How Electrode Junctions Work
What connects the two wires together is a “salt bridge” or liquid junction (Shown in orange on the diagrams). This junction is made of a porous material that allows a small volume of reference solution to flow out of the reference half-cell, essentially acting as a liquid wire. It achieves the difficult goal of isolating the reference solution from the sample solution while still allowing current flow.
Junctions are designed to allow outflow of reference electrolyte to form a circuit and keep the junction unclogged, but they also allow inflow of the solution being measured. Some of the sample solution will enter the reference cell and dilute the potassium chloride or react with the silver elements, respectively resulting in drift over time or a dead electrode. While the electrode must be replaced if the silver reference wire is attacked by cyanides or sulfites, etc., dilution and depletion of the reference electrolyte depends on the ionic strength and pH of solution and is typically solved in one of two ways: refillable or replaceable electrodes.
Comparison of Refillable vs Replaceable Electrodes
One method is to use refillable electrodes, which have a hole that can be opened or closed to add or change electrolyte as needed. The second method is using replaceable or sealed electrodes, which are often filled with electrolyte gels or polymers instead of solution, giving them a lower flow rate. Typical replaceable electrode life is about 2-3 years but depends heavily on how quickly the reference solution is diluted. They will not need to be replaced as often as refillable electrodes must be refilled so they are considered lower maintenance and are generally cheaper than their refillable counterparts. Accuracy can be as high as ±0.01-0.02 pH for both types so the main reason to use refillable electrodes is for samples at extremes (e.g. above gel/polymer melting point, or in strong acids, bases, or low-ionic strength solutions where dilution occurs rapidly). Be aware that refillable electrodes will also last less time in these extreme situations than it otherwise would because the reference wire will become oxidized over a shorter period.