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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.

Making a better beer with chemistry

Making a better beer with chemistry

Lager or ale? Pale ale or stout? Specialty beer, or basic draft? This week, to celebrate the International Beer Day on Friday, August 7th, I have chosen to write about a subject near and dear to me: how to make a better beer! Like many others, at the beginning of my adult life, I enjoyed the beverage without giving much thought to the vast array of styles and how they differed, beyond the obvious visual and gustatory senses. However, as a chemist with many chemist friends, I was introduced at several points to the world of homebrewing. Eventually, I succumbed.

Back in 2014, my husband and I bought all of the accessories to brew 25 liters (~6.5 gallons) of our own beer at a time. The entire process is controlled by us, from designing a recipe and milling the grains to sanitizing and bottling the finished product. We enjoy being able to develop the exact bitterness, sweetness, mouthfeel, and alcohol content for each batch we brew.

Over the years we have become more serious about this hobby by optimizing the procedure and making various improvements to the setup – including building our own temperature-controlled fermentation fridge managed by software. However, without an automated system, we occasionally run into issues with reproducibility between batches when using the same recipe. This is an issue that every brewer can relate to, no matter the size of their operation.

Working for Metrohm since 2013 has allowed me to have access to different analytical instrumentation in order to check certain quality attributes (e.g., strike water composition, mash pH, bitterness). However, Metrohm can provide much more to those working in the brewing industry. Keep reading to discover how we have improved analysis at the largest brewery in Switzerland.

Are you looking for applications in alcoholic beverages? Check out this selection of FREE Application Notes from Metrohm:

Lagers vs. Ales

There are two primary classes of beer: lagers and ales. The major contrast between the two is the type of yeast used for the fermentation process. Lagers must be fermented at colder temperatures, which lends crisp flavors and low ester formation. However, colder processes take longer, and so fermentation steps can last for some months. Ales have a much more sweet and fruity palate of flavors and are much easier to create than lagers, as the fermentation takes place at warmer temperatures and happens at a much faster rate.

Comparison between the fermentation of lagers and ales.

Diving a bit deeper, there are several styles of beer, from light pilsners and pale ales to porters and black imperial stouts. The variety of colors and flavors depend mostly on the grains used during the mash, which is the initial process of soaking the milled grains at a specific temperature (or range) to modify the starches and sugars for the yeast to be able to digest. The strain of yeast also contributes to the final flavor, whether it is dry, fruity, or even sour. Taking good care of the yeast is one of the most important parts of creating a great tasting beer.

Brewing terminology

  • Malting: process of germinating and kilning barley to produce usable sugars in the grain
  • Milling: act of grinding the grains to increase surface area and optimize extraction of sugars
  • Mashing: releasing malt sugars by soaking the milled grains in (hot) water, providing wort
  • Wort: the solution of extracted grain sugars
  • Lautering: process of clarifying wort after mashing
  • Sparging: rinsing the used grains to extract the last amount of malt sugars
  • Boiling: clarified wort is boiled, accomplishing sterilization (hops are added in this step)
  • Cooling: wort must be cooled well below body temperature (37 °C) as quickly as possible to avoid infection
  • Pitching: prepared yeast (dry or slurry) is added to the cooled brewed wort, oxygen is introduced
  • Fermenting: the process whereby yeast consumes simple sugars and excretes ethanol and CO2 as major products

Ingredients for a proper beer

These days, beer can contain several different ingredients and still adhere to a style. Barley, oats, wheat, rye, fruit, honey, spices, hops, yeast, water, and more are all components of our contemporary beer culture. However, in Bavaria during the 1500’s, the rules were much more strict. A purity law known as the Reinheitsgebot (1516) stated that beer must only be produced with water, barley, and hops. Any other adjuncts were not allowed, which meant that other grains such as rye and wheat were forbidden to be used in the brewing process. We all know how seriously the Germans take their beer – you only need to visit the Oktoberfest once to understand!

