Thermometric titration

It is not a new technique, with possibly the first recognizable thermometric titration method reported early in the 20th century (Bell and Cowell, 1913).

In the thermometric titration, titrant is added at a known constant rate to a titrand until the completion of the reaction is indicated by a change in temperature.

The shape of experimentally obtained thermometric titration plots will vary from such idealized examples, and some of the environmental influences listed above may have impacts.

They are manufactured from sintered mixed metal oxides, with lead wires enabling connection to electrical circuitry.

The thermistor is encapsulated in a suitable electrically insulating medium with satisfactory heat transfer characteristics and acceptable chemical resistance.

Prior to derivatization it is therefore necessary to digitally smooth (or "filter") the temperature curve in order to obtain sharp, symmetrical second derivative "peaks" which will accurately locate the correct inflection point.

The degree of digital smoothing is optimized for each determination, and is stored as a method parameter for application every time a titration for that particular analysis is run.

Thermometric titrations generally demand rapid reaction kinetics in order to obtain sharp reproducible endpoints.

Where reaction kinetics are slow, and direct titrations between titrant and titrand are not possible, indirect or back-titrations often can be devised to solve the problem.

Successful applications for thermometric titrations are generally where titrant-titrand reaction kinetics are fast, and chemical equilibria are stoichiometric or nearly so.

A suitable setup for automated thermometric titrimetry comprises the following: Figure 6 illustrates a modern automated thermometric titration system based on the Metrohm 859 Titrotherm interface module with Thermoprobe sensor, Metrohm 800 Dosino dispensing devices and a computer running the operational software.

The range of possible thermometric titration applications far exceeds the actual experience of this writer, and the reader will be referred to the appropriate literature in some instances.

Acid leach solutions from some copper mines can contain large quantities of Fe(III) as well as Cu(II).

Automated potentiometric titration procedures have been granted standard method status, for example by ASTM for TAN and AOAC for FFA.

The electrode must be taken out of service on a regular basis to rehydrate the glass sensing membrane, which is also in danger of fouling by the oily sample solution.

A recent thermometric titrimetric procedure for the determination of FFA developed by Cameiro et al. (2002) has been shown to be particularly amenable to automation.

The speed of this titration coupled with its precision and accuracy makes it ideal for the analysis of FFA in biodiesel feedstocks and product.

In the classical determination of ferrous ion with permanganate, the reaction enthalpy is more than double that of a strong acid/strong base titration:Δ0Hr = −123.9 kJ/mol of Fe.

Thermometric iodometric titrations employing thiosulfate as a titrant are also practical, for example in the determination of Cu(II).

While relatively unstable and requiring frequent standardization, sodium hypochlorite has been used in a very rapid thermometric titration method for the determination of ammonium ion.

Thermometric titrations employing sodium salts of ethylenediaminetetra-acetic acid (EDTA) have been demonstrated for the determination of a range of metal ions.

The breakpoint for the consumption of Mg2+ (blue-tagged endpoint) by EDTA is revealed by upswing in temperature caused purely by the heat of dilution.

Direct EDTA titrations with metal ions are possible when reaction kinetics are fast, for example zinc, copper, calcium and magnesium.

Titrations for cobalt and nickel are carried out in an ammoniacal environment; buffered with ammonia:ammonium chloride solution.

A catalyzed endpoint procedure to determine trace amounts of metal ions in solution (down to approximately 10 mg/L) employs 0.01 mol/L EDTA.

The reaction enthalpies of EDTA with most metal ions are often quite low, and typically titrant concentrations around 1 mol/L are employed with commensurately high amounts of titrand in order to obtain sharp, reproducible endpoints.

At the endpoint, the first excess of metal ion catalyzes a strongly exothermic reaction between a polyhydric phenol (such as resorcinol) and hydrogen peroxide.

For instance, benzalkonium chloride (a quaternary-type cationic surfactant) may be determined in cleaners and algaecides for swimming pools and spas by titrating with a standard solution of sodium dodecyl sulfate.

When an excess of Ba2+ is added to a non-ionic surfactant of the alkyl propylene oxide derivative type, a pseudo-cationic complex is formed.

The titration plot illustrated in Figure 19 shows that the endpoint is quite rounded, suggesting that the reaction might not proceed to stoichiometric equilibrium.

Figs. 1a & 1b. Idealized thermometric titration plots of exothermic (left) and endothermic (right) reactions

Fig. 2. Representation of a thermometric titration curve for a reaction with a non-stoichiometric equilibrium

Fig. 3. Typical thermometric titration plot of an exothermic reaction

Fig. 4b. Thermometric probe for Metrohm 859 Titrotherm thermometric titration system

Fig. 5. Location of a thermometric titration endpoint using the second derivative of a digitally smoothed temperature curve

Fig. 6. Example of modern automated thermometric titration system (employing Metrohm 859 Titrotherm interface module and Metrohm 800 Dosino dispensing devices)

Fig 7. Schematic of relationship between components in automated thermometric titration system. A = dosing device B = thermometric sensor C = stirring device D = thermometric titration interface module E = computer

Fig 8. Titration of NaOH with 1 mol/L HCl

Fig.9. Titration of bicarbonate in company with carbonate by 1 mol/L NaOH

Fig. 10. Titration of a mixture of nitric, acetic and phosphoric acid with 2 mol/L NaOH

Fig. 11. Titration plots for determination of "total caustic", "total soda" and "alumina" in alumina refinery liquors.

Fig. 12. Determination of free H ₂ SO ₄ in copper leach solution by titration in propan-2-ol with 1 mol/L KOH in propan-2-ol

Fig 13. Catalyzed endpoint thermometric titration of free fatty acids in tallow – hydroxyl catalyzed endothermic hydrolysis of paraformaldehyde

Fig. 14. EDTA titration of calcium and magnesium in sea water

Fig. 15. Titration plot of back-titration of excess EDTA with Cu(II) in NH ₃ /NH ₄ Cl buffered solution

Fig. 16. Thermometric EDTA titration determination of trace Cu(II) by Mn(II) catalysis of exothermic reaction between hydrogen peroxide and polyhydric phenol.

Fig. 18. Thermometric titration of non-ionic surfactant in formulation containing anionic surfactant.

Fig. 19. Titration plot of the determination of fluoride with boric acid