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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.

Multiparameter analysis in fertilizers by thermometric titration

Multiparameter analysis in fertilizers by thermometric titration

Agriculture without fertilizers is no longer possible – without them, today’s estimated global population of 7.9 billion people could not be supported. Fertilizers provide plants with much needed nutrients for optimal growth. The ideal fertilizer depends not only on the crop, but the soil as well. To achieve the best results, knowledge of the fertilizer composition is essential.

To learn more about the origins of industrial fertilizers, read about the Haber-Bosch process in our series about the History of Chemistry.

Different fertilizers for different needs

Fertilizers can be classified in various ways, one of which being their origin. Fertilizers derived from plants and/or animals, such as dung or manure, are usually called «organic fertilizers», while fertilizers obtained from mineral salts or ores are called «inorganic fertilizers».

 The most often used classification of inorganic fertilizers is based on their nutrient composition. Classification by nutrient composition allows farmers to select the optimal fertilizer for their soil and crops. Single nutrient or straight fertilizers deliver only one nutrient. Examples are ammonium nitrate or single superphosphate. More common are multi-nutrient fertilizers consisting of two or more nutrients. Examples here include monoammonium phosphate or NPK (nitrogen-phosphate-potassium) fertilizers.

Nutrients for plants

The macronutrients nitrogen, phosphorus, and potassium are the main nutrients required by the plant for its growth. Other secondary nutrients such as sulfur and calcium, or micronutrients like boron are also essential but required in smaller quantities.

Why analyze the fertilizer composition?

Selecting the ideal fertilizer composition is essential for proper plant growth. Crops will suffer from a deficiency in nutrients, however adding an abundance of them can be detrimental, resulting in fertilizer burn for example.

Furthermore, releasing too much fertilizers at once can lead to undesirable environmental pollution. Fertilizer producers are therefore required to specify the amount of nutrients within their products, and various norms from ISO, EN, and AOAC exist for the standardized determination of these nutrients.

Thermometric titration for fertilizer analysis

Traditionally the main nutrients in fertilizers are determined by analytical methods such as gravimetry, photometry, or ICP-OES. These methods require either time-consuming sample preparation or the use of expensive analysis equipment. Thermometric titration provides an inexpensive alternative solution for the analysis of potassium, phosphorus, sulfur, ammoniacal nitrogen, and urea without any time-consuming steps.

Using thermometric titration to analyze fertilizer composition has several benefits: 

  • Analysis of multiple parameters with one device
  • Automation possibility for analyzing multiple samples a day
  • Rapid results for each parameter with titration times under 5 minutes

Want to learn more about the analysis of fertilizers with thermometric titration? Download our free White Paper on this topic: Multiparameter analysis in fertilizers – Fast and easy via thermometric titration. 

What is thermometric titration?

Thermometric titration (TET) is based on the principle of enthalpy change. Each chemical reaction is associated with a change in enthalpy that in turn causes a temperature change. This temperature change during a titration can be measured with a highly sensitive thermistor in order to determine the endpoint of the titration.

If you would like to read more about the basic principles of thermometric titration, click below for our previous  blog post «Thermometric titration – the missing piece of the puzzle».

Metrohm’s maintenance-free Thermoprobe used for fast and reliable indication of thermometric titration endpoints.

How are the analyses performed?

In this section I will explain how the analyses for various macronutrients in fertilizers are done using thermometric titration.

Thermometric titration system consisting of a 859 Titrotherm fully equipped with a Thermoprobe, titration stand and buret, and the tiamo software for the multiparameter analysis of fertilizers.
Phosphorus

Phosphorus is an essential macronutrient for photosynthesis and optimal crop growth, as it provides the energy to extract other nutrients from the soil. Historically, total phosphorus content is determined by gravimetric analysis. Alternatively, spectrophotometric analysis or ICP-OES may be used for the determination. These methods all require time-consuming sample preparation steps or regular calibrations.

The thermometric titration of phosphorus is based on the gravimetric determination, but without long drying times to achieve constant weight. An appropriate aliquot of sample is added to the titration vessel and 5 mL of pH 10 buffer (ammonium chloride / ammonia) as well as 5 mL of an oxalate solution (to precipitate any interfering calcium) are added. The solution is then made up to 30 mL with deionized water and titrated with a magnesium nitrate titrant until after the exothermic endpoint.