Determination of the bitterness compounds in hops, known as «alpha acids», can be easily determined with Metrohm instrumentation. Check out our brochure for more information:

You may have noticed that yeast was not one of the few ingredients mentioned in the purity law, however it was still essential for the brewing process. The yeast was just harvested at the end of each batch and added into the next, and its propagation from the fermentation process always ensured there was enough at the end each time. Ensuring the health of the yeast is integral to fermentation and the quality of the final product. With proper nutrients, oxygen levels, stable temperatures, and a supply of simple digestible sugars, alcohol contents up to 25% (and even beyond) can be achieved with some yeast strains without distillation (through heating or freezing, as for eisbocks).

Improved quality with analytical testing

Good beers do not make themselves. For larger brewing operations, which rely on consistency in quality and flavor between large batch volumes as well as across different countries, comprehensive analytical testing is the key to success.

Metrohm is well-equipped for this task, offering many solutions for breweries large and small.

Don’t take it from me – listen to one of our customers, Jules Wyss, manager of the Quality Assurance laboratory at Feldschlösschen brewery, the largest brewery in Switzerland.

«I have decided to go with Metrohm, because they are the only ones who are up to such a job at all. They share with us their huge know-how.

I can’t think of any other supplier who would have been able to help me in the same way

Jules Wyss

Manager Quality Assurance Laboratory, Feldschlösschen Getränke AG

Previous solutions failed

For a long time, Jules determined the quality parameters in his beer samples using separate analysis systems: a titrator, HPLC system, alcohol measuring device, and a density meter. These separate measurements involved a huge amount of work: not only the analyses themselves, but also the documentation and archiving of the results all had to be handled separately. Furthermore, Jules often had to contend with unreliable results – depending on the measurement procedure, he had to analyze one sample up to three times in order to obtain an accurate result.

A tailor-made system for Feldschlösschen

Jules’ close collaboration with Metrohm has produced a system that takes care of the majority of the necessary measurements. According to Jules, the system can determine around 90% of the parameters he needs to measure. Jules’ new analysis system combines various analysis techniques: ion chromatography and titration from Metrohm as well as alcohol, density, and color measurement from another manufacturer. They are all controlled by the tiamo titration software. This means that bitterness, citric acid, pH value, alcohol content, density, and color can all be determined by executing a single method in tiamo.

Measurement of the overall water quality as well as downstream analysis of the sanitization process on the bottling line is also possible with Metrohm’s line of Process Analysis instrumentation.

Integrated analytical systems with automated capabilities allow for a «plug and play» determination of a variety of quality parameters for QA/QC analysts in the brewing industry. Sample analysis is streamlined and simplified, and throughput is increased via the automation of time-consuming preparative and data collection steps, which also reduces the chance of human error.

Something to celebrate: The Metrohm 6-pack (2018)

In 2018, Metrohm celebrated its 75 year Jubilee. At this time, I decided to combine my experience as a laboratory analyst as well as a marketing manager to brew a series of six different styles of beer for the company, as a giveaway for customers of our Metrohm Process Analytics brand, for whom I worked at the time. Each batch was brewed to contain precisely 7.5% ABV (alcohol by volume), to resonate with the 75 year anniversary. The array of ales was designed to appeal to a broad audience, featuring a stout, porter, brown ale, red ale, hefeweizen, and an India pale ale (IPA). Each style requires different actions especially during the mashing process, based on the type of grains used and the desired outcome (e.g., flavor balance, mouthfeel, alcohol content).

Bespoke bottle caps featuring the Metrohm logo.
The 6 styles of beers brewed as a special customer giveaway to celebrate the Metrohm 75 year Jubilee.

Using a Metrohm Ion Chromatograph, I analyzed my home tap water for concentrations of major cations and anions to ensure no extra salts were needed to adjust it prior to mashing. After some of the beers were prepared, I tested my colleagues at Metrohm International Headquarters in the IC department, to see if they could determine the difference between two bottles with different ingredients:

Overlaid chromatograms from IC organic acid analysis highlighting the differences between 2 styles of the Metrohm 75 year Jubilee beers.

The IC analysis of organic acids and anions showed a clear difference between the beers, allowing them to determine which sample corresponded to which style, since I did not label them prior to shipping the bottles for analysis. As the milk stout contained added lactose, this peak was very pronounced and a perfect indicator to use.

