As you read this blog, thousands of pH measurements are being performed around the world. They often adhere to the same pattern: manual sampling followed by laboratory analysis — meaning inherent disadvantages including increased time expenditure. In contrast, production processes typically require complex analytical techniques and equipment to obtain highly accurate results used for process optimization and control. But what if an exact snapshot of the process is needed to obtain the highest quality information? How do you scale up analysis from the laboratory to the process (i.e., perform analysis under process conditions) and yet still achieve highly accurate results and low measurement uncertainty? The key lies in process analytical technology (PAT) and the use of, e.g., process sensors that enable close and continuous process monitoring and control.
pH measurement… in the process?
pH is probably one of the most widely used measurement parameters. It plays a central role in all areas of chemistry, pharmaceuticals, biotechnology, foods and beverages, and more. All of these industries require the use of suitable process sensors which play an extraordinary role in production.
In practice, the measurement of voltage with pH-sensitive electrodes is an old but still valid principle used daily in laboratories worldwide. But inline pH measurement, i.e., the determination of the pH value directly in the process, also holds great potential for process optimization. It enables real-time process monitoring without prior manual sampling. This improves product yield and quality, saves time and business costs, and increases the safety of the process.
For several years, the trend has been steadily shifting «from the lab to the process» with increasing urgency. The possibility of scaling up production processes and associated qualitative and quantitative measurements allows higher yields, avoidance of out-of-specification batches, real-time release instead of post-analysis, and much more. Figure 1 shows how production can be monitored and controlled close to the process (atline), with automated sampling (online), or directly in the process stream itself (inline). Over the past decades, laboratory analysis (offline) has thus gradually evolved in the direction of process analysis.
When samples are brought from the process sampling point to the laboratory for analysis, they can change during this time due to, e.g., ambient temperature change, absorption of CO2 from the atmosphere in the solution, pressure differences, or (absence of) flow. With inline and online measurement technology, the most up to date parameter values are directly available in the distributed control system (DCS). This connection is what makes reliable process control possible in the first place, and erroneous data due to sampling errors and delays in measurement can be largely ruled out.
Read more about the benefits of process automation in our previous blog post.
The question of whether or not (as well as how) pH determination in the laboratory can be automated and mapped in the process was something of concern for Merck KGaA Darmstadt. Some of the following requirements had to be considered to answer this:
- Request of a fully automated precise pH calibration and adjustment measuring system
- Data should be reproducible to the lab results, with absolute accuracy
- Electrode testing and evaluation possibilities
- Multi-point calibration with testing in a predefined buffer
- Sensor cleaning possibility
- Compliance to GMP requirements
- Automatic certificate generation → the same conditions as in the laboratory—from laboratory to process!
For this feasibility study, a 2060 TI Process Analyzer from Metrohm Process Analytics was configured for atline analysis to simulate the same pH measurements as those performed in their laboratory.
Learn more about the 2060 Process Analyzers from Metrohm Process Analytics here.
Test setup and requirements
Accurate pH measurements in the process with ProTrode sensors
Metrohm is well known as a market leader for supplying high-precision electrodes, especially for the laboratory. Decades of our laboratory expertise have been used to develop a pH sensor that also meets the complex and demanding requirements from the process. Metrohm Process Analytics now offers inline ProTrode pH sensors (Figure 2) that are suitable for all industrial applications where robust and accurate inline pH measurements are required.
ProTrode pH sensors offer many advantages:
- unique sleeve diaphragm suitable for varying sample flows – enables highly precise and reproducible measurements for industrial applications
- various length options available (120 mm, 225 mm, 325 mm, 360 mm, 425 mm) – suitable for all kinds of situations (e.g., vessels, tanks, reactors, pipes)
- special reference system – integrated reference makes pH measurements even easier
Learn more about the family of inline ProTrode pH sensors here.
In the Merck KGaA feasibility study introduced in the previous section, the laboratory analysis was initially replicated with the aid of the 2060 Process Analyzer using a ProTrode pH sensor on a trial scale. In the future, the system can be integrated directly into the process as shown in the illustration below (Figure 3).
ProTrode inline pH sensors simply transmit the acquired data to the 2060 Process Analyzer via analog output or Modbus protocols. The advantage is that by connecting directly to the ProTrode pH sensor itself, an external transmitter is no longer required.
Unique timesaving electrode testing system
Each measurement is only as good as the sensor used. To ensure the most accurate pH measurements in the process with 100% performance, a sensor test is essential. Sensor tests allow qualification of the electrode and can be performed automatically with the assistance of process analyzers. Usually, such tests are performed manually—the disadvantage being this is very time-consuming and error prone.
