pH and ORP Sensors

pH electrodes and sensors are the sensing portions of a pH measurement. Various installation options including retractable, flow thru, immersion, and direct insertion. Proper pH electrode/sensor selection is critical for optimal measurement results.

  • The PH20, FU20 and FU24, all-in-one pH and ORP, sensors show how Yokogawa applies the motto "Simple is best" to sensor technology.

  • Yokogawa's pH and ORP analyzers are highly reliable. They feature advanced functions and are useful for a wide variety of applications including water quality management in a broad range of production processes and in medium-sized wastewater treatment plants, or as general pH and ORP control systems.

  • The SC25V is a pH sensor in a 12 mm design that includes an integral temperature element and a Liquid earth electrode.

  • The cation differential pH and ORP sensors were designed for difficult applications where conventional sensors are ineffective. These include measurements such as brine solutions to applications as diverse as electrolysis processes and cheese manufacturing.

  • The heart of a pH measuring loop is the electrode system. Yokogawa has designed a wide range of electrodes to ensure this heart keeps beating under the most severe conditions.

  • Achieving accurate and reliable readings using a traditional pH analyzer is challenging, however with the right equipment stable and accurate pure water pH measurements can be accomplished. Yokogawa's specialized bellomatic pH sensor is proven best solution for high purity water applications. For those individuals that do not like the maintenance of refilling sensors, then the FU24 which incorporates the successful patented bellow system in an All-in-one body is the ideal solution.

  • The PH8EHP pH sensor and PH8HH holder perform reliable measurement of pH for high purity water used in boilers and semiconductor process applications.

  • PH4/OR4 pH/ORP Sensor Series Allows pH/ORP measurement under severe conditions, such as where the process fluid is heavily contaminated or contains sulfide. PH4/OR4 Sensor Series meets the simple apparatus requirements.

  • The PH71/PH72 pH and/or ORP meter is a Compact, Easy-to-use, Drip proof – Ideal for Field Use. Features "One-Touch Calibration" and Temperature Compensation. Laboratory-Grade Intelligent pH Meters – Sized and Priced to Fit in Your Pocket

  • Yokogawa provides several types of holders, for which cleaners can be added, allowing customers to build reliable and easy-to-maintain measuring systems by selecting the right holders and cleaners for the conditions of applications.

  • Buffer solutions are needed as an indispensable tool for maintaining an accurate pH measurement. Buffer solutions are used as references points for calibration and adjustment of pH measurements to compensate for aging and deterioration.

  • The PH21 is the newest wide body sensor in the Yokogawa portfolio meant for water and wastewater market. 

  • More than 1,000 field devices designed to the PROFIBUS specifications installed in the state-of-the-art water reclamation plant.
  • The high reliability and accuracy of our field devices contributes to the minimization of maintenance costs over the entire plant lifecycle.

Wet scrubbers are used in utilities, paper mills, and chemical plants to remove sulfur dioxide (SO2) and other pollutants from gas streams. Undesirable pollutants are removed by contacting the gases with an aqueous solution or slurry containing a sorbent. The most common sorbents are lime, Ca(OH)2, and limestone, CaCO3

Application Note

For control of batch neutralization, a pH measurement coupled with a timer-controlled chemical feed scheme provides very satisfactory results.

This system can be adapted for either acid waste or alkaline waste neutralization.

Application Note

The term "cooling tower" is used to describe both direct (open circuit) and indirect (closed circuit) heat rejection equipment. Cooling towers are heat-transfer units, used to remove heat from any water-cooled system. The cooled water is then re-circulated (and thus, recycled) back into the system. Since the process water is re-circulated, the mineral concentration increases as a result of the evaporation.

Industry:Refining, Food and Beverage, Power, Oil and Gas, Pulp and Paper, Chemical


The proliferation of microorganisms and the resultant formation of slime is a problem which commonly occurs in aqueous systems. Problematic slime producing microbes may include bacteria, fungi and/or algae. Slime deposits typically occur in many industrial aqueous systems including cooling water systems, pulp and paper mill systems, petroleum operations, clay and pigment slurries, recreational water systems, air washer systems, decorative fountains, food, beverage, and industrial process pasteurizers, sweetwater systems, gas scrubber systems, latex systems, industrial lubricants, cutting fluids, etc.