Metrohm ion chromatography, along with titration, NIRS, and other techniques, allows for reliable, comprehensive beer analysis for all.

In conclusion, I wish you a very happy International Beer Day this Friday. Hopefully this article has illuminated the various ways that beer and other alcoholic beverages can be analytically tested for quality control parameters and more  fast, easy, and reliably with Metrohm instrumentation.

For more information about the beer quality parameters measured at Feldschlösschen brewery, take a look at our article: «In the kingdom of beer The largest brewery in Switzerland gets a made-to-measure system». Cheers!

Read the full article:

«In the kingdom of beer – The largest brewery in Switzerland gets a made-to-measure system»

Post written by Dr. Alyson Lanciki, Scientific Editor (and «chief brewing officer») 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.

Best practice for electrodes in titration

Best practice for electrodes in titration

After my earlier blog post on the topic of «Avoiding the most common mistakes in pH measurement», I will now cover the subject of electrodes that are relevant for titration. Here you will not only find out how to select the right electrode for your application – but also how to clean and to maintain it, and most importantly, how you can assure that your electrode is still ok to be used.

The following topics will be covered (click to go directly to the topic):

 

How to select the right sensor

You might wonder what you need to consider when selecting a suitable sensor for your titration, as a huge variety of different sensors exist. The right sensor needs to be selected based on the type of titration that you want to carry out. For a redox titration, you will need a different sensor than for a complexometric titration.

Furthermore, the sensor selection is highly dependent on matrix, the sample volume, or possible interferences. If you are working in non-aqueous media, you must especially consider any electrostatic effects that might arise. Therefore, I recommend working with an electrode that offers an internal electrical shielding.

The sensor has to show a fast response time, and needs to be robust enough for the application, meaning it needs to be resistant to the chemicals used and the applied cleaning procedure.

Table 1. Overview of suggested sensors for various types of titration (click to enlarge).

For further guidelines regarding how to select the right electrode, either consult our online electrode finder or check out our flyer about «Electrodes in titration» which includes practical tips on care and maintenance.

Maintenance and cleaning

Proper cleaning between your titrations is a key factor for obtaining reliable results. The rinsing step has to assure that neither sample nor titrant contaminates the electrode, leading to carry-over and false results. Therefore, between titrations the electrode (as well as buret tip) has to be rinsed with a suitable solvent, such as deionized water, detergent solution, or any other solvent that removes remaining residues. For non-aqueous titrations, it is furthermore important to condition the glass membrane of the electrode in deionized water after each titration.

Furthermore, both the reference and measuring electrode require regular maintenance. For the reference electrode, it is very important that it is filled up to the opening with the correct (and uncontaminated) electrolyte. A daily check of the electrolyte level should be performed, and if necessary, the reference electrolyte should be topped up. Always refill the reference electrolyte level up to the filler opening. This assures a proper electrolyte outflow and reduced contamination of the electrolyte.

Figure 1. Always refill your electrolyte up to the filler opening for the best performance!

In addition to the regular refilling, the electrolyte should be replaced at least on a monthly basis to guarantee a clean electrolyte with the correct concentration (e.g., evaporation of water can increase the concentration of the electrolyte). Usage of old or contaminated electrolyte leads to an undesired change in the measured potential.

Also ensure that the diaphragm is clean, otherwise you might experience a blockage, leading to an unstable potential caused by the missing contact between electrolyte and sample. Figure 2 shows an example of a contaminated diaphragm. Table 2 suggests some possible cleaning agents to remove sticky substances from the diaphragm. After cleaning the diaphragm, always replace the electrolyte.

Figure 2. Close-up view of a dirty diaphragm.
Table 2. Common electrode contaminants and suggested cleaning agents for each situation. Contact your local Metrohm representative for further questions.

The measuring electrode needs a thorough cleaning at least weekly. Uncoated metal ring or ISE electrodes require regular polishing to maintain a quick response. Glass membranes or polymer membranes must not be polished or cleaned with abrasives. If the electrode is used in oily or sticky samples, degreasing or removing proteins might be necessary by using a suitable solvent.

