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«Analyze This»: 2020 in review

«Analyze This»: 2020 in review

I wanted to end 2020 by thanking all of you for making «Analyze This» – the Metrohm blog for chemists such a success! For our 60th blog post, I’d like to look back and focus on the wealth of interesting topics we have published this year. There is truly something for everyone: it doesn’t matter whether your lab focuses on titration or spectroscopic techniques, or analyzes water samples or illicit substances – we’ve got you covered! If you’re looking to answer your most burning chemical analysis questions, we have FAQs and other series full of advice from the experts. Or if you’re just in the mood to learn something new in a few minutes, there are several posts about the chemical world to discover.

We love to hear back from you as well. Leaving comments on your favorite blog posts or contacting us through social media are great ways to voice your opinion—we at Metrohm are here for you!

Finally, I wish you and your families a safe, restful holiday season. «Analyze This» will return on January 11, 2021, so subscribe if you haven’t already done so, and bookmark this page for an overview of all of our articles grouped by topic!

Stay healthy, and stay curious.

Best wishes,

Dr. Alyson Lanciki, Scientific Editor, Metrohm AG

Quickly jump directly to any section by clicking a topic:

Customer Stories

We are curious by nature, and enjoy hearing about the variety of projects where our products are being used! For some examples of interesting situations where Metrohm analytical equipment is utilized, read on.

From underwater archaeological research to orbiting Earth on the International Space Station, Metrohm is there! We assist on all types of projects, like brewing top quality beers and even growing antibiotic-free shrimp – right here in Switzerland.

Interested in being featured? Contact your local Metrohm dealer for details!

Titration

Metrohm is the global market leader in analytical instruments for titration. Who else is better then to advise you in this area? Our experts are eager to share their knowledge with you, and show this with the abundance of topics they have contributed this year to our blog.

For more in-depth information about obtaining the most accurate pH measurements, take a look at our FAQ about pH calibration or read about avoiding the most common mistakes in pH measurement. You may pick up a few tips!

Choose the best electrode for your needs and keep it in top condition with our best practices, and then learn how to standardize titrant properly. Better understand what to consider during back-titration, check out thermometric titration and its advantages and applications, or read about the most common challenges and how to overcome them when carrying out complexometric titrations

If you are interested in improving your conductivity measurements, measuring dissolved oxygen, or the determination of oxidation in edible fats and oils, check out these blog posts and download our free Application Notes and White Papers!

Finally, this article about comprehensive water analysis with a combination of titration and ion chromatography explains the many benefits for laboratories with large sample loads. The history behind the TitrIC analysis system used for these studies can be found in a separate blog post.

Karl Fischer Titration

Metrohm and Karl Fischer titration: a long history of success. Looking back on more than half a century of experience in KFT, Metrohm has shaped what coulometric and volumetric water analysis are today.

Aside from the other titration blog posts, our experts have also written a 2-part series including 20 of the most frequently asked questions for KFT arranged into three categories: instrument preparation and handling, titration troubleshooting, and the oven technique. Our article about how to properly standardize Karl Fischer titrant will take you step by step through the process to obtain correct results.

For more specific questions, read about the oven method for sample preparation, or which is the best technique to choose when measuring moisture in certain situations: Karl Fischer titration, near-infrared spectroscopy, or both?

Ion Chromatography (IC)

Ion chromatography has been a part of the Metrohm portfolio since the late 1980s. From routine IC analysis to research and development, and from stand-alone analyzers to fully automated systems, Metrohm has provided IC solutions for all situations. If you’re curious about the backstory of R&D, check out the ongoing series about the history of IC at Metrohm.

Metrohm IC user sitting at a laboratory bench.

Common questions for users are answered in blog posts about IC column tips and tricks and Metrohm inline ultrafiltration. Clear calculations showing how to increase productivity and profitability in environmental analysis with IC perfectly complement our article about comprehensive water analysis using IC and titration together for faster sample throughput.

On the topic of foods and beverages, you can find out how to determine total sulfite faster and easier than ever, measure herbicides in drinking water, or even learn how Metrohm IC is used in Switzerland to grow shrimp!

Near-Infrared Spectroscopy (NIRS)

Metrohm NIRS analyzers for the lab and for process analysis enable you to perform routine analysis quickly and with confidence – without requiring sample preparation or additional reagents and yielding results in less than a minute. Combining visible (Vis) and near-infrared (NIR) spectroscopy, these analyzers are capable of performing qualitative analysis of various materials and quantitative analysis of a number of physical and chemical parameters in one run.

Our experts have written all about the benefits of NIR spectroscopy in a 4-part series, which includes an explanation of the advantages of NIRS over conventional wet chemical analysis methods, differences between NIR and IR spectroscopy, how to implement NIRS in your laboratory workflow, and examples of how pre-calibrations make implementation even quicker.

A comparison between NIRS and the Karl Fischer titration method for moisture analysis is made in a dedicated article.

