Introduction to Thermo Orion pH Electrodes

Thermo Orion offers the widest range of pH electrodes to meet the needs of any application. From critical measurements to simple checks, expect quality pH performance. Match any sample type with a ROSS[tm] silver/silver chloride, or calomel internal reference. Exhibited by the many different shapes and sizes, Thermo Orion manufactures pH electrodes for almost any sample size and type. Constructed in glass or epoxy bodies, our pH electrodes are ideal for the lab or field. With a variety of connections, use Thermo Orion pH electrodes on virtually any pH meter. The diversity of Thermo Orion pH electrodes allows you to choose the right electrode for any sample composition, volume, and temperature!

Click on the Highlighted Electrode Family Name below to see detailed information on each electrode in that family.

82-02
PerpHecT Line
PerpHecT® Electrodes, with either ROSS or Ag/AgCl references, offer the most accurate pH measurements possible when used with PerpHecT pH meters containing our patented digital LogR(tm); technology.
See "PerpHecT pH" for more details on digital LogR technology.

81-72
ROSS Line
The ROSS reference, a unique system developed by Thermo Orion, offers stable, fast results regardless of temperature or sample composition. The pH electrodes in the ROSS line contain the patented ROSS reference, giving you the best performance.
ROSS Sure-Flow® Electrodes have a unique reference junction that assures stable, drift-free potentials with an easy-to-clean design.

91-02
Standard Line
Silver/silver chloride, Ag/AgCl, references are widely used because of their accuracy and broad temperature range. Thermo Orion's Standard Line offers a variety of electrodes for any application.

71-02

Tris Line

Calomel, mercurous chloride, references are a value priced alternative to ROSS for biotech samples, especially Tris buffers. Thermo Orion's Tris Line offers three combination calomel electrodes.

91-06

Economy Line

When price is a factor, try the low maintenance Economy Line. Using sealed gel Ag/AgCl references, Economy Line pH electrodes require no filling solution for low maintenance. These pH electrodes are the ideal choice for student and rugged use.

98-03

Micro Line

Micro-pH Electrodes measure samples as small as 0.5 uL and are available in a variety of sizes, including a needle tip to pierce septa and a long bodied electrode for NMR cells. With Ag/AgCl references, Micro-pH electrodes provide fast, stable responses with a minimum of sample.

91-63

Specialty and Titration Line

Thermo Orion's Specialty Electrode Line contains special purpose pH and Titration electrodes.

Examples include the Model 91-63 Combination pH Electrode with Spear Tip (left), Redox/ORP Electrodes and pH and Karl Fischer Endpoint Indicator Electrodes.

90-04

Sure-Flow® Reference Line

Thermo Orion's Sure-Flow Reference Electrode Line includes three

All three incorporate Thermo Orion's unique Sure-Flow reference junction. This junction design allows for easy, "push-button" cleaning of the reference junction, essentially creating a new electrode with each flush and eliminating the number one cause of electrode failure.

pH Electrode Applications Selection Guide

Lab Catalog Index

pH Electrodes

The majority of pH electrodes available are combination electrodes. That is, they combine the reference and pH sensing elements into a single electrode. Separate pH and reference electrodes can still be used, but this tends to be for highest precision, research measurements. Naturally, combination pH electrodes are much more practical to use.

Know your pH electrode!

Key:
A - pH sensor
B - reference half-cell
C - combination pH electrode (A + B)
d - seal
e - internal filling solution
f - internal reference electrode
g - external filling solution
h - pH-sensitive glass membrane
i - internal liquid junction
j - external liquid junction
k - fill hole

The electrochemical circuit between the reference and pH sensing parts of an electrode is connected using a liquid-junction (key = j) that allows a slow flow of a suitable conducting liquid through the junction.

Types of Combination pH Electrode

There are generally three types of pH electrode available - calomel electrodes, silver chloride electrodes and solid-state electrodes.

Calomel Electrodes

Calomel Electrodes are problematic, but the fill solution they contain is less reactive. This means that for certain samples (eg Tris buffers) they are more suitable, since the fill solution does not react with your sample.

- refillable

- low-reactivity filling solution
- toxic
- very temperature sensitive
- suffer from drift

Silver Chloride Electrodes

Non-Refillable: These electrodes are permanently sealed, and contain a gel fill solution instead of the usual liquid fill solution.

- rugged designs

- permanently sealed
- easy to use
- less precise
- shorter life
- inexpensive

Refillable: Refillable electrodes contain a saturated silver chloride fill solution. This solution can occasionally react with certain sample (eg Tris buffer), impairing performance.

- refillable for longer life

- faster response
- wide applicability
- more precise
- reactive filling solution

Solid State Electrodes

Solid-state pH electrodes use a Field Effect Transistor (FET) which is sensitive to pH, instead of the usual glass bulb.

- non-glass sensor

- zero maintenance
- rugged

Final Word on pH Electrodes

Choosing the correct pH electrode for your application is the most important step before taking any pH measurement. The choice and variety may seem baffling, hence if you are bothered about your pH results and the on-going costs, it pays to talk to an expert.

To complicate matters further, there are literally hundreds of associated patents, all claiming to provide better performance over a standard pH electrode. The more significant technologies from the major manufacturers are provided in our guide to choosing a pH electrode.

REFERENCE ELECTRODES

BAS offers several models of Ag/AgCl reference electrodes. Each reference electrode is individually inspected and tested for adherence to an acceptable range relative to a standard calomel electrode. The model RE-6 is used by the epsilon system when configured with any of the standard flowcells. It features glass-body construction with a porous ceramic frit at the solution interface. The filling solution is 3 M NaCl gel that has been saturated with AgCl; the gel is semi-solid and will appear cloudy with occasional particles.

