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Easy moisture determination in fertilizers by near-infrared spectroscopy

Easy moisture determination in fertilizers by near-infrared spectroscopy

Blooms or bombs?

As the global population steadily increases, it is important that sufficient crops are produced each year to provide enough food, clothing, and other products. Crops such as corn, wheat, soy, and cotton receive nutrients from the soil they are grown in. Fertilizers play a crucial role in providing these crops with the nutrients they need to grow properly.

An important ingredient in the production of high quality, effective fertilizers is ammonium nitrate (NH4NO3), a good source of nitrogen and ammonium for plants.

Produced as small beads similar in appearance to kitchen salt, ammonium nitrate is cheap to buy and usually safe to handle – but storing it can be a problem. Over time, the compound absorbs moisture, which leads to clumping of the individual beads into a larger block. When such a large quantity of compacted ammonium nitrate is exposed to intense heat it can trigger an explosion.

Over the last century, ammonium nitrate has been involved in at least 30 disasters and terrorist attacks. One of the most recent occurrences was on the evening of August 4th, 2020 in Beirut, where an ammonium nitrate explosion killed at least 220 people and injured more than 5000. This blast is one of the largest industrial disasters ever linked to NH4NO3.

Moisture analysis methods for fertilizers

During the production process of ammonium nitrate it is important to control the moisture content. A low moisture content is preferable, but unnecessary excess drying leads to additional manufacturing costs.  Regulations for different fertilizers vary across the globe, but local legal limits ensure that the maximum amount of water present must not be exceeded.  Therefore,  rapid, reliable, and accurate methods for the determination of moisture is necessary. Out of those available, Karl Fischer titration is one of the most common; oven drying, for example, cannot be used with fertilizers containing ammonium nitrate.

Compared to these methods, near-infrared spectroscopy (NIRS) offers unique advantages. It is a secondary technique that generates reliable results within seconds without needing any sample preparation. NIRS is a non-destructive measurement technique and at the same time does not create any chemical waste.

Read our previous blog posts below to learn more about NIRS as a secondary technique.

NIRS analysis of solids

The most suitable NIR analyzer to measuring different parameters in fertilizer or ammonium nitrate pellets is the Metrohm DS2500 Solid Analyzer with Large Sample Cup.

Solid samples (e.g., granules and pellets) that are filled in the rotating DS2500 Large Sample Cup must be placed on the analyzer window. While scanning the sample, the Large Sample Cup will rotate in order to compensate for inhomogeneity.

As the DS2500 Solid Analyzer is a pre-dispersive system, the sample is illuminated with monochromatic light in order to keep the energy level as low as possible. Therefore, the instrument lid must be closed prior to starting the analysis so external light does not affect the results. The NIR radiation comes from below and is partially reflected by the sample to the detector, which is also located below the sample vessel plane. After 45 seconds, the measurement is completed, and a result is displayed. As this reflected light contains all the relevant sample information, this measurement technique is called diffuse reflection.

Advantages of using NIRS

The procedure for obtaining the NIR spectrum already highlights its simplicity regarding sample measurement and its speed. Several advantages of NIRS are listed below:

 

  • Fast technique with results in less than 1 minute.
  • No sample preparation required – solids and liquids can be used in pure form.
  • Low cost per sample – no chemicals or solvents needed.
  • Environmentally friendly technique – no waste generated.
  • Non-destructive – precious samples can be reused after analysis.
  • Multiple component analysis – prediction of different constituents in parallel.
  • Easy to operate – inexperienced users are immediately successful.

Overall, near-infrared spectroscopy is a robust alternative technique for the determination of both chemical and physical parameters in solids and liquids. It is a fast method which can also be successfully implemented for routine analysis by staff without any higher laboratory education.

Related Applications

Specialty chemicals have to fulfill multiple quality requirements. One of these quality parameters, which can be found in almost all certificates of analysis and specifications, is the moisture content. The standard method for the determination of moisture content is Karl Fischer titration.

This method requires reproducible sample preparation, chemicals, and waste disposal. Alternatively, near-infrared spectroscopy can be used for the determination of moisture content. With this technique, samples can be analyzed without any preparation and without using any chemicals.

More information about the application details can be found below!

Moisture content is one of the most commonly measured properties of fertilizers. Globally, regulations for different fertilizers vary, but local legal limits ensure that the maximum amount of water must not be exceeded. A number of analytical techniques are available for this purpose. Next to gravimetric methods, Karl Fischer titration is often used for accurate moisture determination.

Compared to these methods, near-infrared spectroscopy offers unique advantages: it generates reliable results within seconds, and at the same time does not create chemical waste. This Application Note explains how NIRS can offer fast, reagent-free analysis of moisture content in various fertilizer products.

