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Best practice for electrodes in Karl Fischer titration

Best practice for electrodes in Karl Fischer titration

Have you ever asked yourself why you need an electrode for the endpoint detection in Karl Fischer (KF) titration? Theoretically, the endpoint of a Karl Fischer titration could be determined based on the color change of the reagent. However, if accuracy and reproducibility are important, endpoint detection with a double Pt electrode is a much better choice.

As the indicator electrode detects the endpoint, you can imagine that the results depend highly on the condition of the electrode. In coulometry, an additional electrode (generator electrode) is used to generate the iodine needed for the titration. Both electrode types (i.e. indicator and generator electrode) need to be kept in good shape to guarantee the correct results. It goes without saying that cleaning, storage, maintenance, and checks of the KF electrodes are important factors for success. This blog post takes a closer look at these topics.

Did you catch our series about frequently asked questions in Karl Fischer titration? Find them here!

Cleaning

Indicator electrode

Double  Pt-wire or double Pt-ring electrodes can be easily cleaned with an abrasive cleaning agent like aluminum oxide powder or toothpaste. After cleaning, rinse the electrode well with water and let it dry before mounting it in a titration cell. Check out our video below for more tips and tricks about the proper cleaning procedure for Karl Fischer titration indicator electrodes.

Take special care not to bend the Pt pins of the double Pt-wire electrode. Bending the pins can lead to tiny cracks in the glass body of the electrode. Over time, reagent can flow into the electrode and lead to corrosion (short circuit). If this happens, the electrode is beyond repair and needs replacement. Alternatively, a double Pt-ring electrode can be used instead. Problems with bent pins are then a thing of the past.
Generator electrode
Without diaphragm
Rinse generator electrodes without diaphragms with water, or if the contaminant is not water soluble, then rinse with a suitable organic solvent. If the anode or the cathode of the generator electrode shows discoloration or deposits that cannot be removed with rinsing, the electrode can then be cleaned with concentrated nitric acid (65%). Be aware that nitric acid is a strong oxidizing agent and must be handled carefully according to relevant safety regulations and instructions. Remember to first mount the green protection cap on the connector to avoid corrosion caused by fumes of nitric acid. Afterwards, rinse the electrode with water and finally with methanol.
With diaphragm
To remove salt-like residues, the generator electrode with diaphragm can be rinsed with water. Oily contamination can be rinsed off with an organic solvent (e.g. hexane). Sticky residues on the diaphragm can be removed in the following way: 

  1. Mount the green protection cap on the connector of the electrode.
  2. Place the electrode in an upright position (e.g. in an Erlenmeyer flask) and add a few milliliters of concentrated nitric acid (65%) in the cathode chamber. Let the acid flow through the diaphragm.
  3. Fill the cathode chamber with water and let it flow through the diaphragm to remove the nitric acid. Repeat this step two or three times. A simple way to see whether another rinsing step is required is by performing a quick check of the pH value at the cathode using pH indication paper.
  4. Finally, fill the cathode chamber with methanol and let it flow out.

Now the generator electrode is as good as new and ready for use in a titration cell again.

Maintenance

Except for the generator electrode with diaphragm, KF electrodes are maintenance free. However, the catholyte filled in the generator electrode with diaphragm can decompose over time. To avoid any influence of the decomposition products on the results, exchange the catholyte on a regular basis according to the manufacturer’s recommendations.

Storage

Unlike pH electrodes, KF electrodes do not contain a glass membrane that could potentially dry out. Therefore, no special solution is required in which to store KF electrodes. If you use the electrodes frequently, it is recommended to keep the electrodes mounted in the titration cell and immersed in the KF reagent. Alternatively, all KF electrodes (indicator and generator electrodes) can be stored dry.

What to check for

It is recommended to check the complete titration setup instead of only the electrode(s).

Volumetry

Carry out a threefold titer determination using either a liquid or a solid water standard suitable for volumetry and calculate the mean value of the titer. Then, determine the water content of a water standard (also via triplicate determination). Make sure that you do not use the same standard as for the titer determination but use a different batch of the standard or even a completely different standard. Calculate the water content and compare it to the certified water content of the standard.

If the recovery is determined to be in the range of 97–103%, the titration system (including the electrode) is working fine. Finding values outside this range means that there is something wrong with the titration system or with the determination procedure. Results of the sample analysis would very likely also deviate from the real water content. Therefore, it is important to find the reason for values that are too high or too low. Sometimes the reason for deviations is just an air bubble in the dosing cylinder or due to an exhausted molecular sieve. However, if you do not find the reason, do not hesitate to contact your local Metrohm agency.

Coulometry

Water standards with lower water contents (0.1%) are available to properly check the health of coulometric titration systems. Carry out a water content determination in triplicate with such a standard. Calculate the recovery with the obtained results and the certified water content of the standard.

A recovery value in the range between 97–103% means that everything is fine with the system and that the electrodes work as expected. As with volumetry, in coulometry it is important to find the reason for any deviating recovery values. Make sure that you find and eliminate the problem to obtain correct results for your samples.