Industry:Refining, Food and beverage, Power, Oil and Gas, Pulp and Paper, Chemical


There are a number of suppliers of oil and fat products used for edible purposes. These products include, but are not limited to olive oil, peanut oil, soybean oil, sunflower oil, lard, shortening, butter, and margarine. The raw materials for these products include animal by-products, fleshy fruits (palm and olive), and oilseeds. 

Industry:Food and Beverage


One of the primary applications for high purity water is for boiler feed water. The measurement of pure water pH can be one of the quickest indicators of process contamination in the production or distribution of pure water. Effective chemical treatment of the feed water is vital in maintaining the useful operating life and minimizing maintenance costs of the boiler.

Industry:Power, Pharmaceutical, Common


Process liquid analyzers such as pH meters, conductivity meters, ORP meters, and density meters play an important role at electrolysis plants in the control of concentrations of various process solutions. This requires both precision and stability under harsh conditions that include highly corrosive substances, high temperatures, and many impurities.


Most zinc are produced at hydrometallurgically, where a high-grade zinc product can be obtained and valuable metals mixed in the raw material can be recovered. In the hydrometallurgy, the raw material of zinc concentrate is roasted and then dissolved in sulfuric acid to remove impurities. The process called leaching and pH control of the leachate is important.

Industry:Chemical, Power


The control of the world's water resource is arguably one of the most important issues. Water demand from industry and domestic users is set to rise throughout the industrialized world. Yokogawa has been applying minimized maintenance measurement systems.


Cyanide-bearing wastewater from mining and electroplating facilities and certain types of chemical plants is toxic and must be treated by oxidation with chlorine or chloride to bring the cyanide concentration within regulatory limits.

Industry:Electrical and Electronics


In flue gas desulfurization systems that use magnesium hydroxide (Mg(OH)2) slurry, the consumption of the desulfurization agent (Mg(OH)2) is controlled by using online pH analyzers. A great concern in the pH measurement is heavy staining of the pH electrodes by the Mg(OH)2 slurry. To ensure accurate measurement, frequent cleaning of the electrodes with an acid is required, adding to both maintenance workload and cost.

Industry:Chemical, Power


The treatment of wastewater from pulp and paper plants is a serious environmental concern. Yokogawa's submersion holder with an ultrasonic+air-jet cleaner (customized product) can reduce the manual cleaning frequency to just once every one or two months.

Industry:Pulp & Paper


Continuous technology improvement is ongoing in the pulp & paper industry to obtain the best possible performance. Problems at the wet end (stock preparation) can rarely be corrected downstream. That is why monitoring and controlling pH in pulp stock is critical to the paper making process. Essentially, at every stage in the manufacture of paper, correct pH values play a vital role.


Reverse osmosis (RO) is a separation process that uses pressure to force a solution through a membrane that retains the solute on one side and allows the pure solvent to pass to the other side. More formally, it is the process of forcing a solvent from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure.


Continuous technology improvement is ongoing in the pulp & paper industry to obtain the best possible performance. The improved plant performance translates to the higher quality improvement and lower cost, and simultaneously environmental friendly plant operation.


Wastewater from electroplating facilities and certain types of chemical plants contains toxic forms of hexavalent chromium such as chromate and dichromate. The hexavalent chromium in this wastewater must be reduced before the water can be discharged. This requires a two-step process: hexavalent chromium (CR6) is reduced to trivalent chromium (CR3); and CR3 is precipitated as chromium hydroxide.

Industry:Electrical and Electronics


Sour Water is the wastewater that is produced from atmospheric and vacuum crude columns at refineries. Hydrogen sulfide and ammonia are typical components in sour water that need to be removed before the water can be reused elsewhere in the plant. Removal of these components is done by sending the sour water from the process to a stripping tower where heat, in the form of steam, is applied.


