pH Analyzer - Frequently Asked Questions (FAQ)
METTLER TOLEDO pH analyzer and ORP analyzer systems are quality process pH analyzers for measuring in-line pH or in-line ORP (redox). Our portfolio offers a variety of different pH and redox probes and analyzers for a range of applications:
- Hygienically designed pH analyzers for food and pharmaceutical
- pH and ORP analyzers for pure and ultrapure water
- ATEX and FM approved industrial pH analyzers
- Digital pH and ORP analyzers with predictive diagnostics
- Non-glass probes for industrial pH analyzers
METTLER TOLEDO has manufactured in-line pH sensors since 1948, and we have been developing our expertise ever since. Some key questions regarding in-line pH analyzer selection include:
What is a pH analyzer?
A pH analyzer is a system used for in-line pH measurement, typically in an industrial process. A pH analyzer generally consists of three components: a pH sensor, a transmitter, and a process adaption or housing unit. Together these three parts make up a process pH analyzer used for control of pH in an industrial environment. METTLER TOLEDO provides a wide range of different pH sensors to meet your in-line pH control needs. Additionally, many of our pH sensors also offer a redox measurement; so that integrating a pH analyzer also lets you integrate an ORP analyzer.
What is pH?
pH (potential of hydrogen) is a figure used to express the acidity or alkalinity of a solution on a logarithmic scale. On this scale 7 is neutral; lower values are more acidic and higher values are more alkaline, with a maximum measurement of 14. In process applications, pH is generally measured with an in-line pH analyzer, thisis most commonly integrated using a glass combination electrode for industrial pH measurement. Additionally, an inline pH probe generally requires a process adaption, cable and transmitter to complete the pH analyzer.
A typical process pH sensoris made up of two separate electrodes built into one, a pH sensing electrode, and a reference electrode. In the simplest terms, a pH sensing electrode uses a special pH sensing glass membrane. H+ ions permeate the membrane creating a charge. The potential between the two electrodes is the measurement of hydrogen ions in the solution, giving the measure of pH. For more details, download the free pH Theory Guide.
Absolutely nothing! The three terms are used interchangeably in the industry. They can be used for pH probes that are used on industrial pH analyzers or in laboratory measurement. You may also hear the term "pH meter". This can be used for a piece of laboratory equipment, or the term pH meter can also be used interchangeably with in-line pH analyzer.
How do I select the right pH probe for my pH analyzer?
For optimal pH measurement it is crucial to choose the right pH probe for each pH analyzer. The most important criteria are: chemical composition, homogeneity, temperature, process pressure, pH range and container size (length and width restrictions). The choice of pH sensor becomes of particular importance for non-aqueous, low conductivity, protein-rich and viscous samples where general-purpose glass electrodes are subject to various sources of error. The response time and accuracy of an in-line pH analyzer is dependent on a number of factors. Measurements at extreme pH values and temperatures, or low conductivity may take longer than those of aqueous solutions at room temperature with a neutral pH value.
How long does a pH probe last?
An inline pH probe lasts for varying amounts of time, depending on the conditions surrounding your in-line pH analyzer. A variety of factors play a role in the life of your pH sensor, including the quality of the pH probe, the media that it is in, the temperature it measures in and how well it is maintained. A well maintained pH probe generally lasts 6 months to 1 year in a routine, moderately clean application. While there are significant variances in the lifetime of pH probes, intelligent, digital sensors with ISM technology use an algorithm to determine the remaining lifetime on your pH sensor. pH analyzers with ISM technology predict sensor malfunction before it happens and can therefore dramatically reduce downtime.
How do I maintain the sensor on my in-line pH analyzer?
Most pH sensors operate maintenance free, but there are some clear examples of where maintenance is required. For example, in certain applications (such as sugar production) where an industrial pH analyzer has its sensor in a viscous solution, clogged diaphragms on the pH sensor need to be cleaned. Certain solutions (such as those used in varnish production) can also coat the pH sensor's membrane glass, causing a reduction in permeability. Without cleaning in these situations, you will get incorrect readings and reductions in sensor lifetime.