Read on for more technical details…

To learn more about how Karl Fischer titration and NIRS complement each other for the analysis of moisture in different products, read our blog post!

For more information

About spectroscopy solutions provided by Metrohm, visit our website!

We offer NIRS for lab, NIRS for process, as well as Raman solutions

Post written by Wim Guns, International Sales Support Spectroscopy at Metrohm International Headquarters, Herisau, Switzerland.

ASTM D6304: Easier determination of moisture in petroleum products

ASTM D6304: Easier determination of moisture in petroleum products

Water in petroleum products, such as lubricating oils, jet fuel, or other similar products can have deleterious effects. Moisture is often associated with corrosion and engine wear. Knowing the water content of petroleum products can prevent damage to costly infrastructure and ensure safer operations.

ASTM D6304 «Standard Test Method for Determination of Water in Petroleum Products, Lubricating Oils, and Additives by Coulometric Karl Fischer Titration» is a standard that is often cited for moisture determination in the specifications of various petroleum products. It has been recently updated (January 2021) and now offers three procedures for accurate moisture determination.

The direct sample injection into the titration cell (Procedure A) is recommended for low viscosity samples without expected interferences. An oven (Procedure B) or water evaporator accessory (Procedure C) can be used to analyze samples that do not readily dissolve in Karl Fischer reagent, viscous samples, and samples with components that are expected to interfere with the Karl Fischer reaction.

In this blog post I want to introduce these three procedures, and then discuss when it is appropriate to use each of them.

Determining the moisture content in petroleum products doesn’t have to be messy. Visit our website to learn more about the new automated measurement capabilities allowed with ASTM D6304.

A coulometric Karl Fischer Titrator such as the 851 Titrando from Metrohm is the basis for all three procedures of ASTM D6304.

Direct injection (Procedure A)

The direct sample injection into the titration cell is recommended for low viscosity samples without expected interferences. An aliquot of known mass or volume is injected into the conditioned titration cell of a coulometric Karl Fischer apparatus, where it is titrated automatically, and the results calculated.

Method D6304 permits the use of coulometric generator electrodes with and without diaphragm. We recommend the use of the generator electrode with diaphragm, due to the low water content of the samples.

Not all petroleum products are soluble in Karl Fischer reagent and phase separation can occur when using Procedure A. If phase separation occurs, the reagents need to be replaced. The number of samples which can be analyzed without phase separation depends on the volume and type of sample, the volume of reagent, and the sample solubility in the reagent.

The generator electrode with diaphragm is recommended for water determination according to ASTM D6304 Procedure A.

However, for these kinds of samples, Procedures B or C are often the better solution. The same is the case if your sample contains interfering substances.

For more information about ASTM D6304 Procedure A, download our free Application Bulletin (AB-209). For more tips and tricks about how to improve your Karl Fischer titration, have a look at our blog series: «Frequently asked questions in Karl Fischer titration».

Water extraction using an oven (Procedure B)

An oven (Procedure B) can be used to analyze samples that do not readily dissolve in Karl Fischer reagent, viscous samples, and samples with components that are expected to interfere with the Karl Fischer reaction.

For the analysis, a representative sample is weighed into a glass vial, which is sealed immediately. The vial is then heated in an oven to extract any water. The vaporized water is carried into the conditioned Karl Fischer titration cell by means of a dry carrier gas where it is titrated.

Schematic drawing of the Karl Fischer oven method.

The ideal temperature used for the evaporation depends on the sample. The 874 Oven Sample Processor can perform a temperature gradient test to determine the optimal temperature for removing water without degrading the sample.

To learn more about the oven method, its working principle and its advantages, check out our blog post: «Oven method for sample preparation in Karl Fischer titration».

Watch our LabCast video below to see the working principle and advantages of using Procedure B.

For more information about using the KF oven method for ASTM D6304 Procedure B, download our free Application Bulletin (AB-209) or free Application Note (AN-K-070).

Just want the highlights? Have a look at our short flyer about how ASTM D6304 has become much easier!

Water extraction using an evaporator (Procedure C)

Instead of using an oven, Procedure C explains how a water evaporator can be used for the water extraction of samples that do not readily dissolve in Karl Fischer reagent, viscous samples, and samples with components that are expected to interfere with the Karl Fischer reaction.

In this procedure, an aliquot of sample is transferred into a heated chamber containing a suitable solvent (most often, toluene). The temperature of the heated chamber depends on the solvent used. The water vaporizes along with the solvent in an azeotrope distillation. The azeotrope is then transferred into the conditioned Karl Fischer titration cell via a dry non-reactive carrier gas. 

Schematic drawing of the evaporator method.