A 2-part FAQ about NIRS has also been written in a collaboration between our laboratory and process analysis colleagues, covering all kinds of questions related to both worlds.

Raman Spectroscopy

This latest addition to the Metrohm family expands the Metrohm portfolio to include novel, portable instruments for materials identification and verification. We offer both Metrohm Raman as well as B&W Tek products to cover a variety of needs and requirements.

Here you can find out some of the history of Raman spectroscopy including the origin story behind Mira, the handheld Raman instrument from Metrohm Raman. For a real-world situation involving methamphetamine identification by law enforcement and first responders, read about Mira DS in action – detecting drugs safely in the field.

Mira - handheld Raman keeping you safe in hazardous situations.

Are you looking for an easier way to detect food fraud? Our article about Misa describes its detection capabilities and provides several free Application Notes for download.

Process Analytics

We cater to both: the laboratory and the production floor. The techniques and methods for laboratory analysis are also available for automated in-process analysis with the Metrohm Process Analytics brand of industrial process analyzers.

Learn about how Metrohm became pioneers in the process world—developing the world’s first online wet chemistry process analyzer, and find out how Metrohm’s modular IC expertise has been used to push the limits in the industrial process optimization.

Additionally, a 2-part FAQ has been written about near-infrared spectroscopy by both laboratory and process analysis experts, which is helpful when starting out or even if you’re an advanced user.

Finally, we offer a 3-part series about the advantages of process analytical technology (PAT) covering the topics of process automation advantages, digital networking of production plants, and error and risk minimization in process analysis.

Voltammetry (VA)

Voltammetry is an electrochemical method for the determination of trace and ultratrace concentrations of heavy metals and other electrochemically active substances. Both benchtop and portable options are available with a variety of electrodes to choose from, allowing analysis in any situation.

A 5-part series about solid-state electrodes covers a range of new sensors suitable for the determination of «heavy metals» using voltammetric methods. This series offers information and example applications for the Bi drop electrode, scTrace Gold electrode (as well as a modified version), screen-printed electrodes, and the glassy carbon rotating disc electrode.

Come underwater with Metrohm and Hublot in our blog post as they try to find the missing pieces of the ancient Antikythera Mechanism in Greece with voltammetry.

If you’d like to learn about the combination of voltammetry with ion chromatography and the expanded application capabilities, take a look at our article about combined analysis techniques.

Electrochemistry (EC)

Electrochemistry plays an important role in groundbreaking technologies such as battery research, fuel cells, and photovoltaics. Metrohm’s electrochemistry portfolio covers everything from potentiostats/galvanostats to accessories and software.

Our two subsidiaries specializing in electrochemistry, Metrohm Autolab (Utrecht, Netherlands) and Metrohm DropSens (Asturias, Spain) develop and produce a comprehensive portfolio of electrochemistry equipment.

This year, the COVID-19 pandemic has been at the top of the news, and with it came the discussion of testing – how reliable or accurate was the data? In our blog post about virus detection with screen-printed electrodes, we explain the differences between different testing methods and their drawbacks, the many benefits of electrochemical testing methods, and provide a free informative White Paper for interested laboratories involved in this research.

Our electrochemistry instruments have also gone to the International Space Station as part of a research project to more efficiently recycle water on board spacecraft for long-term missions.

The History of…

Stories inspire people, illuminating the origins of theories, concepts, and technologies that we may have become to take for granted. Metrohm aims to inspire chemists—young and old—to be the best and never stop learning. Here, you can find our blog posts that tell the stories behind the scenes, including the Metrohm founder Bertold Suhner.

Bertold Suhner, founder of Metrohm.

For more history behind the research and development behind Metrohm products, take a look at our series about the history of IC at Metrohm, or read about how Mira became mobile. If you are more interested in process analysis, then check out the story about the world’s first process analyzer, built by Metrohm Process Analytics.

Need something lighter? Then the 4-part history of chemistry series may be just what you’re looking for.

Specialty Topics

Some articles do not fit neatly into the same groups as the rest, but are nonetheless filled with informative content! Here you can find an overview of Metrohm’s free webinars, grouped by measurement technique.

If you work in a regulated industry such as pharmaceutical manufacturing or food and beverage production, don’t miss our introduction to Analytical Instrument Qualification and what it can mean for consumer safety!

Industry-focused

Finally, if you are more interested in reading articles related to the industry you work in, here are some compilations of our blog posts in various areas including pharmaceutical, illicit substances, food and beverages, and of course water analysis. More applications and information can be found on our website.

Food and beverages
All of these products can be measured for total sulfite content.

Oxidation stability is an estimate of how quickly a fat or oil will become rancid. It is a standard parameter of quality control in the production of oils and fats in the food industry or for the incoming goods inspection in processing facilities. To learn more about how to determine if your edible oils are rancid, read our blog post.

Determining total sulfite in foods and beverages has never been faster or easier than with our IC method. Read on about how to perform this notoriously frustrating analysis and get more details in our free LC/GC The Column article available for download within.