Power plant boiler houses designed to burn coal or high sulfur oil are required by Federal and State pollution regulations to "scrub" (remove) sulfur dioxide from flue gasses to meet emission limits. SO2 in flue gasses is known to be harmful to the environment, as it is one contributor to the formation of acid rain. pH control is critical for the proper functioning of the scrubber system.


Application Description

Many Ethanol plants running today are using a combination style pH electrode with a non-flowing reference to measure pH in the Mash Slurry transfer line from the Mash slurry mix tank to cook.  The Mash is being pumped out of the Mash Slurry tank is at approximately 82 °C and  2 to 4 bar (180 °F and 40 to 60 psig).

Mash Slurry transfer line

The original pH electrode systems that were installed during plant construction are online retractable assemblies and are mounted in orientations from completely horizontal to completely vertical and everywhere in between.

The Problem

The combination probe that is being used will typically drift out of calibration very quickly.  Also, the probe is damaged sometimes from excessive removal from the process.  The reason this probe drifts out of calibration is due to the fact that the non-flowing reference system plugs and becomes fouled by the mash passing by it.  pH measurements are only as good as the reference required to make this measurement.  If the reference is not doing its job, the measurement electrode will drift.

Process Overview

Process Overview

Product Recommendations

Yokogawa manufactures a multi-probe holder called the FF20 – flow through fitting or the FS20, which is pH chamber assembly with ½” NPT process connections.  With these holders we use a combination electrode, part number:  SC21C-AGC55 for measurement and reference and a separate temperature sensor part number: SM60-T1.  The Yokogawa electrode system works due to the fact that the SC21C-AGC55 combination probe uses a pressurized reference system.  By using plant air regulated to a KCl reservoir, the SC21C-AGC55 utilizes a positive flowing reference that does not foul. 

Plants using this system typically check the pH measurement against a grab sample and only make adjustments if the sample and the online measured values are more than 0.2 pH difference from one another.  Typically, the system will not need daily or weekly calibrations.  Most plants will pull the electrodes once a month for cleaning and calibration in a standard 4 and 7 buffer solutions.

Installation Considerations

The Yokogawa pH system is not retractable from the process.  It is usually best to put the Yokogawa pH electrodes in a by-pass or recirculation line that you can add isolation valves for isolating the probes from the process for maintenance and calibration.  The probe assembly should be mounted downstream of the Slurry Tank transfer pump.  Ideally it will be in a recirculation line going back into the tank or into the suction side of the slurry pump.

The picture below shows an installation that is actually flowing from left to right.  The arrows indicate the direction of the mash flow through the recirculation line and back into the suction side of the pump.  You will get an idea of the installation of the Yokogawa probes and the pressurized reference KCl reservoir from this picture.  The reservoir pressure is typically set 1 to 2 psig above the slurry line pressure.  The KCl reservoir will require refilling every 2-3 months for most applications

Installation Example

Note: For additional information on this application contact the local Yokogawa Process Liquid Analyzer Department

Application Note

Wastes have been considered to be a serious worldwide environmental problem in recent years. Because of increasing pollution, these wastes should be treated. However, industrial wastes can contain a number of valuable organic components. Recovery of these components is important economically. Using conventional distillation techniques, the separation of acetic acid and water is both impractical and uneconomical, because it often requires large number of trays and a high reflux ratio. In practice special techniques are used depending on the concentration of acetic acid. 


Background Information

The core of the activated sludge process is primarily dependent on the control of the aeration basin. The most essential component of any activated sludge plant is the biomass, anaerobic and aerobic bacteria, that attaches themselves to the waste, and digest the waste resulting in relatively clean water as the by-product.

There are several types of bugs that are responsible for different duties. There are the carbon eaters (carbonaceous) and chemical eaters like ammonia (nitrogenous). Just like any other living organism they need certain conditions in order to sustain life and reproduce.