Another specific example is in pH analyzers that use a pH probe with refillable-liquid electrolyte. These pH analyzersneed the electrolyte to be topped-up when the level becomes close to being lower than the level of the sample solution. This maintenance prevents a reflux of the sample into the probe. The complete reference electrolyte should also be regularly changed, approximately once a month. This ensures that the electrolyte is fresh and that no crystallization occurs due to evaporation from the open filling port during measurement. It is important not to get any bubbles on the inside of the probe, especially near the junction. If this happens, the measurements will be unstable. To get rid of any bubbles, gently shake the probe in a vertical motion as with a fever thermometer.
How do I clean a process pH probe?
To clean a process pH probe, rinse it with deionized water after each measurement but never wipe it with a tissue. The surface of the paper tissue can scratch and damage the pH-sensitive glass membrane, removing the gel-layer and creating an electrostatic charge on the electrode. This electrostatic charge causes the measured signal to become very unstable. Special cleaning procedures may be necessary after contamination with certain samples. Some process pH analyzers can be integrated with a cleaning automation system that simplifies the cleaning process. This is particularly valuable in industrial pH monitoring applications where access to the sensor is limited.
How often should I calibrate and maintain a pH probe?
Regular maintenance and calibration of your pH probe are critical. How and how often to calibrate and maintain the probe on your pH analyzer depends on both the probe and the usage. In general, the best way to know when your pH analyzer needs calibrated or maintained is to use an intelligent pH sensor and transmitter with ISM technology. These pH analyzers use an algorithm to monitor the sensor's health and provide a measurement of Time to Maintenance and an Adaptive Calibration Timer.
How do I store a pH sensor?
pH sensors should always be stored in reference electrolyte. This allows immediate use of the pH sensor when needed, and ensures a short response time. When stored dry for long periods, many pH sensors must be reactivated by soaking for several hours before installation in an in-line pH analyzer in order to get the optimal measuring results. If these measures are not sufficient, the electrode may be made functional by treating it with a special reactivation solution followed by subsequent conditioning in the reference electrolyte. Do not store a pH sensor in distilled water, as this will cause the pH sensor to have a longer response time.
What is redox potential? What is ORP?
Redox is short for reduction-oxidation reaction. It is a chemical reaction that changes the oxidation states of atoms. ORP stands for oxidation-reduction potential. Redox potential and ORP are two ways of saying the same thing. It is a millivolt measurement of the tendency of a chemical substance to oxidize. While the pH value measures the concentration (more precisely, activity) of hydrogen-ions, the ORP value is determined by the electron activity.
How does an ORP analyzer work?
An ORP analyzer uses an in-line ORP probe or redox probe. At METTLER TOLEDO, many digital pH sensors also provide an ORP output, so the sensor operate as part of both a pH analyzer and OPR analyzer. The ORP sensor works by measuring the difference between electron activity related to a noble metal like platinum or gold and a reference system that provides a stable potential, like those used in a pH measurement.
What is the shelf life of a pH probe or an ORP probe?
Whether used in an in-line pH analyzer or an in-line ORP analyzer, if a pH sensor or ORP sensor is stored at normal room temperature with its original storage cap and solution in place on the end of the electrode, it can last well over a year with no degradation in performance. If the storage solution is allowed to dry or leaks out due to storage at high temperatures, freezing or other causes, the life of the probe may be reduced significantly.
What should I look for in a pH analyzer for pure water applications?
pH measurement in pure water requires a pH electrode that ensures a consistent, stable measurement. Sensors like METTLER TOLEDO's pHure pH sensor feature a special low-resistance pH glass membrane, shielded flow chamber and a continuous outflow of electrolyte. Features like these and others allow for the stable and accurate measurement of pH and ORP that is required in pure water applications at power plants and microelectronics facilities.
pH Probe FAQS
1. What's the lifetime of a pH probe?
The expected lifetime of a correctly used and maintained pH probe is around one to three years. Some factors such as high temperature and measuring at extreme pH values contribute to a reduction of the lifetime even for probes that have been well maintained and properly stored. When a meter starts performing poorly, it may be possible to regenerate the pH-sensitive glass membrane and restore the electrode to its previous level of performance.