If you wish to read more about the three procedures and their advantages and disadvantages, download our White Paper: «Moisture in petroleum products according to ASTM D6304».

When to use which procedure

Procedure A is mainly suited for liquid samples with a low viscosity, such as diesel fuel, jet fuel, or aromatics. A low viscosity is required in order to be able to add the sample easily into the Karl Fischer titration cell. Furthermore, the samples require a good solubility in Karl Fischer reagent. Otherwise phase separation will occur, which requires the replacement of the Karl Fischer reagents. While the reagent exchange can be automated, time is still required until the reagents reach dryness again.

Even if samples are soluble in Karl Fischer reagents, there might still be issues with using Procedure A due to the sample matrix creating side reactions and thus false results. In this case Procedure B or C are the better option.

Procedure B is suitable for all kinds of samples, regardless of their viscosity or matrix composition. It is only the evaporated water that is transferred into the titration cell, leaving the sample as well as interfering matrix components remaining in the sealed vial, which can be simply disposed of after the analysis. For this reason, the reagent exchange frequency is greatly reduced, saving costs, as less reagent is required. Depending on the workload in your lab, it is even possible to fully automate the analysis including reagent exchange using an automated Karl Fischer oven.

The 874 Karl Fischer Oven Processor with an 851 Titrando for a fully automated analysis according to ASTM D6304 Procedure B.

Procedure C, like Procedure B, is suitable for all kinds of samples, regardless of their viscosity or matrix constitution. It is only the evaporated water in an azeotrope with the solvent that is transferred into the titration cell. The sample, as well as interfering matrix components, remain in the evaporation chamber. However, it is necessary to manually empty and refill the evaporation chamber from time to time, which is time consuming, as the chamber needs to cool down before the content can be exchanged. Furthermore, walk-away automation is not possible with this method.

For a more detailed comparison of the various factors for each procedure, download our free White Paper: «Moisture in petroleum products according to ASTM D6304».

Visit our website

Save time with the new automated measurement capabilities allowed with ASTM D6304

Post written by Lucia Meier, Technical Editor at Metrohm International Headquarters, Herisau, Switzerland.

«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.

Moisture Analysis – Karl Fischer Titration, NIRS, or both?

Moisture Analysis – Karl Fischer Titration, NIRS, or both?

In addition to the analysis of the pH value, weighing, and acid-base titration, measurement of water content is one of the most common determinations in laboratories worldwide. Moisture determination is important for nearly every industry, e.g., for lubricants, food and feed, and pharmaceuticals.

Figure 1. Water drops in a spider web

For lubricants, the water concentration is very important to know because excess moisture expedites wear and tear of the machinery. For food and feed, moisture content must be within a narrow range so that the food does not taste dry or stale, nor that it is able to provide a breeding ground for bacteria and fungi, resulting in spoilage. For pharmaceuticals, the water content in solid dosage forms (tablets) and lyophilized products is monitored closely. For the latter, the regulations state that the moisture content needs to be below 2%.

Karl Fischer Titration

Karl Fischer (KF) Titration for water determination was introduced back in the 1930’s, and to this day remains one of the most tried and trusted methods. It is a fast and highly selective method, which means that water, and only water, is determined. KF titration is based on the following two redox reactions.

In the first reaction, methanol and sulfur dioxide react to form the respective ester. Upon addition of iodine, the ester is oxidized to the sulfate species in a water-consuming reaction. The reaction finishes when no water is left.

Figure 2. Manual sample injection for volumetric KF Titration

KF titration can be used for the determination of the water content in all sample types: liquids, solids, slurries, or even gases. For concentrations between 0.1% and 100%, volumetric KF titration is the method of choice, whereas for lower moisture content between 0.001% and 1%, coulometric KF titration is recommended.

Depending on the sample type, its water content, and its solubility in the KF reagents, the sample can either be added directly to the titration vessel, or would first need to be dissolved in a suitable solvent. Suitable solvents are those which do not react with the KF reagents — therefore aldehydes and ketones are ruled out. In case the sample is dissolved in a solvent, a blank correction with the pure solvent also needs to be performed. For the measurement, the sample is injected directly into the titration vessel using a syringe and needle (Fig. 2). The endpoint is detected by a polarized double Pt pin electrode, and from this the water concentration is directly calculated.

Insoluble or hygroscopic samples can be analyzed using the gas extraction technique with a KF Oven. Here, the sample is sealed in small vial, and the water is evaporated by heat then is subsequently carried to the titration cell.

Figure 3. Fully automated KF Titration with the Metrohm 874 KF Oven Sample Processor

For more information, download our free Application Bulletins: AB-077 for volumetric Karl Fischer titration and AB-137 for coulometric Karl Fischer analysis.

