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Thermometric titration – the missing piece of the puzzle

Thermometric titration – the missing piece of the puzzle

Titration is a well-established analysis technique taught to each and every chemistry student. Titration is carried out in nearly every analytical laboratory either as manual titration, photometric titration, or potentiometric titration. In this blog entry, I would like to present an additional kind of titration you may  not have heard of before – thermometric titration – which can be considered the missing piece of the titration puzzle.

Here, I plan to cover the following topics:
  1. What is thermometric titration?
  2. Why consider thermometric titration?
  3. Practical application examples

What is thermometric titration?

At first glance, thermometric titration (TET) looks like a normal titration and you won’t see much (or any) difference from a short distance. The differences compared to potentiometric titration are in the details.

TET is based on the principal of enthalpy change (ΔH). Each chemical reaction is associated with a change in enthalpy which in turn causes a temperature change. During a titration, analyte and titrant react either exothermically (increase in temperature) or endothermically (decrease in temperature).

During a thermometric titration, the titrant is added at a constant rate and the change in temperature caused by the reaction between analyte and titrant is measured. By plotting the temperature versus the added titrant volume, the endpoint can be determined by a break within the titration curve. Figure 1 shows idealized thermometric titration curves for both exothermic and endothermic situations.

Figure 1. Illustration of exothermic and endothermic titration curves showing clear endpoints where the temperature of the solution changes abruptly.

What happens during a thermometric titration?

During an exothermic titration reaction, the temperature increases with the titrant addition as long as analyte is still present. When all analyte is consumed, the temperature decreases again as the solution equilibrates with the atmospheric temperature and/or due to the dilution of the solution with titrant (Figure 1, left graph). This temperature decrease results in an exothermic endpoint.

On the contrary, for an endothermic titration reaction, the temperature decreases with the titrant addition as long as analyte is still available. When all analyte is consumed, the temperature stabilizes or increases again as the solution equilibrates with the atmospheric temperature and/or due to the dilution of the solution with titrant (Figure 1, right graph). This temperature decrease results in an endothermic endpoint.

Knowing the absolute temperature, isolating the titration vessel, or thermostating the titration vessel is thus not required for the titration.

Figure 2. Metrohm’s maintenance-free Thermoprobe used for the reliable indication of thermometric endpoints.

In order to measure the small temperature changes during the titration, a very fast responding thermistor with a high resolution is required. These sensors are capable of measuring temperature differences of less than 0.001 °C, and allow the collection of a measuring point every 0.3 seconds (Figure 2). 

Visit the Metrohm website to learn more about the fast, sensitive Thermoprobe products available even for aggressive sample solutions.

If you would like to learn more about the theory behind TET, then download our free comprehensive monograph on thermometric titration.

Why consider thermometric titration?

Potentiometric and photometric titration are already well established as instrumental titration techniques, so why should one consider thermometric titration instead?

 

TET has the same advantages as any instrumental titration technique:
  • Inexpensive analyses: Titration instruments are inexpensive to purchase and do not have high running and maintenance costs compared to other instruments for elemental analysis (e.g., HPLC or ICP-MS).
  • Absolute method: Titration is an absolute method, meaning it is not necessary to frequently calibrate the system.
  • Versatile use: Titration is a universal method, which can be used to determine many different analytes in various industries.
  • Easy to automate: Titration can be easily automated, increasing reproducibility and efficiency in your lab.
In comparison to classical instrumental titration, thermometric titration has several additional advantages:
  • Fast titrations: Due to the constant titrant addition, thermometric titrations are very fast. Typically, a thermometric titration takes 2–3 minutes.
  • Single sensor: Regardless of the titration reaction (e.g., acid-base, redox, precipitation, …), the same sensor (Thermoprobe) can be used for all of them.
  • Maintenance-free sensor: Additionally, the Thermoprobe is maintenance free. It requires no calibration or electrolyte filling and can simply be stored dry.
  • Less solvent: Typically, thermometric titrations use 30 mL of solvent or even less. The small amount of solvent ensures that the dilution is minimized, and the enthalpy changes can be detected reliably. As a side benefit, less waste is produced.
  • Additional titrations possible: Because enthalpy change is universal for any chemical reaction, thermometric titration is not bound to finding a suitable color indicator or indication electrode. This allows the possibility of additional titrations which cannot be covered by other kinds of titration.
  • Easier sample preparation: As TET uses higher titrant concentrations it is possible to use larger sample sizes, reducing weighing and dilution errors. Tedious sample preparation steps such as filtration can be omitted as well.
Figure 3. The Metrohm 859 Titrotherm with 801 Stirrer and notebook with tiamo™ software.

