Refex pH & ORP Electrodes

At the heart of every Refex electrode is non-porous reference junction technology.

Basically, most pH/ORP sensors have a porous junction—ceramic or Teflon frit/diaphragm, wood stack, ‘solid’/immobilized gel, even paper—that creates an intimate connection between process fluid and the actual reference electrode. This porous junction is the Achille’s heel of traditional electrodes.

Refex pH/ORP combination sensors have an impermeable barrier between the process liquid that you’re measuring and the electrode’s reference half-cell and electrolyte—essentially, no liquid-liquid contact. The barrier surrounding the Ag/AgCl reference half-cell is a solid polymeric casing that has a large immobilized KCl loading, making it electrically conductive.

So, why is this important? Because porous junctions in pH/ORP probes cause all sorts of problems and inaccuracies.

Traditional technology pH/ORP sensors generally work by having a porous ceramic frit or micro-capillary sleeve acting as a semi-permeable membrane between an internal salt bridge and ions in the process stream/sample.

While this works relatively well, it can be prone to a number of problems, especially over time and in harsh conditions. First, it often has a very small surface area, which means that there can be issues from sensor fouling. With something so small (and it has to be small or the salt bridge will be too exposed to the sample and be washed away, leading to inaccurate and drifting readings), it’s prone to being covered with something akin to an oil slick. The process sticks to the sensor, preventing proper measurement and causing an incorrect pH measurement value.

Second, it’s essentially a semi-permeable membrane, so anything that affects the speed at which the salt bridge ions diffuse through it will affect the measurement. Slow diffusion rates create sluggish readings while rapid diffusion depletes the cell prematurely. Thirdly, porous membranes allow the process to get inside the reference half-cell, diluting or reacting with the reference electrolyte to create drift and destruction of the electrode. For example, under high pressure, electrolyte will simply be diluted or replaced by process fluid being pushed through the junction.

So, there are many circumstances where traditional sensors either don’t work for long (or at all) or don’t work as accurately as needed.

One of the most significant benefits is probe lifetime. Refex electrodes last much longer in areas where traditional pH/ORP sensors are commonly used, and can be used in applications where traditional electrodes are quickly destroyed.

For example, in the oil and gas industry, the fluids that you’re measuring the pH or ORP of are often reactive, volatile, and corrosive (and definitely not something you want to get on yourself). Refex electrodes have been reported to last five times as long (or longer) in these extreme applications, compared to typical electrodes. In some cases where traditional porous junction reference electrodes last only days—such as chemical production or bioreactors where it’s necessary to measure pH under pressure—Refex electrodes deliver excellent performance.

In the long term, you'll spend much less money on replacement probes and spend much less time maintaining them. Plus, with more accurate readings over a longer period of time, you can count on faster corrections to chemical imbalances, helping improve process efficiency by preventing correction chemical overdosing (reducing costs further).

Refex sensors can also dramatically reduce plant downtime, and for any company focused on manufacturing, reducing downtime is absolutely essential to running an economically efficient operation.

To summarize, the benefits of using Refex sensors include:

• Measurement is accurate, repeatable and above all, reliable
• Confidence in meeting statutory and regulatory measurement requirements
• Low maintenance saves manpower—no need to calibrate every week!
• Maximize plant throughput by dramatically reducing downtime
• Long life—fewer sensor changes means real cost savings
• Reduced cost of ownership from greater reliability & reduced maintenance
• Improve process quality control through reliable measurement
• Fast response provides real savings by preventing chemical over-dosing
• Are virtually maintenance free
• Have exceptionally long lifespans (>5 times longer than traditional probes)
• Have large surface contact area
• Work in vacuum without electrolyte reservoirs and at pressures up to 300 PSI
• Do not leak electrolyte into the process (critical in low-ionic and UPW applications)
• Do not become clogged (important where precipitates are present)
• Do not become poisoned (perfect for H2S and chemical process applications)
• Much faster response times prevent control overshoot
• Greater accuracy and repeatability

Electrode fouling is one of the major problems requiring frequent maintenance and recalibration. The problem in conventional electrodes is the porous liquid junction for the reference electrode. This porous junction, whether ceramic, Teflon, paper, or even wood, can over time become clogged by process medium, increasing impedance and affecting performance.

