Perfect Liquid Junction

Leakage of the electrolyte from inside the reference electrode compartment to the sample and diffusion of the sample into the reference electrode are real problems facing the reference electrode. Would it be possible to design a reference electrode in which the open liquid junction channel would be replaced by a Pt wire sealed to the end of the capillary Such a junction would not leak and the problems would be solved. It could be represented, for example, as 

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Compared with the hydrogen electrode reference electrodes are more advantageous as they are easy to prepare, easier to work with, and give perfectly reproducible potentials their electrolytes generally have so small liquid junction potentials that they can be disregarded. Since their potentials, compared with the standard hydrogen electrode are exactly known, the in-... 

OCp]3 can be calculated directly if the concentrations of all ligands and of all competing cations are known (21). Where this information is not available, ISE s can in principle enable conventional single-ion activites to be measured directly. The limited sensitivity of present-day ISE s precludes their use in natural waters,/ although they Ccin be. used in experimental systems involving elevated concentrations of trace metals. Provided that the salinity remains constant, a cell without liquid junction, composed of perfectly selective chloride and lead ISE s could be used. The difference between the emfs measured in the sample (E ) and in a standard solution with the same tanperature and major ion composition (Eg) would be given by... 

In the field of electroanalysis the problem of the operative mechanism of glass electrodes may be mentioned. Thin membranes of several types of glass act as perfect Nemst-type electrodes for protons or other ions over many decades of concentration. The basic question is whether a glass electrode reacts to an ion i because 1 is charge-determining or because a liquid junction potential is set up with the transport dominated by i. In the former case Nernst s law for the electrode potential, 11.5.5.1], applies, in the latter [1.6.7.8] is valid, with t = = 1... 

Measurements made by calibration of electrodes with lUPAC aqueous RVS or PS standards to obtain pH(X) are perfectly valid. However, the interpretation of pH(X) in terms of the activity of hydrogen ion is complicated by the non zero residual liquid junction potential as well as by systematic differences between electrode pairs, principally attributable to the reference electrode. For 35%o salinity seawater (S = 0.035) calculated from pH(X) is typically 12% too low. Special seawater pH scales have been devised to overcome this problem ... 

The attractive feature of regenerative cells are the perfect phase contact by the liquid junction 37) elimination of the energy loss due to absorption in the... 

As described in qualitative terms in section 2.1.1, when there is a salt bridge or a liquid junction through a porous membrane, it takes an extremely long time to reach a state of equilibrium where a perfect mix of solutions is achieved. Generally a quasi-steady state is observed at open circuit (i.e., at zero current) where the molar fluxes are low but not zero (see appendix A1.1). 

If one considers a sample solution and bridge electrolyte of any desired composition, one must make assumptions about the concentration gradient within the liquid junction in order to arrive at simplified expressions. While it is possible to numerically simulate these profiles on the basis of the Nemst-Planck equation,for practical purposes it is perfectly acceptable to use the Henderson equation as approximation. It assumes linear concentration gradients across the Jimction and is written as ... 

The liquid junction potential adds to the membrane potential, so the potential readings are too high at high pH and too low at low pH. If the pH electrode is calibrated between pH 4 and 10 and one does not correct for the liquid junction potential, one expects an acid error of up to 0.25 pH units and an alkaline error up to 0.15 pH units. These considerations are with a perfectly selective pH electrode. 

As shown in Fig. 3-9, flow over the packing surface between junctions is assumed to be laminar. At each junction a fraction, q, of the flow enters the perfectly mixed regions, corresponding to the static holdup region, while the remainder of the flow bypasses the mixing. The probability of an element of fluid being mixed at each junction is taken as q. Michell and Furzer66 derived the expression for the mean residence time for the liquid as... 

The movement of material from cell wall to cell junctions is believed to occur due to perturbations of closed cell geometry from that of perfect sphere. Since the material inside the closed cells (solvent-rich material) has better wetting properties than that around the cells (polymer-rich material), these perturbations are unstable. The tendency of the system is to form larger spherical domains from the smaller solvent-rich spherical cells (as in the case of gas bubble coalescence in a liquid medium). Clearly, as a first step toward this process, closed cells would coalesce with one another the smaller cells tending to coalesce with the bigger ones. During the removal of the solvent to form the solid product, the wall thinning and rupture processes continue to occur. The extent of this second mechanism depends on how efficient is the solvent removal process. 

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