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To automate or not to automate? Advantages of PAT: Part 1

To automate or not to automate? Advantages of PAT: Part 1

I have to admit that the technological world of process analysis seemed foreign for me for a while. When I first heard about process automation, I imagined futuristic robots that do the work, similar to modern science fiction films. Perhaps many people might have the same impression.

There is often a great deal of uncertainty about what the expression «we automate your process» actually means. In this blog series, I want to show you that process analytical technology (PAT) is less complicated than expected and offers several advantages for users.

What does process analytical technology (PAT) mean? 

I was once told in conversation:

«Process analytics is for everyone who believes that they don’t need it.»

There is definitely truth in this statement, and it certainly shows the abundance of application possibilities. At the same time, it should be considered that in the future, users of process analytical technology will not only invest in conventional measurement technologies (e.g., direct measurement, TDLAS, GC), but also increasingly in the determination of substance properties and material compositions.

Pollution (gases and aerosols) in ambient air are especially harmful to human health. These substances can continuously and reliably be monitored by process analyzers.

PAT serves to analyze, optimize, and ultimately control processes and their critical parameters. This control makes a major contribution to quality assurance and the overall process reliability at the manufacturer. Thinking back to some well-known chemical disasters (e.g. Minamata, Toulouse, or Tianjin) in which poisonous substances were released, causing immense damage to people and the environment, the importance regarding regular monitoring of critical parameters becomes abundantly clear. The list of analytes that can and must be monitored is long, ranging from contamination in wastewater due to municipal or industrial wastewater treatment plants, to pharmaceutical agents, to gases and aerosols in the ambient air.

From Lab to Process

Considering the history of manufacturing and other industrial processes, it is clear that the ultimate goal is to increase throughput in ever-shorter timeframes, with an eye on safety measures and minimization of costs where possible. Independence through automation and fast, reliable data transfer is a high priority.

In order to make the process economically viable along the entire value chain, the resulting products should be manufactured at the highest quality in a short time and with minimal raw material and energy usage. For 24/7 operations in particular, knowledge of the composition of the starting materials and intermediate products (or rather, any impurities) is essential for optimal process control and reliability.

How can reliable process monitoring be ensured around the clock? Very few companies have company laboratories with an actual 3-shift operation, and often send their samples to external laboratories. Additionally, the samples are sometimes taken with longer time intervals between them. This carries various risks.

On one hand, the time lost between the sampling event and receiving the results from the analysis is enormous. It is only possible to react to fluctuations and deviations from target concentrations or limit values ​​with a certain delay. On the other hand, working and environmental conditions are not comparable and can lead to changes in the sample. Oxidation, pressure or temperature changes, introduction of moisture, and many other factors can change a sample’s original properties during transport, waiting periods, and manual laboratory analysis.

Example trend graph comparing process deviations mitigated by manual control (grey) and fully automatic process control (orange) via PAT.

Process analyzers: automated operation around the clock

Analyses, which are usually carried out manually, are automated by using industrial process analyzers. The samples are automatically removed from critical points in the production process and processed further. The information obtained is used to control the process without any delay, as the data can be transferred immediately to a central computing system at the plant. Automated analysis right at the sample point allows for increased accuracy and reproducibility of the data.

In practice, this entails rerouting a partial stream from the process in question to be fed to the analyzer by means of valves, peristaltic pumps, or bypass lines. Each sample is therefore fresh and correlates to the current process conditions. Probes can also be integrated directly into the process for continuous inline measurement.

The analysis is performed using common titration, spectroscopy, ion chromatography, or electrochemical methods known from the laboratory, which are optimally integrated into the process analyzer for each individual application requirement. The methods can be used in combination, allowing several measuring points to be monitored in parallel with one system. Thanks to the process analyzers that are specifically configured and expandable for the application, the optimal conditions for stable process control are obtained.