What you should avoid

  • Do not use solvents that contain ketones or aldehydes (e.g. denatured ethanol) to clean KF electrodes or any KF accessories.
  • Do not treat KF electrodes in an ultrasonic bath. This might destroy the electrode.
  • For drying, use a maximum temperature of 50 °C. Higher temperatures might damage the electrode.
  • Do not bend the Pt pins of the double Pt-wire electrode.

Summary

As you can see, keeping your KF electrodes in good shape is actually very simple. Regular cleaning helps to avoid erroneous results and ensures that your Karl Fischer electrodes will work for a long time.

Best practice for electrodes in titration

Treat your sensors right!
Post written by Michael Margreth, Sr. Product Specialist Titration (Karl Fischer Titration) at Metrohm International Headquarters, Herisau, Switzerland.
Coffee: serving up chemistry in every cup

Coffee: serving up chemistry in every cup

International Coffee Day is October 1st, not that many of us need a day to celebrate the drink we enjoy all year round. What makes a high quality cup of coffee? There are several factors at hand from the origin of the beans and the climate they grow in, to how the beans are processed, roasted, and packaged, and finally how the roasted beans are ground and brewed. In this blog post, I will discuss a bit of the history of coffee, how it is processed, and how to accurately determine the quality parameters in order to brew the most flavorful cup.

Origins of our favorite brew

The word «coffee» was introduced in 1582, derived from the Dutch «koffie». This traces back even further to the Arabic word for coffee, «qahwah», which has been speculated to come from «quwwa» (defined as power or energy), or even from Kaffa (also spelled as Kefa) which was a medieval Ethiopian kingdom that exported coffee plants to Arabia. It is believed that coffee was first discovered by a goat herder in Ethiopia who noticed the energy of his goats increase after consuming the coffee fruit (known as «cherries»). From Ethiopia, coffee consumption spread through the Arabian Peninsula and Middle East during the 15th and 16th centuries.

Coffee cherries ripening.
Coffee is now the most consumed beverage, other than water, around the globe. From the thickest espressos all the way to transparent drip coffee, the world is truly hooked! This beverage is so deeply ingrained in all cultures that the top places for annual World Barista Championships (2019) were taken by contestants from South Korea, Greece, and Canada!

Good coffee: not as straightforward as you might think

Coffee comes in many forms, with niche roasters looking to discover new flavors daily. Since the gourmet coffee market is growing more, specialty coffee is also in high demand. Global Brands Magazine reported the price of Black Ivory Coffee at $500 per pound in 2020.

Why so expensive — is the taste that good? In order to make Black Ivory Coffee, the coffee cherries are fed to and digested by elephants, in a similar manner to Kopi Luwak (or civet coffee), another expensive coffee type created by fermentation of coffee cherries in the gut of civet cats. The resulting coffee beans are then cleaned, dried, and roasted.

Civet cats can digest coffee beans to create a unique coffee experience with a heavy price tag.
Aside from these high-priced small batches, other types of coffee beans are roasted in large quantities for mainstream consumption (arabica, robusta, and liberica). Arabica beans make up 60% of the global market with 2.5 million tons exported per year from Brazil alone. Robusta beans account for a bit less than 40% of the market and are mostly produced in Vietnam. Robusta beans exhibit more bitter flavors, contain more caffeine, and are used more often to create instant coffee. Liberica beans have high levels of sugars but low concentrations of caffeine compared to the other two major species. Very low yields (between two and four times lower than the others) and larger plant size make this type more difficult to mass produce and therefore it only accounts for approximately 2% of the global coffee market. From these major coffee species, several varieties have been produced with a large range of different flavor characteristics and caffeine content.

Typically, coffee is grown in (sub)tropical areas, but the ideal climate differs depending on the species. Some prefer higher altitudes and are more suitable for mountainous regions. Others need hot and dry conditions to produce the best quality beans. Now there are over 70 countries that produce coffee. That’s a good thing, because global coffee consumption in 2020/2021 is estimated to be 167.23 million 60 kg bags, which is more than 10 million tons of coffee!

Map showing the different coffee-producing countries around the world.

Changes in coffee consumption practices

The adoption of the pod coffee machine (e.g., Keurig, Nespresso) over the past decade has pushed the consumption of coffee from something generally enjoyed in a café, restaurant, or on the go, to a much higher rate of consumption at home. With this significant shift to pod coffee, the ability to adjust grind size, water temperature, or extraction time used by the best baristas to counter changes in flavor and strength is no longer a possibility. In fact, the ease of pressing a single button and receiving hot, fresh coffee within seconds is exactly why pod coffee is so popular. This puts new pressures on coffee roasters to maintain the flavor and caffeine strength expected of their brand and varieties.

Though many people may think that the largest contribution to a good cup of coffee is due to the coffee brewing process, many other quality parameters such as acidity, roast temperature, and water quality contribute even more. Two of the main factors in an optimal cup of coffee, the acidity (taste) and the amount of caffeine, are mainly affected by the bean type, region of origin, and roasting temperature.
Progression of the coffee bean roasting process.

