Free-diffusion junction

Free diffusion junction electrodes are designed to minimize junction potentials. The junction is a Teflon capillary tube containing electrolyte that is periodically renewed by syringe. 

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 strict separation of the components of Eq. (12) is rather difficult, but Izutsu [37] claims that under appropriate conditions, using the free diffusion junction, the separation of these components may be possible. 

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. 

FIGURE 23-18 A combinalion pH electrode system wilh a free diffusion junction, (Courtesy of Hach Company,... 

Variation in junction potential. We reemphasize that variation in the junction potential between standard and sample leads to a fundamental uncertainty in the measurement of pH that is not correctable. Absolute measurements more reliable than 0.01 pH unit are generally unobtainable. Even reliability to 0.03 pH unit requires tremendous care. On the other hand, it is often possible to delect pH differences between similar solutions or pH changes in a single solution that are as small as 0.001 unit. For this reason, many pH meters are designed to produce readings to less than 0.01 pH unit. The free diffusion junction described in Section 23H-5 and illustrated in Figure 23-18 can minimize the variation in the junction potential. 

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

The LJP between different solvents contains three components i.e., (a) a component related to electrolyte concentrations and ionic mobilities, (b) a component related to ion solvation (and ionic mobilities), and (c) a component related to solvent-solvent interactions. Component (a) is somewhat similar to the LJP between solutions in the same solvent, but components (b) and (c) are specific to the LJP between different solvents. Here, a free-diffusion junction with the same electrolyte on the two sides (ci MX(Si)/c2 MX(S2)) is considered (see [221] for the case with different electrolytes on the two sides). We found that, under appropriate conditions, we can experimentally measure the variation in each of the three... 

At a free-diffusion junction, the LJP reaches a steady value within a few seconds after its formation, is reproducible within 1 mV, and is very stable, although the thickness of the junction expands with time by the mumal diffusions of both the solvents and the electrolytes, as far as the electrolytes on the two sides are the same the drift is within 1 mV h even when the LJP is near to 200 mV [218]. Even when the electrolytes on the two sides are of different kinds, if the concentration on one side is more than 20 times that on the other and if the junction is a free-diffusion type, the LJP is almost decided by the more concentrated electrolyte and it is stable with time [218, 219]. This justifies, to some extent, the use of aqueous reference electrodes for the measurements in nonaqueous solutions. In reality, however, the junctions between different solvents are usually not free-diffusion type but restrained with a diaphragm. The situation is thus complicated the composition of solvents and electrolytes in the diaphragm is indefinite and sometimes a clog of electrolyte is formed, making the LJP less reproducible and less stable. This applies to the case when an aqueous reference electrode is inserted into a nonaqueous solution. 

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