If you would like some deeper insight, download our free monograph: “Water determination by Karl Fischer Titration”. 

Near-infrared spectroscopy

Near-infrared spectroscopy (NIRS) is a technique that has been used for myriad applications in the areas of food and feed, polymers, and textiles since the 1980’s. A decade later, other segments began using this technique, such as for pharmaceutical, personal care, and petroleum products.

NIRS detects overtones and combination bands of molecular vibrations. Among the typical vibrations in organic molecules for functional groups such as -CH, -NH, -SH, and -OH, it is the -OH moiety which is an especially strong near infrared absorber. That is also the reason why moisture quantification is one of the key applications of NIR spectroscopy.

For a further explanation, read our previous blog entry on this subject: Benefits of NIR spectroscopy: Part 2.

NIR spectroscopy is used for the quantification of water in solids, liquids, and slurries. The detection limit for moisture in solids is about 0.1%, whereas for liquids it is in the range of 0.02% (200 mg/L), However, in special cases (e.g., water in THF), moisture detection limits of 40–50 mg/L have been achieved.

This technique does not require any sample preparation, which means that samples can be used as-is. Solid samples are measured in high quality disposable sample vials, whereas liquids are measured in high quality disposable cuvettes. Figure 4 displays how the different samples are positioned on the analyzer for a measurement.

Detailed information about the NIRS technique has been described in our previous blog article: Benefits of NIR spectroscopy: Part 1.

Figure 4. Solid (left) and liquid (right) sample positioning for NIR measurements

NIRS is a secondary technique, meaning it can only be used for routine analysis for moisture quantification after a prediction model has been developed. This can be understood by an analogy to HPLC, for which measuring standards to create a calibration curve is among the initial steps. The same applies to NIRS: first, spectra with known moisture content must be measured and then a prediction model is created.

The development of prediction models has been described in detail in our previous blog article: Benefits of NIR spectroscopy: Part 3.

The schematic outline is shown in Figure 5.

Figure 5. Workflow for NIR Method implementation for moisture analysis

For creation of the calibration set, around 30–50 samples need to be measured with both NIRS and KF titration, and the values obtained from KF titration must be linked to the NIR spectra. The next steps are model development and validation (steps 2 and 3 in Figure 5), which are quite straightforward for moisture analysis. Water is a strong NIR absorber, and its peaks are always around 1900–2000 nm (combination band) and 1400–1550 nm (first overtone). This is shown in Figure 6 below.

Figure 6. NIR Spectra of moisturizing creams, showing the absorptions related to H2O at 1400–1550 nm and 1900–2000 nm

After creation and validation of the prediction model, near-infrared spectroscopy can be used for routine moisture determination of that substance. The results for moisture content will be obtained within 1 minute, without any sample preparation or use of chemicals. Also, the analyst does not need to be a chemist, as all they need to do is place a sample on the instrument and press start.

You can find even more information about moisture determination by near-infrared spectroscopy in polyamides, caprolactam, lyophilized products, fertilizers, lubricants, and ethanol/hydrocarbon blends below by downloading our free Application Notes.

Your choice for moisture measurements: KF Titration, NIRS, or both!

As summarized in Table 1, KF Titration and NIR Spectroscopy each have their advantages. KF Titration is a versatile method with a low level of detection. Its major advantage is that it will always work, no matter if you have a sample type that you measure regularly or whether it is a sample type that you encounter for the first time.

Table 1. Overview of characteristics of moisture determination via titration and NIR spectroscopy

NIR spectroscopy requires a method development process, meaning it is not suitable for sample types that always vary (e.g., different types of tablets, different types of oil). NIRS however is a very good method for sample types that are always identical, for example for moisture content in lyophilized products or for moisture content in chemicals, such as fertilizers.

For the implementation of a NIR moisture method, it is required that samples are measured with KF titration as the primary method for the model development. In addition, during the routine use of a NIR method, it is important to confirm once in a while (e.g., every 50th or every 100th sample) with KF Titration that the NIR model is still robust, and to ensure that the error has not increased. If a change is noticed, extra samples need to be added to the prediction model to cover the observed sample variation.

In conclusion, both KF Titration and NIR spectroscopy are powerful techniques for measuring moisture in an array of samples. Which technique to use depends on the application and the individual preference of the user.

For more information

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Karl Fischer titration and near-infrared spectroscopy in perfect synergy

Post written by Dr. Dave van Staveren (Head of Competence Center Spectroscopy), Dr. Christian Haider (Head of Competence Center Titration), and Iris Kalkman (Product Specialist Titration) at Metrohm International Headquarters, Herisau, Switzerland.