Science—brewing up your perfect cup

Not all coffee beans are created equal, but luckily science allows us to define many of the key quality parameters that result in the taste and caffeine strength we expect from our favorite brand of coffee. Coffee is generally acidic with a pH of around five. Highly acidic coffee displays a sour, harsh flavor. While there are ways to counteract this on the consumer side, for manufacturers it is even more important to identify that there is an issue to begin with. A simple identifier is the titratable acidity of the coffee, and this has a direct correlation with the taste you associate with your favorite brew.

Of equal importance is the «kick» you may get from your preferred caffeine fix. Whether you drink one cup per day, or four, the recommended daily limit for adults is suggested at 400 mg caffeine. Of course, decaffeinated coffee is also an option for those who are sensitive to its effects or are looking for ways to reduce their intake (but can’t stay away from coffee).

Traditionally, caffeine has been analyzed by titration, liquid chromatography (LC), or spectrophotometry after a long sample preparation procedure. Now, the analysis of key coffee quality parameters like caffeine content can be done simply and effectively using a single titration system.

Example titration curve for caffeine analysis with OMNIS (click to enlarge).
The pH and acidity of coffee samples are analyzed using a robust pH electrode during titration against standardized sodium hydroxide. Caffeine is determined through a redox back titration after a known excess of iodine is added to the sample and left to react. After the reaction period the sample is filtered and titrated with sodium thiosulfate.

Find out more about this analysis in our free Application Note!

Metrohm has the solution for your analysis needs

The OMNIS platform from Metrohm provides laboratory analysts the automation they need to make each sample determination significantly simpler, faster, and more reproducible thanks to minimal manual sample preparation steps. Key steps in the analysis process that have required manual interactions, reagent addition, filtration, and accurate volume transfers are now completed accurately and automatically. Learn more about the OMNIS titration platform on our website.
Hopefully this article has given you some insight into coffee’s long journey from the farm to your cup, and that you have learned about the chemistry behind the way your coffee tastes! Enjoy International Coffee Day, whether you celebrate on October 1st or every other day of the year.

Download our free Application Note:

Analysis of caffeine, pH, and acidity in coffee – Fully automated determination including filtering, reagent addition, and sample pipetting using OMNIS
Post written by Isaac Rogers (Titration Product Manager at Metrohm Australia & New Zealand) and Dr. Alyson Lanciki (Scientific Editor at Metrohm International Headquarters).
Chemical analysis of sourdough: pH and total titratable acidity (TTA)

Chemical analysis of sourdough: pH and total titratable acidity (TTA)

Like many, I am fascinated by the chemistry behind baking, and in this blog I want to talk about bread—sourdough in particular. There is a well-known saying in the baking industry: «The pH value bakes and the total titratable acidity tastes». Why are these two parameters important for baking bread, and how can they be determined in the best way? This is what I want to discuss here.

A brief history of sourdough

Bread has been part of the human diet for several thousand years, although not necessarily in the forms we are familiar with today. One exception to this is sourdough bread. Wild yeast and bacteria (lactobacilli) ferment the dough naturally, creating a tangy loaf full of crevices. Despite originating in the Fertile Crescent, one of the oldest physical examples (at nearly 6,000 years old) was excavated in Switzerland, showing how widely it spread by that point already.

Currently, one of the places most well-known for its sourdough bread is San Francisco, in California. Why California? Bakers from France brought their techniques there during the Gold Rush in the mid 1800’s, and it has since become ubiquitous with the city. In fact, San Francisco has its own eponymous strain of sourdough bacteria: Fructilactobacillus sanfranciscensis.

Sourdough bread loaf full of crevices
Figure 1. Cross section of a sourdough bread loaf.

Many home bakers try to make sourdough at some point, since the ingredients are simple and no leavening agent is used, except for what nature provides. However, with so many people at home during 2020–2021, it was an ideal time for many people to see what they could produce. The development of the starter is of key importance—if there is not sufficient wild yeast and bacteria (or they do not have enough nutrients), then the dough will not rise, and you are left with a dense, chewy result. (While much has been written about how to make the best homemade sourdough, I cannot contribute to this topic, as my own baking spree focused on the Swiss Butterzopf.)

Click here to download the recipe and try it out yourself!

Lactobacilli: helpful bacteria

As their name suggests, lactobacilli produce lactic acid (Figure 2) and also acetic acid, and these give the sourdough bread its characteristic tangy, sour taste. The sourness of the bread also has positive effects on its shelf life, making it possible for our ancestors to preserve the bread for a longer time to supplement their diet.

There is another reason why the presence of this helpful bacteria is important. Without the lactic and acetic acid, it would be impossible to bake bread made from rye flour, which is commonly used in sourdough bread of northern Europe. How come?

Figure 2. Chemical structure of lactic acid.

