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Mettler-Toledo AG Process Analytics

Good Operating Proceduresfor pH Sensors

Quality Measurements for In-Line Applications

2 Good Operating Procedures for pH SensorsMETTLER TOLEDO

pH M

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ent Introduction

pH is the most common liquid process analytical parameter. Its mea-surement and control are important across the process industries in-cluding pharmaceuticals and chemicals production. Typically, pH is measured using glass pH sensors, an instrument that has been avail-able for almost 100 years. Over this time, many developments have been introduced to improve sensor measurement accuracy, durability and ease of use. Today, pH sensors for industrial use are high perfor-mance devices, tailored for specific applications.

This Good Operating Procedures guide explains how to correctly use and maintain pH sensors. The guide also describes how greater mea-surement accuracy and performance are possible from METTLER TOLEDO’s Intelligent Sensor Management (ISM®) digital sensor platform.

Urdorf, Switzerland, February 2015

3Good Operating Procedures for pH Sensors METTLER TOLEDO

pH sensor structure

Modern industrial pH sensors comprise a pH glass electrode in combi-nation with a reference electrode. The transmitter measure the potential between the pH glass and the reference electrode and calculates the pH values.

The most important component of a pH sensor is its pH-sensitive glass, also called the glass membrane. A thin gel layer forms on the outside of this glass when it is in contact with a solution. A similar gel layer is also formed on the inside of the pH-sensitive glass, where the inner buffer is present.

The hydronium ions in the measurement solution can either diffuse into or out of the outer gel layer, and by doing so alter the charge of the layer. This charge generates a potential difference between the outer and inner gel layers, which is used to calculate the pH of the solution. There is no hydronium ion transport through the glass and therefore the expression “glass membrane” is a little misleading.

The reference electrode is in direct contact with the measuring solution via its liquid junction. A common liquid junction is a ceramic dia-phragm with a defined pore size.

Referenceelement

Lead-offelement

Referenceelectrolyte

Inner buffer

Socket

Membrane

Junction


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ent Sensor placement

When a pH measurement system is installed close to a motorized valve, pump or strong electromagnetic field, a high noise to signal ratio can be generated in the sensor cable. This will result in a fluctuating measurement on the connected instrument (transmitter). Therefore, the measurement system should be installed away from such equipment and fields.

Cable length

The high impedance mV signal output by pH sensors will degrade over long cable lengths. Keeping the distance between the sensor and the transmitter short (less than 10 m) will prevent loss of signal integrity.

Calibration

Sensor aging and / or clogging on the sensor due to process condi-tions, will result in a decrease in measurement system accuracy. To keep a pH sensor measuring reliably, periodic calibration is required.

Calibration is the comparison of a calibration standard of known accu-racy (e.g. pH buffer solution) with another instrument of unknown ac-curacy (e.g. a pH sensor) to detect, correlate, report or correct any vari-ation in the accuracy of the item being compared (e.g. a pH transmitter). Consequently, the term “pH sensor calibration” is a misno-mer as it is the transmitter attached to the sensor that is calibrated. However, as we will see, the advent of digital technology has changed this and in ISM systems it is indeed the sensor that is calibrated.


Accurate calibration requires the use of a traceable set of buffer solu-tions with a pH established by international agreement. The National Institute of Standards and Technology (NIST) buffers are widely recog-nized as “the standard” for pH measurement system calibration.

NIST

Primary Standards

Secondary Standards

Working Standards

Process Instrument

However, due to the cost and short shelf life of NIST standards, working standards (typically buffers that contain colored dye to identify them) are commonly in use in industrial applications. Buffer manufacturers, such as METTLER TOLEDO, take care that working standards are trace-able to secondary standards, which are traceable to primary standards and finally traceable to NIST.

The output of a pH sensor is temperature dependent; hence, most in-dustrial pH sensors contain a temperature sensor. This allows the con-nected transmitter to appropriately adjust the displayed pH value during calibration as well as when the sensor is measuring in the process.