Measuring the true sodium content in foodstuff directly and inexpensively is possible using thermometric titration, which is discussed in more detail here. To find out the best way to determine moisture content in foods, our experts have written a blog post about the differences between Karl Fischer titration and near-infrared spectroscopy methods.

To determine if foods, beverages, spices, and more are adulterated, you no longer have to wait for the lab. With Misa, it is possible to measure a variety of illicit substances in complex matrices within minutes, even on the go.

All of these products can be measured for total sulfite content.

Making high quality products is a subject we are passionate about. This article discusses improving beer brewing practices and focuses on the tailor-made system built for Feldschlösschen, Switzerland’s largest brewer.

Pharmaceutical / healthcare

Like the food sector, pharmaceutical manufacturing is a very tightly regulated industry. Consumer health is on the line if quality drops.

Ensuring that the analytical instruments used in the production processes are professionally qualified is a must, especially when auditors come knocking. Find out more about this step in our blog post about Analytical Instrument Qualification (AIQ).

Moisture content in the excipients, active ingredients, and in the final product is imperative to measure. This can be accomplished with different analytical methods, which we compare and contrast for you here.

The topic of virus detection has been on the minds of everyone this year. In this blog post, we discuss virus detection based on screen-printed electrodes, which are a more cost-effective and customizable option compared to other conventional techniques.

Water analysis

Water is our business. From trace analysis up to high concentration determinations, Metrohm has you covered with a variety of analytical measurement techniques and methods developed by the experts.

Learn how to increase productivity and profitability in environmental analysis laboratories with IC with a real life example and cost calculations, or read about how one of our customers in Switzerland uses automated Metrohm IC to monitor the water quality in shrimp breeding pools.

If heavy metal analysis is what you are interested in, then you may find our 5-part series about trace analysis with solid-state electrodes very handy.

Unwanted substances may find their way into our water supply through agricultural practices. Find out an easier way to determine herbicides in drinking water here!

Water is arguably one of the most important ingredients in the brewing process. Determination of major anions and cations along with other parameters such as alkalinity are described in our blog post celebrating International Beer Day.

All of these products can be measured for total sulfite content.
Illicit / harmful substances

When you are unsure if your expensive spices are real or just a colored powder, if your dairy products have been adulterated with melamine, or fruits and vegetables were sprayed with illegal pesticides, it’s time to test for food fraud. Read our blog post about simple, fast determination of illicit substances in foods and beverages for more information.

Detection of drugs, explosives, and other illegal substances can be performed safely by law enforcement officers and first responders without the need for a lab or chemicals with Mira DS. Here you can read about a real life training to identify a methamphetamine laboratory.

Drinking water regulations are put in place by authorities out of concern for our health. Herbicides are important to measure in our drinking water as they have been found to be carcinogenic in many instances.

Post written by Dr. Alyson Lanciki, Scientific Editor at Metrohm International Headquarters, Herisau, Switzerland.

Oven method for sample preparation in Karl Fischer titration

Oven method for sample preparation in Karl Fischer titration

Maybe you have experienced one of the following situations in the laboratory. You need to determine the water content of a sample using Karl Fischer titration and you realize one or more of these issues:

  • The sample does not dissolve in the KF reagent. No solubilizer helps, the sample is still not dissolving, and the results are far from reproducible.
  • The sample reacts with the KF reagent. The titration does not stop, and there is no endpoint detected.
  • The sample contaminates the titration cell and the electrode(s). Even if you replace the reagent after every measurement, the obtained results are out of specification.

There is a way to solve the above-mentioned problems. Trust me—it’s fantastic!!

The solution is the oven method or gas extraction technique.

What is Karl Fischer titration? Download our free Monograph to learn more from the experts.

What is the oven method?

The oven method is a sample preparation technique used in Karl Fischer titration to analyze samples…

For more help, take a look at our frequently asked questions in Karl Fischer titration under the section «Sample Handling» here on our website:

The principle is very simple.

The sample is weighed into a headspace vial and closed with a septum cap. When placed in an oven, the water evaporates and a carrier gas (usually air or nitrogen) dried with a molecular sieve transports the released water into the titration cell, where the determination of the water content takes place. The water is separated from the sample matrix, avoiding side reactions and contamination.

The temperature of the oven is chosen according to the temperature stability of the sample. This leads to the question to which temperature the sample should be heated. What is the optimal oven temperature?

Finding the optimal oven temperature

Using a suitable oven temperature to analyze a sample is crucial to obtain the correct results. The oven temperature should be as high as possible, within reason. This guarantees a fast and complete release of the water and subsequently, short titration times. However, you should avoid choosing a temperature that is too high. Decomposition of the sample usually leads to the formation of unwanted substances that can falsify the water content. Therefore, as a rule of thumb, I recommend choosing an oven temperature 20 °C below the decomposition temperature of the sample.