Many components in a process must be in balance in order to obtain complete synergy; example biomass blends, air, return activated sludge (RAS), waste activated sludge (WAS) and throughput. The aeration basin is a holding and/or treatment pond that everything cycles through. It is essential to monitor and control several factors that can influence the efficiency of the biological conditions in the basin; for example:

Temperature: Normally the temperature will be between 10- 40°C. Most biomass bugs achieve optimum efficiency in this range. Increasing or decreasing the temperature can result in the increasing or decreasing the rate at which the bugs eat and reproduce. Along with this all chemical reactions that are taking place at the same time are affected by the process temperature as well.

pH: For most systems the pH should be kept between 6.5 to 8.5 pH, when the pH is too high or too low, the biomass losses the ability to convert the food to energy and raw materials. A pH below 6.5 may cause the growth of fungi and fungal bulking, and will have to be adjusted using a caustic, lime or magnesium hydroxide.

Low Nutrients: If nitrogen and phosphorus are not presented in sufficient amount it can limit the growth rate of the biomass. A sign of nutrient deficiency includes foam on the aeration basin.

Dissolved Oxygen: DO is one of the most critical points of measurement; for most processes the target concentration will be between 1-3 mg/L. The concentration amount is an indication of the basin environment; whether it is in denitrification (excess nitrate, NO3) or nitrification (excess ammonium, NH4) environment. Essentially the DO measurement is set to a level to minimize the ammonium breakthrough. It is not uncommon to see NH4 and DO measurements together.

The DO measurement should be maintained at the point of greatest oxygen demand in the system. Normally this is near the intake portion of the aeration basin, because when the process is in the secondary clarifier no oxygen is added and the biomass bugs are starved of oxygen. When the process is returned to the aeration basin via the RAS pumps the biomass is returned to an oxygen rich area and the bug consume vast amounts of oxygen right away.

Septicity/Toxicity: Septic wastes contain elevated amounts of sulfides and organic acids (such as acetic acid).Other organic materials and heavy metals are also toxic to the biomass, reducing their efficiency or even destroying them.



Having too much oxygen in the process is not a problem for the biological system; however the cost for generating the oxygen is one of the largest expenses. By obtaining a good representative average of the dissolved oxygen present in the basin could save the plant large amounts of money. For this reason multiple measurements points are sometimes put into place.

Application Note


In Bioscience company's pH is used at various places including glass lined reactor for product efficiency. One of such measurement of pH in reactor as explained below.

Application Information

Typical example of process is:

Reactor used is GLASS LINED REACTOR and length is 1950mm; and flange size 100mm split flange with 8 holes

PCB (fixing hole center to center) 190mm; flange outer to outer 225mm; flange extension pipe (Nozzle) ID 95mm.

Nature of fluid: Aqueous media
Operating temp & pressure: 30°C temp & atmosphere pressure
Area classification - Example Zone 1 / Zone 2
Nozzle ID: 95mm.


Product Recommendation

Measurement System

Process Liquid Analyzer: 

  • 2-wire FLEXA pH/ORP Analyzer
  • Dual sensor measurement on 2-wire type analyser
  • Indication of sensor wellness


  • 4-wire PH450G pH/ORP Analyzer



  • Easy touchscreen operation
  • Trending display up to 2 weeks
  • Advanced Process Temperature Compensation

Sensor Selection:




  • Best measurement accuracy both in high-alkali processes and in samples with very low conductivity.
  • Sterilizable and autoclavable
  • SINGLE PORE‘s for clog-free contact of electrolyte with measurement medium
  • HAMILTON “H” pH glass
  • Serialized with batch number

Sensor Accessories

5mtrs Cable, Model No.: WU10-V-S-05 ,

Immersion sensor fitting (Customised): 1900mm Length, 4" Flange Connection with 8 holes PCB (fixing hole centre to centre) 190mm;  flange outer to outer 225mm; pipe (Nozzle) ID95mm. MOC : PTFE.

Tangible benefit

More reliable and accurate analysis of pH which helps to improve end product quality.

