Diffusion potential, liquid junction potentials

The influence of interfaeial potentials (diffusion or liquid junction potentials) established at the boundary between two different electrolyte solutions (based on e.g. aqueous and nonaqueous solvents) has been investigated frequently, for a thorough overview see Jakuszewski and Woszezak [68Jak2]. Concerning the usage of absolute and international Volt see preceding chapter. 

This is known as the Planck-Henderson equation for diffusion or liquid-junction potentials. 

It is important to emphasize here the difference between cells without transfer and the cell with transfer. A cell with transfer has two additional potential differences between the salt bridge and the electrolytes at each end of the bridge. These potentials can be minimized and almost eliminated in a number of ways. The additional potentials are referred to as the diffusion or liquid junction potentials, which will be discussed in Chapter 3. 

Liquid Junction Potentials A liquid junction potential develops at the interface between any two ionic solutions that differ in composition and for which the mobility of the ions differs. Consider, for example, solutions of 0.1 M ITCl and 0.01 M ITCl separated by a porous membrane (Figure 11.6a). Since the concentration of ITCl on the left side of the membrane is greater than that on the right side of the membrane, there is a net diffusion of IT " and Ck in the direction of the arrows. The mobility of IT ", however, is greater than that for Ck, as shown by the difference in the... 

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. 

Sometimes the term normal hydrogen electrode (and respectively normal potential instead of standard potential) has been used referring to a hydrogen electrode with a platinized platinum electrode immersed in 1 M sulfuric acid irrespectively of the actual proton activity in this solution. With the latter electrode poorly defined diffusion (liquid junction) potentials will be caused, thus data obtained with this electrode are not included. The term normal hydrogen electrode should not be used either, because it implies a reference to the concentration unit normal which is not to be used anymore, see also below. 

The foregoing text highlights the fact that at the interface between electrolytic solutions of different concentrations (or between two different electrolytes at the same concentration) there originates a liquid junction potential (also known as diffusion potential). The reason for this potential lies in the fact that the rates of diffusion of ions are a function of their type and of their concentration. For example, in the case of a junction between two concentrations of a binary electrolyte (e.g., NaOH, HC1), the two different types of ion diffuse at different rates from the stronger to the weaker solution. Hence, there arises an excess of ions of one type, and a deficit of ions of the other type on opposite sides of the liquid junction. The resultant uneven distribution of electric charges constitutes a potential difference between the two solutions, and this acts in such a way as to retard the faster ion and to accelerate the slower. In this way an equilibrium is soon reached, and a steady potential difference is set up across the boundary between the solutions. Once the steady potential difference is attained, no further net charge transfer occurs across the liquid junction and the different types of ion diffuse at the same rate. 

Here, x is the coordinate normal to the diaphragm, so that d — q—p. The liquid junction potential A0L is the diffusion potential difference between solutions 2 and 1. The liquid junction potential can be calculated for more complex systems than that leading to Eq. (2.5.31) by several methods. A general calculation of the integral in Eq. (2.5.30) is not possible and thus assumptions must be made for the dependence of the ion concentration on x in the liquid junction. The approximate calculation of L. J. Henderson is... 

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. 

Planck s solution for the liquid junction potential [30, 31] is based on the assumption of stationary state transport, through diffusion and migration, and... 

Examination of the behaviour of a dilute solution of the substrate at a small electrode is a preliminary step towards electrochemical transformation of an organic compound. The electrode potential is swept in a linear fashion and the current recorded. This experiment shows the potential range where the substrate is electroactive and information about the mechanism of the electrochemical process can be deduced from the shape of the voltammetric response curve [44]. Substrate concentrations of the order of 10 molar are used with electrodes of area 0.2 cm or less and a supporting electrolyte concentration around 0.1 molar. As the electrode potential is swept through the electroactive region, a current response of the order of microamperes is seen. The response rises and eventually reaches a maximum value. At such low substrate concentration, the rate of the surface electron transfer process eventually becomes limited by the rate of diffusion of substrate towards the electrode. The counter electrode is placed in the same reaction vessel. At these low concentrations, products formed at the counter electrode do not interfere with the working electrode process. The potential of the working electrode is controlled relative to a reference electrode. For most work, even in aprotic solvents, the reference electrode is the aqueous saturated calomel electrode. Quoted reaction potentials then include the liquid junction potential. A reference electrode, which uses the same solvent as the main electrochemical cell, is used when mechanistic conclusions are to be drawn from the experimental results. 

Diffusion or liquid junctions are thus not clearly defined since the potential difference is dependent on the sharpness of the junction. Uniformly sharp junctions have been obtained by Lamb and Larson J.A.G.k XLii. 229,1920) by allowing one liquid to flow against the other, whilst Bjerrum f. Elektroohem. Liii. 428, 1906), has obtained a relatively diffuse junction by the insertion of a sand diaphragm between the two electrolytes. 