But what can you do if you have no idea at which temperature your sample should be analyzed? No worries! There are several ways to find the optimal oven temperature.

One possibility is to search in the literature. The more information on temperature stability of the sample you find, the better off you will be. If you are able to find a decomposition temperature, it will help immensely to define the optimal oven temperature. Maybe you are lucky and someone else has already analyzed the same sample; then you may also find a recommended oven temperature. A good start is reading our free Application Bulletin AB-280, which lists several substances.

Are you searching for Karl Fischer titration oven applications? Look no further – the Metrohm Application Finder contains several applications you can download for free! Check them out here:

If literature research does not reveal a suitable oven temperature, you must determine it yourself. How this is done depends upon the type of instrument you are using.

Some instruments offer you the possibility to run a so-called temperature gradient or temperature ramp. The sample is heated at a constant rate (e.g., 0.5 °C or 2 °C per minute) in a defined temperature range (e.g., 50 to 250 °C). At the same time, the released water is determined. In the end, the software will display a curve, showing you the released water as a function of the temperature. The following graph shows an example of such a temperature gradient curve.

The blue line corresponds to the determined water content, whereas the orange line indicates the drift value. An increasing drift signals the release of water, but it can also be a sign for decomposition, especially if the drift no longer decreases to a low level. In this graph, the drift peak at 50 °C corresponds to the blank value and free water. Between 120 and 200 °C, the drift value increases again, meaning the sample releases water. Then the drift decreases and remains low and stable up to 250 °C. There are no signs of decomposition up to 250 °C. As we do not know what would happen at temperatures above 250 °C, the optimal oven temperature for this sample is 230 °C (250 °C – 20 °C = 230 °C).

In case the instrument you use does not offer the option to run a temperature gradient, you can manually increase the temperature and measure the sample at different temperatures. In an Excel spreadsheet, you can display the curve (released water against temperature). If there is a temperature range where you see reproducible water contents, then you have found the optimal oven temperature.

Here is an example of a sample which started to decompose at temperatures above 106 °C (left sample vial) and thus is turning brown. An optimal temperature would therefore be 85 °C.

Sample analysis with a KF oven – step by step

After you have found the optimal oven temperature, water content determination in the sample can begin.

  • First, I recommend to run a system preparation. This means running a determination, but with an empty sample vial. During this preparation step, all tubes in the system are purged with dried carrier gas, and any traces of water are removed.
  • Next, you need to determine the blank value. The sample vials and the caps contain some residual moisture. With the blank determination, the amount of water contained in an empty sample vial is determined. The mean value of e.g. 3 blank value determinations is then subtracted from the water content obtained for the samples.
  • Finally, you can analyze the samples.

Please keep in mind that the same parameters for the system preparation, the blank value determination, and the sample determination must be used. This is of importance if you want to measure a check standard before and/or after the sample analysis or sample series. If the optimal oven temperature for the standard is different from the one for the sample, I recommend that you determine a blank value for the standard as well.

Checking an oven system

There are special, solid water standards available to check the performance of an oven system. These water standards are perfect to inspect the complete oven system and to ensure that the evaporated water reaches the titration cell and is determined there. Such standards include a certificate stating the water content.

Using the certified value, you can calculate the recovery when determining the water content of the standard with the oven. If the recovery value is between 97–103%, everything is fine. However, if the recovery is outside this range, the oven system should be checked for leaks or water deposits. It might be that only the molecular sieve needs to be exchanged. Possibly, the reagent is exhausted and needs to be replaced.

There are other reasons which explain recovery values which are too high or too low. The reason must be found, as incorrect recovery values also mean that the determined sample water content is wrong. Have a look at our free Application Bulletin 280 for detailed information on troubleshooting an oven system.

Summary

The oven method is a simple and convenient way to analyze difficult samples. Side reactions are reduced to a minimum. The titration cell and the reagent are not contaminated with sample. In case you have to analyze a large series of samples, automation of the oven method is possible. Have a look at the available instruments for the oven method on our website!

Want to learn more

about Karl Fischer titration

Watch our free webinars here!

Post written by Michael Margreth, Sr. Product Specialist Titration (Karl Fischer Titration) at Metrohm International Headquarters, Herisau, Switzerland.

Frequently asked questions in Karl Fischer titration – Part 2

Frequently asked questions in Karl Fischer titration – Part 2

Since I started working at Metrohm more than 15 years ago, I have received many questions about Karl Fischer titration. Some of those questions have been asked repeatedly from several people in different locations around the world. Therefore, I have chosen 20 of the most frequent questions received over the years concerning Karl Fischer equipment and arranged them into three categories: instrument preparation and handling, titration troubleshooting, and the oven technique. Part 1 covered instrument preparation and handling, and Part 2 will now focus on titration troubleshooting and the KF oven technique.

