Liquid junction free diffusion

Fig. 2.13 Examples of liquid junctions (A) liquid junction with free diffusion is formed in a three-way cock which connects the solution under investigation with the salt bridge solution (B) liquid junction with restrained diffusion is formed in a ceramic plug which connects the salt bridge with the investigated solution...

Fig. 2.3. Liquid junction with free diffusion (a) the test solution is drawn (in the direction of the arrow) into the stopcock (h) the saturated KCl solution from the liquid bridge is drawn into the stopcock (c) by turning the stopcock the test solution/liquid-bridge liquid junction is formed. (After Mattock and Band.)...

While the Planck liquid-junction model corresponds to a junction with restrained flow , for example in a porous diaphragm, fig. 2.2, the Hendersoii model approaches a liquid junction with free diffusion (fig. 2.3). Ives and Janz [13] give inaccuracies in measuring liquid-junction potentials between 1 and 2 mV. 

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. ... 

Major problems could be encountered due to errors associated with the liquid junction. It is recommended that either a free diffusion junction is used or it is verified that the junction is working correctly using dilute solutions as follows. For commercial electrodes calibrated with lUPAC aqueous RVS or PS standards, the pH(X) of dilute solutions should be within 0.02 of those given in Table 1. The difference in determined pH(X) between a stirred and unstirred dilute solution should be < 0.02. The characteristics of glass electrodes are such that below pH 5 the readings should be stable within 2 min, but for pH 5 to 8.8 or so minutes may be necessary to attain stability. Interpretation of pH(X) measured in this way in terms of activity of hydrogen ion, is subject to an uncertainty of 0.02 in pH. 

Operational standards (OS) are also defined which are traceable to the Reference Value Standard (RVS). Values are assigned by means of the operational cells I and II where the Uquid junctions are the free diffusion type reproducibly formed in 1 mm vertical capillary tubes. These operational standards are not restricted in number provided certain preparation criteria are met, and pH(OS) values for 16 solutions are given in Table 4. These OS represent an alternative procedure and are in no way to be regarded as inferior to the primary standards. As a consequence of their definition, all pH(OS) values fall on the line with slope given by the slope factor value forthe appropriate temperature in Table 1. Any ddference in liquid junction potential between the solutions of cells I and II and KCl is subsumed into the assigned value of pH(OS). 

Hickman HJ (1970) The liquid junction potential - the free diffusion junction. Chem Eng Sci 25 381-398... 

Most of the electrochemical cells have liquid junction (diffusion) potential. The electrochemical cells without transfer are free of it. The example of the electrochemical cell without transfer is the famous Harned cell. 

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