Starch is the key component within bread and influences the shape, crumb consistency, and overall flavor. During the baking process, gelatinization occurs between the starch within the flour and the water added to the dough. However, flour also contains the enzyme amylase, which catalyzes the hydrolysis of starch into sugar. During the gelatinization process, starch is more prone to hydrolysis by amylase. Strong amylase activity at this point will have detrimental effects on the bread crumb. For wheat, the amylase is already denatured at the temperature gelatinization begins within the dough. This is not the case for rye, which gelatinizes at a lower temperature when amylase activity happens to be the highest [1]. By making an acidic (sour) dough, the amylase activity is inhibited and it becomes possible to bake bread made from rye flour.

So how much acid is necessary and when is it too much? This question brings us back to the two key parameters, pH value and total titratable acidity (TTA), I mentioned in the introduction.

Figure 3. Fermenting sourdough starter in a glass jar.

pH value regulates enzyme activity

The pH value is important to inhibit amylase in an  optimal manner. Every enzyme has an optimal pH range in which it functions the most efficiently. For amylase, the optimal pH value (highest enzyme activity) ranges from pH 5.4 to 5.8. At a lower pH value its activity will be reduced.

The pH value can be easily measured using a pH electrode. For dough analysis, an electrode such as the Spearhead electrode which can pierce into the sample is the best sensor. As the pH value is temperature dependent, the sensor measures the temperature as well.

What is the pH value?

The pH value is the negative logarithm of the hydronium concentration. Therefore, the smaller the pH value, the higher the hydronium concentration.

Pure water itself contains a small amount of free hydronium ions, and its pH value is therefore 7.

As acids release hydronium ions when they are in solution (dissociation), acidic solutions have pH values between 0 and 7. 

Contrary to this, alkaline solutions and products have even less hydronium ions than pure water. They have pH values ranging from 7 to 14. An example of an alkaline solution is lye, which is used to produce lye rolls.

For more information on pH measurement check out our other blog posts «Avoiding the most common mistakes in pH measurement» and «FAQ: All about pH calibration».

Total titratable acidity helps assess the taste

Why do we need to determine the total titratable acidity (TTA) if measuring and controlling the pH value is sufficient to regulate the amylase activity? This is because the pH value does not provide any information about the ratio of lactic acid and acetic acid present in the dough. While the amylase activity is not dependent on the ratio of the two, the composition is important for the taste. For optimal sourdough flavor, the ratio of lactic acid to acetic acid should lie between 3:1 and 4:1. If the ratio shifts towards containing more acetic acid, the taste usually becomes too sour.

Weaker acids such as lactic acid and acetic acid do not completely dissociate, meaning not all acid molecules present will release their hydrogen ion. As lactic acid is a stronger acid in comparison to acetic acid, more lactic acid will dissociate and thus contribute more to the pH value. By determining the TTA, it is possible to find out what the total amount of acids is within the dough.

For the determination of the TTA, dough is homogenized with water to obtain a suspension. It is then titrated to a pH value of 8.5 with 0.1 molar sodium hydroxide solution. The use of an automated titrator provides reliable results without human interference (Figure 4).

Figure 4. A robust and compact titrator for the determination of the total titratable acidity

Before the titration starts, the pH value of the suspension can be determined easily, so you get both parameters (pH value and TTA) without needing to do double the work. For more detailed information on how the analysis is done, download our free Application Note.

pH value and TTA for the perfect sourdough quality

By combining the information about the pH value and TTA it becomes possible to assess the quality of sourdough and thus maintain a constant quality in the final product, especially if delays in the production process occur.

It also becomes possible to detect changes in the sourdough starter which might occur if storage conditions cannot be maintained, and thus provide critical information when it is time to prepare a new starter.

Table 1. Common pH and TTA values for various kind of breads [1].
Bread type pH value TTA
Wheat bread 5.4–6.0 4–6
Wheat mixed bread 5.0–5.3 6–8
Rye mixed bread 4.5–4.8 7–9
Rye bread 4.3–4.7 8–10
Rye bread (coarsely ground) 4.2–4.6 9–14

Lessons learned

I hope this blog post on the chemistry of sourdough has given you some new insights on this fascinating kind of bread.

As for myself, I will probably not venture into the sourdough baking arena but stick with my homemade Butterzopf.

Figure 5Butterzopf (made by the author): a traditional Swiss bread usually consumed on Sundays for brunch.

If you are interested in other blog articles related to yeast, check out our post about beer brewing: «Making a better beer with chemistry». If you have more of a sweet tooth then read our blog post on the «Chemistry of chocolate».

The chemistry of bread

Straightforward determination of pH value and total titratable acidity (TTA) in dough

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

Nonaqueous acid-base titrations – Common mistakes and how to avoid them

Nonaqueous acid-base titrations – Common mistakes and how to avoid them

Nonaqueous acid-base titrations are widely used in several industries, including the petrochemical  and pharmaceutical sectors. Whether you are determining the acid or base number (AN or BN) in oils or fats, titrating substances that are insoluble in water, or quantifying products with different strengths of acidity or alkalinity separately, nonaqueous acid-base titration is the method of choice.