Summary of questions in the FAQ (click to go directly to each question):

Titration troubleshooting

1.  If the drift value is 0, does this mean that the titration cell is over-titrated?

A drift of zero can be a sign that the cell might be over-titrated. In combination with the mV signal (lower than end-point criteria) and the color of the working medium (darker yellow than usual), it is a clear indicator for over-titration. However, volumetric titrations sometimes exhibit a zero drift for a short time without being over-titrated. If you have a real excess of iodine in the titration cell, the result of the next determination will most likely be erroneous. Therefore, over-titration should be avoided. There are various possible reasons for over-titration, like the sample itself (e.g., oxidizing agents which generate iodine from the working medium), the electrode (coating or invisible depositions on the Pt pins/rings), the reagent, and method parameters (e.g., the titration is rate too high), to name just a few.

2.  Should I discard the Karl Fischer reagent immediately if it turns brown?

Different factors can cause over-titration, however, the reagent is not always the reason behind this issue. The indicator electrode can also be the reason for overshooting the endpoint. In this case, regular cleaning of the electrode can prevent over-titration (see also questions 7 to 9 from Part 1 in this series on cleaning).

A low stirring speed also increases the risk of over-titration, so make sure the solution is well mixed. Depending on the type of reagent, the parameters of the titration need to be adjusted. Especially if you use two-component reagents, I recommend decreasing the speed of the titrant addition to avoid over-titration. Over-titration has an influence on the result, especially if the degree of over-titration changes from one determination to the next. So over-titration should always be avoided to guarantee correct results.

3.  What is drift correction, and when should I use it?

I recommend using the drift correction in coulometric KF titration only. You can also use it in volumetric titration, but here the drift level is normally not as stable as for coulometric titrations. This can result in variations in the results. A stabilization time can reduce such an effect. However, compared to the absolute water amounts in volumetry, the influence of drift is usually negligible.

4.  My results are negative. What does a negative water content mean?

Negative values do occur if you have a high start drift and a sample with a very low water content. In this case, the value for drift correction can be higher than the absolute water content of the sample, resulting in a negative water content.

If possible, use a larger sample size to increase the amount of water added to the titration cell with the sample. Furthermore, you should try to reduce the drift value in general. Perhaps the molecular sieve or the septum need to be replaced. You can also use a stabilizing time to make sure the drift is stable before analyzing the sample.

Karl Fischer oven

5.  My samples are not soluble. What can I do?

In case the sample does not dissolve in KF reagents and additional solvents do not increase the solubility of the sample, then gas extraction or the oven technique could be the perfect solution.

The sample is weighed in a headspace vial and closed with a septum cap. Then the vial is placed in the oven and heated to a predefined temperature, leading the sample to release its water. At the same time, a double hollow needle pierces through the septum. A dry carrier gas, usually nitrogen or dried air, flows into the sample vial. Taking the water of the sample with it, the carrier gas flows into the titration cell where the water content determination takes place.

6.  Can all types of samples be analyzed with the oven method?

Many samples can be analyzed with the oven. Whether an application actually works for a sample strongly depends on the sample itself. Of course, there are samples that are not suitable for the oven method, e.g., samples that decompose before releasing the water or that release their water at higher temperatures than the maximum oven temperature.

7.  How do I find the optimal oven temperature for water extraction?

Depending on the instrument used, you can run a temperature gradient of 2 °C/min. This means it is possible to heat a sample from 50 to 250 °C within 100 minutes. The software will then display a curve of water release against temperature (see graph).

From such a curve, the optimal temperature can be determined. Different peaks may show blank, adherent water, different kinds of bound water, or even decomposition of the sample.

This example curve shows the water release of a sample as it has been heated between 130 and 200 °C. At higher temperatures, the drift decreases to a stable and low level.

Generally, you should choose a temperature after the last water release peak (where the drift returns to the base level) but approximately 20 °C below decomposition temperature. Decomposition can be recognized by increasing drift, smoke, or a color change of the sample. In this example, there are no signs of decomposition up to an oven temperature of 250 °C. Therefore, the optimal oven temperature for this sample is 230 °C (250 °C – 20 °C).

In case the instrument you use does not offer the option to run a temperature gradient, you can manually increase the temperature and measure the sample at different temperatures. In an Excel spreadsheet, you can display the curve plotting released water against temperature. If there is a plateau (i.e., a temperature range where you find reproducible water contents), you have found the optimal oven temperature.

8.  What is the highest possible water content that can be measured with a Karl Fischer oven?

Very often, the oven is used in combination with a coulometric titrator. The coulometric titration cell used in an oven system is filled with 150 mL of reagent. Theoretically, this amount of reagent allows for the determination of 1500 mg of water. However, this amount is too high to be determined in one titration and it would lead to very long titration times and negative effects on the results. We recommend that the water content of a single sample (in a vial) should not be higher than 10 mg, ideally around 1000–2000 µg water. For samples with water contents in the higher percentage range, you should consider the combination with a volumetric titrator.