Spectroscopic methods have become particularly well-established in recent years for process analysis and optimization purposes. In contrast to conventional analysis methods, near-infrared (NIR) spectroscopy shows a number of advantages, especially due to the analysis speed. Results can be acquired within a few seconds and transferred directly to the chemical control system so that production processes can be optimized quickly and reliably. Samples are analyzed in situ, completely without the use of chemicals, in a non-destructive manner, which means further added value for process safety.

The many advantages of PAT

Automation in the context of process analysis technology does not always have anything to do with futuristic robots. Instead, PAT offers companies a number of advantages:

 

  • Fully automatic, 24/7 monitoring of the process
  • Timely and automatic feedback of the analysis results to the system control for automatic process readjustment
  • Reduction in fluctuations of product quality
  • Increased process understanding to run production more efficiently
  • Independent of your own laboratory (or contract lab)
  • Complete digital traceability of analysis results
  • Total solution concepts including sample preconditioning, saving time and increasing safety

What’s next?

In our next post in this series, you will discover the role process analysis technology plays in digital transformation with regard to «Industry 4.0».

Want to learn more about the history of process analysis technology at Metrohm? Check out our previous blog post:

Read what our customers have to say!

We have supported customers even in the most unlikely of places⁠—from the production floor to the desert and even on active ships!

Post written by Dr. Kerstin Dreblow, Product Manager Wet Chemical Process Analyzers, Deutsche Metrohm Prozessanalytik (Germany), with contributions from Dr. Alyson Lanciki, Scientific Editor at Metrohm International Headquarters (Switzerland).

How to Transfer Manual Titration to Autotitration

How to Transfer Manual Titration to Autotitration

Maybe you’ve read our earlier blog post on the main error sources in manual titration and are now wondering what you have to do in order to convert your manual titration to autotitration. In this blog entry, I want to give you a step by step guideline on how to proceed and what you have to consider.

Let’s jump right in with the following topics (click to go directly to each topic):

Choice of sensor

The first and most crucial step in transferring a manual titration to autotitration is the choice of the sensor for the indication of the equivalence point.

One of the simplest choices is to use a photometric sensor, effectively replacing the human eye with a sensor, especially when norms or standards stipulate the use of color indicators. It is often simpler to use a potentiometric electrode for indication purposes, because indicator solution is not needed and you can even combine multiple titrations into one to save time.

The electrode choice depends on the type of reaction, the sample, and the titrant used. Acid-base titrations require a different electrode than redox or precipitation titrations. Additionally, the sample matrix can have a significant influence on the electrode. The more complex the matrix is, the more crucial the choice. For example, you must use a different pH electrode for non-aqueous titrations than for aqueous titrations.

To help you select the best electrode for your titrations, we’ve prepared a free flyer, which you can download below. If you prefer, our Electrode Finder is even easier to use. Select the reaction type and application area of your titration and we will present you the best solution.

Optrode: optical sensor for photometric titrations.

Optimizing the sample size and solution volumes

If you’ve performed manual titrations before, then you know that many of these methods have endpoints requiring the use of up to 30 mL or even 40 mL of titrant. Autotitrators are most commonly equipped with 10 mL or 20 mL burets. Since refilling the buret during the titration causes errors, you should reduce the sample size for autotitrations. In general, for autotitration it is recommended that the equivalence point lies between 10% and 90% of the total buret volume. The second step when transferring a manual titration to autotitration is thus the optimization of the sample size.

Don’t forget: decreasing the sample size additionally reduces the waste produced  since you need less titrant, contributing to greener chemistry and cost savings with each titration.

When you transfer a manual titration to autotitration, it might be necessary to adjust the amount of used diluent (water or solvent) for the analysis. To obtain accurate results it is imperative that the glass membrane (for measurement) and diaphragm (for reference) of the sensor are fully immersed into the solution, as shown here.

Selecting the right titration mode

Depending on the reaction type, some titration reactions are completed faster than others. For this reason, autotitrators are equipped with different titration modes.