Clogging can become so significant that the electrode stops responding at all. Lime scale, manganese, precipitation between hydrogen sulfides and AgCl, and protein/fat build up in sewage and industrial effluents are all examples of material that can affect electrode performance in this way. Fine particle clogging—pigments and dye stuffs, etc.—are particularly bad and have been known to shorten conventional electrode life considerably.

To review, reference electrodes are low impedance sensors (typically 10 kΩ), while the pH glass element is a high impedance sensor (typically 100 MΩ). Coating and deposits on sensors can increase impedance by 1 MΩ, but this is not a major problem for the pH glass electrode. The impedance changes to 101 MΩ, an increase of only 1%. However, this same effect on the reference side (across the porous junction) changes the impedance from 10 kΩ to 1010 kΩ, orders of magnitude of change. This is where the problem lies.

One solution to this problem has been to use a flowing junction electrode where liquid electrolyte is pressurized to create a positive KCl outflow through the junction, keeping it clear of contaminants. While this helps, it is maintenance and consumable intensive, and still allows poisoning of the reference element with resultant precipitation within the sensor body. The pressurization and flow of the electrolyte then seals the electrode’s fate by blocking the junction from the inside.

Refex electrodes are far more resistant to fouling and buildup. Being non-porous, there is nothing to block, and provided any coating on the electrode is conductive, it will continue to operate as if clean. Note that eventually, it will be necessary to clean off heavy buildup from the electrode. It is difficult to accurately measure the process with the electrode buried in its own ‘micro-environment’ of buildup.

As an example, raw water (such as water sourced from reservoirs, lakes, and rivers) can be bitterly cold with low ionic strength and trace quantities of magnesium and iron. Conventional pH sensors with porous reference junctions can quickly foul because of the trace metals, requiring frequent cleaning and recalibration. Because of the difference in salt concentration between the reference electrolyte (typically KCl 3 mol/l) and the water being measured, diffusion occurs through the porous junction and the reference cell electrolyte becomes depleted. This causes a diffusion potential error—which is significant when a tight pH value must be maintained. When conventional electrodes with porous junctions are installed in low ionic strength water applications, they often exhibit instability in the form of continuous drift.

The opposite is true when working with process solutions with high ionic strength. Here, diffusion occurs in the opposite direction with a consequent change to the electrolyte properties.

Refex probes, being non-porous, eliminate measurement error caused by diffusion potential. There is no electrolyte loss or dilution while the electrode is in service, producing an extremely stable reference output, with drift of less than 1mV/month. Ingress of poisoning material is eliminated, greatly extending electrode life.

The Refex solid state polymeric reference system allows sensors to respond instantaneously to pH change. This is a very important quality when titration and chemical dosing is required. The entire outside wetted surface of the Refex reference electrode is electrochemically active without diffusion potential or flow errors, ensuring rapid response to pH changes and preventing dosing over-shoot and the unnecessary use of expensive dosing chemicals. This response characteristic can result in significant cost reduction through chemical savings.

Conventional electrodes respond far more slowly due to ion diffusion time through the porous junction. The use of tortuous paths and double junction techniques to extend electrode life only adds to the slowing of response, and as the porous junctions become obstructed, response time slows further. Slow response inevitably results in set point overshoot.

There are dozens of practical applications for Refex electrodes in heavy industry and oil and gas production. However, you might be surprised where else you would find Refex electrodes.

For example, in the food and beverage industry, Refex electrodes are used to test pH during production. Refex probes are favored in this application because they have protein-resisting properties, leading to better accuracy and enhanced longevity.

They’re also used where sulfides are present (e.g. flue gas desulphurization). Sulfides will destroy a porous pH sensor by causing silver precipitation, thus rendering the measurement useless.

Refex sensors are also used to measure pH in oil and gas boilers. These systems generally use ultrapure water (UPW) mixed with anti-corrosion agents. Maintaining the right pH is essential to keeping the ratio of water to anti-corrosion agent stable. However, when you submerge a porous pH sensor into the UPW, it bleeds KCl, creating drift and eventual failure of the sensor. With a Refex non-porous pH sensor, this issue is avoided.

Refex also offers direct replacement electrodes to upgrade and improve installations where other brands are already in use. Replacements are available for:

• ABB/TBI
• BAT (Barben Analytical Technologies)
• E+H
• Ingold/Mettler Toledo
• Yokogawa
• Sensorex
• Rosemount Analytical
• Foxboro
• Van London/Phoenix

If you have specific compatibility requirements, contact South Fork Instruments for assistance.