9.  What is the maximum sample size that can be used with the oven? If I use too much sample, will the needle be blocked?

The standard vial for the oven method has a volume of approximately 9 mL. However, we do not recommend filling the vial completely. Do not fill more than 5–6 mL of sample in a vial. We offer the possibility to customize our oven systems, allowing you to use your own vials. Please contact your local Metrohm agency for more information on customized oven systems.

For liquid samples, we recommend using a long needle to lead the gas through the sample. Solid samples and especially samples that melt during analysis require a short needle. The tip of the needle is positioned above the sample material to avoid needle blockage.

Additionally, you should use a «relative blank value», i.e., taking only the remaining air volume into account for blank subtraction. You can find more information about the relative blank and how to calculate it in Application Note AN-K-048.

10.  What is the detection limit of the oven method, and how much sample is required to analyze a sample with 10 ppm (mg/L) water content?

We recommend having at least 50 µg of water in the sample, if analyzed with coulometry. However, if conditions are absolutely perfect (i.e., very low and stable drift plus perfect blank determination), it is possible to determine even lower water contents, down to 20 µg of absolute water. For a sample with a water content of < 10 ppm (mg/L), this would correspond to a sample size of at least 2 g.

11.  How do I verify an oven method?

For the verification of an oven system, you can use a certified water standard for oven systems. With such a standard, you can check the reproducibility and the recovery. There are a few types of standards available for different temperature ranges.

I hope this collected information helps you to answer some of your most burning KF questions. If you have further unanswered questions, do not hesitate to contact your local Metrohm distributor or check out our selection of webinars.

Automate thermal sample preparation

It’s easy with an oven sample changer from Metrohm!

Post written by Michael Margreth, Sr. Product Specialist Titration (Karl Fischer Titration) at Metrohm International Headquarters, Herisau, Switzerland.

Frequently asked questions in Karl Fischer titration – Part 1

Frequently asked questions in Karl Fischer titration – Part 1

Since I started working at Metrohm more than 15 years ago, I have received many questions about Karl Fischer titration. Some of those questions have been asked repeatedly from several people in different locations around the world. Therefore, I have chosen 20 of the most frequent questions received over the years concerning Karl Fischer equipment and arranged them into three categories: instrument preparation and handling, titration troubleshooting, and the oven technique. Part 1 will cover instrument preparation and handling, and Part 2 will cover the other two topics.

Summary of questions in the FAQ (click to go directly to each question):

Instrument preparation and handling

1.  How can I check if the electrode is working correctly?

I recommend carrying out a volumetric or coulometric Karl Fischer titration using a certified water standard as sample. In volumetry, you can carry out a threefold titer determination followed by a determination of a different standard. Then, you can calculate the recovery of the water content determination of the standard.

To check a coulometric system, carry out a threefold determination with a certified water standard and calculate the recovery. If the recovery is between 97–103%, this indicated that the system, including the electrode, is working fine.

The color of the working medium is an additional indicator as to whether the indication is working properly.

Pale yellow is perfect, whereas dark yellow or even pale brown suggests indication problems. If this happens, then the indicator electrode should be cleaned.

Check out questions 7 and 8 for tips on the cleaning of the indicator electrode.

2.  How long can an electrode be stored in KF reagent?

Karl Fischer electrodes are made from glass and platinum. Therefore, the KF reagent does not affect the electrode. It can be stored in reagent as long as you want.

3.  Can the molecular sieve be dried and reused, or should it be replaced?

The molecular sieve can of course be dried and reused. I recommend drying it for at least 24 hours at a temperature between 200–300 °C. Afterwards, let it cool down in a desiccator and then transfer it into a glass bottle with an airtight seal for storage. 

4.  How long does conditioning normally take?

Conditioning of a freshly filled titration vessel normally takes around 2–4 minutes for volumetry, depending on the reaction speed (type of reagent), and around 15–30 minutes for coulometry. In combination with an oven, it might take a bit longer to reach a stable drift owing to the constant gas flow. I recommend stabilizing the entire oven system for at least 1 hour before the first titration.

Between single measurements in the same working medium, conditioning takes approximately 1–2 minutes. Take care that the original drift level is reached again.

5.  When conditioning, many bubbles form in the coulometric titration cell with a very high drift, also when using fresh reagent. What could be the reason for this effect?

At the anode, the generator electrode produces iodine from the iodide-containing reagent. The bubbles you see at the cathode are the result of the reduction of H+ ions to hydrogen gas.

After opening the titration cell or after filling it with fresh reagent, the conditioning step removes any moisture brought into the system, avoiding a bias in the water content determination of the sample. Removing the water results in an increased drift level. During conditioning, the aforementioned H2 is generated. The gas bubbles are therefore completely normal and not a cause for concern. Generally, the following rule applies: The more moisture present in the titration vessel, the higher the drift value will be, and the more hydrogen will form.