If you already have some experience performing nonaqueous acid-base titrations, you may remember that there are several challenges to overcome in comparison to aqueous acid-base titrations.

In this blog post, I would like to cover some of the most typical issues that could pop up during nonaqueous acid-base titrations and discuss how to best avoid them. An important point to note is that there is no single solution regarding how to perform any nonaqueous acid-base titration correctly. The right procedure depends highly on the solvent and titrant used.

What is a nonaqueous acid-base titration?

Before discussing nonaqueous titrations, first let’s talk a little bit about aqueous acid-base titrations.

Here, a sample is dissolved in water, and depending of the nature of the sample (whether it is acidic or basic) a titration is performed either using aqueous base or aqueous acid as titrant. For indication, a glass pH electrode is used.

However, sometimes due to the nature of the sample, aqueous titration is not possible. Nonaqueous acid-base titration is used when:

  • the substance of interest is not soluble in water
  • samples are fats or oils
  • components of mixtures of acids or bases have to be determined separately by titration

In these cases, a suitable organic solvent is used to dissolve the sample instead of water. The solvent:

  • should dissolve the sample and not react with it
  • permits the determination of components in a mixture
  • if possible, should not be toxic

The solvents that are most often used include ethanol, methanol, isopropanol, toluene, and glacial acetic acid (or a mixture of these). Titrants are not prepared with water but rather in solvent. Frequently used nonaqueous basic titrants are potassium hydroxide in isopropyl alcohol or sodium hydroxide in ethanol, and a common nonaqueous acidic titrant is perchloric acid in glacial acetic acid.

Due to the nature of nonaqueous solvents, they are normally poor conductors and do not buffer well. This makes indication a bit challenging because the electrode must be suitable for such sample types. Therefore, Metrohm offers the Solvotrode which is developed specifically for nonaqueous titrations.

This pH electrode offers the following advantages over a standard pH electrode:

  • Large membrane surface and a small membrane resistance for accurate reading, also in poorly buffered solutions
  • A flexible ground-joint diaphragm which can easily be cleaned even when contaminated with oily or sticky samples, additionally it offers a symmetrical outflow for outstanding reproducibility
  • The electrode is shielded and is therefore less sensitive to electrostatic interferences
  • It can be used with any nonaqueous electrolyte such as lithium chloride in ethanol

In the following sections I will discuss the most common mistakes when performing potentiometric nonaqueous acid-base titrations and how you can avoid them.

Electrostatic effects

The influence of electrostatic effects during analysis is normally negligible. However, maybe you have once seen a curve like the one below which looks relatively normal until suddenly a spike occurs.

Figure 1. Titration curve with a spike which might have occurred from an electrostatic interference.

This is then an indication of an electrostatic effect. However, where does it come from and how can we overcome this?

Electrostatic charge can be generated from many sources, such as friction. For example, while walking across a surface you will generate an electrostatic charge which will be stored in your body. You have probably touched the doorknob after walking across a carpeted space in your socks and obtained a small electric shock—this is the discharge of built up electrostatic charge. If we now assume that you are electrostatically charged and then you approach an electrode that is currently measuring (in use), this will result in a spike (Figure 1). Therefore, it is essential to make sure that you are either properly discharged or that you do not approach the electrode during measurement. You can avoid this issue by wearing the appropriate clothes. ESD (electrostatic discharge) clothes and shoes are mostly recommended when performing nonaqueous titrations.

Blocked diaphragm

A blocked diaphragm is another point which occurs more regularly during nonaqueous titrations. Due to the oily and sticky sample, you might have seen that the electrode diaphragm is clogged and cannot be opened anymore. What should you do then?

In most cases, you can place the electrode in a beaker of warm water overnight. This treatment often helps to loosen the diaphragm. To completely prevent the diaphragm from clogging, a Solvotrode with easyClean technology should be used. With this electrode, electrolyte is released by pressing the head ensuring that the diaphragm is not blocked.

Choice of electrolyte and storage solution

We recommend two types of electrolyte for nonaqueous titrations.

For titrations with alkaline titrants: tetraethylammonium bromide c(TEABr) = 0.4 mol/L in ethylene glycol

For titrations with acidic titrants: lithium chloride c(LiCl) = 2 mol/L in ethanol

Please make sure to store the electrode in the same electrolyte with which it is filled.

Checking the electrode according to ASTM D664

To check whether the Solvotrode is still in good working condition, perform a test according to ASTM D664 using aqueous buffer solutions of pH 4 and 7. The procedure is as follows:

  • Measure the potential of buffer pH 4.0 while stirring and note the value after 1 minute
  • Remove the electrode and rinse it well with deionized water
  • Measure the potential of buffer pH 7.0 while stirring and note the value after 1 minute
  • Calculate the mV difference between the reading of buffers 4.0 and 7.0
  • The difference must be larger than 162 mV (20–25 °C) to indicate an electrode in good shape

If the measured potential difference is less than 162 mV, the electrode requires maintenance. Lift the flexible sleeve of the ground-joint diaphragm to let some electrolyte flow out. Repeat the measurement according to the steps above. If the value is still less than 162 mV, clean the electrode or replace it.