2. How do you select the correct pH probe?
For optimal pH measurement it is crucial to choose the right pH probe for each application. The most important sample criteria are: chemical composition, homogeneity, temperature, process pressure, pH range and container size (length and width restrictions). The choice of sensor becomes of particular importance for non-aqueous, low conductivity, protein-rich and viscous samples where general purpose glass electrodes are subject to various sources of error. The response time and accuracy of an electrode is dependent on a number of factors. Measurements at extreme pH values and temperatures, or low conductivity may take longer than those of aqueous solutions at room temperature with a neutral pH value.
3. How should you maintain/clean a probe?
Regular maintenance is very important for prolonging the lifetime of any pH probe. Probes with liquid electrolyte need the electrolyte to be topped-up when the level threatens to become lower than the level of the sample solution. This maintenance prevents a reflux of the sample into the probe. The complete reference electrolyte should also be regularly changed, approximately once a month. This ensures that the electrolyte is fresh and that no crystallization occurs due to evaporation from the open filling port during measurement. It is important not to get any bubbles on the inside of the probe, especially near the junction. If this happens the measurements will be unstable. To get rid of any bubbles, gently shake the probe in a vertical motion as with a fever thermometer.
To clean the probe, rinse it with deionized water after each measurement but never wipe it with a tissue. The surface of the paper tissue can scratch and damage the pH-sensitive glass membrane, removing the gel-layer and creating an electrostatic charge on the electrode. This electrostatic charge causes the measured signal to become very unstable. Special cleaning procedures may be necessary after contamination with certain samples.
pH Probe Selection and Handling
For optimal pH measurements, the correct probe must first be selected. The most important sample criteria to be considered are:
- chemical composition
- process pressure
- pH range
- container size (length and width restrictions)
The choice becomes particularly significant for non-aqueous, low conductivity, protein-rich and viscous samples where general purpose glass probes are subject to various sources of error.
The response time and accuracy of a pH probe is dependent on a number of factors. Measurements at extreme pH values and temperatures, or in low conductivity samples may take longer than those of aqueous solutions at room temperature with a neutral pH and high conductivity. The significance of the different types of samples is explained below by taking the different probe characteristics as a starting point.
Different kinds of junction
The opening that the reference part of a pH probe uses to maintain contact with the sample can have several different forms. These forms have evolved through time because of the different demands put on the probes when measuring diverse samples. The “standard” junction is the simplest one and is made from a ceramic material. It consists of a porous piece of ceramic which is pushed through the glass shaft of the probe. This porous nature allows the electrolyte to slowly flow out of the probe, but stops it from streaming out freely.
This kind of junction is very suitable for standard measurements in aqueous solutions. The METTLER TOLEDO InPro 325x series is an example of a pH probe with a ceramic junction. Even though this is probably the most widely used junction because of its simplicity of use with aqueous solutions, it has one main drawback: Because of the porous structure of the junction it is relatively easy for particles in a sample to block the junction, especially if the sample is viscous or if it is a suspension.
You also have to be careful with some aqueous samples such as those with a high protein concentration, as proteins may precipitate within the ceramic junction if they come in contact with the reference electrolyte, which is often KCl. This reaction will cause the porous structure to become filled with protein debris, blocking the junction and rendering the pH probe useless. Measurements are not possible if the electrolyte cannot flow freely since the reference potential will no longer be stable.
The same problem can also be caused if the inner electrolyte reacts with the sample solution being measured and the two meet in the junction. This reaction can create a precipitate which may block the junction, for example if KCl electrolyte saturated with AgCl is used with samples containing sulfides, the silver and sulfides react to form Ag2S which then blocks the ceramic junction. Factory-filled, prepressurized liquid / gel electrolyte pH probes are suited to a wide scope of applications in the biotechnology, pharmaceutical and chemical process industries. This design ensures the best possible measurement performance under the most diverse operating conditions.
PTFE annular diaphragm
An annular PTFE diaphragm, instead of a ceramic junction, increases the surface exposed to the media to prevent clogging of the diaphragm. Highly contaminated process conditions makes pH measurement and control a complicated issue. An annular PTFE reference diaphragm (e.g. as in METTLER TOLEDO‘s InPro 480x series) is designed for service in tough environments. It resists fouling from hydrocarbon contaminants and sulfides, ensuring high accuracy and fast response throughout its long life. For process media containing particles and aggressive chemicals, the optional flat glass membrane pH probe is the optimal solution.