6.  What is the best frequency to clean the Karl Fischer equipment?

There is no strict rule as to when you should clean the KF equipment. The cleaning intervals strongly depend on the type and the amount of sample added to the titration cell. Poor solubility and contamination of the indicator electrode (deposition layer on its surface) or memory effects due to large amounts of sample can be good reasons for cleaning the equipment.

The drift can be a good indicator as well. In case you observe higher and unstable drift values, I would recommend cleaning the titration cell or at least refilling the working medium.

7.  How do I clean the Karl Fischer equipment?

For a mounted titration vessel, it can be as simple as rinsing with alcohol. For an intense cleaning, the vessel should be removed from the titrator. Water, solvents like methanol, or cleaning agents are fine to clean the KF equipment. Even concentrated nitric acid can be used as an oxidizing agent, e.g. in case of contaminated indicator electrodes or coulometric generator electrodes.

All of these options are fine, but keep in mind that the last cleaning step should always be rinsing with alcohol followed by proper drying in a drying oven or with a hair dryer at max. 50 °C to remove as much adherent water as possible.

You should never use ketones (e.g., acetone) to clean Karl Fischer equipment, as they react with methanol. This reaction releases water. If there are still traces of ketones left in the titration cell after cleaning, they will react with the methanol in the KF reagent and might cause the drift to be too high to start any titration.

8.  Is it also possible to use a cleaning agent like «CIF» or toothpaste to clean the double Pt electrode?

Normally, rinsing with alcoholic solvents and polishing with paper tissue should be enough to clean the indicator electrode. You may also use detergents, toothpaste, or the polishing set offered by Metrohm! Just make sure that you rinse the electrode properly after the cleaning process to remove all traces of your chosen cleaning agent before using the electrode again.

Cleaning instructions can also be found in our video about metal and KF electrode maintenance:

9.  How do I clean a generator electrode with a diaphragm?

After removing the generator electrode from the titration vessel, dispose the catholyte solution, then rinse the electrode with water. Place the generator electrode upright (e.g., in an Erlenmeyer flask) and cover the connector with the protection cap to prevent corrosion. Fill the generator electrode with some milliliters of concentrated nitric acid, and let the acid flow through the diaphragm. Then fill the cathode compartment with water, and again allow the liquid to flow through the diaphragm. Repeat the rinsing step with water several times to make sure that all traces of nitric acid are washed out of the diaphragm.

Please note that the nitric acid treatment can be left out if the level of contamination does not require it.

Finally, pour some methanol into the generator electrode to remove the water. Repeat this step a few times to remove all traces of water. The last step is properly drying the electrode in a drying oven or with a hair dryer at max. 50 °C. After this cleaning procedure, the electrode is as good as new and can be used again for titrations.

In Part 2, I cover the topics of KF titration troubleshooting and the Karl Fischer oven technique. You can click directly to the post below!

Post written by Michael Margreth, Sr. Product Specialist Titration (Karl Fischer Titration) at Metrohm International Headquarters, Herisau, Switzerland.

Titer determination in Karl Fischer titration

Titer determination in Karl Fischer titration

In a recent post, we have discussed the importance of titer determinations for potentiometric titrations.

Without a titer determination, you will not obtain correct results. The same applies for volumetric Karl Fischer (KF) titrations. In this blog post, I will cover the following topics (click to jump directly to each):

Why should I do titer determinations?

Why is a titer determination necessary? Well, the answer is quite simple. Without knowing the titer of a KF titrant, the water content of the sample cannot be calculated correctly. In Karl Fischer titration, the titer states how many mg of water can be titrated with one mL of titrant. Therefore, the KF titer has the unit «mg/mL».

You might say: “Now, ok, let’s determine the titer. That isn’t too much work and afterwards, I know the titer value and I don’t need to repeat the titer determination.

I agree this would be very nice. However, reality is somewhat different. You must carry out titer determinations on a regular basis. In closed bottles, KF titrants are very stable and the titer does not change appreciably. Once you open the bottle, the KF titrant starts to change significantly. Air will enter the bottle, and considering that 1 L of air contains several milligrams of water, you can imagine that this moisture has an influence on the titer. To prevent moist air from getting into the titrant, the bottle must be either tightly closed after use with the original cap, or should be protected with an absorber tube filled with a molecular sieve (0.3 nm).

Please be aware that temperature changes also have an influence on the titer. A temperature increase of the titrant by 1 °C leads to a titer decrease of approximately 0.1% due to volume expansion. Consider this, in case the temperature in your laboratory fluctuates during the working day.

Do not forget: if your titration system is stopped overnight, the reagent in the tubes and in the cylinder is affected and the titer is no longer equal to the titrant in the bottle. Therefore, I recommend first running a preparation step to flush all tubes before the first titration.

How often should I perform titer determinations?