Learn more about the 859 Titrotherm system for the most reliable TET determinations on the Metrohm website.

Practical application examples

In this section I would like to present you some practical examples where TET can be applied.

Acid number and base number

The acid number (AN) and base number (BN) are two key parameters in the petroleum industry. They are determined by a nonaqueous acid-base titration using KOH or HClO4, respectively, as titrant.

During such determinations, very weak acids (for AN analysis) and bases (for BN analysis) are titrated with only small enthalpy changes. Using a catalytic indicator, these weak acids and bases can also be determined by TET.

ASTM D8045 describes the analysis of the AN by thermometric titration. The benefits of carrying out this titration are:

  • Less solvent (30 mL instead of 60 or 120 mL), meaning less waste
  • Fast titration (1–3 minutes)
  • No conditioning of the sensor

If you wish to learn more about how well the results of the AN determination according to ASTM D8045 correlate with ASTM D664, download our free White Paper WP-012 as well as our brochure below.

For more detailed information about the titration itself, download the free Application Bulletin AB-427 (AN) and Application Bulletin AB-405 (BN) below.

Sodium

Using conventional titration, the salt content in foodstuff is usually determined based solely on the chloride content. However, foods usually contain additional sources of sodium, e.g. monosodium glutamate (also known as «MSG»). With TET it becomes possible to titrate the sodium directly, and thus to inexpensively determine the true sodium content in foodstuff, as stipulated in several countries.

If you wish to learn more about the sodium determination, watch our Metrohm LabCast video: «Sodium determination in food: Fast and direct thanks to thermometric titration».

Fertilizer analysis

Fertilizers consists of various nutrients, including phosphorus, nitrogen, and potassium, which are important for plant growth. TET enables the analysis of these nutrients by employing classical gravimetric reactions as the basis for the titration (e.g., precipitation of sulfate with barium). This allows for a rapid determination, without needing to wait hours for a result, as with conventional procedures based on drying and weighing the precipitate.

Nutrients which can be analyzed by TET include:
  • Phosphate
  • Potassium
  • Ammoniacal nitrogen
  • Urea nitrogen
  • Sulfate

Want to learn more about the analysis of fertilizers with thermometric titration? Download our free White Paper WP-060 on this topic. For more detailed information regarding the different fertilizer applications, check out the Metrohm Application Finder, or find a curated selection here.

Metal-organic compounds

Metal-organic compounds, such as Grignard reagents or butyl lithium compounds, are used for synthetizing active pharmaceutical ingredients (APIs) or manufacturing polymers such as polybutadiene. With TET, the analysis of these sensitive species can be performed rapidly and reliably by titrating them under inert gas with 2-butanol.

If you wish to learn more about this topic, check out our news article and download the free corresponding Application Note AN-H-142.

These were just a few examples about the possibilities of thermometric titration, to demonstrate its versatile use. For a more detailed selection, have a look at our Application Finder.

To summarize:

  • TET is an alternative titration method based on enthalpy change
  • A fast and sensitive Thermoprobe is used to determine exothermic and endothermic endpoints
  • Thermometric titration is a fast analysis technique providing results in less than 3 minutes
  • Thermometric titration can be used for various analyses, including titrations which cannot be performed otherwise (e.g., sodium determination)

I hope this overview has given you a better idea about thermometric titration – the missing piece of the titration puzzle.

For more information

Download our free Monograph:

Practical thermometric titrimetry

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

Frequently asked questions in near-infrared spectroscopy analysis – Part 1

Frequently asked questions in near-infrared spectroscopy analysis – Part 1

Whether you are new to the technique, a seasoned veteran, or merely just curious about near-infrared spectroscopy (NIRS), Metrohm is here to help you to learn all about how to perform the best analysis possible with your instruments.