Correct storage of your electrode

Another important point to consider is the right storage for your electrode. Incorrect storage reduces the lifetime of an electrode, and therefore it needs replacement more frequently. Unfortunately, there is not one single storage solution which covers all electrode types. The right storage solution highly depends upon the electrode type.

If it is a separate indicator or only a reference electrode, then it is much easier to determine the correct storage solution, as the perfect solution for only one part must be found. For combined electrodes, the situation is a bit more complicated. Combined electrodes contain a reference electrode and a measuring electrode that each have different preferences. Therefore, sometimes a compromise is necessary. The reference electrode prefers to be stored in reference electrolyte to remain ready to use, whereas an indicator glass membrane prefers deionized water. On the other hand, a metal indicator electrode prefers to be stored dry.

For combined pH electrodes with c(KCl) = 3 mol/L as reference electrolyte, a special storage solution was developed by Metrohm, which maintains the glass membrane as quickly as possible without impairing the performance of the reference system. All other pH electrodes are stored in their respective reference electrolyte (normally indicated on the head of the electrode, see Figure 3).

Figure 3. Different reference electrolytes for different electrode types.

Metal electrodes are also stored differently, depending on the type. Combined metal ring electrodes are stored in reference electrolyte to maintain the diaphragm properly, whereas Titrodes are stored in deionized water, as these electrodes contain a pH glass membrane that needs to be kept hydrated. Always fill the storage vessel of your electrode with approximately 12 mL of storage solution and exchange it regularly as it might be contaminated by sample or cleaning solution.

The table below shows typical storage conditions depending on the type of the electrode. 

Table 3. Storage conditions for various electrode types.

If you are not sure how to store your electrode correctly, check the information in the electrode flyer or on the Metrohm website.

Check your electrode

The easiest way to check the performance of your electrode is to monitor it during a standardized titration (e.g., titer determination) which is performed regularly (e.g., weekly) and where prerequisites such as sample size, concentration of titrant, and volume of added water are always very similar. Otherwise, you can also follow a procedure recommended by Metrohm. To check metal electrodes, you can find a test procedure in application bulletin AB-048, for surfactant electrodes in application bulletin AB-305 and for ion selective electrodes, a check procedure is given in the ISE manual.

As an example, I will explain the test procedure of a silver electrode a bit more in detail. Silver electrodes can easily be checked by a standardized titration using hydrochloric acid (c(HCl) = 0.1 mol/L) as sample, and silver nitrate (c(AgNO3) = 0.1 mol/L) as titrant. Perform a threefold determination with the recommended titration parameters and sample size.

The following parameters are evaluated and compared to optimal values:

  • added volume of titrant at equivalence point (EP)
  • time until equivalence point is reached
  • potential jump (potential difference) between the potential measured at 90% and 110% of the EP volume
Figure 4. Example testing procedure for evaluation of electrode performance.

If the evaluated data cannot meet the specified values, clean the electrode thoroughly and repeat the test. If no improvement is observed, the sensor must be replaced.

Further symptoms can indicate a necessary replacement: sluggish response, unstable or drifting signal, longer duration of titration, smaller potential jumps, and worse shape of the titration curve. In Figure 5 below,  two different titration curves for calcium and magnesium in water are shown using a combined Calcium ISE. In Figure 5, the upper curve is obtained with a new Ca-ISE; the titration is fast and you will obtain 2 equivalence points: one each for calcium and magnesium. In the lower curve, an old electrode was used. The titration takes much longer and the second equivalence point for magnesium cannot be recognized anymore due to the lack of sensitivity of the electrode.

Figure 5. Comparison of the response of a new ISE vs. an aged ISE.

To summarize

  • Select the right indication for your titration type.
  • The quality of the electrode highly influences the quality of your titration results.
  • Proper maintenance and storage can increase the lifetime of the electrode.
  • Check the electrode performance regularly or monitor the titration performance (duration, potential jump) over time to reduce downtime of your instrumentation.

 

If you would like to get some more information and practical tips on electrodes in titration, please check out our white paper on «Basics of potentiometry» or our webinar «Avoid titration mistakes through best practice sensor handling». 

