Liquid junction potentials minimization

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 potential developed is determined by the chloride concentration of the inner solution, as defined by the Nemst equation. As can been seen from the above reaction, the potential of the electrode remains constant as long as the chloride concentration remains constant. Potassium chloride is widely used for the inner solution because it does not generally interfere with pH measurements, and the mobility of the potassium and chloride ions is nearly equal. Thus, it minimizes liquid-junction potentials. The saturated potassium chloride is mainly used, but lower concentrations such as 1M potassium chloride can also be used. When the electrode is placed in a saturated potassium chloride solution, it develops a potential of 199 mV vs the standard hydrogen electrode. 

If the carrier solution is identical to that used in the salt bridge of the reference electrode, the liquid junction potential is minimized, and so are its fluctuations. 

The sds (]) will include (Fig. 7.174) the liquid-junction potential that always arises whenever two solutions of different composition are in contact. But it will be assumed throughout this treatment that the liquid-junction potential has been minimized by the... 

It is often necessary to compare the potentials obtained with one reference electrode to those obtained with another reference electrode. It is important that the electrodes in question use the same solvent and that the contribution from the liquid junction potential is minimized. Otherwise, we have to use the internal reference redox couple. The comparison can be done according to the diagram shown in... 

It follows that for the more rigorous work, it is worth developing reference electrodes which minimize the liquid junction potentials. This is probably best achieved by using the RTIL under study as the solvent in the reference system. Building on published experiments, the latter is probably most securely based on the Ag/Ag+ system. Thus, for example, in an RTIL in which the anion is [BF4]-,... 

Minimization of the liquid junction potential is commonly carried out using a salt bridge in which the ions have almost equal mobilities. One example is potassium chloride (t+ = 0.49 and t =0.51) and another is potassium nitrate (t+ = 0.51 and / = 0.49). If a large concentration of electrolyte is used in the salt bridge this dominates the ion transport through the junctions such that the two values of have the same magnitude but opposing polarities. The result is that they annul each other. In this way values of E can be reduced to 1-2 mV. 

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),... 

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... 

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,... 

Component Ey (a) may be estimated in a way similar to that used for the calculation of the liquid junction potential in aqueous solutions. It may be added that the KCl saturated calomel electrode which minimizes the liquid junction potential in aqueous solutions produces rather large Lj values when used for boundaries with aprotic solvents [38-40]. 

In order to minimize liquid junction potentials, Parker and coworkers [32, 38, 39, 44, 45] advocated the use of 0.1 M tetraethylammonium picrate in acetonitrile for the construction of a salt bridge. This approach, confirmed by Izutsu [37], is based on the finding that both of the ions Et4N and picrate have similar mobilities in many solvents, but not in other solvents such as methanol. Also, in the case of mixed solvents formed by water and a solvent of high acidity this bridge does not work properly [46]. 

The presence of erythrocytes in the sample may also affect the magnitude of the residual liquid junction potential in a less predictable manner. For example, erythrocytes in blood of normal hematocrit are estimated to produce approximately 1.8mmol/L positive error in the measurement of sodium by ISEs when an open, unrestricted liquid-liquid junction is used. This bias may be minimized if a restrictive membrane or frit is used to modify the liquid-liquid junction. 

For the practical application of ISEs for measurement of ion activities in real samples there exist several concerns. We have already addressed the importance of liquid junction potentials and the need to either minimize these values or keep them constant between the calibrating solutions and unknowns. Also, in using ISEs one must fully realize the difference in measuring ion activities versus ion concentrations. The activity of an ion in solution is given by the expression... 

Attempts to Minimize Liquid Junction Potentials by Use of Salt Bridges, Since in many researches involving galvanic cells liquid junctions have appeared, not as interesting subjects for study, but as troublesome variables, attempts have been made to eliminate or minimize them. The most usual device for attaining this end is to introduce a strong solution of some electrolyte, known as a "salt bridge, between the solutions otherwise in direct contact. Thus the liquid junction... 

We minimize the liquid-junction potential by using a high concentration of a salt whose ions have nearly equal mobility, for example, KCl. 

A second limitation in the accuracy is the residual liquid-junction potential. The cell is standardized in one solution, and then the unknown pH is measured in a solution of a different composition. We have mentioned that this residual liquid-junction potential is minimized by keeping the pH and compositions of the solutions as near as possible. Because of this, the cell should be standardized at a pH close to that of the unknown. The error in standardizing at a pH far removed from that of the test solution is generally within 0.01 to 0.02 pH unit but can be as large as 0.05 pH unit for very alkaline solutions. 

What is the liquid-junction potential Residual liquid-junction potential How can these be minimized ... 

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