Proper rinsing and cleaning

Proper rinsing is essential if you want to obtain reliable results. Otherwise, the curve might flatten and the equivalence points are no longer recognizable. Figure 2 illustrates this phenomenon well.

Figure 2. Different determinations according to ASTM D664. With time, the start potential of the curves shifts which indicates an unsuitable cleaning procedure.

The sample is the same, however, you see that the equivalence point and starting potential begin to shift and the curves become flatter. This indicates an improper cleaning procedure between measurements. The corresponding electrode is shown in Figure 3.

Figure 3. Appearance of the electrode used in Figure 2 after five measurements.

This electrode was certainly not cleaned properly! Anyone who performs a nonaqueous titration must consider which solvent might best dissolve the residue—this is not an issue that other analysts can easily solve due to the nature of each individual sample. However, do not ignore an electrode with such an appearance.

Conditioning the glass membrane correctly

As you may remember from our previous blog post about pH measurement, it is essential that the hydration layer of the glass membrane stays intact. Nonaqueous solvents dehydrate the glass membrane rather quickly. A change in the hydration layer can have an impact on the measured potential, therefore it is important that the hydration layer is always in the same state before starting a titration to achieve the most reproducible results.

Proper electrode immersion depth

This can be established with a conditioning step of the glass membrane to rebuild the hydration layer. However, if the solvent is able to remove the hydration layer faster than it takes to perform a titration, this can lead to ghost equivalence points. Therefore, the electrode should be completely dehydrated and kept like this for all further titrations.

 

Polar solvents (e.g., ethanol, acetone, isopropyl alcohol, or mixtures with toluene)

Water-free solvents (e.g., dimethylformamide, acetonitrile, acetic anhydride, or mixtures of these)

Preparation of electrode

Store only the pH membrane (not the diaphragm) in deionized water overnight to build up a proper hydration layer.

Lift the flexible sleeve to allow some electrolyte to flow out.

Dehydrate the pH membrane by placing only the pH membrane (not the diaphragm) in the solvent you will use afterwards for titration.

Lift the flexible sleeve to allow some electrolyte to flow out.

Conditioning of glass membrane Place the pH membrane (bulb only) into deionized water for 1 minute. Place the pH membrane (bulb only) into the corresponding solvent for 1 minute.
Rinsing procedure Rinse electrode with 50–70% ethanol. If this does not help, use a suitable solvent to rinse the electrode and then clean afterwards with 50–70% ethanol. Rinse electrode with glacial acetic acid. If this does not help, use a suitable solvent to rinse the electrode and then clean afterwards with glacial acetic acid.
Remarks Make sure to always keep the bulb of the electrode in deionized water for the same time duration, otherwise the thickness of the hydrated layer (and therefore the response) may vary. Avoid any contact of the electrode with water as this can induce a reaction with the solvent causing ghost equivalence points and irreproducible results.

Maintenance of burets

It is not only the electrode that needs some special attention when performing nonaqueous titrations, but also the electrical buret. Some special maintenance is required since alkaline nonaqueous titrants are especially aggressive and they tend to crystallize, therefore leakage of the buret is likely.

The buret must be maintained on a regular basis according to the manufacturer’s instructions. Metrohm recommends the following procedure:

  • For shorter titration breaks, it is recommended to refill the cylinder with titrant (especially with OMNIS)
  • Clean the buret with deionized water at the end of the day
  • Lubricate the cylinder unit on the centering tube and on the cylinder disc

Also check the corresponding manual of the buret. The most important points are mentioned there which will lead to a longer working life of the buret.

Thermometric titration as an alternative

One alternative to using potentiometric nonaqueous acid-base titration is thermometric titration (TET), depending on the sample and analyte to be measured. Thermometric titration monitors the endothermic or exothermic reaction of a sample with the titrant using a very sensitive thermistor.

The benefit of TET over potentiometric titration is clearly the maintenance-free sensor which does not require any conditioning nor electrolyte refilling. More information about thermometric titration can be found in our previous blog posts below.

Summary

Hopefully this article has provided you with information about the main problems encountered during nonaqueous titrations. First, make sure that all electrostatic influences are eliminated. This will save a significant amount of troubleshooting. Then prepare and treat your electrode correctly before, during, and after titration. Make sure to condition the electrode right before your first measurement!

Of special importance here is the solvent you plan to use. If it is a polar solvent, the electrode should be conditioned in deionized water. If nonpolar solvents like acetic anhydride are used, the electrode should be dehydrated first. Between measurements, the electrode should be cleaned with a suitable solvent and the diaphragm should be opened on occasion.

Last but not least, take care of your buret. Maintain it regularly and replace it whenever necessary. With this advice, performing nonaqueous titrations should be a breeze!

For more information

about nonaqueous titrations, download our monograph:

Nonaqueous titration of acids and bases with potentiometric endpoint indication

Post written by Iris Kalkman (Product Specialist Titration at Metrohm International Headquarters, Herisau, Switzerland) and Dr. Sabrina Gschwind (Head of R&D at Metroglas, Affoltern, Switzerland).