Figure 1. Exothermic titration curve of the phosphate determination in an NPK fertilizer (blue = titration curve, pink = second derivative showing the endpoint).

For more detailed information about thermometric titration of phosphorus, download one of the following free application documents:

Ammoniacal nitrogen and urea

Nitrogen is an essential macronutrient as it is a component of amino acids (protein building blocks) and nucleic acid (building blocks of DNA). In inorganic fertilizers, nitrogen is usually present as ammonium, nitrate, or urea. Ammonia is usually determined after alkaline distillation by acid-base back-titration, while other nitrogen species are usually first converted to ammonia via digestion prior to analysis.

With thermometric titration, a different approach is used. Ammonium ions as well as urea react exothermically with hypochlorite in a redox reaction. This reaction is further catalyzed in the presence of bromide ions in a slightly alkaline solution. 

Figure 2. Exothermic titration curve of ammoniacal nitrogen and urea in an NPK fertilizer (blue = titration curve, pink = second derivative showing the endpoint). The first endpoint (left) corresponds to ammonia and the second one (right) to urea.

To analyze ammoniacal nitrogen and urea, an appropriate aliquot of sample is added to the titration vessel and then 10 mL of a bromide/bicarbonate solution is added. The solution is then made up to 50 mL with deionized water and titrated with hypochlorite until after the exothermic endpoint.

For more detailed information about thermometric titration of ammonium and urea, check out the following free application notes:

Potassium

Potassium is an essential macronutrient for crops, needed for regulating their water and making them more resistant to droughts. Historically, potassium content is determined by gravimetric analysis. More recently, ICP-OES is used for this determination, but the instrumentation is very expensive.

The thermometric titration of potassium is based on the precipitation of potassium with sodium tetraphenyl borate (STPB). It is a quick titration and for this reason has already been integrated in various Chinese standards on fertilizers (HG/T 2321 for potassium dihydrogen phosphate, GB/T 20784 for potassium nitrate, and GB/T 37918 for potassium chloride).

An appropriate aliquot of sample is added to the titration vessel. The solution is then made up to 30 mL with deionized water and titrated with STPB until after the exothermic endpoint is reached.

Figure 3. Exothermic titration curve of the potassium determination in potash (blue = titration curve, pink = second derivative showing the endpoint).

For more detailed information about thermometric titration of potassium, download our free application notes:

Sulfur

Sulfur is a secondary macronutrient and plays an important role in chloroplast growth as well as acting as a catalyst for nitrogen uptake. Sulfur is usually provided in the form of sulfate. Sulfuric acid also influences the wet phosphoric acid production process, and therefore knowledge of its content is crucial.

Conventionally, sulfur is determined by gravimetry. The same precipitation reaction with barium is also used for the thermometric titration, without time-consuming drying to weight.

For the analysis, an appropriate aliquot of sample is added to the titration vessel and acidified (if necessary). The solution is then made up to 30 mL with deionized water and titrated with barium chloride until after the exothermic endpoint. For improved method sensitivity, the samples can be spiked with a standard sulfuric acid solution.

Figure 4. Exothermic titration curve of the sulfate determination in an NPK fertilizer spiked with a known amount of sulfuric acid for better endpoint recognition (blue = titration curve, pink = second derivative showing the endpoint).

For more detailed information about thermometric titration of sulfur, download our free application documents:

Summary

Thermometric titration is an inexpensive analysis method without the need for costly maintenance or calibrations. It provides a rapid and robust solution for the determination of multiple parameters in fertilizers. If you wish to learn more about thermometric titration and its potential to solve application challenges do not hesitate to contact your local Metrohm representative!

For more information, read our White Paper

Multiparameter analysis in fertilizers – Fast and easy via thermometric titration

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

Supercharge your battery research – Part 1

Supercharge your battery research – Part 1

Replacing traditional fuel-powered vehicles with battery-powered options is essential to reduce carbon dioxide (CO2) emissions. This greenhouse gas results from the combustion of fossil fuels, therefore limiting its input into the atmosphere will also influence global warming. Battery research therefore focuses on discovering new materials with higher energy and power density as well as a more efficient energy storage.

