under different conditions of solution composition and dynamics. This is an intrinsic feature of the glass electrode-reference electrode combination used for pH measurements. This uncertainty does not arise in conductance measurements, and can be neglected in measurements of hydronium ion activity only by the use of reversible electrochemical (electrometric) cells that are constructed without liquid junction potentials. These approaches require considerable technical expertise and attention to detail in order to obtain results with maximal accuracy and precision. 

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... 

It is not a simple task to present precise pH and concentration dependence measurements, because changes in the pH and concentration of electrolyte refer to the solution phase, and thermodynamic relationships with respect to the film can only be obtained if an equilibrium exists between the film and solution phases. Constant ionic strength and constant pH should be maintained. Similarly, a liquid junction potential can distort observations. The formation of gas bubbles at extreme potential limits may cause ohmic drop and may also destroy the film. The pH values and ionic strength further assumes importance in light of experimental observations by Asturias et al. [73]. That polyaniline acts as an anion exchanger at pH 2 and 3 in its conductive state and its Donnan potential becomes... 

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. 

It is difficult to measure accurately the hydrogen ion activity of high purity water having a low conductivity such as less than 10 micromhos. The problem arises from the high resistance and unbuffered nature of high purity water and from the liquid junction potential that is developed that is, the measurement is likely to be noisy because of the high sample resistance, it is likely to drift because of carbon dioxide adsorption, and it is likely to require considerable stabilization time or be in error because of a large liquid junction potential. 

In order to prevent the zinc sulfate and the copjjer sulfate mixing, a porous disc, or a conducting salt bridge of potassium sulfate in a tube, or a wick can be used to connect the solutions. This helps to reduce the liquid junction potential, which occurs when two solutions of unequal concentration, or containing dissimilar ions are placed in contact. Because of the different rates of diffusion of the ions, a liquid junction (or diffusion) potential, E, is set up and this affects the total cell emf. For example, between a solution of 0.1 M HCl and a solution of 0.01 M HCl, E is about 38 mV. 

The form of eqn (4.10) can be rationalised as follows. First, the transport numbers are proportional to the ionic conductivity and hence the diffusion coefficients of the two ions as explained in Chapter 3. Thus the larger the difference in the transport numbers of the two ions the greater will be the liquid junction potential expected. Secondly, the liquid junction potential depends on the ratio C2/C1 if the two concentrations are equal the potential is predicted to disappear as would be anticipated. 

Because the gold nanoporous membrane is electrically conductive, charge injection by applying an external potential was also shown to provide means for inducing either anionic or cationic permselectivity. The potentiometric response was derived using as starting point the expression of the liquid junction potential (p) between two miscible electrolytes composed of 1 1 salts ... 

Distilled or high-purity water has a very low conductivity, such as less than 10 micro-ohms. Therefore samples are likely to show noise caused by high resistance, the unbuffered nature of pure water and also the liquid junction potential. The sample may need a considerable time to stabilise, during which it may absorb carbon dioxide from the atmosphere. In order to reduce these problems the following methods may be used ... 

If one wants to carry out precise concentration determinations, a constant ionic strength must be established in both electrode compartments so that the same activity coefficients apply to both solutions. This also reduces the liquid junction potential at the contact point of the two solutions, further increasing the accuracy. If it is not possible to introduce any foreign substances into the sample solution (as in physiological measurements or with samples which are to be used further), then a capillary connection between the two solutions is used, and the final reference solution is given a composition similar to that of the sample matrix. In doubtful cases a non-destmctive, nonpolluting conductance measurement can be made on the sample solution and the same conductance is established in the reference solution. In favorable cases a number of... 

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