Chemistry of Fireworks

Chemistry of Fireworks

Developed nearly two millennia ago in ancient China, fireworks are increasingly used in cultural celebrations around the world and enjoyed by nearly all ages. As one of the most entertaining forms of chemistry, fireworks appeal to our senses of sight and sound, offering a staggering variety of colors, sizes, shapes, sounds, and so on. We love to watch fireworks because they take our breath away with their magnificence and mystery.

However it is not all fun and games. The business of fireworks (and the field of pyrotechnics in general) is very serious since they should be made as safe as possible to use and also environmentally friendly. Beyond fireworks, other pyrotechnics are found in all kinds of entertainment, like in concerts, movies, and more serious applications for defense and security (e.g., safety measures like flares and signal lights).

What are fireworks made of?

Early fireworks were quite dangerous and were used for protection rather than for celebrations, and hardly resemble the ones we are now familiar with.

It all began back in Ancient China with the invention of gunpowder, which was created from a mixture of charcoal, sulfur, and saltpeter (potassium nitrate). Eventually, as new developments were made to increase the safety and predictability of using these early fireworks, experimentation with colors began and people started using them more for nonviolent purposes. Now there is an entire industry devoted to the development of all kinds of fireworks for consumers and professionals alike.

Learn more about the history of fireworks in the links below:

A firework, or aerial shell as it is also known, basically consists of three main parts aside from the housing: gunpowder and an igniter to make the rocket explode, and inside of the transported capsule on the top there are small garniture pods usually called «stars» (despite being shaped like spheres or cylinders) that include various chemicals for the desired effects. Stars consist of a colorant, a fuel, an oxidizer (oxygen providing substance, e.g., chlorates or nitrates), and a binder to hold the ingredient mixture together in a compact briquette.

The industry has spent a significant amount of time in development to make fireworks explode in shapes like stars and stripes, hearts, or even more complex forms like a cartoon figure, or letters and numbers if timed correctly.

Cross-sectional diagram of a firework capsule filled with star garnitures (72) and igniter (70). [1]

Forming a rainbow of colors

The vibrant colors of fireworks come from the combustion of metal ions which make up to 20% of the components. Metals have been used to color flames even before the invention of modern fireworks (e.g. Bengal fire). Chemically speaking, these metal ions change their electronic state by heating (addition of energy) and then going back to a lower energy state before emitting light of a certain color.

Table 1. List of metals used in pyrotechnics and their colors [2].

Color

Metal

Example compounds

Red

Strontium (intense red)

SrCO3 (strontium carbonate)

Lithium (medium red)

Li2CO3 (lithium carbonate)

LiCl (lithium chloride)

Orange

Calcium

CaCl2 (calcium chloride)

Yellow

Sodium

NaNO3 (sodium nitrate)

Green

Barium

BaCl2 (barium chloride)

B3N3 (boron nitride)

Blue

Copper halides

CuCl2 (copper chloride), at low temperature

Indigo

Cesium

CsNO3 (cesium nitrate)

Violet

Potassium

KNO3 (potassium nitrate)

Rubidium (violet-red)

RbNO3 (rubidium nitrate)

Gold

Charcoal, iron, or carbon black

 

White

Titanium, aluminum, beryllium, or magnesium powders

 

Very prominent here is the yellow color from sodium which is also seen in older street lightbulbs in some countries. Unfortunately, the most vibrant colors formed are also the most toxic for the environment, like strontium (red) and barium (green). These contaminants can be measured in the air, water, and even in the soil—but more on that later.

Find out more information about how fireworks get their colors in the links below:

Safety first

Safety is always a critical issue when discussing fireworks, whether concerning their construction, their use, or their storage. Too many serious accidents have happened over the years involving fireworks.

Learn more about how to handle fireworks in a safe manner here:

Among one of the largest fireworks disasters recorded in Europe was in Enschede (The Netherlands) in 2000. This explosion occurred in the warehouse of the S.E. Fireworks factory, which was located in the center of a residential area as the city grew and continued to build homes around it. An entire neighborhood was razed and the largest of the explosions was felt up to 30 kilometers away.

Because of this incident, sales of larger fireworks in most European countries is only allowed outdoors. Accumulating fireworks at home in preparation for celebrations should be avoided at least in confined environments like basements or apartments. It is better to store them in a ventilated shed or car parking to avoid problems in the case of a fire. Also do not store fireworks for long periods, since most of commercial fireworks are meant to be used within 3–6 months after production because the paper contents can get humid, ionic substances can dissolve and recrystallize, and therefore the likelihood of a failure increases.

In the event of a firework failure: Never have a look immediately! Wait at least 15 minutes at a proper distance and then use a tool to confine it afterwards—never touch it with your bare hands, especially when dealing with exploding fireworks or rockets.