Note: For additional information on this application contact the local Yokogawa Process Liquid Analyzer Department

White Paper

ORP (Oxidation-Reduction Potential) is the measurement, in millivolts, of a solution's capacity for electron transfer (oxidation or reduction). ORP measurement may also be called REDOX for REDuction OXidation. The name reflects that fact that within a chemical reaction reduction and oxidation are complementary; one cannot occur without the other.


To facilitate the accurate measurement of pH, and its presentation as a scale, a range of "standard liquids" or "buffer solutions" are used. These liquids, whose constituents are accurately defined, have known stable values. Although in the preceding text the relationship to hydrogen ions has been made, research has shown, that the activity of hydroxonium ions (H30+) is more relevant.

Media Publication

The different combinations available can make selecting the right pH sensor for your particular needs seem overwhelming. Because pH measurement points within a process vary, a sensor suitable for one application may not work well elsewhere in the plant or provide the same operational life there. This article discusses some important items for determining the optimal sensor for the process.

The first thing to remember is that a complete pH sensor consists of four mandatory elements: temperature electrode; solution ground; pH measuring glass electrode; and reference electrode. These elements either can be individual electrodes or combined into one sensor body. For a better understanding of the different options available and where one option better suits an application than another, you should consider the individual parts independently.

Temperature Electrode and Solution Ground

These are the simplest components of a pH sensor. A temperature electrode is essential to accurately and automatically compensate for temperature variations in the liquid being measured and temperature-dependent effects on the contact potentials in the glass and the reference systems.

The existence of a solution ground also is vital. However, in both cases, knowing the materials utilized isn't that important. Concentrate instead on ensuring the pH measuring glass and reference system used are the best ones for the process conditions.

Applicability of Common pH-Sensitive Glass: Thickness of the glass as well as its composition determine service range.

Figure 1. Applicability of Common pH-Sensitive Glass: Thickness of the glass as well as its composition determine service range.

pH Measuring Glass Electrode

The correct choice of a glass electrode for a particular application requires knowledge of the components of the measuring loop and their significant properties. Selection must consider not only the "pH glass" chemical makeup but also the mechanical design shape. 

Normally, the glass electrode has a bulb-shaped membrane of specific pH glass that is welded to the glass tube. (Alternatives to the standard bulb version of the pH-sensitive glass membranes are available and are better suited for certain process conditions.) The most common bulb constructions are:

Ball shape (shockproof) - universal electrode suitable for most pH applications;

Dome shape (heavy duty) - extremely durable pH membrane (≈1-mm thickness) best suited for measurements in aggressive or abrasive media; and 

Flat shape - used in combined sensors for applications involving a considerable solids' component.

The H+ ions in the liquid being measured interact with the surface (texture) of the pH-sensitive glass to form a stable balance at a specific active H+ concentration. This texture, which provides a high affinity for such a reaction, stems from the specific alkali metal ions used in the composition. Each manufacturer has its own "recipe."

To facilitate the interaction, the pH-sensitive glass membrane must be "conditioned" by allowing it to absorb a film of water or gel. Conditioning is achieved by soaking the electrode in water for a minimum of 24 hours. Most sensors come pre-conditioned and can be used immediately without soaking.

The suitability of an electrode depends on its type of glass membrane. Every manufacturer has its own unique formulation; however, some common types of glass are available:

  • "G" or general-purpose glass is used in processes where the nominal pH value varies around pH 7 and the operating temperature is from -20° to 70°C.
  • "L" or chemical-resistant glass is designed for measurements in alkaline media with process temperatures up to 130°C.
  • "Na" or sodium glass is utilized where good interaction with alkaline (sodium, potassium, calcium and magnesium) ions, rather than H+, is necessary. This glass serves as the reference element in differential sensors or as the measurement sensor for applications that determine/monitor salt concentration.

Figure 1 illustrates the general applicability of G and L glasses. As it shows, the range of any particular glass type also depends on the membrane thickness.