The two most often used modes (monotonic and dynamic mode) can be distinguished by the way the titrant is added. When using the monotonic mode, the same amount of titrant is always added with each addition. In the dynamic mode, the amount of titrant added differs depending on how close you are to the equivalence point. The closer you are to the equivalence point, the smaller the additions—similar to a manual titration.

As a rule of thumb, use the dynamic mode for fast titrations such as acid-base titration and the monotonic mode for slower titrations, where the equivalence point is suddenly reached (e.g. vitamin C determination).

Learn more about the different titration modes in this free webinar available on our website.

Optimizing the titration setup

The stirring speed and sensor placement within your titration beaker will influence the accuracy of your results.

Depending on the sample beaker and stirrer, choose the stirring speed in such a way that the mixing is fast enough but no splashing occurs. Also be sure that no vortex is created in the solution due to the speed of mixing, leaving your electrode hanging in dry air. Make sure to place the electrode close to the beaker wall and upstream of (behind) the buret tip as displayed in this example. This allows for an ideal mixing of the titrant with the sample and improves your accuracy.

Optimizing the titration method

In this last step, I want to present some options about how you can optimize your titration in regards to titration speed and titrant consumption.

One efficient way to speed up titrations, especially monotonic titrations, is to utilize a start volume. This would be similar to a case where you pre-dose titrant to your sample before you beginning the manual titration. Don’t forget to add a pause step after the addition! This way, the added titrant can mix well with the sample before the titration starts.

To save titrant and to reduce waste, I recommend to use stop criteria. The simplest stop criterion is the stop volume. If your equivalence point always occurs at the same volume, this is the easiest way to go. If the volume of your equivalence point varies, you can use the option to define a volume which should be added after the value for the expected equivalence point has been reached. In general, I recommend a stop volume of approximately 1 mL after the equivalence point.

To summarize:

  • Select the right sensor for your titration
  • Adjust sample size and diluent volume
  • Select the titration mode depending on your reaction
  • Optimize your titration for speed and titrant consumption

 

You see, changing from manual to autotitration is as easy as it sounds – maybe even easier!

If you want to learn even more about practical aspects of modern titration, have a look at our monograph.

Want to learn more?

Download our free monograph:

Practical aspects of modern titration

Post written by Lucia Meier, Product Specialist Titration at Metrohm International Headquarters, Herisau, Switzerland.

Avoiding the most common mistakes in pH measurement

Avoiding the most common mistakes in pH measurement

If you’re reading this, then I’m sure you have already performed at least one pH measurement in your lifetime, since it is one of the most important parameters in analytical chemistry. I remember my first contact with a potentiometric pH meter and a pH electrode – and I can still remember how I felt back then.

I was young and completely unsure how I should handle the instrument and the electrode. Was I doing everything correctly? Consequently I had many questions about the best practices.

Today, I am much more confident! Therefore, I would love to share with you some of the most common uncertainties and mistakes I see during my daily work when potentiometric pH measurements are performed. By the end of this article, I am certain that you will agree with me: pH measurement can be just as easy as it looks. I will cover the following topics (click to go directly to each topic):

Is this the correct electrode for your application?

Troubleshooting already starts before you put the sensor into your sample solution. A wide variety of electrodes are available on the market, and it can be quite difficult to determine which electrode is the best for your application. Many different diaphragm types as well as glass membrane materials exist:

We’ve prepared a flyer for you to help find the perfect electrode for your application. Additionally, we have provided valuable information about maintenance and storage. You can download the flyer in several languages: English, German, French, or Spanish.

What’s most important when preparing the electrode for calibration or measurement?

Before starting your measurement, check the electrode for cracks or contaminations. Open the plug to ensure that the electrolyte can flow out (otherwise you may observe unstable results), and check the level of the electrolyte.

The electrolyte should always be filled up to the opening in order to ensure an outflow from the hydrostatic pressure. If the level of the sample is higher than the level of electrolyte within the sensor, then sample will enter the reference system of your electrode. This causes the reference potential to shift, and results are no longer reproducible.