Having said this, fireworks have integrated some safety features over the last several years to work more properly and reliably. For instance, the propellants have been modified from containing black powder to using technology from rockets such as plasticizers for better burning performance during launch, also resulting in less smoke and dust on the ground. A dedicated chain of reactions has to be followed, otherwise it will burn in a harmless way.

Knowledge is power: Prevent accidents with proper analytical testing

In order to help prevent fireworks accidents such as the one in Enschede and countless others, it is crucial to closely monitor different quality parameters including the water content of paper-based fireworks, grain size of the metal particles, and the purity and composition of the colorant, just to mention a few. Adequate quality control provides an entertaining, but safe fireworks experience even in the hands of the general public, when proper protocols are followed.

Metrohm offers several analytical technologies and related applications for this area of research. Analyses can be performed for a wide variety of substances and quality parameters as well as trace materials in the laboratory, on the street, and in the air either via wet chemical methods (e.g., Karl Fischer titration, ion chromatography, voltammetry) or spectroscopic techniques (e.g., near-infrared spectroscopy [NIRS] and Raman spectroscopy).

As mentioned earlier, moisture is an important quality parameter when discussing the safety of explosive materials. Metrohm offers two different techniques for accurate analysis of water content in a variety of matrices which are outlined in the following blog posts.

When it comes to determining the individual concentrations of the main constituents, some wet chemical techniques really stand out. Suppressed anion chromatography is ideal for measuring the ionic components of e.g., firecracker powder, other explosive material, and even in explosion residues for forensic purposes. Coupling an ion chromatograph to a mass spectrometer (IC-MS) opens up even more analysis possibilities. Read more about these studies (and more) by downloading our free Application Notes.

The use of several different metal salts to create the vibrant colors of fireworks can be beautiful but also harmful to our health and that of our environment. Voltammetry (VA) is an electrochemical method suitable for the determination of trace and ultratrace concentrations of heavy metals and other electrochemically active substances. Not only is VA excellent at determining these substances in the laboratory, but also in the field such as for measuring the after effects of a fireworks display or an undesired event. Check out our selection of VA instruments and applications on our website.

Spectroscopic techniques like Raman can help to determine the presence of dangerous explosive materials even when keeping a safe distance by using different instrument attachments. Read our free White Paper about how to use MIRA DS from Metrohm Raman for the purpose of identifying explosives safely.

Environmentally friendly fireworks – a contradiction?

Although fireworks are a very spectacular form of entertainment, there is quite an environmental impact after big cultural events or national holidays. The general atmospheric pollution after a fireworks display has been set off can be seen in an increase of dust and smoke, but also heavy metal content in the air as most contemporary fireworks use these for coloring.

The unburnt material still contains a significant amount of heavy metals. After falling to the ground, this material can dissolve and enter the ground water after it rains. Plastics materials that covered the fireworks for safety reasons are found again as broken shell shrapnel or as microplastics. The combustion of the compounds inside the fireworks leads to increased air pollution in form of aerosols that can be measured and evaluated resulting in heavy metals in the air, fine dust, and even nanoparticles which are extremely harmful for our lungs.

Metrohm Process Analytics has developed the 2060 MARGA (Monitor for AeRosols and Gases in ambient Air) which is used by official agencies and research bodies worldwide to monitor the air quality fully autonomously. This instrument is based on the analytical technique of ion chromatography and can be used as a dedicated continuous air monitoring device that can be left unattended for several weeks at a time, or as a research instrument that can be used for other projects when not monitoring the air quality.

Learn more about the 2060 MARGA and its capabilities in our blog post.

To find out more about the use of Metrohm instruments to monitor the air quality, check out this selection of peer-reviewed articles.

A new «green» firework generation is being developed for both professional and indoor use to try to minimize the heavy metal content and also reduce aerosol forming agents. This makes them more suitable for indoor pyrotechnic shows and for movie production. In regular outdoor shows (e.g. at theme parks), the gunpowder for transport of the capsule has mostly been substituted with an air pressure gun mechanism.

A significant amount of research has gone into substituting heavy metal-based colorants with more environmentally benign substances by increasing the luminosity of lithium derivatives by substituting them for strontium, or by using boron instead of barium or chlorinated compounds.

Finally, the plastic parts commonly used to surround fireworks are planned to be substituted by microcrystalline cellulose mixtures with better plasticizing binders. This leads to a similar stability compared to the current plastic materials, but the cellulose-based containers burn up completely and do not leave harmful materials scattered on the ground.

The future of fireworks shows

All safety measures increase the joy of fireworks not only during, but also after the event—being green and being safe. Foretelling the future, some of these celebrations may now use a cadre of lighted drones in a choreographed dance. This has been happening more steadily as drones fall in price and increase in their handling and programming capabilities. However, fireworks have already been with us for a couple of thousand years, and probably will not disappear any time soon.

Download our free Application Notes

and White Papers related to explosives and propellants

Post written by Dr. Norbert Mayr (Ph.D. in the field of HEDM, pyrotechnics, propellants, and oxidizers), Marketing Specialist & Product Training at Metrohm International Headquarters, Herisau, Switzerland.