THERMOMETRIC TITRATIONS Prof Anees Ahmad Department of Chemistry

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THERMOMETRIC TITRATIONS Prof. Anees Ahmad Department of Chemistry

THERMOMETRIC TITRATIONS Prof. Anees Ahmad Department of Chemistry

INTRODUCTION � Each chemical reaction involves an enthalpy change that leads to change in

INTRODUCTION � Each chemical reaction involves an enthalpy change that leads to 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.

INTRODUCTION (CONTD. ) In thermometric titration, the reagent solution (titrant) is added to the

INTRODUCTION (CONTD. ) In 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 -5 K resolution, even minute changes in enthalpy can be monitored reliably. �

TITRATION (CONTD) � Consider the titration reaction: � a. A + b. B =

TITRATION (CONTD) � Consider the titration reaction: � a. A + b. B = p. P Where: � A = the titrant, and a = the corresponding number of moles reacting � B = the analyte, and b = the corresponding number of moles reacting � P = the product, and p = the corresponding number of moles produced

TITRATION (CONTD) � At completion, the reaction produces a molar heat of reaction ΔHr

TITRATION (CONTD) � At completion, the reaction produces a molar heat of reaction ΔHr which is shown as a measurable temperature change ΔT. � In an ideal system, where no losses or gains of heat due to environmental influences are involved, the progress of the reaction is observed as a constant increase or decrease of temperature depending respectively on whether ΔHr is negative (indicating an exothermic reaction) or positive (indicating an endothermic reaction).

IDEALIZED TITRATION Figs. 1 a & 1 b. Idealized thermometric titration plots of exothermic

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

TITRATION (CONTD) � In this context, environmental influences may include: � � � �

TITRATION (CONTD) � In this context, environmental influences may include: � � � � Figs. 1 a & 1 b. Idealized thermometric titration plots of exothermic (left) and endothermic (right) reactions Heat losses or gains from outside the system via the vessel walls and cover; Differences in the temperature between the titrant and the titrand; Evaporative losses from the surface of the rapidly mixed fluid; Heats of solution when the titrant solvent is mixed with the analyte solvent; Heat introduced by the mechanical action of stirring(minor influence); and Heat produced by thermistor itself (very minor influence).

TITRATION (CONTD) � If the equilibrium for the reaction lies far to the right

TITRATION (CONTD) � If the equilibrium for the reaction lies far to the right (i. e. a stoichiometric equilibrium has been achieved), then when all analyte has been reacted by the titrant continuing addition of titrant will be revealed by a sharp break in the temperature/volume curve. � Figures 1 a and 1 b illustrate idealized examples.

EXPERIMENTAL TITRATION � The shape of experimentally obtained thermometric titration plots will vary from

EXPERIMENTAL TITRATION � 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. � Curvature at the endpoint might be observed. � This can be due to insensitivity of the sensor or where thermal equilibrium at the endpoint is slow to occur. � It can also occur where the reaction between titrant and titrand does not proceed to stoichiometric completion.

EXPERIMENTAL TITRATION (CONTD) � The determinant of the degree to which a reaction will

EXPERIMENTAL TITRATION (CONTD) � The determinant of the degree to which a reaction will proceed to completion is the free energy change. � If this is favourable, then the reaction will proceed to be completion and be essentially stoichiometric. � In this case, the sharpness of the endpoint is dependent on the magnitude of the enthalpy change. � If it is unfavourable, the endpoint will be rounded regardless of the magnitude of the enthalpy change.

EXPERIMENTAL TITRATION (CONTD) � Reactions where non-stoichiometric equilibria are evident can be used to

EXPERIMENTAL TITRATION (CONTD) � Reactions where non-stoichiometric equilibria are evident can be used to obtain satisfactory results using a thermometric titration approach. � If the portions of the titration curve both prior to and after the endpoint are reasonably linear, then the intersection of tangents to these lines will accurately locate the endpoint. � This is illustrated in Figure 2.

EXPERIMENTAL TITRATION (CONTD) Fig. 2. Representation of a thermometric titration curve for a reaction

EXPERIMENTAL TITRATION (CONTD) Fig. 2. Representation of a thermometric titration curve for a reaction with a non-stoichiometric equilibrium

EXAMPLE TITRATION (CONTD) � Consider the titration reaction: � a. A + b. B

EXAMPLE TITRATION (CONTD) � Consider the titration reaction: � a. A + b. B = p. P Where: � A = the titrant, and a = the corresponding number of moles reacting � B = the analyte, and b = the corresponding number of moles reacting � P = the product, and p = the corresponding number of moles produced

EXAMPLE TITRATION (CONTD) If reaction in this equation: a. A + b. B =

EXAMPLE TITRATION (CONTD) If reaction in this equation: a. A + b. B = p. P is non-stoichiometric at equilibrium. � Let A represent the titrant, and B the titrand. At the beginning of the titration, the titrand B is strongly in excess, and the reaction is pushed towards completion. Under these conditions, for a constant rate of titrant addition the temperature increase is constant and the curve is essentially linear until the endpoint is approached. In a similar manner, when the titrant is in excess past the endpoint, a linear temperature response can also be anticipated. Thus intersection of tangents will reveal the true endpoint.