Make sure that you insert your sensor deep enough into the sample. At least the glass membrane and the diaphragm need to be covered, as shown in this example.

Calibration: When is it necessary, and what must I consider?

Calibrations must be performed on a regular basis. Depending on the number of measurements and the sample matrix, I recommend calibrating at least weekly. If used often, or if the sample matrix is contaminating the sensor, then you should calibrate daily or even more frequently. Of course you should always calibrate your sensor if you have received a new one, after maintenance, or after a longer storage period.

For calibration, consider the following points:

  • Always use fresh (not expired) buffers – the calibration can only be as good as the buffers used!
  • Perform at least a 2-point calibration.
  • Your sample pH should be within the calibration buffer pH value.
  • Always measure the temperature, as the pH value is temperature-dependent.
  • Most manufacturers already include buffer table templates with their instruments. Make sure that you select the correct one.

How should you store the pH electrode?

The correct storage of the pH electrode can increase its lifetime significantly. Never store the pH electrode dry! The glass membrane builds up a hydration layer, which is necessary for proper pH measurement. If you store the electrode dry, this hydration layer will be destroyed. Even though the layer can be recovered by conditioning the sensor in deionized water, the sensor will become slower.

For electrodes filled with potassium chloride (c(KCl) = 3 mol/L) as reference electrolyte, we have developed a dedicated storage solution which keeps the glass membrane in top quality without impairing the performance of the diaphragm.

The figure above shows how quickly the sensor responds when placed in a sample after a storage period. You can clearly see that storing the sensor in the dedicated solution leads to a much faster response time in comparison to storage in c(KCl) = 3 mol/L. This means even more productivity and less waiting.

All electrodes which are filled with another reference electrolyte than c(KCl) = 3 mol/L are stored in their reference electrolyte.

How should the pH electrode be cleaned?

Between the measurements, the electrode must be rinsed well with deionized water. If the sample is sticky or contains proteins, use a suitable solvent to remove the contamination. From time to time, it is important to give the electrode a «special treat» and clean it with the pHit Kit, shown below. This set includes everything that is necessary to gently and efficiently clean the electrode.

Very important: Never wipe the sensor off with a tissue! Similar to rubbing the surface of a balloon, you will charge the surface of the glass membrane. The built-up electrostatic energy will influence your measurement, which will get significantly longer. Additionally, you can scratch the sensitive glass membrane surface, thus destroying it.

To stir or not to stir?

Depending on the electrode type you are using, it is recommended to always stir constantly, at the same speed, during analysis. The following graph illustrates why:

The upper curve shows the measurement with an electrode having a fixed ground-joint diaphragm, and the lower curve utilized a very common electrode with a ceramic pin diaphragm. 

Not only does the top electrode show less signal noise, the signal remains nearly unchanged once the stirrer is switched off. However, there is a significant signal drop for the ceramic pin diaphragm (bottom). Therefore, the stirring speed should be identical for all buffers and samples to minimize such effects. 

Is my electrode still ok to use?

To get an idea about whether your electrode is still ok to use or not, it is generally enough to check the slope and the pH(0) after calibration. The slope should be between 95–103%, whereas the pH(0) should lie between pH 6.8–7.2. Further information can be gained if a pH electrode test is performed, which is implemented in some of Metrohm’s instruments, or a test according to application bulletin AB-188.

If the electrode does not meet the specifications, clean it according to the instructions and perform the test again. If the sensor still does not pass, a replacement is inevitable.

Check out our webinars:

«Basic of pH measurements» or «Troubleshooting of pH measurement

You can also download our whitepaper WP-003 «pH measurement: Six technical tips»  for free: 

Post written by Dr. Sabrina Gschwind, Jr. PM Titration (Sensors) at Metrohm International Headquarters, Herisau, Switzerland.

Why your titration results aren’t reproducible: The main error sources in manual titration

Why your titration results aren’t reproducible: The main error sources in manual titration

In the practical course in Analytical Chemistry during my first semester at university, I had to titrate a lot. Thinking back on it, I remember carefully dosing titrant with the glass buret, the cumbersome process of refilling the buret, and the constant suspicion that I hadn’t correctly chosen the endpoint.