Various critical parameters need to be determined to develop viable new batteries. In this first of two blog posts, I want to highlight a few of the analytical parameters which can be determined using high precision analytical instruments from Metrohm and provide some free downloads in this research area.

What’s in a lithium battery?

Today, lithium ion batteries are the most common rechargeable batteries available on the market. A battery consists of an anode (negative pole) and cathode (positive pole). An electrolyte facilitates charge transfer in the form of lithium ions between these two poles. Meanwhile a separator placed between anode and cathode prevents short-circuits. An example cross section can be seen in Figure 1.

Figure 1. Cross-section illustration of a lithium ion battery. While the battery is being charged, lithium ions migrate from the cathode to the anode (from right to left), and during discharging they move from the anode to the cathode (from left to right).

The anode is made from graphite containing intercalated lithium applied to a copper foil, while the cathode consists of metal oxides dotted with lithium ions applied to an aluminum foil. The most common transition metals used in cathode materials are cobalt, nickel, manganese, or iron. The electrolyte is an anhydrous aprotic solvent containing a lithium salt (e.g., lithium hexafluorophosphate) to facilitate charge transfer. The separator is prepared from a porous material, acting as an insulator to prevent short-circuits. The composition of all of these components has a significant influence on the battery characteristics.

After this brief overview about the composition of a lithium battery, let’s take a look at selected key parameters and how they can be analyzed.

Water content in battery raw materials

Lithium-ion batteries should be free of water (concentration of H2O less than 20 mg/kg), because water reacts with the conducting salt (e.g., LiPF6) to form toxic hydrofluoric acid. Sensitive coulometric Karl Fischer titration is the ideal method for determining water content at trace levels. Water determination for solids is carried out using the Karl Fischer oven method – the residual moisture in the sample is evaporated and transferred to the titration cell where it is subsequently titrated. The working principle and advantages of the KF oven method are described in more detail in our blog post «Oven method for sample preparation in Karl Fischer titration».

For more details on how to carry out the water determination in one of the following battery components, download our free Application Bulletin AB-434:

 

  • raw materials for the manufacture of lithium-ion batteries
  • electrode coating preparations (slurry) for anode and cathode coating
  • the coated anode and cathode foils as well as in separator foils and in packed foil layers
  • electrolytes for lithium-ion batteries

Transition metal composition of cathode materials

The cathode of a lithium-ion battery is usually made from metal oxides derived from cobalt, nickel, manganese, iron, or aluminum. To produce the cathode, solutions containing the desired metal salts are used. For an optimized production process, the exact content of the metals present in the solution must be known. Additionally, the metal composition within the obtained cathode material should be determined. Potentiometric titration is a suitable technique to determine the metal content in starting solutions and the finished cathode materials.

The following mixtures of metals or metal oxides can be analyzed potentiometrically:

  • Nickel, cobalt, and manganese in solutions
  • Nickel, cobalt, and manganese in cathode materials such as cobalt tetraoxide (Co3O4), lithium manganite, or lithium cobaltite

For more details about the potentiometric analysis of a mixture of nickel, cobalt, and manganese download our free Application Note AN-T-218.

Analysis of lithium salts

Potentiometric titration is also ideally suited for determining the purity of lithium salts. For lithium hydroxide and lithium carbonate, the purity is determined using an aqueous acid-base titration. It is also possible to determine carbonate impurity within lithium hydroxide using this method.

For more details about performing the assay of lithium hydroxide and lithium carbonate, download our free Application Note AN-T-215.

For the assay of lithium chloride and lithium nitrate, the lithium is directly titrated using the precipitation reaction between lithium and fluoride in ethanolic solutions. For more details about how to carry out the assay of lithium chloride, download Application Note AN-T-181 and for lithium nitrate download AN-T-216.

The knowledge of other cations which might be present in lithium salts (and their concentration) is also of interest. Various cations (e.g., sodium, ammonium, or calcium) can be determined using ion chromatography (IC). IC is an efficient and precise multi-parameter method to quantify anions and cations over a wide concentration range.

The chromatogram in Figure 2 shows the separation of lithium, sodium, and calcium in a lithium ore processing stream.

Figure 2. Ion chromatogram of the lithium ore processing stream (1: lithium, 23.8 g/L; 2: sodium, 1.55 g/L; 3: calcium, 0.08 g/L).