The third type of junction is the open junction. This means the reference electrolyte is completely open to the environment and is in direct contact with the sample solution. This is only possible with a solid polymer reference electrolyte.
The great advantage of this junction type is clearly the fact that it is unlikely to clog. Open junctions can easily cope with very dirty samples and constantly provide good measurements. However, the solid polymer reference electrolyte which is used for this open junction has a slower reaction time and low electrolyte flow. This necessitates that the samples measured have a high enough ion concentration for stable measurements to be possible. Nevertheless, these probes are suitable for most samples and are very robust.
Dual-membrane without junction
The cell membrane chlor-alkali process is very tough on conventional pH probes. It exposes them to high temperatures, and clogging and poisoning from a variety of compounds. This is particularly true in the anode side of the process' electrolysis cell. Chlorine diffuses through the probe’s diaphragm and attacks the reference system. This results in incorrect pH measurement and shorter sensor lifetime.
Reliable pH measurement can be achieved with sensors such as the InPro 4850 i from METTLER TOLEDO. This is a dual-membrane pH probe that has been designed specifically to provide long-term accurate measurement in chlor-alkali applications. The main difference in measuring technology between dual-membrane pH probes and conventional pH probes is the presence of a sodium-reference (pNa) system. Such probes feature a sodium-sensitive glass membrane which is charged by the sodium ions in the process medium. The sodium concentration in the brine is used as the reference. The pNa reference system is hermetically sealed: there is no diaphragm; therefore, no oxidants can enter the probe to attack the reference system. These probes also feature a high-alkali-resistant pH membrane glass for pH measurement. It is the amalgamation of pH measurement and pNa reference that is the reason that this kind of probe is highly suited to chlor-alkali processes.
In applications where precise measurements are important, refillable pH and ORP probes deliver high accuracy and long lifetimes.
Sterilizable, hygienically designed pH and redox (ORP) sensors offering high measurement accuracy in pharmaceutical, biotech and food applications.
Specialized, highly durable pH and redox (ORP) probes designed to perform reliably under harsh conditions in chemical applications.
Unique pH probes designed for reliable pH measurement in low conductivty, ultrapure and pure waters.
Refillable electrolyte for longer sensor lifetime
Ability to select application specific electrolyte
Silver-ion trap prevents sulfide poisoning
ISM diagnostics indicate maintenance timelines
Available in pH and ORP/Redox models
Designed for regular SIP and CIP processes
For use in bioreactors with high pressures
Diagnostics help prevent batch interruptions
Unique models for upside-down installation
Liquid, gel and polymer reference electrolytes
For use in difficult industrial environments
Withstands abrasive, sticky or oxidizing media
Diagnostics help manage sensor cleaning efforts
Titanium models prevent sensors from breaking
Liquid, gel and polymer reference electrolytes
Designed for water treatment applications
Free-flowing liquid electrolyte for reliability
Eliminates need for flow bowl for high accuracy
Plug and measure technology for fast set-up
Available in pH and ORP/Redox models
ISFET pH probes offer a non-glass solution for industries such as food or biotechnology, where safety and hygienic design are paramount.
Easy-to-use, portable pH meters for industrial applications. They offer high measurement accuracy in a robust, weatherproof design.
ISM accessories such as software or transmission solution allow you to take full advantage of ISM technology.
Gamma-sterilizable sensors in single-use format for measuring pH and DO. Designed for biotech companies that manufacture single-use bags.
Unbreakable, solid-state ISFET technology
FDA and 3-A compliant materials
Hygienic design is EHEDG tested
One-handed operation and an intuitive interface
Weatherproof and robust for demanding environments
Rigorously drop tested to ensure durability
Easy data management with 2,000 stored data points
Calibrate your sensors away from the process
Monitor sensor history, diagnostics & inventory
Easy system verification with sensor simulators
Connect sensors to a computer or mobile
Suitable for use in a wide range of applications
Compatible with standard METTLER TOLEDO transmitters
Long-term measurement stability and performance
Designed for 1" Eldon James port disc