Liquid contact potentials

Surface-potential measurements are more difficult to interpret quantitatively in terms of molecules, but there is now little doubt about certain important points. The contribution of each molecule to the potential is found by dividing the surface potential (the change in the air-liquid contact potential caused by the presence of the film) by the number of molecules. It is customary to draw the analogy between the electrical double layer at the surface and a plane parallel condenser in this evaluation, and most writers1 express the results in terms of /x, where AV, the surface potential, is given by 4nnjx>... 

Following experiments of L. Nobili on liquid contact potentials, Fechner took four glass vessels a, ft, A, B a and ft were filled with the same liquid and contained metal plates, usually platinum, connected with a galvanometer. A and B contained two liquids to be tested for contact electrification. The liquids were connected by siphon tubes i, 2 and 3, with capillary ends. The metal plates were cleaned and immersed in a liquid till no current between them was detectable by a galvanometer, when Fechner assumed complete homogeneity of the plates in a and ft. This precaution was not taken by Becquerel... 

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. ... 

The importance of liquid contact potentials in physiology was clearly recognised by Emil du Bois-Reymond, who made measurements by the potentiometer method (see p. 688), without arriving at any systematic generalisations. These were first reached in a research by his pupil Worm-Muller." He found... 

G. Meyer and S. W. J. Smith pointed out that Planck s theory of liquid contact potentials (see p. 705), since the ions of KCl and KI have practically equal mobilities, shows that there should be no contact potential between their solutions, and hence a cell formed from dropping electrodes in each should have practically no e.m.f. But Meyer found 0 284 volt and Smith 0 256 to 0 262 volt. Hence part of the potential difference at the electrode depends on the anion and is not eliminated. 

Many experiments with voltaic cells ( piles ) were made by M. Berthelot. He tried to compare affinity and electromotive force by finding the e.m.f. of a cell which caused electrolysis with evolution of bubbles of gas, and attempted to connect this with the heat of reaction. He later recognised the significance of entropy changes, and experimented with liquid cells and liquid contact potentials, oxidation and reduction, acid-base neutralisation, etc., also the effect of superposition of an alternating current. Berthelot was not really at home in electrochemistry and his work is hardly ever mentioned. Experiments with several types of cells made by Hittorf gave appreciable differences between the chemical and electrical energies. 

Braun — Oxidation and Reduction — Gibbs. Helmholtz — Contact Potentials — Liquid Contact Potentials — Electrode Potentials — Polarisation — Single Potentials. 

The succeeding material is broadly organized according to the types of experimental quantities measured because much of the literature is so grouped. In the next chapter spread monolayers are discussed, and in later chapters the topics of adsorption from solution and of gas adsorption are considered. Irrespective of the experimental compartmentation, the conclusions as to the nature of mobile adsorbed films, that is, their structure and equations of state, will tend to be of a general validity. Thus, only a limited discussion of Gibbs monolayers has been given here, and none of such related aspects as the contact potentials of solutions or of adsorption at liquid-liquid interfaces, as it is more efficient to treat these topics later. 

Electrode Potential (E) the difference in electrical potential between an electrode and the electrolyte with which it is in contact. It is best given with reference to the standard hydrogen electrode (S.H.E.), when it is equal in magnitude to the e.m.f. of a cell consisting of the electrode and the S.H.E. (with any liquid-junction potential eliminated). When in such a cell the electrode is the cathode, its electrode potential is positive when the electrode is the anode, its electrode potential is negative. When the species undergoing the reaction are in their standard states, E =, the stan-... 

The small potential difference produced at the contact between the two solutions (the so-called liquid-junction potential) is neglected. 

If the two Cu cables are short circuited while the cell is broken into two parts by splitting the liquid phase, it can easily be proved that the same AE as for cell (12a) is measured as a contact potential difference (cpd) between the two solutions. In fact... 

For correlating relative Eamo values with values in the UHV scale (0 values), two quantities must be known 0 and A0. Contact potential measurements at metal/solution interfaces can be measured.4 In that case the interfacial structure is exactly that in the electrochemical situation (bulk liquid phase, room temperature). However, 0 to convert E into 0 must be independently known. It may happen that the metal surface state is not exactly the same during the measurements of 0 and A0. 

Japaridze et al.m 323 have studied the interface between Hg and a number of vicinal and nonvicinal diols such as 1,2-, 1,3-, 2,3- and 1,4-butanediol (BD), ethanediol (ED), and 1,3-propanediol. KF and LiC104 were used as surface-inactive electrolytes. The potential of zero charge was measured by the capacitance method against an SCE in water without correction for the liquid junction potential at the solvent/H20 contact (such a potential drop is estimated to be in the range of 20 to 30 mV). The potential of the capacitance minimum was found to be independent of the electrolyte concentration while capacitance decreased with dilution. Therefore, Emin was taken to measure E . These values are reported in Table 4. 