On-demand Metrohm webinar

«Avoid titration mistakes through best practice sensor handling»

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

A History of Chemistry – Part 4

A History of Chemistry – Part 4

This article is the final installment in our four-part series on the history of chemistry. Did you miss the others? Don’t worry – you can find them all here:

The industrialization of electrochemistry

Michael Faraday (1791–1867) had a modest upbringing. He was 14 years old when he began his bookbinding apprenticeship. The young Faraday read a multitude of works that he received for binding and thus educated himself in the sciences as well as in literature and art. A customer at the bookbinding workshop noticed the curious apprentice and mentioned him to his father, who then took Faraday with him to several lectures given by electrochemistry pioneer Humphry Davy. Shortly after, Faraday began working for Davy.

As his assistant, Faraday traveled with Davy across Europe, as they carried out experiments together and met numerous influential scientists. Back in England, Faraday continued training as a chemist and in 1833 became a professor of chemistry. During this time, he investigated the basic laws of electrolysis. These formed the basis of electrochemistry and, in the second half of the century, enabled the development of an electrochemical industry which manufactured products such as chlorine, hydrogen, aluminum, magnesium, sodium, and potassium in its plants located at hydroelectric power stations.

Solvay’s soda ash

The industrial production of soda ash (sodium carbonate) had been possible since the development of the Leblanc process at the end of the 18th century. However, the synthesis required expensive raw materials and produced large amounts of the byproduct hydrogen chloride, which is toxic to the environment in which it is introduced. The produced hydrogen chloride escapes from industrial stacks and kills surrounding vegetation, and is also lethal to aquatic life when added to water.

During the second half of the 19th century, the Belgian Ernest Solvay (1838–1922) occupied himself with the issue. Solvay, who came from an industrialist family, had little formal education but was familiar with chemical procedures thanks to his work at his uncle’s and father’s factories. He developed the process for manufacturing soda ash, which was named after him and only has one byproduct – the harmless calcium chloride (CaCl2).

In 1861, Ernest Solvay and his brother Alfred began soda ash production in their own small factory in Brussels. By continuously adjusting the process, they became increasingly successful and continued to expand. Solvay, who had become very wealthy, became active in furthering scientific research and charitable causes. He also showed his sense of social responsibility in his factories: he established an eight-hour workday, paid holiday leave, a social security system, and a pension for his employees –long before it was legally mandatory.

The majority of the soda ash produced today is still created using the Solvay process.

Would you like to learn more?

Visit our site to read more about the Solvay process and the associated analysis techniques:

The periodic table of elements

There had already been 64 chemical elements had discovered by 1868. However, there was as yet no clear system of regulating which particular atom combinations formed new molecules. Sorting the elements based on their atomic mass had not offered a solution up to this point.

Dmitri Mendeleev (1834–1907) recognized a pattern here: when elements are sorted by their atomic mass, certain elemental properties are periodically repeated – specifically, after every eight elements. Mendeleev therefore retained the arrangement in ascending order of atomic mass, but then also sorted the elements that had the same properties below one another. Whenever properties were repeated after fewer than eight elements, he left open gaps to be filled with elements that had not yet been discovered. Mendeleev arranged the transition elements, which did not fit with his «octet rule», into their own column. This resulted in the first periodic table of elements in 1869.

From aniline to aspirin

Organic chemistry, which now went far beyond the synthesis of artificial urea, had become a significant and rapidly growing industry. The tar dye companies BASF, Bayer, and Hoechst, all of which were founded in the 1860s, grew so rapidly that they were employing thousands of people even before the turn of the century. From the end of the 19th century, the tar dye industry also developed synthetic organic medicinal products. Bayer, for example, patented the byproduct-free synthesis of acetylsalicylic acid in 1898 and marketed the product under the name «Aspirin» from the beginning of the 20th century.

In basic research, chemists began devoting themselves to increasingly complex organic molecules. Emil Fischer (1852–1919) investigated biologically significant molecules such as sugars and amino acids. In 1890, he used glycerin as a basis for synthesizing three sugars: glucose, fructose, and mannose. He later researched proteins. During this period, he discovered new amino acids and shed light on the type of bond which connects them to one another: an amide bond which he gave the name «peptide bond» [1].