In this series, we will cover several frequently asked questions regarding both our laboratory NIRS instruments as well as our line of Process Analysis NIRS products.

1. What is the difference between IR spectroscopy and NIR spectroscopy?

IR (infrared) and NIR (near-infrared) spectroscopy utilize different spectral ranges of light. Light in the NIR range is higher in energy than IR light (Figure 1), which affects the interaction with the molecules in a sample.

Electromagnetic Spectrum
Figure 1. The electromagnetic spectrum.

This energy difference has both advantages and disadvantages, and the selection of the ideal technology depends very much on the application. The higher energy NIR light is absorbed less than IR light by most organic materials, broadening the resulting bands and making it difficult to assign them to specific functional groups without mathematical processing.

However, this same feature makes it possible to perform analysis without sample preparation, as there is no need to prepare very thin layers of analyte or use ATR (attenuated total reflection). Additionally, NIRS can quantify the water content in samples up to 15%.

Want to learn more about how to perform faster quality control at lower operating costs by using NIRS in your lab? Download our free white paper here: Boost Efficiency in the QC laboratory: How NIRS helps reduce costs up to 90%.

The weaker absorption of NIR light leads to using long pathlengths for liquid measurements, which is particularly helpful in industrial process environments. Speaking of such process applications, with NIR spectroscopy, you can use long fiber optic cables to connect the analyzer to the measuring probe, allowing remote measurements throughout the process due to low absorbance of the NIR light by the fiber (Figure 2).

Electromagnetic Spectrum
Figure 2. Illustration of the long-distance measurement possibility of a NIRS process analyzer with the use of low-dispersion fiber optic cables. Many sampling options are available for completely automated analysis, allowing users to gather real-time data for immediate process adjustments.

For more information, read our previous blog post outlining the differences between infrared and near-infrared spectroscopy.

2. NIR spectroscopy is a «secondary technology». What does this mean?

To create prediction models in NIR spectroscopy, the NIR spectra are correlated with parameters of interest, e.g., the water content in a sample. These models are then used during routine quality control to analyze samples.

Values from a reference (primary) method need to be correlated with the NIR spectrum to create prediction models (Figure 3). Since NIR spectroscopy results depend on the availability of such reference values during prediction model development, NIR spectroscopy is therefore considered a secondary technology.

Electromagnetic Spectrum
Figure 3. Correlation plot of moisture content in samples measured by NIRS compared to the same samples measured with a primary laboratory method.

For more information about how Karl Fischer titration and NIR spectroscopy work in perfect synergy, download our brochure: Water Content Analysis – Karl Fischer titration and Near-Infrared Spectroscopy in perfect synergy.

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

3. What is a prediction model, and how often do I need to create/update it?

In NIR spectroscopy, prediction models interpret a sample’s NIR spectrum to determine the values of key quality parameters such as water content, density, or total acid number, just to name a few. Prediction models are created by combining sample NIR spectra with reference values from reference methods, such as Karl Fischer titration for water content (Figure 3).

A prediction model, which consists of sufficient representative spectra and reference values, is typically created once and will only need an update if samples begin to vary (for example after a change of production process equipment or parameter, raw material supplier, etc.).

Want to know more about prediction models for NIRS? Read our blog post about the creation and validation of prediction models here.

4. How many samples are required to develop a prediction model?

The number of samples needed for a good prediction model depends on the complexity of the sample matrix and the molecular absorptivity of the key parameter.

For an «easy» matrix, e.g., a halogenated solvent with its water concentration as the measurement parameter, a sample set of 1020 spectra covering the complete concentration range of interest may be sufficient. For applications that are more complex, we recommend using at least 40–60 spectra in order  to build a reliable prediction model.

Find out more about NIRS pre-calibrations built on prediction models and how they can save time and effort in the lab.

5. Which norms describe the use of NIR in regulated and non-regulated industries?

Norms describing how to implement a near-infrared spectroscopy system in a validated environment include USP <856> and USP <1856>. A general norm for non-regulated environments regarding how to create prediction models and basic requirements for near-infrared spectroscopy systems are described in ASTM E1655. Method validation and instrument validation are guided by ASTM D6122 and ASTM D6299, respectively.