For more information on how this analysis was carried out, download our free Application Note AN-C-189.

Eluated ions and decomposition products

In the development and optimization of lithium-ion batteries, one of the items of special interest is the content of ions (e.g., lithium, fluoride, and hexafluorophosphate) in the electrolyte or in eluates of different components. Ion chromatography allows the determination of decomposition products in electrolyte, or anions and cations eluated for example from finished batteries. Additionally, any sample preparation steps that might be required (e.g., preconcentration, dilution, filtration) can be automated with the Metrohm Inline Sample Preparation («MISP») techniques.

For more detailed information about selected IC applications for battery research, check out our Application Notes:

  • Cations in lithium hexafluorophosphate (AN-C-037)
  • Trace cations in lithium hexafluorophosphate (AN-CS-011)
  • Anions in electrolyte (AN-N-012)
  • Decomposition products of lithium hexaflurophosphate (AN-S-372)

Summary

This blog post contains only part of the analyses for battery research which are possible using Metrohm’s analytical instruments. Part 2 discusses the electrochemical characterization of batteries and their raw materials. Click below to read it!

Battery research

Positive experiences with top quality Metrohm equipment!

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

ASTM D6304: Easier determination of moisture in petroleum products

ASTM D6304: Easier determination of moisture in petroleum products

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

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

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

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

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

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

Direct injection (Procedure A)

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

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

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

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

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

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

Water extraction using an oven (Procedure B)

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

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

Schematic drawing of the Karl Fischer oven method.

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

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

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

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

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

Water extraction using an evaporator (Procedure C)

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

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

Schematic drawing of the evaporator method.

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

When to use which procedure

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

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

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

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

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

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

Visit our website

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

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

Fast determination of acid and base number by thermometric titration

Fast determination of acid and base number by thermometric titration

Acid number (AN) and base number (BN) are critical parameters in the quality control of petroleum products as they are often stipulated by product specifications. Traditionally both parameters can be determined by potentiometric or photometric titration according to various standards such as ASTM D664 (Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration), ASTM D2896 (Standard Test Method for Base Number of Petroleum Products by Potentiometric Perchloric Acid Titration), or ASTM D974 (Standard Test Method for Acid and Base Number by Color-Indicator Titration). However, there is a rapid and reliable alternative titration method – thermometric titration.

Why determine the acid and base number?

The acid number is an indication for the amount of acids present in petroleum products. Weak acids present in crude oils (e.g. naphthenic acid) can be linked to corrosion of refinery equipment. For petroleum products, aging can lead to the buildup of acids, which increases the risk of corrosion to pipes and holding tanks.

To prevent such an acidic buildup, basic additives are added to refined petroleum products, such as lubricating oil. These basic additives neutralize the weak acids and can prevent corrosion. The amount of basic additives can be characterized using the base number.

What is thermometric titration?

Thermometric titration (TET) is based on the principle of enthalpy change. Each chemical reaction is associated with a change in enthalpy that in turn causes a temperature change. This temperature change during a titration can be measured with a highly sensitive thermistor (Figure 1) in order to determine the endpoint of the titration.

Figure 1. Metrohm’s maintenance-free Thermoprobe used for fast and reliable indication of thermometric titration endpoints.

If you would like to read more about the basic principles of thermometric titration, click below for our previous  blog post «Thermometric titration – the missing piece of the puzzle».

TET: the best choice for AN and BN determination

If you’ve performed a potentiometric titration of the acid and base number, you probably know that not all samples are soluble in the solvent mixture. Even if they are soluble, several cleaning steps (including conditioning of the electrode after each titration) are necessary in order to achieve good reproducibility.

While photometric titration provides an alternative indication method for samples which are not colored, the solubility issue remains. Thermometric titration of the AN according to ASTM D8045 provides the ideal solution to all of these issues.

  • The xylene/IPA (3/1) solution allows better solubility of many samples, especially crude oils
  • Endpoint indication is not affected by colored samples
  • The Thermoprobe requires no conditioning or additional cleaning steps – only a rinse with solvent
  • The Thermoprobe is maintenance-free – no electrolyte refilling necessary, just store it dry

There are even more benefits if compared to the potentiometric titration according to ASTM D664 or ASTM D2896.