Everyone in class kept getting different results—but we were never quite sure why. At the time, I wasn’t as experienced as I am now. Today, after 10 years of experience in titration, I’ve learned that the results of manual titration depend quite a lot on the person carrying it out. Here are the top error sources in manual titration and how you can avoid them.  

Choosing the right indicator I’m sure you’ve learned at some point that the pH value of the titration endpoint depends on the acid dissociation constant (Ka) of the acid and base that are used. If a strong base is titrated by a strong acid, the pH value at the endpoint is around 7. The titration of a strong base with a weak acid shifts the endpoint towards the alkaline range. The titration of a strong acid with a weak base will result in an endpoint in the acidic range. This explains why several different indicators are used in acid-base titrations. But which is the right one to choose?
The chart above shows some of the most frequently used pH indicators. You can probably imagine that you won’t get correct results when the pH of your endpoint is around 7, but you use crystal violet or methyl orange as the indicator. Luckily, most standards and SOPs specify an indicator. Follow the instructions, and you’re on the safe side!

Endpoint recognition is subjective

The problems really start when you try to recognize the endpoint. Have you ever thought about the nuances of the color change?

Above, you see five stages of an acid-base titration of c(HCl) = 1 mol/L with c(NaOH) = 1 mol/L. The only difference between each image and its predecessor is one additional drop of titrant. Where would you choose the endpoint in this case?

Is the endpoint reached in picture 1, where only a faint pink is visible? Or is it reached in picture 3 where the color becomes more intense? Or even in picture 5, at which point the pink color is most vibrant? Between picture 1 and picture 5, just four drops of titrant were added. With the pharmaceutical definition of a drop as a volume of 50 µL, this corresponds to 200 µL of titrant or about 7.3 mg of hydrochloric acid—an enormous error.

Reading the buret volume

Do you remember how to correctly read the buret? You have to stand on a footstool and make sure that you read the meniscus value horizontally. Do you know why?

The volume reading depends upon the angle from which you view the buret. In the case shown here, the readings vary up to 0.2 mL (200 µL) from the actual value, depending on the reading angle. The more your line of sight deviates from the horizontal, the more inaccurate the reading—and the result. You can assume an average error of 200 µL. This is a lot for a titration, as I showed in the previous example!

Improving objectivity and accuracy

How can you eliminate these errors? The easiest one to overcome is the reading error. The solution for this is to use an electronic buret. When using an electronic buret, all you need to do is fill it with the titrant and then you press a button. The device automatically measures the volume and gives you a digital readout. Using an electronic buret ensures already a high level of objectivity for your results.

It also improves the accuracy of your results. I don’t have to tell you how important accuracy is in analytical chemistry, but I’ll give an example. Imagine you determined the purity of gold at 90%, but in reality, it’s 99% pure. You would lose a lot of money when selling your gold under this pretense!

Earlier, I showed that visual endpoint recognition using a color indicator can result in errors of up to 200 µL. An inaccurate buret reading can lead to an additional 200 µL error. While using an electronic buret doesn’t help you achieve a more objective endpoint recognition, it does reduce the minimum volume addition per drop: it’s no longer 50 µL, but can be as small as 0.25 µL depending on the cylinder volume you use. This substantially lowers the error resulting from endpoint recognition. The following minimum volume additions are common:

The next step: Automated titration

If you want to overcome all sources of error described in this post, you’ll have to switch to automated titration, or autotitration. In this case, you will use a sensor to measure pH change in the sample and a mathematical algorithm to detect the endpoint—an indicator isn’t required anymore. Additionally you have the same precision as with the electronic buret.

Want to learn more?

Download our free White Paper:

Manual vs. Automated Titration: Benefits and Advantages to Switching

Post written by Iris Kalkman, Product Specialist Titration at Metrohm International Headquarters, Herisau, Switzerland.