Figure 4. Different steps for the successful development of quantitative methods according to international standards.

For specific measurements, e.g. RON and MON analysis in fuels, standards such as ASTM D2699 and ASTM D2700 should be followed.

For further information, download our free Application Note: Quality Control of Gasoline – Rapid determination of RON, MON, AKI, aromatic content, and density with NIRS.

6. How can NIRS be implemented in a production process?

Chemical analysis in process streams is not always a simple task. The chemical and physical properties such as viscosity and flammability of the sample streams can interfere in the analysis measurements. Some industrial processes are quite delicate—even the slightest changes to the process parameters can lead to significant variability in the properties of final products. Therefore, it is essential to measure the properties of the stream continuously and adjust the processing parameters via rapid feedback to assure a consistent and high quality product.

Figure 5. Example of the integration of inline NIRS analysis in a fluid bed dryer of a production plant.

Curious about this type of application? Download it for free from the Metrohm website!

The use of fiber optic probes in NIRS systems has opened up new perspectives for process monitoring. A suitable NIR probe connected to the spectrometer via optical fiber allows direct online and inline monitoring without interference in the process. Currently, a wide variety of NIR optical probes are available, from transmission pair probes and immersion probes to reflectance and transflectance probes, suitable for contact and non-contact measurements. This diversity allows NIR spectroscopy to be applied to almost any kind of sample composition, including melts, solutions, emulsions, and solid powders.

Selecting the right probe, or sample interface, to use with a NIR process analyzer is crucial to successful process implementation for inline or online process monitoring. Depending upon whether the sample is in a liquid, solid or gaseous state, transflectance or transmission probes are used to measure the sample, and specific fitting attachments are used to connect the probes to the reactor, tank, or pipe. With more than 45 years of experience, Metrohm Process Analytics can design the best solutions for your process. 

Visit our website to find a selection of free Application Notes to download related to NIRS measurements in industrial processes.

7. How can product quality be optimized with process NIRS?

Regular control of key process parameters is essential to comply with certain product and process specifications, and results in attaining optimal product quality and consistency in any industry. NIRS analyzers can provide data every 30 seconds for near real-time monitoring of production processes.

Figure 6. The Metrohm Process Analytics NIRS XDS Process Analyzer, shown here with multiplexer option allowing up to 9 measuring channels. Here, both microbundle (yellow) and single fiber (blue) optical cables are connected, with both a reflectance probe and transmission pair configured.

Using NIRS process analyzers is not only preferable for 24/7 monitoring of the manufacturing process, it is also extremely beneficial for inspecting the quality of raw materials and reagents. By providing data in «real-time» to the industrial control system (e.g., DCS or PLC), any process can be automated based on the NIRS data. As a result, downtimes are reduced, unforeseen situations are avoided, and costly company assets are safeguarded.

Furthermore, the included software on Metrohm Process Analytics NIRS instruments has a built-in chemometric package which allows qualification of a product even while it is still being produced. A report is then generated which can be directly used by the QC manager. Therefore, the product quality consistency is improved leading to potential added revenues.

Do you want to learn more about improving product quality with online or inline NIRS analysis? Take a look at our brochure!

In the next part of this FAQ, we will cover even more of your burning questions regarding NIRS for lab and process measurements. Don’t forget to subscribe to the blog so you don’t miss out on future posts!

Want to learn more about NIR spectroscopy and potential applications? Have a look at our free and comprehensive application booklet about NIR spectroscopy.

Download our Monograph

A guide to near-infrared spectroscopic analysis of industrial manufacturing processes

Post written by Dr. Nicolas Rühl (Product Manager Spectroscopy at Metrohm International Headquarters, Herisau, Switzerland) and Dr. Alexandre Olive (Product Manager Process Spectroscopy at Metrohm Applikon, Schiedam, The Netherlands).

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.

Keep on the lookout for our next installment in this two-part series, or subscribe to the blog below so you’re sure not to miss it! In Part 2, I will cover the topics of KF titration troubleshooting and the Karl Fischer oven technique.

