Titrage thermométrique - la pièce manquante du puzzle

Le titrage est une technique d'analyse bien établie, enseignée à tous les étudiants en chimie. Il est effectué dans presque tous les laboratoires d'analyse sous la forme d'un titrage manuel, d'un titrage photométrique ou d'un titrage potentiométrique. Dans cet article de blog, j'aimerais présenter un autre type de titrage dont vous n'avez peut-être pas entendu parler auparavant - le titrage thermométrique - qui peut être considéré comme la pièce manquante du puzzle du titrage.

J'ai l'intention d'aborder les sujets suivants :

Qu'est-ce que le titrage thermométrique ?
Pourquoi envisager le titrage thermométrique ?
Exemples d'applications pratiques

Qu'est-ce que le titrage thermométrique ?

À première vue, le titrage thermométrique (TET) ressemble à un titrage normal et vous ne verrez pas beaucoup (ou pas du tout) de différence à courte distance. Les différences par rapport au titrage potentiométrique se situent dans les détails.

Figure 1. Illustration des courbes de titrage exothermique et endothermique montrant des points finaux clairs où la température de la solution change brusquement.

Le TET est basé sur le principe du changement d'enthalpie (ΔH). Chaque réaction chimique est associée à un changement d'enthalpie qui, à son tour, entraîne un changement de température. Au cours d'un titrage, l'analyte et le réactif de titrage réagissent soit de manière exothermique (augmentation de la température), soit de manière endothermique (diminution de la température).

Au cours d'un titrage thermométrique, le réactif titrant est ajouté à un rythme constant et le changement de température provoqué par la réaction entre l'analyte et le réactif titrant est mesuré. En traçant la température en fonction du volume de titrant ajouté, le point final peut être déterminé par une rupture dans la courbe de titrage. La figure 1 montre des courbes de titrage thermométrique idéalisées pour des situations exothermiques et endothermiques.

Que se passe-t-il lors d'un titrage thermométrique ?

Au cours d'une réaction de titrage exothermique, la température augmente avec l'ajout du réactif de titrage tant que l'analyte est encore présent. Lorsque tout l'analyte est consommé, la température diminue à nouveau car la solution s'équilibre avec la température atmosphérique et/ou en raison de la dilution de la solution avec le réactif de titrage (figure 1, graphique de gauche). Cette baisse de température se traduit par un point final exothermique.

Au contraire, pour une réaction de titrage endothermique, la température diminue avec l'ajout de réactif de titrage tant que de l'analyte est encore disponible. Lorsque tout l'analyte est consommé, la température se stabilise ou augmente à nouveau car la solution s'équilibre avec la température atmosphérique et/ou en raison de la dilution de la solution avec le réactif de titrage (figure 1, graphique de droite). Cette diminution de la température se traduit par un point final endothermique.

Il n'est donc pas nécessaire de connaître la température absolue, d'isoler le récipient de titrage ou de le thermostater pour effectuer le titrage.

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

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

Learn more about our fast, sensitive Thermoprobe products below – available even for aggressive sample solutions.

Metrohm Thermoprobe

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

Monograph: Practical thermometric titrimetry

Why consider thermometric titration?

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

TET has the same advantages as any instrumental titration technique:

Inexpensive analyses: Titration instruments are inexpensive to purchase and do not have high running and maintenance costs compared to other instruments for elemental analysis (e.g., HPLC or ICP-MS).
Absolute method: Titration is an absolute method, meaning it is not necessary to frequently calibrate the system.
Versatile use: Titration is a universal method, which can be used to determine many different analytes in various industries.
Easy to automate: Titration can be easily automated, increasing reproducibility and efficiency in your lab.

Find out more information about how automating titrations can help your laboratory workload in our previous blog article.

Why consider automation – even for simple titrations

In comparison to classical instrumental titration, thermometric titration has several additional advantages:

Fast titrations: Due to the constant titrant addition, thermometric titrations are very fast. Typically, a thermometric titration takes 2–3 minutes.
Single sensor: Regardless of the titration reaction (e.g., acid-base, redox, precipitation, …), the same sensor (Thermoprobe) can be used for all of them.
Maintenance-free sensor: Additionally, the Thermoprobe is maintenance free. It requires no calibration or electrolyte filling and can simply be stored dry.
Less solvent: Typically, thermometric titrations use 30 mL of solvent or even less. The small amount of solvent ensures that the dilution is minimized, and the enthalpy changes can be detected reliably. As a side benefit, less waste is produced.
Additional titrations possible: Because enthalpy change is universal for any chemical reaction, thermometric titration is not bound to finding a suitable color indicator or indication electrode. This allows the possibility of additional titrations which cannot be covered by other kinds of titration.
Easier sample preparation: As TET uses higher titrant concentrations it is possible to use larger sample sizes, reducing weighing and dilution errors. Tedious sample preparation steps such as filtration can be omitted as well.

Figure 3. The Metrohm 859 Titrotherm with 801 Stirrer and notebook with tiamo™ software.

Learn more about the 859 Titrotherm system for the most reliable TET determinations below.

859 Titrotherm complete with tiamo™ software

Practical application examples

In this section I will show some practical examples where TET can be applied.

Acid number and base number

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

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

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

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

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

White Paper: Avoid corrosion—A new method for TAN determination in crude oil and petroleum products

Brochure: Determination of AN in crude oil and in petroleum products in accordance with ASTM D8045 – Precise and reproducible results in 60 seconds

For more detailed information about the titration itself, download the following free Application Bulletins.

AB-427: Acid number in raw oils and petroleum products through thermometric titration in accordance with ASTM D8045

AB-405: Determination of the total base number in petroleum products

Sodium

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

If you wish to learn more about the sodium determination, watch our Metrohm LabCast video:

For more detailed information on the titration itself, download the free Application Bulletin here.

AB-298: Automated sodium determination in various foods with 859 Titrotherm

Fertilizer analysis

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

Nutrients

which can be analyzed by TET include:

Phosphate
Potassium
Ammoniacal nitrogen
Urea nitrogen
Sulfate

Want to learn more about the analysis of fertilizers with thermometric titration? Download our free White Paper below on this topic.

White Paper: Multiparameter analysis in fertilizers – Fast and easy via thermometric titration

For more detailed information regarding the different fertilizer applications, check out the Metrohm Application Finder:

Metrohm Application Finder: TET applications for fertilizers

Metal-organic compounds

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

If you wish to learn more about this topic, check out our free corresponding Application Note.

Determination of metal-organic compounds

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

Download free TET applications here

To summarize:

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

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