First World War: Artificial fertilizer and warfare agents

The use of fertilizer had been common practice throughout agriculture ever since Liebig proved that it would improve yield. The nitrogen needed by plants for growth was added to fertilizers largely in the form of guano. This consists of the weathered excrement of seabirds which forms meter-thick layers over many years, particularly on the beaches of South America, where there are low levels of precipitation. In order to meet the high demand for food – and thus for fertilizer – entire shiploads of guano were being imported to Europe.

However, the import of guano could not keep pace with the rapid growth of population indefinitely, so at the end of the 19th century, researchers began looking for a way to fix nitrogen from the air. The German chemist Fritz Haber (1868–1934) eventually found a solution in 1909 and, with his ammonia synthesis, prevented the famine prophesied in the western world. Unfortunately, this development also enabled Germany’s production of warfare agents during the First World War, as ammonia could be used to create ammonium nitrate, which was then used in ammunition.

Fritz Haber
Carl Bosch

In the Haber-Bosch process, ammonia is produced as a result of a reaction between hydrogen and nitrogen. Fritz Haber achieved synthesis at a high temperature and a high pressure level, and with the aid of a catalyst. Carl Bosch (1874–1940) developed the industrial implementation of the process. For this purpose, he developed specific equipment made of state-of-the-art materials which could withstand both high pressure and temperature levels.

In 1914, the First World War broke out. The nations involved, as well as neutral states, faced blockades in their trade routes and had to become largely self-sufficient. Thanks to governmental structuring and aid, this led to a boom in industrial research across the globe. Numerous reputable scientists were actively involved in the war or supported it, including Fritz Haber, Walther Nernst, and Emil Fischer. In addition to the Haber-Bosch process, the pressure to create innovations that prevailed before and during the war also resulted in the first synthetic rubber as well as mustard gas and the toxic gas phosgene. Chlorine gas, which is produced during ammonia synthesis, was also used as a warfare agent during World War One.

What if . . .

. . . the Haber-Bosch process didn’t exist? Without the nitrogen fertilizer produced using the Haber-Bosch process, there would likely be a lot fewer people on Earth: the population growth of around 1.6 billion in 1900 to nearly 8 billion today would not have been possible without yield improvements brought about by artificial nitrogen fertilizers. Agriculture is still dependent on it today: without this process, the planet would only be able to provide enough food for half the population [2].

Chemistry since WWI

Following the armistice agreement in 1918, the German chemical industry – which had been world-leading until then – lost all of its patents and had to reveal numerous production secrets in order to satisfy the reparation demands of the victorious Allied Powers [3]. The German chemical industry, which had been the world’s largest at the time, had to relinquish its place at the top. Although it experienced another upswing toward the beginning of the Second World War, today’s leading lights in the chemical industry are the USA and France. During the post-war period, polymer chemistry and pharmaceutical chemistry were the fields that saw particular advancement and brought about countless products which are still essentials today. Among these are polymers, including synthetic fibers such as nylon and polyester, and artificially produced vitamins and hormones.

The time around the turn of the 20th century saw rapid advancement in chemistry, both in fundamental research as well as in industry – and to a great extent, it is the relationship between the two which enabled this progress. Numerous processes developed during this time period, including the Haber-Bosch and Solvay processes, have remained the methods of choice in the production of chemicals – in this case ammonia and soda ash, respectively – to this very day. 

References

[1] The Components of Life: From Nucleic Acids to Carbohydrates; 1st ed., Rogers, K., Ed.; Britannica Educational Publishing/Rosen Educational Services: New York, 2011; p 59.

[2] Erisman, J. W., Sutton, M. A., Galloway, J., Klimont, Z., and Winiwarter, W. (2008) Nat. Geosci. 1, 636–639.

[3] Kricheldorf, H. R. Menschen und ihre Materialien: Von der Steinzeit bis heute; 1st ed., Wiley-VCH Verlag & Co. KGaA: Weinheim, 2012; p 111.

Post written by Dr. Alyson Lanciki, Scientific Editor at Metrohm International Headquarters, Herisau, Switzerland. Primary research and content contribution done by Stephanie Kappes.

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.