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

Comprehensive water analysis: combining titration, IC, and direct measurement in one setup

Comprehensive water analysis: combining titration, IC, and direct measurement in one setup

If you perform water analyses on a regular basis, then you know that analyzing different parameters for drinking water can be quite time-consuming, expensive, and it requires significant manual labor. In this article, I’d like to show you an example of wider possibilities in automated sample analysis when it comes to combining different analytical techniques, especially for our drinking water.

Water is the source and basis of all life. It is essential for metabolism and is our most important foodstuff.

As a solvent and transporting agent it carries not only the vital minerals and nutrients, but also, increasingly, harmful pollutants, which accumulate in aquatic or terrestrial organisms.

Within the context of quality control and risk assessment, there is a need in the water laboratory for cost-effective and fast instruments and methods that can deal with the ever more complex spectrum of harmful substances, the increasing throughput of samples, and the decreasing detection limits.

Comprehensive analysis of ionic components in liquid samples such as water involves four analytical techniques:

  • Direct measurement
  • Titration
  • Ion chromatography
  • Voltammetry

Each of these techniques has its own particular strengths. However, applying them one after the other on discrete systems in the laboratory is a rather complex task that takes up significant time.

Back in 1998, Metrohm accepted the challenge of combining different analytical techniques in a single fully automated system, and the first TitrIC system was introduced.

What is TitrIC?

The TitrIC system from Metrohm combines direct measurement, titration, and ion chromatography in a fully automated system.

Direct measurements include temperature, conductivity, and pH. The acid capacity (m and p values) is determined titrimetrically. Major anions and cations are quantified by ion chromatography. Calcium and magnesium, which are used to calculate total hardness, can be determined by titration or ion chromatography.

The results are displayed in a common table, and a shared report is given out at the end of the analysis. All methods in TitrIC utilize the same liquid handling units and a common sample changer.

For more detailed information about the newest TitrIC system, which is available in two predefined packages (TitrIC flex I and TitrIC flex II), take a look at our informative brochure:

Efficient: Titrations and ion chromatography are performed simultaneously with the TitrIC flex system.

Figure 1. Flowchart of TitrIC flex II automated analysis and data acquisition.

How does TitrIC work?

Each water sample analysis is performed fully automated at the push of a button—fill up a sample beaker with the sample, place it on the sample rack, and start the measurement. The liquid handling units transfer the required sample volume (per measurement technique) for reproducible results. TitrIC carries out all the work, and analyzes up to 175 samples in a row without any manual intervention required, no matter what time the measurement series has begun. The high degree of automation reduces costs and increases both productivity and the precision of the analysis.

Figure 2. The Metrohm TitrIC flex II system with OMNIS Sample Robot S and Dis-Cover functionality.

To learn more about how to perform comprehensive water analysis with TitrIC flex II, download our free application note AN-S-387:

Would you like to know more about why automation should be preferred over manual titration? Check out our previous blog post on this topic:

Calculations with TitrIC

With the TitrIC system, not only are sample analyses simplified, but the result calculations are performed automatically. This saves time and most importantly, avoids sources of human error due to erroneously noting the measurement data or performing incorrect calculations.

Selection of calculations which can be automatically performed with TitrIC: 

  • Molar concentrations of all cations
  • Molar concentrations of all anions
  • Ionic balance
  • Total water hardness (Ca & Mg)
  • … and more

Ionic balances provide clarity

The calculation of the ion balance helps to determine the accuracy of your water analysis. The calculations are based on the principle of electro-neutrality, which requires that the sum in eq/L or meq/L of the positive ions (cations) must equal the sum of negative ions (anions) in solution.

TitrIC can deliver all necessary data required to calculate the ion balance out of one sample. Both anions and cations are analyzed by IC, and the carbonate concentration (indicative of the acid capacity of water) is determined by titration.

If the value for the difference in the above equation is almost zero, then this indicates that you have accurately determined the major anions and cations in your sample.