 

  • Less solvent used: 30 mL instead of 60 mL or 120 mL saves additional costs and reduces waste
  • Faster titrations: half the time of potentiometric titrations, saving about 2 minutes per analysis
  • Robust sensor: the Thermoprobe is completely maintenance-free and needs no conditioning, further reducing analysis time.

For a comprehensive comparison between the AN determination according to ASTM D8045 (thermometric titration) and ASTM D664 (potentiometric titration), check out Table 1 below. While the titrant and solvent mixtures differ if you perform a base number determination, the values for solvent volume, titration time, electrode conditioning, and sensor maintenance reflect the comparison between thermometric base number determination and potentiometric determination according to ASTM D2896 very well. Discussions for an ASTM standard on thermometric BN determinations are currently ongoing within the respective committee.

Table 1. Comparison between ASTM D664 and ASTM D8045 concerning various parameters.

Since you are titrating faster, using less solvent, and do not have to perform complicated sensor maintenance, you can save quite a bit of money by switching to thermometric titration.

Not convinced yet? Then listen to one of our customers, Thomas Fischer from Oel Check GmbH, Germany, about his positive experiences with Metrohm thermometric titration.

«Thermometric titration has several advantages compared to potentiometric titration. It is much faster and more robust. A typical thermometric titration takes just about 2 minutes. Moreover, the electrode does not need to be regenerated between determinations.»

Thomas Fischer

Laboratory Manager, Oel Check GmBH

Additionally, I suggest downloading our related white paper on this topic: «Avoid corrosion: A new method for TAN determination in crude oil and petroleum products», which contains comparison data between ASTM D664 and ASTM D8045.

How to perform the analysis

During the AN or BN determination, very weak acids or bases (respectively) are titrated, resulting in small enthalpy changes. By using a catalytic endpoint indicator, these weak acids and bases can also be determined by TET.

What is catalyzed endpoint indication?

Endpoint indication becomes difficult for titrations with small enthalpy changes, such as with weak acids or bases. In these situations, a catalytic endpoint indicator is used. The catalytic endpoint indicator undergoes a strongly exothermic or endothermic reaction during the titration. As with an indicator which changes color when all analyte has been titrated, the catalytic endpoint indicator only starts its reaction with the titrant after all analyte has been consumed. In this way, the indication of the endpoint becomes possible.

Figure 2. Thermometric titration system consisting of a 859 Titrotherm fully equipped with a Thermoprobe, titration stand and buret, and the tiamo software for the TAN or TBN determination.

Acid number

An appropriate amount of the sample (depending on the expected AN) is weighed into the titration vessel, then 30 mL solvent mixture (isopropanol:xylene 1:3) and 0.5 g paraformaldehyde are added. After dissolution of the sample, the solution is then titrated with alcoholic KOH to a single exothermic endpoint.

Here, the paraformaldehyde acts as the catalytic endpoint indicator. As soon as there is an excess of KOH available it will de-polymerize in a strongly endothermic reaction, resulting in an exothermic endpoint.

Figure 3. Thermometric titration curve of an acid number determination, resulting in a single, well-defined exothermic endpoint.

 For more detailed information about this application, download our free Application Bulletin AB-427.

Base number

An appropriate amount of the sample (depending on the expected BN) is weighed directly into the titration vessel, then 1 mL isobutyl vinyl ether and 40 mL toluene are added. After dissolution of the sample, the solution is then titrated with HClO4 in glacial acetic acid to a single endothermic endpoint.

In this situation, the isobutyl vinyl ether serves as the catalytic endpoint indicator. When an excess of HClO4 is present, it will polymerize in a strongly exothermic reaction, resulting in an endothermic endpoint.

Figure 4. Thermometric titration curve of a base number determination, resulting in a single, well-defined endothermic endpoint.

For more detailed information about this application, download our free Application Bulletin AB-405.

Summary

Thermometric titration provides a rapid and robust solution for the determination of the acid and base number in comparison to potentiometric or photometric titration. The method solves the issue of sample solubility by using more suitable solvents. Furthermore, less solvent is needed, and the analysis time is reduced. All this results in considerably lower costs per analysis, making it a viable alternative for the acid and base number determination.

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