Salt Bridge Electrolytes

Opening this stopcock allows salt bridge electrolyte to... 

In that diagram A and B represent both electrodes. Q is the concentration of the electrolyte 1 in contact with the electrode A. C2 is the salt bridge electrolyte concentration. C3 is the concentration of the electrolyte 3 in contact with the electrode B. The electrodes are joined through metallic conductors MA and MB connected to a - potentiostat. The cell under study A-Q is kept at a temperature TA and the reference half-cell B-C3 is maintained at a temperature To. For the determination of the temperature coefficient, the temperature in the half-cell A-Ci is varied, while the temperature T0 is kept constant. 

Many of the problems associated with the liquid junction are common to all reference electrodes, while others are confined primarily to the microcapillary electrodes and/or measurements in fluids containing polyelectrolytes and colloidal or suspended components. There are several factors which affect the liquid junction potential of most reference electrodes. One of the most important factors is the mobility and concentration of the bridge electrolyte vis-a-vis the sample electrolyte. Usually the salt bridge solution is chosen to have a high concentration of equi-transferent ions. In this way, conditions are established for the transport of charge at the liquid junction by the salt bridge electrolyte, which, if equitransferent, results in a minimal diffusion potential. 

Another factor which may influence the liquid junction potential is termed the "suspension effect" in which the presence of colloids or suspended particles, e.g., red blood cells, produce an anomalous liquid junction potential. It has been suggested that this phenomenon is caused by the effect of colloidal particles on the relative rates of diffusion, i.e., transference numbers, of the salt bridge electrolyte. Another possibility is that colloids with ion-exchange properties give rise to a Donnan potential across the suspension/supernatant liquid interface. Whatever the cause, the effect may be significant and must be avoided in accurate studies with electrodes. 

Equation 5.7 also shows that using a salt bridge with an electrolyte that has similar cation and anion conductivities is beneficial for minimizing the diffusion potential. This is why KCl(aq) is suggested to be used as the salt bridge electrolyte, in which the limiting ionic conductivities of K+(aq) and Cl (aq) are very similar and, respectively, 73.48 and 76.31 cm S mol- [Chapter 10, Table 10.12]. 

A ance at Eq. (27) shows what is to be done in order to keep the liquid junction potential small. Aside from the trivial case in which a = a 2 (same electrolyte and same concentration on both sides), its magnitude depends above all on the factor in front of the logarithm on the difference in transport numbers of the cation and anion. One can attempt to minimize its value by choosing cations and anions with similar mobilities for the salt bridge electrolyte. Potassium chloride (fK+ — 0.49, tQ - = 0.51), for example, is a suitable salt. KNO3, NH4NO3, RbCl, etc. also exhibit equally favorable salt bridge electrolyte properties. 

Construction of the Salt Bridge Electrolyte/Sample Solution Contact Zone... 

A flow of the salt bridge electrolyte into the test solution is desired for a number of reasons First, the contact zone is continually regenerated, thus retaining its characteristic features second, the diffusion limited penetration of ions from the test solution into the reference electrode compartment is reduced (precisely the reason for using cells with transport in the first place). 

In view of this last possibility, care should be taken that the upper filUhole for the salt bridge electrolyte is not closed with an air-tight seal, so that the hydrostatic pressure of the column of electrolyte is sufficient to allow the solution to flow out. For the same reason, the reference electrode should not be immersed in the test solution so far that the liquid level in the sample solution is higher than that in the reference electrode. There may be ions present in the test solution which upon penetration into the reference electrode are able to alter the equilibrium Galvani potential of the reference electrode half-cell (for example, Br or 1 with an Ag/AgCl internal element). 

In order to avoid any precipitation and the associated problems discussed above, the tip of the salt bridge should be kept in a pure, somewhat diluted solution of the salt bridge electrolyte when not in use. 

The Ag/AgCl micropipette reference electrodes usually employed in physiological measurements of cell membrane potentials can be used as reference electrodes for intracellular ion activity measurements as well. The salt bridge electrolyte must be chosen with care so that the intracellular measured ion concentration is not influenced. 

Cells without liquid junction are to be employed whenever possible. For measurements with salt bridges, sleeve diaphragms should be chosen for their ability to provide a stable liquid junction potential (with double salt bridge reference electrodes the pH of the outer electrolyte should be adjusted to match that of the sample). In addition, the sample and salt bridge electrolyte solutions should have similar ionic strengths. Cells with liquid junction seldom allow accuracies better than 0.01 pa units. 

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