WHAT IS A REFERENCE ELECTRODE?
REMOVING THE SHEATH
STORAGE
ROTATION
TESTING
UNIJET REFERENCE ELECTRODE

WHAT IS A REFERENCE ELECTRODE?

The purpose of the reference electrode is to provide a stable, reproducible voltage to which the working (detector) electrode potential may be referenced (see PRINCIPLES). A reference electrode may be considered a small battery whose voltage (potential) is determined by the chemistry taking place between a solid conductor (usually a metal salt) and the electrolytic solution around it. Ideally, if a small current is passed through the electrode, the potential change is negligible, and in any case, returns to the initial value when the current ceases. In addition, the potential value should not vary with time and should be reproducible from electrode to electrode. The most common reference electrodes meeting these criteria are the mercury/mercurous chloride (calomel) and the silver/silver chloride (Ag/AgCl) electrodes.

REMOVING THE SHEATH

Every Ag/AgCl reference electrode is shipped with a colored plastic sheath that covers the porous tip and retards drying. Immediately upon receipt, remove this sheath by rolling it down from the glass body to the tip. The plastic will roll down and slide off the end of the electrode. Do not tug at the sheath or hold the electrode by its lead while you are doing this. If you have trouble removing the sheath, make a small cut at the upper edge of the sheath using small scissors, and try again.

BE CAREFUL NOT TO BREAK OFF THE END OF THE ELECTRODE!

ONCE THE SHEATH IS REMOVED, STORE THE ELECTRODE TIP IN 3 M NaCl AS SHOWN BELOW.

The Ag/AgCl reference electrodes are easily ruined by drying. Keep the tips wetted at all times and store in 3 M NaCl when not in use.

STORAGE

The Ag/AgCl reference electrodes are easily ruined by drying. Keep the tips wetted at all times and store in 3 M NaCl when not in use. A reference electrode storage vial is available from BAS (MR-5275. Be sure that you check the electrodes periodically and replace the solution in the storage vessel with fresh 3 M NaCl to keep the tips wet. DO NOT ENTIRELY IMMERSE REFERENCE ELECTRODES. Keep the connecting pins dry, or they will corrode and contaminate the reference electrode.

Reference electrodes will naturally change with use due to the transport of ions and solvent across the junction. The rate of change is a function of the difference in composition between the sample solution (i.e., mobile phase in liquid chromatography) and the filling solution (3 M NaCl gel). In LCEC usage, it is advisable to change to a new reference electrode frequently, at least once per month. Storing the reference electrode in 3 M NaCl between experiments will extend its lifetime. In spite of all attempts to extend their lifetimes, reference electrodes are still expendable items, so be certain to have spares on hand as needed.

ROTATION

We recommend that three reference electrodes be rotated in your LCEC system. Keep one electrode in your system for about two weeks. Turn the detector to STANDBY before removing and replacing the electrode. (Failure to do so can ruin a glassy carbon working electrode.) Rinse excess 3 M NaCl storage solution off the replacement reference electrode before inserting it into the cell. Replace the bushing and O-ring if they show signs of wear. Turn the cell on. Place the first electrode into the storage container. In another two weeks, replace the reference electrode with the third reference electrode provided in the kit. By rotating the three reference electrodes provided with your detector on a continuous basis, you can maximize their lifetimes. Depending on the mobile phase conditions and detector use, the reference electrodes can last from 3-6 months. When you replace reference electrodes, replace all three of them at the same time.

TESTING

If you are concerned about the viability of a particular Ag/AgCl reference electrode, you can test it using a simple voltmeter, additional reference electrodes of the same type (or a calomel reference electrode), and a small beaker of 3 M NaCl.

Read the potential difference between the electrodes on the voltmeter. Ideally the difference between two electrodes of the same type would be zero. However, in actual practice there is commonly some variation. If the two electrodes are of the same type (e.g., Ag/AgCl vs. Ag/AgCl, or calomel vs. calomel) the meter should read 0 ± 20 mV. If your reading for any pair of electrodes is significantly different, you should have another electrode of the same type handy to help distinguish which of the two is bad. When comparing an Ag/AgCl reference electrode to a calomel electrode, make the calomel the black (negative) input on the voltmeter. The meter should then read -35 ± 20 mV. A single-probe combination pH electrode is not suitable for this test.

UNIJET REFERENCE ELECTRODE

The UniJet detector is shipped with an embedded silver wire, which gets coated with AgCl to form the Ag/AgCl reference electrode. It should be regenerated each time the mobile phase is changed or the electrode is polished . After polishing and rinsing with water and methanol, apply a drop of the reference electrode coating solution (CF-2200) to the electrode surface:

Do not allow the solution to get on the working electrode or on your skin. Leave the solution on the electrode for 5 minutes. Rinse the electrode with water. The reference electrode should be a dull bronze color and uniform in appearance.

The UniJet reference electrode is a pseudo-Ag/AgCl reference electrode. The reference electrode potential is determined by the quantity of Cl^- in the mobile phase. The higher the Cl^- concentration, the closer the reference will become to a standard Ag/AgCl. We recommend using 10 mM NaCl in any mobile phase to increase the stability of the reference electrode and prevent alterations in the potential. The UniJet reference electrode is about 100 mV more positive than the standard RE-6 Ag/AgCl reference electrode. Thus, the potential set on the detector should be 100 mV less than was used with an RE-6.