Thermometric titrimetry puts heat back into the titration game

Thermometric titrimetry is versatile, robust, fast and reliable, ideally suited to the demands of modern process and quality control laboratories. The analytical chemistry technique of titration is over 200 years old. It has its roots in the very beginnings of quality control in industrial chemistry in the 18th century and it is still a mainstay in many industrial laboratories for raw material analysis, process control and quality control of finished products.

Automated titrators made their appearance over 40 years ago and have supplanted manual titration (characterised by graduated glass burettes and visual indicators) in tasks where accuracy, precision and freedom from the subjective judgement of the analyst are critical. Most developments in automated titrimetry have been driven by the manufacturers of the electrochemical sensors used to detect titration endpoints. However 'mainly' does not mean 'exclusively' because there is another automated titrimetric technique which is making a re-appearance on the analytical scene and once more providing the industrial analyst with a choice. This technique is thermometric titrimetry.

Thermometric titrimetry is distinguished from other analytical solution calorimetric techniques in that no attempt is made to quantify the amount of heat associated with a chemical reaction. Instead, a plot of temperature versus volume of titrant is made. In modern thermometric titration systems, the endpoint is located by the rate of change of temperature. The titrant is delivered at a constant rate, and when the reaction between the titrant and titrand is complete, an inflection in the temperature curve results.

Whereas potentiometric titrations require electrochemical sensors specific to the types of reactions they are required to monitor (and also require a reference electrode), thermometric titrations simply require a device which is able to sensitively monitor changes in the temperature of the solution. The thermistor has proved to be an ideal sensor for thermometric titrimetry. Thermistors are solid state devices that exhibit a large change in electrical resistance for small changes in temperature. Thermistors used in thermometric titrimetry exhibit very fast response times, typically an order of magnitude faster than glass membrane pH electrodes. Unlike potentiometric probes, thermometric probes employing thermistors are essentially maintenance-free and do not require calibration.

Thermometric titrimetry can be used for virtually all titration reactions where potentiometric titrimetry has been traditionally employed (Table 1). In fact, thermometric titrimetry extends into analytical areas where potentiometric titrimetry is precluded. Potentiometric titrations have always been difficult in non-aqueous solutions and impossible in non-electrically conducting media. Thermometric titrations may be conducted in solution media which, by nature of their composition, precludes the use of electrochemical sensors. For instance, titrations in non-aqueous and electrically non-conducting media may be performed easily. Further, thermometric titrations may be performed in matrices which can cause fouling of potentiometric sensors and reference electrodes.

The advent of cheap computers and powerful software to handle the titration data has overcome the barriers which have previously prevented a wider acceptance among analysts. Thermometric titrimetry also has advantages over potentiometric titrimetry in instances where it is desired to titrate analytes with small equilibrium constants (eg, the titration of boric acid with sodium hydroxide). It is also suitable for the sequential titration of different analytes in the same solution where equilibrium constants differ only by a small amount. An example in this latter instance is the sequential titration of calcium and magnesium by EDTA (Y4-).

Because the same thermometric probe may be employed for a wide variety of reaction types, otherwise difficult analytical problems are easily solved. The traditional method for the determination of the sulphuric and nitric acid contents involves boiling off the nitric acid component, a lengthy, corrosive and polluting process. The modern thermometric titration approach is to link the titration of the sulphuric acid component (with barium chloride as titrant) with a 'total acid' titration with sodium hydroxide. The sulphuric and nitric acids contents are computed automatically in a linked spreadsheet. The entire automated titration sequence is complete in less than five minutes.

Other 'hybrid' thermometric techniques have been developed, for instance to analyse etchants in the semiconductor industry and for electrolysis plating baths. Thermometric titrimetry has generally been regarded as a technique best suited to industrial analysis where analytes are present in the g/L or percent range. This is because a certain concentration of analyte and titrant is necessary for a reasonable temperature change. An exception had been the analysis of chloride in natural waters, where accurate results down to 20 ppm have been demonstrated.

However, the use of elegant endpoint amplification techniques can permit the determination of some analytes into the parts per million range. An example is the determination of low levels of metal ions in solution by an EDTA back-titration. The titrant is a solution of a transition metal ion (eg, Mn(II) or Cu(II)). The first trace of titrant in excess catalyses the oxidation by hydrogen peroxide of a polyhydric phenol such as resorcinol.

Conclusion

Thermometric titrimetry is a powerful analytical technique which has the ability to perform acid/base, redox, precipitation, EDTA and moisture titrations with a wide range of analytes in different matrices and over a wide range of concentrations.

Titration type, or analyte	Thermometric, sensor	Electrochemical, sensor
Acid/base	Thermistor	pH
Redox	Thermistor	Pt pin or plate
EDTA	Thermistor	ISE(1)
Sulphate	Thermistor	Ba ISE(1)
Chloride	Thermistor	Cl ISE or Ag billet
Phosphate	Thermistor	?
Surfactants	Thermistor	Surfactrode
Water	Thermistor	Pt KF(2) electrode

(1) ISE = Ion Selective Electrode

(2) KF = Karl Fischer

Table 1: Sensor requirements for potentiometric and thermometric titrations.

Reaction	pKeq	DHR kJ/mol
Ca2+ + Y4-CaY2-1	0.6	-25.2
Mg2+ + Y4- MgY2-	8.7	+16.8

Table 2: Sequential titration of calcium and magnesium by EDTA.