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ent 2-point calibration

A pH sensor is primarily characterized by its zero point (its mV output when in a solution of pH 7) and its slope (its mV output plotted against the pH scale). To ensure a transmitter is displaying accurate measure-ments, calibration is required using two buffer solutions (2-points). Most pH measurement ranges are either above or below neutral. Therefore, one of the buffers should be pH 7 (to adjust the zero point) and the other should be at or beyond the extreme of the range expected in the process.

mV pH7

Two-point calibration at 7.0 and 9.21

UncalibratedCalibrated

Zero Point

Calibrated range7.0 – 9.21

Multi-point calibration

For greater accuracy, and particularly for situations with a wide ex-pected pH measurement range, a multi-point (three or more points) calibration should be performed. Of course, this requires the use of a transmitter that allows such calibrations.


1-point calibration

A 1-point calibration means adjusting the sensor’s off-set. As this type of calibration does not account for a sensor’s slope, measurements may not be as accurate as they would after a 2-point calibration.

Process calibration

A sample of the media can be measured in the laboratory and, based on the lab pH measurement, the transmitter can be adjusted. A prob-lem in process calibrations can arise if there is a temperature difference between the process temperature and the temperature of the sample in the lab. The temperature dependence of the process pH sensor as well as the lab pH sensor can be compensated for according to the Nernst equation, but the temperature dependence of the particular media is of-ten not known. Therefore, it is very important to execute the lab mea-surement at the same temperature as the process. Failure to account for a temperature difference can result in an error of up to 0.2 pH units.

Further information can be found in our Accuracy Guide c www.mt.com/pro-accuracy-guide

Calibration and USP < 791 >

The pharmaceutical industry has the strongest regulations of all the process industries and the monographs published by the United States Pharmacopeia (USP) are the most widely used. In December 2014 it revised its < 791 > pH general chapter.


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ent Automatic buffer recognition and buffer pH temperature correction are

acknowledged in the updated chapter, and a new section on calibra-tion has the most changes from the previous version. The updated method requires a 2-point calibration utilizing appropriate buffers, and after calibration / adjustment, a third pH point is then verified using a buffer between the two calibration points, with an allowed accuracy of ± 0.05 pH at the verification point.

After completion of the 2-point calibration process, verify that the pH slope and offset are within acceptable parameters. Typical acceptable parameters are slopes ranging from 90 % – 102 % and an offset of ± 30 mV (0.5 pH units at 25 °C). Depending on the pH instrumentation, the pH slope and offset may be determined in software or by manual methods. If these parameters are not within acceptable parameters, the sensor should be properly cleaned, replenished, serviced, or replaced, and the 2-point calibration process repeated.

USP37-NF32 S2

Diagnostic functions such as glass or reference electrode resistance measurement may be available to determine equipment deficiencies. Refer to electrode supplier for diagnostic tools to ensure proper elec-trode function.

USP < 791 > is more focused on lab pH measurement, but on-line pH measurement is now recognized. Finally, this revised monograph describes elements which are already implemented in modern pH measuring systems, such as:

• Automatic buffer recognition and automatic temperature compensa-tion, as implemented in METTLER TOLEDO transmitters and iSense software (see below).

• Diagnostic tools are recognized as valuables features. • Digital pH sensors are now mentioned.


Calibration best practice

To achieve measurement accuracy, it is important to note the following:• Only calibrate a clean pH sensor. Check the liquid junction for clog-

ging.• Only use buffer solution within its shelf life.• Rinse the sensor with deionized water and carefully dry it with tissue

paper.• Pour fresh buffer solution into a beaker and dispose of it after cali-

bration. Never put the sensor into the buffer bottle.

pH sensor maintenance

Clean after useRinsing the sensor with water (cold or warm) will do the job in many cases. Using a soft brush can be used to support the rinsing. If an in-soluble coating is on the sensor, a very dilute acid or alkaline solution can help remove it. In cases of insoluble organic coatings, a solvent can be used.

Dry storage of a pH sensor after cleaning must be avoided. Store the sensor in 3 M KCl solution or in the particular reference electrolyte used for liquid-filled sensors.

Check the junction (diaphragm)Today’s sensors are equipped with a silver-ion barrier which prevents a sensor’s diaphragm turning black due to the presence of Ag sulfide. A junction that is blocked with proteins can be cleaned by immersing the sensor in a pepsin / HCl solution for several hours. Pepsin is a product of animal origin and the use of this cleaning solution might be ex-cluded in certain pharmaceutical applications.

Sensor regenerationThe gel layer that builds up on pH-sensitive glass becomes thicker (which slows measurement response) due to normal sensor aging.