This question is asked frequently, and unfortunately has no simple answer. In other words, I cannot recommend a single fixed interval for titer determinations. The frequency depends on various factors:

  • the type of reagent (two-component titrants are more stable than single-component titrants)
  • the tightness of the seals between the titration vessel and the titrant bottle
  • how accurate the water content in the sample must be determined

In the beginning, I would recommend performing a titer determination on a daily basis. After a few days, it will become apparent whether the titer remains stable or decreases. Then you can decide to adjust the interval between successive titer determinations.

What equipment do I need for a titer determination?

You need a fully equipped titrator for volumetric KF titration, as well as the KF reagents (titrant and solvent). Another prerequisite for accurate titer determinations is an analytical balance with a minimal resolution of 0.1 mg. Last but not least, you need a standard containing a known amount of water and some tools to add the standard to the titration vessel. These tools are discussed in the next section.

How to carry out a titer determination

Three different water standards are available for titer determinations. There are both liquid and solid standards available from various reagent suppliers. The third possibility is available in every laboratory: distilled water. Below, we will take a closer look at the individual handling of these three standards. For determination of appropriate sample sizes, you can download our free Application Bulletin AB-424, Titer determination in volumetric Karl Fischer titration.

1. Liquid water standard

For the addition of a liquid water standard, you need a syringe and a needle.

There are two possibilities to add liquid standard. One is to inject it with the tip of the needle placed above the reagent level. In this case, aspirate the last drop back into the syringe. Otherwise, it will be dropped off at the septum. The droplet is included in the sample weight, but the water content in the drop is not determined. This will lead to false results.

If the needle is long enough, you can immerse the tip in the reagent during the standard addition. In this case, there is no last droplet to consider, and you can pull the needle out of the titration vessel without any additional aspiration step.

Step-by-step – how to carry out a titer determination:

  1. Open the ampoule containing the standard as recommended by the manufacturer.
  2. Aspirate approximately 1 mL of the standard into the syringe.
  3. Remove the tip of the needle from the liquid and pull the plunger back to the maximum volume. Sway the syringe to rinse it with standard. Then eject the 1 mL of standard into the waste.
  4. Aspirate the remaining content of the ampoule into the needle.
  5. Remove any excess liquid from the outside of the needle with a paper tissue.
  6. Place the needle on a balance, and tare the balance.
  7. Then, start the determination and inject a suitable amount of standard through the septum into the titration vessel. Please take care that the standard is injected into the reagent and not at the electrode or the wall of the titration vessel. This leads to unreproducible results.
  8. After injecting the standard, place the syringe again on the balance.
  9. Enter the sample weight in the software.
2. Solid water standard

It is not possible to add the solid water standard with a syringe. For this, different tools are required. Here, examples are shown of a weighing boat and the Metrohm OMNIS spoon for paste.

Place the weighing boat on the balance, then tare the balance. Weigh in an appropriate amount of the solid standard, and tare the balance again. Start the titration, quickly remove the stopper with septum, add the solid standard and quickly replace the stopper. When adding the standard, take care that no standard sticks to the electrode or the walls of the titration vessel. In case that happens, gently swirl the titration vessel to wash down the standard. After the addition of the standard, place the weighing boat on the balance again and enter the sample weight in the software.

3. Pure water

Pure water can be added to the titration vessel either by weight or by volume.

For a titer determination with pure water, only a few drops are required. Such small volumes can be difficult to add precisely, and results strongly depend on the user. Moreover, addition by weight requires a balance capable of weighing a few milligrams. I personally prefer using water standards, and suggest that you use them as well.

By weight

Fill a small syringe (~1 mL) with water. Due to the very small amounts of pure water added for the titer determination, I recommend using a very thin needle to more accurately add small volumes. After filling the syringe, place it on a balance and tare the balance. Then start the titration, and inject an appropriate amount of water through the septum into the titration vessel. Aspirate the last droplet back into the syringe. Remove the needle, place the syringe on the balance again, and enter the sample weight in the software.

By volume

Fill a microliter syringe with an appropriate volume of water. Make sure there are no air bubbles in the syringe, as they will falsify the result. Begin the titration and inject the syringe contents through the septum into the titration vessel. Enter the added sample size in the software.

Acceptable results

During trainings, I am often asked if the obtained result is acceptable. I recommend carrying out a threefold titer determination. Ideally, the relative standard deviation of those three determinations is smaller than 0.3%.

How long can the reagent be used?

As long as you carry out regular titer determinations, the titer change will be considered in the calculation, and the results will be correct. Just keep in mind: the lower the titer, the larger the volume needed for the determination.

I hope I was able to convince you that titer determination is essential to obtain correct results in volumetric Karl Fischer titration, and that it is not that difficult to perform.

In case you still have unanswered questions, please download Metrohm Application Bulletin AB-424 to get additional information, tips, and tricks on performing titer determination.

Still have questions?

Check out our Application Bulletin: Titer determination in volumetric Karl Fischer titration.

Post written by Michael Margreth, Sr. Product Specialist Titration (Karl Fischer Titration) at Metrohm International Headquarters, Herisau, Switzerland.