Reference Electrodes

Reference electrodes for potentiometric analysis come in a variety of options which can be characterised according to chemistries; electrolyte movement and junction configuration. All combinations of these groups are possible. In early days each of these options had their distinct advantages, but several advances in design have neutralised these advantages; notably the double junction design and the renewable electrolyte. Regrettably, there are a few standard methods which have not kept up with the times and continue to specify obscure reference electrodes. We would like to promote the IJ as remarkably versatile, durable and a sensible alternative to these obscure electrodes.

1. Chemistries.

Silver/Silver chloride.

This is by far the most common reference type used today by almost all pH electrode manufacturers because it is simple, inexpensive, very stable and non toxic. It is mainly used with saturated potassium chloride electrolyte, but can be used with lower concentrations such as 1M potassium chloride. Note that changing the electrolyte concentration changes the potential. Silver chloride is slightly soluble in strong potassium chloride solutions, so it is sometimes recommended the potassium chloride be saturated with silver chloride to avoid stripping the silver chloride off the silver wire. This is unnecessary with the IJ types because the electrolyte in the primary reference chamber is gelled.

Calomel (mercury/mercurous chloride).

The calomel electrode was the most popular reference until about 1960 and most older publications on pH refer to it. It has similar stability to silver/silver chloride and a better thermal hysteresis. It is less prone to contamination because the mercury/mercurous chloride interface is protected inside a tube not in direct contact with the electrolyte. This advantage is negated by a double junction silver/ silver chloride construction as is used in the IJ. It has an advantage over a flowing single junction silver/silver chloride reference used in TRIS buffer samples and sulphide samples because the silver chloride in the electrolyte (either added or stripped) reacts with TRIS and sulphides, producing drift and errors. Again, this advantage is negated with the double junction Silver/silver chloride reference.

Recently, calomel has fallen into disfavour because of its toxicity. It offers virtually no performance advantages over a double junction silver/silver chloride and is more difficult to construct. It lingers on because many standard methods written prior to 1960 have not been updated to reflect contemporary electrochemical practice.

Mercury/Mercurous Sulphate.

In early times this reference was widely used where chloride leaching from conventional flowing silver/silver chloride or calomel references was incompatible with the sample e.g. a silver titration. Again, the double junction silver/silver chloride works just as well, but mercury/mercurous sulphate persists in various standard methods.
2. Electrolyte Movement

Conventional

"Conventional" references utilise a flow of electrolyte out of a porous constriction into the sample. This is intended to maintain a clean contact zone between the reference electrolyte and sample so that a stable and reproducible "liquid junction potential" is formed at the interface. While this system works well with ideal samples, i.e. free from suspended solids, oils fats and proteins etc, its use in non-ideal samples eventually causes contamination and consequent errors as reported by Illingworth (1).

Gel references do not flow and have the advantage they require no maintenance in that they don’t have to be topped up with electrolyte. However, the restriction to ideal samples is even more imperative than with the flowing electrolyte system because there is no flushing to maintain a clean contact zone.

Free Diffusion

Free diffusion is a static system as in the gel system, but allows unimpeded movement of ions. This is the system recommended by Illingworth for highest accuracy. However the open capillary tube he describes has to be recharged for every sample measurement and is not practical for routine use.
3. Junction Configuration

Ceramic Frit

Mainly used with glass body electrodes (and our PBFC). Once the pores in the frit become contaminated, cleaning is very difficult if not impossible. This was the type used in the pH electrodes tested by Illingworth.

Fibrous wick

Similar to the ceramic type and commonly used in Gel references.

Ground glass or ground plastic sleeve

The annular interface is known to produce highly stable liquid junctions and is commonly used for ISE measurements where small potentials necessitate precise measurements. Can be disassembled for cleaning, but not conveniently. Used in top of the range models from respected manufacturers such as Orion (Sure flow)

Annular teflon or polypropylene plugs.

Claimed to be "Non-fouling", the hydrophobic nature of teflon may prevent some contamination. However, electrolyte has to be present within the pores of the teflon, so contamination is still possible.

Double Junction.

A second electrolyte chamber is used to separate the primary reference from the sample. The second chamber isolates the primary reference electrolyte with another electrolyte so that the contamination and incompatibility problems discussed above are eliminated.
The IJ reference is a double junction sleeve type silver/silver chloride reference Instead of a permanent sleeve the system employs a slightly flexible polypropylene sleeve which fits over a spear membrane. This allows the sleeve to be easily removed for cleaning and renewal of the electrolyte. The primary reference chamber is gelled, but does not contact the sample. The second reference chamber is the annular space between the plastic sleeve and the glass stem. It terminates with the contact zone comprising a ground glass section on the stem, and a collar on the plastic sleeve. Contact between the second electrolyte and the sample is relatively open, so the system approximates the free diffusion system of Illingworth.

(1) Illingworth, J.A.; Biochem. J. (1981) 195, 259-262.