Process conditions greatly influence the chemical resistance of the glass membrane. Elevated temperatures and high salt concentrations (NaCO3 and CaCO3 dissolve the glass) or applications in strongly alkaline liquids generally shorten electrode life. Additives can be included during the manufacturing of the glass to make it more resistant to attack; electrodes suitable for measurements in either strongly acidic or basic liquids can be produced. In aggressive solutions, a heavy duty electrde with a thick dome-shaped glass membrane is preferable.

Reference Electrode

The accuracy of the pH measurement also depends on the properties of the reference electrode, which must be considered to make the correct selection.

A good reference electrode satisfies the following requirements:

  • The probe is insensitive to expected changes in the process liquid composition.
  • According to Nernst's Law, the output voltage of the internal reference probe must be the same as the internal reference probe on the pH measuring glass sensor.
  • The output voltage is stable.

Reference electrodes use a variety of reference systems, flow diaphragms and electrolytes.

Generally, reference electrodes rely on a silver/silver-chloride-potassium chloride (Ag/AgCl-KCI) system. The reference system consists of a silver wire elecrolytically coated with silver chloride (Figure 2). This metal-metal salt combination is dipped in a KCI solution.

Normal Reference System: This relies on a silver wire coated with silver chlorine.

Figure 2. Normal Reference System: This relies on a silver wire coated with silver chlorine.

An alternative construction more suitable for high-temperature applications dips the silver wire into a paste of Ag, AgCl and KCI (Figure 3). A plug of wadding soaked in KCI seals the paste into a tube. This reference assembly simlarly is dipped in a KCI solution.

The electrolyte used in the reference electrode must:

  • be chemically inert and neutral;
  • have a constant activity of ions;
  • be equi-transferent (i.e. the ions of the electrolyte must pass the diaphragm at the same speed); and
  • have a low electrical resistance

The most common electrolytes used in reference electrodes are: 1-M KCI solution (with or without gel); 3.3-M KCI solution; and saturated KCI solution. High viscosity KCI solutions suit high-temperature (>70°C) applications.

The conductivity of the 3.3-M KCl electrolyte solution is roughly three times higher than that of the 1-M KCI electrolyte solution. So, the same reference electrode will have a lower reference impedance. The high conductivity solution is an advantage for some ultrapure water applications that someimes show a high impedance error when using 1-M KCI to 3.3-M KCI solution, you must recalibrate the sensor. Without recalibration, readings will be off by approximately 0.5 pH.

High Temperature Reference System: Such a system uses of a plaste containing silvre, silver chloride and potassium chloride.

Figure 3. High Temperature Reference System: Such a system uses of a plaste containing silvre, silver chloride and potassium chloride.

Junction Issues

Selecting the correct type of junction for a reference electrode also is important because the junction maintains contact between the reference system in the electrode and the process liquid. The choice depends on the process conditions under which the electrode must function. The process liquid must not penetrate the electrode because this could cause poisoning and unstable reference potential.

Figure 4 shows the four available types of junction: two versions of a ceramic, polytetrafluoroethylene (PTFE), and glass-sleeve capillary element. In the first two types, the KCI solution flows slowly into the process. The flow rate depends on the overpressure in the electrode and the process temperature. The electrolyte flow rate rises with increasing temperature.

Non-flowing reference electrodes with a porous PTFE junction suit many dirty liquid applications. The dirt-resistant properties of PTFE will prevent complete fouling of the diaphragm.

However, with very dirty liquids, a glass-sleeve capillary element is preferred because of its larger flow surface. The sleeve easily can be cleaned by first moving the ground ring upwards and then wiping the ground faces.

To provide a stable reference value, tjhe electrolyte in the reference electrode must not be changed by penetration of the process liquid. However, some applications contain ions - e.g., mercury (Hg2+), copper (Cu+), lead (Pb2+) and silver (Ag+) - that will react with the KCI solution if process ingress occurs. The leads to an internal differential that results in errors in pH measurements.

Solving this problem requires separating the KCI solution and the process liquid using a double junction electrolyte, resulting in a reference electrode with a built-in salt bridge.

