Thermometric Titration Applications

Each chemical reaction involves an enthalpy change that leads to a change in temperature. The amount of substance converted during the reaction determines the increase (i.e. exothermic reaction) or decrease (i.e. endothermic reaction) in temperature.

In a thermometric titration, the reagent solution (titrant) is added to the sample at a constant rate until attaining the endpoint. The change in temperature of the reaction solution is plotted against the volume of titrant that is added. The titration endpoint can be identified by a break in the titration curve and can be accurately determined by means of the second derivative. As the temperature sensor (Thermoprobe) has a 0.3 second response time and a 10^-5K resolution, even minute changes in enthalpy can be monitored reliably.

Following are the most salient advantages of thermometric titration:

• Easy-to-learn and carry out, and is completely supported by the tiamo™ titration software
• Results can be obtained rapidly
• Solves the issue of titrating difficult samples that cannot be titrated potentiometrically
• Single sensor for all applications
• Sensor calibration is not required
• Sensor is maintenance-free
• No membrane or diaphragm issues
• A robust technique for routine work
• Highly suitable for aggressive media

Metrohm has been actively involved in the development of novel applications (Figure 1) for thermometric titration. 

Figure 1. 859 Titrotherm from Metrohm for the Thermometric Titration

AB298: Automated Sodium Determination in Various Foods

Regulatory agencies and consumers are working harder for the precise analysis of sodium content in food products. Direct measurement techniques, such as inductively coupled plasma spectroscopy (ICP) and atomic absorption spectroscopy (AAS), are quite expensive and time-consuming for testing labs. As a result, a number of food companies indirectly analyze sodium by using the analysis results of chloride. Subsequently, the sodium content is typically calculated by presuming that the molar ratio of chloride ions to sodium ions in the food is 1:1. However, this is not necessarily the case, especially when there is a presence of chloride-containing ingredients (potassium chloride) or common sodium-containing food ingredients (monosodium glutamate and sodium benzoate) in the food matrix.

The determination of sodium content in foodstuffs does not involve any complicated manual sample preparation steps. The food is initially homogenized in a completely automatic manner using a high-frequency homogenizer Polytron 1300D that can be mounted on a USB Sample Processor. Subsequently, ammonium hydrogen difluoride is added to the sample, and then thermometric titration of the sample is carried out using a standardized aluminum solution containing a stoichiometric excess of potassium ions. Insoluble NaK₂AlF₆ is acquired from this exothermic reaction:

AB307, AB308, AB314, AB316: Determination of Sulfate and Phosphate in Various Fertilizers

This series of Application Bulletins cover the determination of phosphate and sulfate in liquid fertilizers and granular fertilizers such as triple superphosphate (TSP), diammonium phosphate (DAP), and monoammonium phosphate (MAP). Thermometric titration of sulfate can be carried out in a fast and easy way using a standard Ba²⁺ solution as a titrant. This process is already in use for determining sulfate in wet-process phosphoric acid. These results are presented as the percentage of elemental sulfur (%S).

Thermometric titration of phosphate can be performed in a fast and easy way using a standard Mg²⁺ solution as a titrant. This technique for determining titrimetric phosphate is based on a classical gravimetric method. The phosphate-containing solution is buffered by using NH₄Cl/NH₃ solution instantly before the titration. Insoluble MgNH₄PO₄ is formed as a result of an exothermic reaction. These results are presented as the percentage of P and P₂O₅. The amount of phosphoric acid in liquid fertilizers can be thermometrically titrated easily using a standardized NaOH solution c (NaOH) = 2 mol/L. The interfering calcium content present in the phosphoric fertilizer can be removed by the addition of a saturated oxalate solution.

AB313: Determination of Total Caustic, Total Soda, and Alumina in Bayer Process Liquors

Total caustic (total hydroxide content of the liquor), total soda (the sum of the total caustic and carbonate contents), and the aluminum content (present as alumina) of aluminate liquors generated from the Bayer process have to be established. This can be achieved with high speed and better precision using a thermometric acid-base titration. It takes approximately 5 minutes to perform a complete titration to determine all of the three parameters. This process is an automated adaptation of the conventional Watts-Utley method,¹ and is identical to the Van Dalen-Ward thermometric titration method,² but it has the additional benefit of the ability to analyze the carbonate content of the liquor.

An aliquot of Bayer liquor is transferred into a titration vessel and potassium sodium tartrate solution is added to make an aluminate ion complex. Hydroxide ions are released at the rate of one mole per each mole of aluminate ion. The already present hydroxide ions and the added hydroxide ions are titrated using standardized hydrochloric acid. The titration is allowed to continue so that the carbonate ions present in the liquor are also titrated. The initial titration is then automatically terminated, and a potassium fluoride solution is added to break the alumino-tartrate complex for the release of 3 moles of hydroxide ions for each mole of aluminum. The released hydroxide ions are then titrated using the standardized hydrochloric acid and established as the alumina content in the liquor.

AB315: Determination of Free Fatty Acids (FFA) in Edible Oils

While performing a titration, the added titrant reacts with the analyte in an exothermic (liberating heat) or endothermic (absorbing heat) manner. The change in temperature during the titration is measured by the Thermoprobe. When the entire analyte content reacts with the titrant, there is a difference in the temperature change of the reaction solution, and this is evident as a temperature curve inflection.

Whereas in specific cases, this inflection in the temperature curve is not adequate enough to analyze a thermometric titration. While performing titration of samples that consist of very weak acids, such as rape oil, the titration endpoint is highly difficult to recognize.

A breakthrough can be found in such cases by employing catalytically enhanced titrations, as they are dependent on a catalyzed indicator reaction to obtain a sharp inflection in the titration curve. An example of such a catalyzed indicator reaction is the endothermic hydrolysis of paraformaldehyde in the existence of excess hydroxide. This process is employed to determine FFA in edible oils and involves adding a little amount of paraformaldehyde to the sample solution before performing the titration.

Once all the acidic components of the reaction solution are neutralized, the addition of more titrant causes a surplus of hydroxide ions which in turn initiate the endothermic hydrolysis of paraformaldehyde. As a result, there is a sharp inflection in the temperature curve.

A mixture of 2-propanol and toluene in the ratio of 1:1 is used to dissolve edible oils, and the dissolved mixture is titrated using a standardized solution of tetrabutylammonium hydroxide that is dissolved in 2-propanol (c(TBAH) = 0.01 mol/L) to achieve a catalytically enhanced endpoint. The respective tiamo™ method has the ability to display the results either as a total acid number (TAN) (mg KOH/g) or as a percentage of free fatty acids (% FFA).

AB341: Determination of TAN in Engine Oil with 859 Titrotherm

The technique for determining TAN in engine oil or other mineral oils is similar to the technique employed to determine free fatty acids in edible oils. TAN is the amount of base (in mg KOH per gram oil) needed to neutralize the acids in the oil. The TAN is a measure of the acidity of the oil caused by the existence of acidic compounds formed as a result of the introduction of acids, degradation of additives, or oxidation.

References and Further Reading

1. H. L. Watts and D. W. Utley, Anal. Chem. 28, 1731 (1956).
2. E. Van Dalen and L. G. Ward, Anal. Chem. 45, 2248 (1973).
3. 

This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.

For more information on this source, please visit Metrohm AG.