EXAMPLE TITRATION (CONTD) Fig. 3. Typical thermometric titration plot of an exothermic reaction

EXAMPLE TITRATION (CONTD) Fig. 3. Typical thermometric titration plot of an exothermic reaction

THERMISTORS � Thermistors respond quickly to small changes in temperature such as temperature gradients

THERMISTORS � Thermistors respond quickly to small changes in temperature such as temperature gradients in the mixed titration solution, � Signal can exhibit a small amount of noise. � 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. � See Fig 5 (next)

THERMISTORS This is illustrated in Figure 5. The degree of digital smoothing is optimized

THERMISTORS This is illustrated in Figure 5. 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. Fig. 5. Location of a thermometric titration endpoint using the second derivative of a digitally smoothed temperature curve Fig. 4 b. Thermometric probe for Metrohm 859 Titrothermometric titration system

APPLICATIONS � Because enthalpy change is a universal characteristic of chemical reactions, thermometric endpoint

APPLICATIONS � Because enthalpy change is a universal characteristic of chemical reactions, thermometric endpoint sensing can be applied to a wide range of titration types, e. g. � Acid/base � Redox � Complexometric (EDTA) and � Precipitation � Since the sensor is not required to interact with the titration solution electrochemically, titrations in nonconducting media can be performed, as can titrations using reactions for which no convenient or costeffective potentiometric sensor is available.

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

APPLICATIONS 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. � Catalytically enhanced endpoints can be used in some instances where the temperature change at the endpoint is very small and endpoints would not be detected satisfactorily by the titration software. �

APPLICATIONS � � The suitability of a particular chemical reaction as a candidate for

APPLICATIONS � � The suitability of a particular chemical reaction as a candidate for a thermometric titration procedure can generally be predicted on the basis of the estimated amount of analyte present in the sample and the enthalpy of the reaction. However, other parameters such as the kinetics of the reaction, the sample matrix itself, heats of dilution and losses of heat to the environment can affect the outcome. A properly designed experimental program is the most reliable way of determining the viability of a thermometric titration approach. Successful applications for thermometric titrations are generally where titrant-titrand reaction kinetics are fast, and chemical equilibria are stoichiometric or nearly so.

EXAMPLES OF THERMOMETRIC TITRATION DETERMINATIONS � � The analyst wishes to simplify the conduct

EXAMPLES OF THERMOMETRIC TITRATION DETERMINATIONS � � The analyst wishes to simplify the conduct of a variety of titrations by using one sensor for all. For example, a laboratory might conduct routinely acid/base, redox, complexometric, sulfate and chloride titrations. A single thermometric sensor in conjunction with an autosampler will enable all titrations to be performed in the same carousel load without having to change titration sensors. After preparation of the samples and placing in the carousel, the analyst assigns the appropriate thermometric method to the beaker position in the carousel.

EXAMPLE (CONTD) � � � The titration environment is considered unsuitable for conventional titration

EXAMPLE (CONTD) � � � The titration environment is considered unsuitable for conventional titration sensors. For example, glass membrane p. H electrodes must be kept adequately hydrated for properation. The use of such electrodes in substantially non-aqueous media as in the determination of trace acids in lipids and lubricating oils can lead to loss of performance as the membrane fouls and dehydrates, and/or if the reference junction is partly or completely blocked. It is often necessary to keep a number of electrodes cycling through a rejuvenation program in order to keep up with an analytical workload. Thermometric sensors have no electrochemical interaction with the titrating solution, and therefore can be used on a continuous basis with essentially no maintenance. Similarly, the potentiometric titration of sulfate with barium chloride in various industrial samples can lead to rapid degradation of the indicating barium ion selective electrode.

EXAMPLE (CONTD) �A thermometric titration methodology which cannot be emulated using other types of

EXAMPLE (CONTD) �A thermometric titration methodology which cannot be emulated using other types of titration sensors will deliver superior or results otherwise unobtainable by other techniques. � Examples are the determination of fluoride by titration with boric acid, the analysis of orthophosphate by titration with magnesium ions, and the direct titration of aluminium with fluoride ions

APPARATUS AND SETUP FOR AUTOMATED THERMOMETRIC TITRIMETRY �A suitable setup for automated thermometric titrimetry

APPARATUS AND SETUP FOR AUTOMATED THERMOMETRIC TITRIMETRY �A suitable setup for automated thermometric titrimetry comprises the following: � Precision fluid dispensing devices – “burettes” – for adding titrants and dosing of other reagents � Thermistor-based thermometric sensor � Titration vessel � Stirring device, capable of highly efficient stirring of vessel contents without splashing � Computer with thermometric titration operating system � Thermometric titration interface module – this regulates the data flow between the burettes, sensors and the computer

