Electrolyte Key – Salt Bridge

Commercially available reference electrodes rely on salt bridges to connect to the test solution.

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Electrolyte Key - Salt Bridge

An electrolyte bridge, also known as a salt bridge, is used for the contacting of various electrolytes. Commercial reference electrodes rely on salt bridges.

The diaphragm acts here as the ions conducting contact point.

The salt bridges of commercially available reference electrodes include diaphragms made of ceramics, platinum twine, grindings, slightest bores, capillaries such as the Haber-Luggin capillary, fibers, fabrics such as Pellon or robust Teflon.

Commercially available reference electrodes rely on salt bridges. The connection to the measuring solution is made via a so-called diaphragm, which is usually made of ceramic, platinum twine, or grindings. — Commercially available reference electrodes rely on salt bridges.

Properties of Typical Diaphragm Materials

Characteristics of the most used materials, composed out of Galster, pH-Messung, Wiley-VCH, 1990

Material / Type	Properties
Porous ceramic rod	good chemical resistance reproducible electrolyte output contamination of measurement solution contaminates and clogs fast
Platinum rod	Good chemical resistance Not prone to contaminate Less electrolyte output than ceramic Less contamination of measurement solution
Joints	easy to clean no clogging very high electrolyte output higher contamination of measurement solutione electrolyte output depends on the joint’s roughness and location joint can detach
Capillary	very high electrolyte output higher contamination of measurement solution electrolyte output reproducible one source for clogging
Fiber	low electrolyte output low contamination of measurement solution

Electrolyte Key and Half Cells

Of course, separated half cells must be connected by an electrolyte key or a salt bridge. The easiest way is filter paper soaked in potassium nitrate. The better way is to use an u-shaped glass tube filled with potassium nitrate or chloride. Both ends of the tube are plugged with porous plugs.

An electrolyte key is neccessary to connect two half cells.

Leak Rates depending on the Diaphragm

The various materials have different rates of flow, which depend on the porosity of the material and the size of the diaphragm. Also, the workmanship in the production of the reference electrodes, damages, and contaminations in the diaphragm can affect the rates of flow.

Values adopted from: Galster, pH-Messung, Wiley-VCH, 1990

Diaphragm	Diameter in mm	Leak Rate in ml/dm @ 1 m electrolyte height
Ceramic	0.6	0.1
Ceramic	1.0	0.5
Ceramic	6.0	30
Glas sleeve	NS 7/16	1.0 - 10.0

Remarks concerning Salt Bridges

When using electrolyte bridges, it should generally be noted that depending on the type of diaphragm, different amounts of interior electrolyte will flow out into the measuring solution. The choice of the reference electrode therefore also depends on the measuring solution in which the electrode is to be used.
To avoid contaminations of the interior electrolyte in the reference electrode caused by diffusing measuring solution, the effusion of the interior electrolyte into the measuring solution is necessary. Therefore, the filling hole should be open or at least not be closed airtight during the measurements. The filling level of the interior electrolyte should be significantly higher than the level of the measuring solution.
In difficult measuring media such as highly contaminated or highly viscous samples, ground diaphragms are preferably used because of their high rates of flow.

Reference electrodes with a solid interior electrolyte do not have any electrolyte outflow. However, the diaphragm allows the measuring solution to enter the solid electrolyte and contaminate, dissolve or dilute it. Interior electrolyte from the reference electrode contaminates your measuring solution.
Reference electrodes have to be refilled repeatedly. The liquid level in the reference electrode must always be higher than the level of the measuring solution.
Reference electrodes with a solid electrolyte become inaccurate over time because the solid electrolyte changes due to the entering ions from the measuring solution.
At each diaphragm a diffusion voltage arises (diffusion or liquid junction potential), leading to deviations in the measured potential.

Diffusion Potential - Liquid Junction Potential

The diaphragm is the contact point between the bridging electrolyte and the measuring electrolyte. At this point, the diffusion voltages arise because the anions and cations of different electrolytes have different migration velocities. Diffusion voltages can be reduced by choosing the bridging electrolyte and the type of the salt bridge.

Maybe this online calculator is helpful.

Diffusion potentials adopted from R. Holze, Leitfaden der Elektrochemie, 1998

Electrolyte	Diffusion Potential to saturated KCl
1 mol/L HCl	14.1 mV
0.1 mol/L HCl	4.6 mV
0.01 mol/L HCl	3.0 mV
0.1 mol/L KCl	1.8 mV
Buffer pH 1.68	3.3 mV
Buffer pH 4.01	2.6 mV
Buffer pH 7.00	1.9 mV
Buffer pH 10.01	1.8 mV
0.01 mol/L NaOH	2.3 mV
0.1 mol/L NaOH	-0.4 mV
1 mol/L NaOH	-8.6 mV

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