Crucial parameters for automatic electrode evaluation (i.e., evaluation of the glass membrane and reference system) are integrated in process analyzers from Metrohm Process Analytics. These include:
- checking the slope and the asymmetry potential
- checking the response
- check of the diaphragm (flow potential)
- freely configurable evaluation criteria and limit values
- liquid junction potential / checking of the reference system
This concept is a practically applicable method to improve the accuracy of pH measurements based on "liquid junction" basic research.
→ Following the electrode test, an automatic evaluation of the electrode condition is performed, which is absolutely unique!
With online process analyzers, calibration, adjustment, and cleaning routines are fully automatic. The condition of the electrode is continuously monitored by the system. Between measurements, the electrode is immersed in a membrane-preserving storage solution that prevents it from drying out and simultaneously regenerates the swelling layer. The electrode is therefore always ready for use and does not need to be removed from the process for maintenance activities.
Fully automatic sensor cleaning in one system
The service life of the electrode used is limited by several factors. Frequently, changes due to aging processes or contamination of the diaphragm remain undetected. If measurements are performed with online process analyzers, the sensor is cleaned and rinsed at freely determined time intervals. Cleaning fluid and water are automatically pumped into the calibration chamber. This eliminates any need to remove and manually clean the sensor. The use of such automated cleaning systems can significantly extend sensor service life and avoid downtime.
A successful scale-up process with the 2060 Process Analyzer and the ProTrode
For industries like chemical and pharmaceutical manufacturing, product and process developments still take place in laboratories and pilot plants. The challenge is that the findings from these studies are hardly suitable for implementation in a full-scale production process. This is an important topic for the future, especially for modular plant concepts. By using the 2060 TI Process Analyzer, nearly identical measurement conditions were able to be transferred from the laboratory to the process.
It is important to emphasize that in the future it will be increasingly important that the technologies from both laboratory and process are comparable in order to better understand the actual process chemistry.
«Using the same technology in the laboratory and in the process is a crucial factor in this.»
(«Gleiche Technologie in Labor und Prozess ist hier ein essenzieller Faktor.») 
A process analyzer that replicates laboratory conditions
When transferring any analysis method from the laboratory to the process floor, a fixed and defined measurement setup is necessary to be able to fully reproduce the corresponding conditions. In this case, the accurate measurement of the pH value is subject to several chemical, physical, and mechanical variables, which places high demands on the analysis and the sensor technology used.
The 2060 TI Process Analyzer was used for this project, which allowed the possibility to automate the measurement thanks to its modular and flexible design and the intelligent software interface.
Download our free brochure below to find out more information about the line of 2060 Process Analyzers from Metrohm Process Analytics.
The ultimate requirement for the experimental setup (shown in Figure 4) was to reproduce the same conditions as those found in the laboratory. A trace heating system ensured a defined sample temperature from the sampling point into the measuring vessel. In addition, a thermostatic measuring vessel was used so that the sample temperature was identical to the process temperature.
In the experimental setup, the analysis was transferred to an external measuring vessel with a predefined stirring speed. Even stirring speeds of 0–2 m/s behind the sensor tip exhibited deviations of ± 0.1 pH. On one hand, this illustrates the importance of defined stirring speeds, as enabled by the 2060 Process Analyzer. On the other hand, thanks to the sleeve diaphragm, the ProTrode pH sensor facilitates highly precise measurements even with varying flow rates in order to respond to challenging and variable conditions.
A deeper view into the process
The eminent artist Henri Matisse once remarked that «accuracy is far from being the truth». Inaccurate measurements and resulting errors can be disastrous for industrial production processes. Product quality must be ensured within very narrow tolerance limits at all times. Inferior quality and faulty batches can result in significant economic damages and avoidable downtimes.
Together with Merck KGaA, we have succeeded in transferring nearly identical conditions from the laboratory to the process in this impressive project. Especially when it comes to applications in challenging process environments, using a process analyzer for high-precision pH measurement offers the following advantages, and it can clearly contribute to all aspects and phases of an economical and safe production process:
- precisely predefined measurement conditions (e.g., setting times, drift behavior of the sensor, temperature, stirring speed)
- integrated sensor testing (because calibration alone is not sufficient to get information about the condition of the sensor)
- individual calibration and adjustment (5-point calibration according to FDA specifications is possible)
- programmable cleaning intervals (without needing to remove the electrode from the measuring point)
 User Association of Automation Technology in Process Industries (NAMUR), Technology Roadmap “Process Sensors 2027+”, November 2021, p. 18 (in German) https://www.namur.net/fileadmin/media_www/Dokumente/Roadmap_Prozesssensoren_2027.pdf