Advantages of a combined system like TitrIC

  • Utmost accuracy: all results come from the same sample beaker

  • Completely automated, leaving analysts more time for other tasks

  • One shared sample changer saves benchtop space and costs

  • Save time with parallel titration and IC analysis

  • Flexibility: use titration, direct measurement, or IC either alone or combined with the other techniques

  • Single database for all results and calculation of the ionic balance, which is only possible with such a combined system, and gives further credibility to the sample results

Even more possibility in sample analysis

TitrIC has been developed especially for automated drinking water analysis but can be adapted to suit any number of analytical requirements in food, electroplating, or pharmaceutical industries. Your application determines the parameters that are of interest.

If the combination of direct measurement, titration, and IC does not suit your needs, perhaps a combination of voltammetry and ion chromatography in a single, fully automatic system might be more fitting. Luckily, there is the VoltIC Professional from Metrohm which fulfills these requirements.

Check out our website to learn more about this system:

As you see, the possibility of combining different analysis techniques is almost endless. Metrohm, as a leading manufacturer of instruments for chemical analysis, is aware of your analytical challenges. For this reason, we offer not only the most advanced instruments, but complete solutions for very specific analytical issues. Get the best out of your daily work in the laboratory!

Discover even more

about combined analytical systems from Metrohm

Post written by Jennifer Lüber, Jr. Product Specialist Titration/TitrIC at Metrohm International Headquarters, Herisau, Switzerland.

What to consider during back-titration

What to consider during back-titration

Titrations can be classified in various ways: by their chemical reaction (e.g., acid-base titration or redox titration), the indication method (e.g., potentiometric titration or photometric titration), and last but not least by their titration principle (direct titration or indirect titration). In this article, I want to elaborate on a specific titration principle – the back-titration – which is also called «residual titration». Learn more about when it is used and how you should calculate results when using the back-titration principle.

What is a back-titration?

In contrast to direct titrations, where analyte A directly reacts with titrant T, back-titrations are a subcategory of indirect titrations. Indirect titrations are used when, for example, no suitable sensor is available or the reaction is too slow for a practical direct titration.

During a back-titration, an exact volume of reagent B is added to the analyte A. Reagent B is usually a common titrant itself. The amount of reagent B is chosen in such a way that an excess remains after its interaction with analyte A. This excess is then titrated with titrant T. The amount of analyte A can then be determined from the difference between the added amount of reagent B and the remaining excess of reagent B.

As with any titration, both involved reactions must be quantitative, and stoichiometric factors involved for both reactions must be known.

Figure 1. Reaction principle of a back-titration: Reagent B is added in excess to analyte A. After a defined waiting period which allows for the reaction between A and B, the excess of reagent B is titrated with titrant T.

When are back-titrations used?

Back titrations are mainly used in the following cases:

  • if the analyte is volatile (e.g., NH3) or an insoluble salt (e.g., Li2CO3)
  • if the reaction between analyte A and titrant T is too slow for a practical direct titration
  • if weak acid – weak base reactions are involved
  • when no suitable indication method is available for a direct titration

Typical examples are complexometric titrations, for example aluminum with EDTA. This direct titration is only feasible at elevated temperatures. However, adding EDTA in excess to aluminum and back-titrating the residual EDTA with copper sulfate allows a titration at room temperature. This is not only true for aluminum, but for other metals as well.

Learn which metals can be titrated directly, and for which a back-titration is more feasible in our free monograph on complexometric titration.

Other examples include the saponification value and iodine value for edible fats and oils. For the saponification value, ethanolic KOH is added in excess to the fat or oil. After a determined refluxing time to saponify the oil or fat, the remaining excess is back-titrated with hydrochloric acid. The process is similar for the iodine value, where the remaining excess of iodine chloride (Wijs-solution) is back-titrated with sodium thiosulfate.

For more information on the analysis of edible fats and oils, take a look at our corresponding free Application Bulletin AB-141.

How is a back-titration performed?

A back titration is performed according to the following general principle:

  1. Add reagent B in excess to analyte A.
  2. Allow reagent B to react with analyte A. This might require a certain waiting time or even refluxing (e.g., saponification value).
  3. Titration of remaining excess of reagent B with titrant T.

For the first step, it is important to precisely add the volume of reagent B. Therefore, it is important to use a buret for this addition (Fig. 2).