In contrast to the case of the 8CB liquid crystal, no contact potential differences between first and second layers were observed with these lubricants. This indicates that there is no special orientation of the dipole active end groups, or perhaps that the end groups form hydrogen-bonded pairs with neighboring molecules so as to give no net dipole moment. 

This theory will be demonstrated on a membrane with fixed univalent negative charges, with a concentration in the membrane, cx. The pores of the membrane are filled with the same solvent as the solutions with which the membrane is in contact that contain the same uni-univalent electrolyte with concentrations cx and c2. Conditions at the membrane-solution interface are analogous to those described by the Donnan equilibrium theory, where the fixed ion X acts as a non-diffusible ion. The Donnan potentials A0D 4 = 0p — 0(1) and A0D 2 = 0(2) — 0q are established at both surfaces of the membranes (x = p and jc = q). A liquid junction potential, A0l = 0q — 0P, due to ion diffusion is formed within the membrane. Thus... 

Liquid-solid contact Liquid-liquid contact Statistical distribution due to ion concentration fluctuations Double layer (zeta potential) disruption Volta potential (for electron conducting materials) Electrolytic (galvanic) potential (for ionic systems)... 

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. 

In the general case of several electrolytes present in the solutions in contact with the liquid junction, no simple result analogous to (2.6.10) can be obtained. A basic problem stems from the fact that the electrolyte distribution in the liquid junction is dependent on time, so that the liquid-junction potential is also time-dependent. Because of these complications, further discussion will consider only those liquid-junction models where a stationary state has been attained, so that the liquid-junction potential is independent of time. This condition is notably fulfilled in liquid junctions in porous diaphragms. 

Another less precise but frequently used method employs a liquid bridge between the analysed solution and the reference electrode solution. This bridge is usually filled with a saturated or 3.5 m KCl solution. If the reference electrode is a saturated calomel electrode, no further liquid bridge is necessary. Use of this bridge is based on the fact that the mobilities of potassium and chloride ions are about the same so that, as follows from the Henderson equation, the liquid-junction potential with a dilute solution on the other side has a very low value. Only when the saturated KCl solution is in contact with a very concentrated electrolyte solution with very different cation and anion mobilities does the liquid junction potential attain larger values [2] for the liquid junction 3.5 M KCl II1 M NaOH, A0z, = 10.5 mV. 

In the presence of colloidal solutions in contact with a liquid junction, anomalous liquid-junction potentials are often measured. This suspension or Palmarm effect [14] has not yet been satisfactorily explained. It is probably a Donnan-type potential with the electrically-charged colloidal species acting as indiffusible ions (cf. section 5.1.3). 

Figure 4.20.A shows a more recent cell reported by Cobben et al. It consists of three Perspex blocks, of which two (A) are identical and the third (B) different. Part A is a Perspex block (1) furnished with two pairs of resilient hooks (3) for electrical contact. With the aid of a spring, the hooks press at the surface of the sensor contact pads (4), the back side of which rests on the Perspex siuface, so the sensor gate is positioned in the centre of the block, which is marked by an engraved cross as in the above-described wall-jet cell. Part B is a prismatic Perspex block (2) (85 x 24 x 10 mm ) into which a Z-shaped flow channel of 0.5 mm diameter is drilled. Each of the wedges of the Z reaches the outside of the block. The Z-shaped flow-cell thus built has a zero dead volume. As a result, the solution volume held between the two CHEMFETs is very small (3 pL). The cell is sealed by gently pushing block A to B with a lever. The inherent plasticity of the PVC membrane ensures water-tight closure of the cell. The closeness between the two electrodes enables differential measurements with no interference from the liquid junction potential. The differential signal provided by a potassium-selective and a sodium-selective CHEMFET exhibits a Nemstian behaviour and is selective towards potassium in the presence of a (fixed) excess concentration of sodium. The combined use of a highly lead-selective CHEMFET and a potassium-selective CHEMFET in this type of cell also provides excellent results.

Undoubtedly, the mercury/aqueous solution interface, was in the past, the most intensively studied interface, which was reflected in a large number of original and review papers devoted to its description, for example. Ref. 1, and in the more recent work by Trasatti and Lust [2] on the potentials of zero charge. It is noteworthy that in view of numerous measurements of the double-layer capacitance at mercury brought in contact with NaF and Na2S04 solutions, the classical theory of Grahame [3] stiU holds [2]. According to Trasatti [4], the most reliable PZC value for Hg/H20 interface in the absence of specific adsorption equals to —0.433 0.001 V versus saturated calomel electrode, (SCE) residual uncertainty arises mainly from the unknown liquid junction potential at the electrolyte solution/SCE reference electrode boundary. 

Drift Liquid junction potential at the reference not constant/Change the reference electrode. Loose contact/Rectify the fault. Electrode not plugged in properly/Rectify the fault. 

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... 

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