Liquid junction potential with salt bridge

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.)... 

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. 

Many of the problems associated with the liquid junction are common to all reference electrodes, while others are confined primarily to the microcapillary electrodes and/or measurements in fluids containing polyelectrolytes and colloidal or suspended components. There are several factors which affect the liquid junction potential of most reference electrodes. One of the most important factors is the mobility and concentration of the bridge electrolyte vis-a-vis the sample electrolyte. Usually the salt bridge solution is chosen to have a high concentration of equi-transferent ions. In this way, conditions are established for the transport of charge at the liquid junction by the salt bridge electrolyte, which, if equitransferent, results in a minimal diffusion potential. 

Cells without liquid junction are to be employed whenever possible. For measurements with salt bridges, sleeve diaphragms should be chosen for their ability to provide a stable liquid junction potential (with double salt bridge reference electrodes the pH of the outer electrolyte should be adjusted to match that of the sample). In addition, the sample and salt bridge electrolyte solutions should have similar ionic strengths. Cells with liquid junction seldom allow accuracies better than 0.01 pa units. 

To avoid contamination of the solution under study, and to minimise the liquid-junction potential, it is usual to use a salt bridge, but in many cases this can be dispensed with thus if corrosion in a chloride-containing solution is being studied a Ag/AgCl electrode immersed directly in the solution could be used similarly a Pb/PbOj electrode could be used for studies of corrosion in H2SO4. 

An electrode potential varies with the concentration of the ions in the solution. Hence two electrodes of the same metal, but immersed in solutions containing different concentrations of its ions, may form a cell. Such a cell is termed a concentration cell. The e.m.f. of the cell will be the algebraic difference of the two potentials, if a salt bridge be inserted to eliminate the liquid-liquid junction potential. It may be calculated as follows. At 25 °C ... 

Henderson or Plank formalisms. Mobilities for several ions can be seen in Table 18a. 1. Liquid junction potentials can become more problematic with voltammetric or amperometric measurements. For example, the redox potentials of a given analyte measured in different solvent systems cannot be directly compared, since the liquid junction potential will be different for each solvent system. However, the junction potential Ej can be constant and reproducible. It can also be very small (about 2-3 mV) if the anion and cation of the salt bridge have similar mobilities. As a result, for most practical measurements the liquid junction potential can be neglected [9]. 

When a constant ionic strength of the test solution is maintained and the reference electrode liquid bridge is filled with a solution of a salt whose cation and anion have similar mobilities (for example solutions of KCl, KNO3 and NH4NO3), the liquid-junction potential is reasonably constant (cf. p. 24-5). However, problems may be encountered in measurements on suspensions (for example in blood or in soil extracts). The potential difference measured in the suspension may be very different from that obtained in the supernatant or in the filtrate. This phenomenon is called the suspension (Pallmann) effect [110] The appearance of the Pallmann effect depends on the position of the reference electrode, but not on that of ISE [65] (i.e. there is a difference between the potentials obtained with the reference electrode in the suspension and in the supernatant). This effect has not been satisfactorily explained it may be caused by the formation of an anomalous liquid-junction or Donnan potential. It... 

The space charge in the liquid junction [1]. By liquid junction or the liquid junction potential we mean the diffusion potential developing in an electrically insulated electrolyte solution with differing ionic diffu-sivities and an initial concentration discontinuity. Besides its conceptual importance as probably the simplest nonequilibrium electro-diffusional situation, the dynamics of liquid junction is important to understand for applications, such as salt bridges, etc. 

Aqueous reference electrodes, such as SCE and Ag/AgCl electrodes, are often used in noil-aqueous systems by dipping their tips into lion-aqueous solutions of the salt bridge. The tip should not be dipped directly into the solution under study, because the solution is contaminated with water and the electrolyte (usually KC1). When we use such aqueous reference electrodes, we must take the liquid junction potential (LJP) between aqueous and non-aqueous solutions (Table 6.2) into account. If we carefully reproduce the composition of the solutions at the junction, the LJP is usually reproducible within 10 mV. This is the reason why aqueous reference electrodes are often used in non-aqueous systems. However, the LJP sometimes exceeds 200 mV and it is easily influenced by the electrolytes and the solvents at the junction (Section 6.4). The use of aqueous reference electrodes should be avoided, if possible. 

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. 

A salt bridge also helps to minimize problems associated with liquid junction potentials (Eijp). Such potentials in combination with the ohmic potential term resulting from the presence of uncompensated resistance in the electrochemical cell (Rn) may alter the potential applied between the working and reference electrode (Eapp). so that the measured potential (Eceii) is given by (6),... 

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,... 

Figure 3.28. Illustration of a seminal colloid titration result obtained after pioneering work by E.J.W. Verwey and H. de Bruyn. Silver iodide in (l-l) electrolytes drawn curves 7 1 KNOg + NaNOg mixture, O NaClO, A NaNOg. The surface charge could not be exactly established because the surface area was not well known. pAg and units are convertible because Nemst s law applies. The (7 l)-KNOg + NaNOg mixture was chosen to suppress the liquid Junction potential (sec. F5.5d) with the salt bridge. Source Redrawn from data by J.A.W, van Laar, PhD Thesis. State Unlv. Utrecht (1952) E.L. Mackor. Rec. Trau. Chim. 70 (1951) 763, as collated by J.Th.G. Overbeek In Colloid Science Vol. 1, H.R. Kruyt, Ed., Elsevier (1952) 162. Older references include E.J.W. Verwey, H.R. Kruyt, Z. Phys. Chem. A167 (1933) 149 E.J.W. Verwey. Rec. Trav. Chim. 60 (1941) 887 and H. De Bruljn. Rec. Trav. Chim. 61 (1942) 5, 21.

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. 

The pH scale has been defined operationally, and standard reference solutions based on a conventional scale of hydrogen ion activity have been selected (i, 2). Measurements of the pH of seawater made with different electrodes and instruments are satisfactorily reproducible when standardized in the same way (3). The results obtained, however, do not always have a clear interpretation. Formally, this diflSculty can be attributed to the residual liquid junction potential involved in the measurement. The primary standards are necessarily dilute buffer solutions (ionic strength, I 0.1) whereas seawater normally has an ionic strength exceeding 0.6. This difference in the concentrations and mobilities of the ions coming in contact with the concentrated solution of potassium chloride of which the salt bridge-liquid junction is composed gives rise to a potential difference that is indeterminate. Consequently, the meas-m ed pH is in error by an unknown amount and does not fall exactly on the scale fixed by the primary standards. 

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