Here D, is the diffusion coefficient of species i, and Ej is the liquid junction potential that develops along the channel. The electrolytic mobility n, is the limiting velocity of an ion in the electric field of unit strength. It has dimensions of cm2 s 1 V 1 and... 

The total potential difference across the terminals of a cell is the sum of the potential differences, arising at the boundaries of two different phaseH. The most important boundary is that one between the electrode and the electrolyte. At the junction of two solutions of the same electrolyte but of different concentrations or solutions containing different electrolytes a potential difference will also arise, i. e. the so called liquid junction potential or diffusion potential which is, however, rather small (it will be dealt with in more detail later on). 

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. 

The formula (VI-29) is valid for any uni-univalent electrolyte. It is evident from this formula that the sign of the liquid junction potential and also the orientation of the diffusion double layer, in respect to the double layer at the electrodes, depends on the relative magnitude of the anion and cation transference numbers. Should the anion transference number exceed that of cation... 

Due to the different mobilities, concentration gradients and thus potential gradients will be established. In actual measurements these potentials will be added to the electrode potentials. A calculation of liquid junction potential is possible with the -> Henderson equation. As liquid junction potential is an undesired addition in most cases, methods to suppress liquid junction potential like -> salt bridge are employed. (See also -> diffusion potentials, -> electrolyte junction, -> flowing junctions, and -> Maclnnes.)... 

Liquid-junction potential -> liquid-junction potential, and -> diffusion potential... 

This liquid junction potential or diffusion potential is caused by slight separation of ionic charges that results from the tendencies of the various ions to diffuse at unequal rates across the boundary of the solutions. The effects of the liquid junction potential can be minimized by using a... 

II. The Constrained Diffusion Jimction.—The assumption made by Planck in order to integrate the equation for the liquid junction potential is equivalent to what has been called a constrained diffusion junction this is supposed to consist of two solutions of definite concentration separated by a layer of constant thickness in which a steady state is reached as a result of diffusion of the two solutions from opposite sides. The Planck type of junction could be set up by employing a membrane whose two surfaces are in contact with the two electrolytes which are continuously renewed in this way the concentrations at the interfaces and the thickness of the intermediate layer are kept constant, and a steady state is maintained within the layer. The mathematical treatment of the constrained diffusion junction is complicated for electrolytes consisting entirely of univalent ions, the result is the Planck equation,... 

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. ... 

In many instances, however, it has not yet been found possible to avoid a junction involving different electrolytes. If it is required to know the e.m.f. of the cell exclusive of the liquid junction potential, two alternatives are available either the junction may be set up in a reproducible manner and its potential calculated, approximately, by one of the methods already described, or an attempt may be made to eliminate entirely, or at least to minimize, the liquid junction potential. In order to achieve the latter objective, it is the general practice to place a salt bridge, consisting usually of a saturated solution of potassium chloride, between the two solutions that w ould normally constitute the junction (Fig. 70). An indication of the efficacy of potassium chloride in reducing the magnitude of the liquid junction potential is provided by thf. data in Table XLVII 3 the values iucorded are the e.m.f.of the cell, with free diffusion junctions,... 

The stability and reproducibility of the liquid junction potential should also be briefly discussed. When the free-diffusion junction is used with an MA electrolyte on both sides, this potential settles within a few seconds and is later stable to 2mVh and reproducible to 2 mV [52], although due to mutual diffusion the interface region does expand in time. However, such good reproducibility is observed only when there is the same electrolyte, MA, on both sides of the interface, even at different concentrations. When two different electrolytes AM (Ci) and MB (C2) are used in both solvents, relatively stable potentials are observed only when Cj > C2 or 2 C. ... 

When immersed in solution, the reference electrode in Fig. 10.5 usually fulfills its role of simply completing the electrical circuit. In a colloidal suspension, however, the colloid may cause K+ and CP to diffuse at different rates. Because of attraction or repulsion by the charged colloid, one ion moves ahead of the other. Ion separation at the junction between the electrode solution and the suspension produces a charge separation or electrical potential, the liquid-liquid junction potential ( j). Accurate... 

If the two electrode systems that compose a cell involve electrolytic solutions of different composition, there will be a potential difference across the boundary between the two solutions. This potential difference is called the liquid junction potential, or the diffusion potential. To illustrate how such a potential difference arises, consider two silver-silver chloride electrodes, one in contact with a concentrated HCl solution, activity = the other in contact with a dilute HCl solution, activity = Fig. 17.7(a). If the boundary between the two solutions is open, the and Cl ions in the more concentrated solution diffuse into the more dilute solution. The ion diffuses much more rapidly than does the Cl ion (Fig. 17.7b). As the ion begins to outdistance the Cl ion, an electrical double layer develops at the interface between the two solutions (Fig. 17.7c). The potential difference across the double layer produces an electrical field that slows the faster moving ion and speeds the slower moving ion. A steady state is established in which the two ions migrate at the same speed the ion that moved faster initially leads the march. 

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