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ent With a regeneration solution containing ammonium bifluoride, the gel

layer can be removed. This etching process is based on the reaction of diluted hydrofluoric acid with the glass and the gel layer. Do not leave a sensor in a regeneration solution for longer than two minutes. After treatment, allow the sensor to rebuild a new gel layer by immersing it in 3 M KCl / L for at least 24 hours.

Site audits

Periodic auditing of pH measurement systems will identify any issues regarding cables, connections, etc. The use of pH simulators and digi-tal calibrators for digital measurement systems will verify that transmit-ters are operating correctly.

Intelligent Sensor Management (ISM)

ISM is METTLER TOLEDO’s digital platform for process analytics sys-tems. ISM sensors contain a microprocessor in the sensor head. This feature allows a number of significant advantages which analog sys-tems cannot provide. Some of these are outlined below.


Robust digital signalUnlike analog systems where the pH value is determined in the trans-mitter, ISM pH sensors calculate the value directly in the sensor itself (resulting in greater measurement accuracy) then output this digitally for display by the transmitter or control system. As the signal is digital, it is unaffected by electromagnetic fields and is reliable over long cable runs.

Sensor calibration in the labCalibrating pH sensors at the measurement point is inconvenient and can compromise staff safety. Using iSense PC software (see below), a pH sensor can be accurately calibrated in any convenient location. The sensor’s microprocessor retains the calibration data, so once calibrated an ISM pH sensor can be stored until it is required.

Plug and MeasureWhen a calibrated ISM pH sensor is connected to an ISM transmitter, the stored calibration data is automatically uploaded to the transmitter, which configures itself without any operator intervention. This not only reduces the time an instrument engineer has to spend at a measure-ment point, it also significantly reduces the possibility of human error.

Predictive diagnostics for efficient maintenanceKnowing when to calibrate, clean or replace a pH sensor is difficult to determine. To be on the safe side, instrument engineers will often con-duct sensor maintenance even though it may not be required.

On the other hand, the failure of a pH sensor during a batch process can be hugely detrimental. Operators need to know that a sensor will operate reliably until the process is finished. ISM sensors contain ad-vanced algorithms that provide expert diagnostics information that is particular to each application.


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ent The key ISM diagnostics

tool is the Dynamic Lifetime Indicator (DLI). The DLI provides operators and instrument engineers with a clear indication of how much the exposure to a process has altered a sensor’s condition. In the case of pH sensors, by continuously analyzing the process conditions and other factors, the DLI con-stantly calculates the re-maining reliable lifetime of the sensor. If process con-ditions become more or less harsh, the DLI rapidly

responds appropriately. In addition, the DLI actually adapts to process conditions to ensure diagnostics data is always reliable.

Through observing the DLI (or via transmitter alarms), pH sensors with a short remaining lifetime can be replaced preemptively before they fail during operation, resulting in improved safety, higher process integrity and fewer product quality fluctuations.

Using information from the DLI, two other ISM diagnostic tools inform users when sensor calibration (Adaptive Calibration Timer) and cleaning (Time To Maintenance) will be required.

ISM diagnostics allow a measurement point to be optimized on an on-going basis and for all critical situations to be predicted, so that instru-ment engineers can respond before production is affected. And be-cause measurement point maintenance only happens when it is required, operators can be certain that resources are not being wasted.


Understanding ISM’s diagnostics tools does not require the interpretation of complex data. The iMonitor diagnos-tics display on the M800 transmitter and iSense software uses traffic light color coding to give an at-a-glance overview of sensor “health” in the field or in the maintenance shop.

As well as being viewable on ISM transmitters and iSense, the diag-nostic tools can also be integrated into asset management systems for remote monitoring.

iSense – a digital sensor expert

iSense is Windows-based software for ISM sensors that runs on PCs, laptops and mobile devices. It is the hub for all ISM sensor activities, including calibration and maintenance, and provides a fully controlla-ble method of managing sensors and maximizing their use.

All calibration and maintenance routines on iSense are accompanied by easy-to-follow animations. These reduce the requirement for training and ensure that every procedure is performed without mistakes or missed steps.

A few of iSense’s main features are outlined below.

Greater diagnostics accuracyWhen a pH sensor is connected to iSense, detailed information (e.g. sensor slope and zero point) are used to fine-tune the DLI. This pro-vides greater diagnostics accuracy when the sensor is reinstalled in a process.