Figure 2. Example of a Titrator equipped with an additional buret for the addition of reagent B.

Furthermore, it is important that the exact molar amount of reagent B is known. This can be achieved in two ways. The first way is to carry out a blank determination in the same manner as the back-titration of the sample, however, omitting the sample. If reagent B is a common titrant (e.g., EDTA), it is also possible to carry out a standardization of reagent B before the back-titration.

In any case, as standardization of titrant T is required. This then gives us the following two general analysis procedures:

Back-titration with blank
  1. Titer determination of titrant T
  2. Blank determination (back-titration omitting sample)
  3. Back-titration of sample
Back-titration with standardizations
  1. Titer determination of titrant T
  2. Titer determination of reagent B
  3. Back-titration of sample

Be aware: since you are performing a back-titration, the blank volume will be larger than the equivalence point (EP) volume, unlike a blank in a direct titration. This is why the EP volume must be subtracted from the blank or the added volume of reagent B, respectively.

For more information on titrant standardization, please have a look at our blog entry on this topic.

How to calculate the result for a back-titration?

As with direct titrations, to calculate the result of a back-titration it is necessary to know the involved stoichiometric reactions, aside from the exact concentrations and the volumes. Depending on which analysis procedure described above is used, the calculation of the result is slightly different.

For a back-titration with a blank, use the following formula to obtain a result in mass-%:

VBlank:  Volume of the equivalence point from the blank determination in mL

VEP Volume at the equivalence point in mL

cTitrant:  Nominal titrant concentration in mol/L

fTitrant Titer factor of the titrant (unitless)

r:  Stoichiometric ratio (unitless)

MA Molecular weight of analyte A in g/mol

mSample Weight of sample in mg

100:  Conversion factor, to obtain the result in %

The stoichiometric ratio r considers both reactions, analyte A with reagent B and reagent B with titrant T. If the stoichiometric factor is always 1, such as for complexometric back-titrations or the saponification value, then the reaction ratio is also 1. However, if the stoichiometric factor for one reaction is not equal to 1, then the reaction ratio must be determined. The reaction ratio can be determined in the following manner:

 

  1. Reaction equation between A and B
  2. Reaction equation between B and T
  3. Multiplication of the two reaction quotients
Example 1

Reaction ratio: 

Example 2

Reaction ratio: 

Below is an actual example of lithium carbonate, which can be determined by back-titration using sulfuric acid and sodium hydroxide.

The lithium carbonate reacts in a 1:1 ratio with sulfuric acid. To determine the excess sulfuric acid, two moles of sodium hydroxide are required per mole of sulfuric acid, resulting in a 1:2 ratio. This gives a stoichiometric ratio r of 0.5 for this titration.

 For a back-titration with a standardization of reagent B, use the following formula to obtain a result in mass-%:

VB Added volume of the reagent B in mL

cB:  Nominal concentration of reagent B in mol/L

fB:  Titer factor of reagent B (unitless)

VEP:  Volume at the equivalence point in mL

cT:  Nominal concentration of titrant T in mol/L

fT Titer factor of the titrant T (unitless)

sBT Stoichiometric factor between reagent B and titrant T

sAB:  Stoichiometric factor between analyte A and reagent B

MA:  Molecular weight of analyte A in g/mol

mSample:  Weight of sample in mg

100:  Conversion factor, to obtain the result in %

Modern titrators are capable of automatically calculating the results of back-titrations. All information concerning the used variables (e.g., blank value) are stored together with the result for full traceability.

To summarize:

Back-titrations are not so different from regular titrations, and the same general principles apply. The following points are necessary for a back-titration: 

  • Know the stoichiometric reactions between your analyte and reagent B, as well as between reagent B and titrant T.
  • Know the exact concentration of your titrant T.
  • Know the exact concentration of your reagent B, or carry out a blank determination.
  • Use appropriate titration parameters depending on your analysis.

If you want to learn more about how you can improve your titration, have a look at our blog entry “How to transfer manual titration to autotitration”, where you can find practical tips about how to improve your titrations.

If you are unsure how to determine the exact concentration of your titrant T or reagent B by standardization, then take a look at our blog entry “What to consider when standardizing titrant”.

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