Processes containing cyanides, bromides, iodides or sulfides are a second example of where selecting the KCI solution is critical. (Many biological process liquids contain sulfur compounds.) In most cases, a black diaphragm indicates the reference electrode lacks a double junction. The black is a deposit of silver sulfide in or directly after the flow diaphragm.

Glass-in-Glass Reference Method

Such deposits can result in:

  • long response of the pH measuring circuit;
  • non-reproducible diffusion voltages and, consequently, drift in the indication;
  • extreme difficulty in calibration (because the formed diffusion voltage can be pH dependent); and
  • increased resistance of the diaphragm (slower measurement).

In processes with pressure variations, the composition of the electrolyte may change as a result of process liquid penetration into the electrode. Such a change may cause a measurement error or even poisoning of the reference system of the electrode. To alleviate this problem, consider an electrode with an integral pressure-compensation system. Various designs exist but ones using a bellows are most common.

In that design, a compressed bellows is positioned within the electrolyte vessel of the electrode. One side of the bellows is connected ot the outside pressure via a ceramic junction. The pressure inside the bellows equals the pressure outside; only the elasticity of the bellows itself causes the over-pressure that results in a flow of electrolyte. When the bellows is fully expanded, the electrolyte is exhausted and refilling is required. The bellows msut be compressed before refilling.

Pressure-compensated systems also can be used in processes with pressures below atmosphere; some designs can allow for 360° mounting capabilities.

Differential Measuring System

All designs of KCI reference electrodes are susceptible to the process coating or plugging the junction, causing an inaccurate or unstable reference voltage that results in pH measurement errors. Over time, depletion of internal electrolyte and poisoning by the process will affect a standard reference. This issue has led to development of an alternative reference design known as a differential reference system. There are two common approaches:

1. Glass-in-glass (Figure 5). This uses a standard glass electrode as the measuring electrode to generate a potential proportional to the process pH. The second glass electrode serves as the reference electrode and consists of an internal measurement electrode immersed in a stable buffer solution. The internal electrode makes electrochemical contact with the process via a salt bridge chamber (double junction) and generates a standard reference potential. Both glass electrodes have a common potential developed at the solution ground electrode.

This design eliminates the stability problems experienced with conventional references due to process poisoning of the reference element. However, because of the liquid junction interface with the process, plugging and coating problems still can occur.

Glass-in-Glass Reference Method

Figure 5. Glass-in-Glass Reference Method: Internal electrode reference electrode makes contact with process via a salt bridge chamber. Note: Reference Electrode (left) and Measuring Electrode (right)

2. Sodium or cation references (Figure 6). Its reference portion is made entirely of a glass that provides a mV output corresponding to the sodium or salt (cation) concetration of the process or solution it's in. Therefore, as the sodium or cation salt concentrations change, so will the reference voltage it generates.

This design avoids many of the challenges encountered in pH applications. There's no liquid junction to be coated or clogged and no path for the process to affect the electrolyte or the internal Ag/AgCl element. This makes the sensor truly maintenance free.

Cation Reference Method

Figure 6. Cation Reference Method: The reference portion in the reference electrode is made completely of glass. Note: Reference Electrode (left) Measuring Electrode (right)

Choose Correctly

Careful consideration of the demands of the specific application undperpin achieving an accurate, reliable measurement with a sensor that provides reasonable life expectancy while minimizing required routine maintenance. If you're frequently calibrating or replacing a pH sensor, then there's a good chance it's not the right combination for the process. Many general-purpose wide body (all-in-one) electrodes will work very well for many applications. However, some services do better if you choose individual electrodes with unique glass design and reference systems not available in an all-in-one design. Use the quick reference charts in Figure 7 as a best practice guide to find the recommended sensor based on the specifics of a particular application.

Selection Guide: These charts highlight factors that suggest a particular type of sensor for a given application.

Figure 7. Selection Guide: These charts highlight factors that suggest a particular type of sensor for a given application.



The lifetime of a pH sensor has a significant impact on the overall annual costs of a pH measuring loop. Optimizing four key factors will decrease these costs and optimize process control and overall plant efficiency.

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