Further, the DLI and other vital sensor data are also viewable and re-cordable on iSense. This gives users a single point which archives the condition of the installed sensor base.


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Transfer knowledge between sensorsIn some applications process conditions mean that it can take a few days for sensor diagnostics to stabilize.

An ISM sensor can learn from another sensor that has already been used in an application. For example, when a pH sensor is removed from a process and is connected to iSense, information on the condi-tions of that particular process can be stored as an application profile. This profile can then be transferred into a different pH sensor.

When this second sensor is installed in the same process, because of the transferred knowledge, it does not need time to acclimatize and al-ready knows when it will need to be serviced or replaced. If conditions in the process alter, the sensor diagnostics adjust themselves appropri-ately.

Convenient calibration, easy traceabilityDue to the location of measurement points, sensor calibration is often inconvenient and it can even compromise staff safety through possible exposure to hazardous environments. iSense offers accurate sensor verification and calibration in any convenient location such as a lab or maintenance shop. And because ISM sensors retain their own calibra-tion data, sensors can be easily and quickly exchanged in the field (see Plug and Measure above).


iSense’s electronic user management logbook allows control and tracking of all sensor activities, ensuring complete documentation of ISM sensors over their lifetime, to meet internal or external require-ments.

For the stringent document requirements of the pharmaceutical indus-try, iSense is available in a 21 CFR Part 11 compliant version which incorporates electronic signatures, an unalterable database as well as an audit trail.

Guided servicingAll calibration and maintenance procedures on iSense are accom-panied with animations on the screen. These show step by step which tasks the instrument engi-neer needs to perform. This not only provides assistance for new users but also ensures reproduc-

ible results with experienced operators.

Selecting the best pH sensor for your processes

When choosing a pH sensor for an application, the following points should be considered:• Process conditions: pH range, temperature, pressure• Membrane glass: A41 / HA / LOT / HF• Type of reference electrolyte: liquid/liquid pre-pressurized / gel / poly-

mer• Type of liquid junction: ceramic diaphragm / open aperture / PTFE• Outer dimension: must fit to housing• Analog or digital: must fit to transmitter, RTD must also fit to trans-

mitter inputs


pH M

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ent It should be noted that the glass membrane of an ISM sensor is identi-

cal to its analog counterpart. One noticeable difference between the two sensor types is that every ISM pH sensor includes a solution ground. This is required for many diagnostic parameters but also en-ables the measurement of the redox potential (ORP) of a solution.

Table showing some METTLER TOLEDO pH sensors

Application pH sensor Membrane Reference Comments glass

Pharma / Biotechnology InPro 3253 i A41 Liquid, High performance fermentation / cell cultures pre-pressurized low maintenance

Industrial biotechnology InPro 2000 i A41 * Liquid, Problem solver pressurized in electrode housing

Pharma buffer preparation InPro 3250 i HA Liquid, Fast response time chromatography pre-pressurized

Chemical industry InPro 2000 i HA * Liquid, Problem solver pressurized in electrode housing

Chemical industry InPro 4260 i HA * Polymer, Open junction open junction eliminate clogging

Chemical industry InPro 4800 i HA * Gel, PTFE For harsh annular junction environment

Chlor-alkali InPro 4850 i HA Na+ sensitive Uses sodium glass concentration in brine as a reference

* Other membrane glasses available

List is not complete. Ask our sales engineers for further options.


Conclusions

Many processes in the pharmaceutical and chemical industries require the measurement and control of pH. Dependable pH measurement is reliant on pH sensors being used and maintained correctly. Good oper-ating practices, including regular calibration, are vital to ensuring mea-surement reliability.

Developments in sensor design mean that almost all applications are compatible with pH sensors. The advent of digital technology offers significant benefits in respect of sensor maintenance, ease of handling and measurement accuracy.

Regardless of whether analog or digital sensors are being used, follow-ing a few basic points will provide a stable base for reliable pH mea-surement:• Keep the liquid junction clean• Only calibrate clean sensors• Use fresh buffer for calibration• Store sensors in reference electrolyte

For more informationwww.mt.com/pro

Mettler-Toledo AGProcess AnalyticsPhone +41 44 729 62 11Fax +41 44 729 66 36

Subject to technical changes© 04 / 2015 Mettler-Toledo AGPrinted in Switzerland.

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