METROHM THERMOMETRIC TITRATION SYSTEM

METROHM THERMOMETRIC TITRATION SYSTEM

SALIENT ADVANTAGES OF THERMOMETRIC TITRATION: Easy-to-learn and carry out, and is completely supported by

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 � Robust technique for routine work � Highly suitable for aggressive media �

AUTOMATED THERMOMETRIC TITRATION SYSTEM Schematic of the relationship between components in automated thermometric titration

AUTOMATED THERMOMETRIC TITRATION SYSTEM Schematic of the relationship between components in automated thermometric titration system. A = dosing device B = thermometric sensor C = stirring device D = thermometric titration interface module E = computer

TYPES OF THERMOMETRIC TITRATION � Applications for thermometric titrimetry are drawn from the major

TYPES OF THERMOMETRIC TITRATION � Applications for thermometric titrimetry are drawn from the major groupings, namely: � Acid-base titration � Redox titration � Precipitation titration � Complexometric titration

ADVANTAGES Because the sensor does not interact electrically or electrochemically with the solution, electrical

ADVANTAGES Because the sensor does not interact electrically or electrochemically with the solution, electrical conductance of the titrating medium is not a prerequisite for a determination. � Titrations may be carried out in completely nonconducting, non-polar media if required. � Further, titrations may be carried out in turbid solutions or even suspensions of solids, and titrations where precipitates are reaction products can be contemplated. � The range of possible thermometric titration applications are many more �

MORE APPLICATION (EXAMPLES) � Acid-base titrations � Determination of fully dissociated acids and bases

MORE APPLICATION (EXAMPLES) � Acid-base titrations � Determination of fully dissociated acids and bases � Titration of weak acids � Titration of acid mixtures � Titration of complex alkaline solutions � Non-aqueous acid-base titrations � Catalyzed endpoint thermometric acid-base titrations

MORE APPLICATION (EXAMPLES) � Acid-base titrations (Na. OH vs HCl) • The heat of

MORE APPLICATION (EXAMPLES) � Acid-base titrations (Na. OH vs HCl) • The heat of neutralization: 56 k. J/mol. • The reaction is strongly exothermic. Titration of Na. OH with 1 mol/L HCl

MORE APPLICATION (EXAMPLES) � Titration of weak acids • Weakly dissociated acids yield sharp

MORE APPLICATION (EXAMPLES) � Titration of weak acids • Weakly dissociated acids yield sharp thermometric endpoints when titrated with a strong base. • For instance, bicarbonate can be unequivocally determined in the company of carbonate by titrating with hydroxyl (Δ 0 Hr=40. 9 k. J/mol). Titration of bicarbonate in company with carbonate by 1 mol/L Na. OH

MORE APPLICATION (EXAMPLES) � Titration of acid mixtures • Mixtures of complex acids can

MORE APPLICATION (EXAMPLES) � Titration of acid mixtures • Mixtures of complex acids can be resolved by thermometric titration with standard Na. OH in aqueous solution. • In a mixture of nitric, acetic and phosphoric acids used in the fabrication of semi-conductors, three endpoints could be predicted on the basis of the dissociation constants of the acids: Titration of a mixture of nitric, acetic and phosphoric acid with 2 mol/L Na. OH

MORE APPLICATION (EXAMPLES) � Titration of acid mixtures Titration of a mixture of nitric,

MORE APPLICATION (EXAMPLES) � Titration of acid mixtures Titration of a mixture of nitric, acetic and phosphoric acid with 2 mol/L Na. OH Endpoint 1 Endpoint 2 Endpoint 3 HNO 3 HOAc HNO 3 (p. Ka = -1. 3) (p. Ka = 4. 75) (p. Ka = -1. 3) H 3 PO 4 (p. Ka 1 = 2. 12) (p. Ka 2 = 7. 21) (p. Ka 3 = 12. 36)

MORE APPLICATION (EXAMPLES) � Redox titrations � Titrations with permanganate and dichromate � Titrations

MORE APPLICATION (EXAMPLES) � Redox titrations � Titrations with permanganate and dichromate � Titrations with thiosulfate � Titrations with hypochlorite

MORE APPLICATION (EXAMPLES) � Complexometric (EDTA) titrations EDTA titration of calcium and magnesium in

MORE APPLICATION (EXAMPLES) � Complexometric (EDTA) titrations EDTA titration of calcium and magnesium in sea water

MORE APPLICATION (EXAMPLES) � Complexometric (EDTA) titrations Titration plot of back-titration of excess EDTA

MORE APPLICATION (EXAMPLES) � Complexometric (EDTA) titrations Titration plot of back-titration of excess EDTA with Cu(II) in NH 3/NH 4 Cl buffered solution

MORE APPLICATION (EXAMPLES) � Complexometric (EDTA) titrations Thermometric EDTA titration determination of trace Cu(II)

MORE APPLICATION (EXAMPLES) � Complexometric (EDTA) titrations Thermometric EDTA titration determination of trace Cu(II) by Mn(II) catalysis of exothermic reaction between hydrogen peroxide and polyhydric phenol.

MORE APPLICATION (EXAMPLES) � Precipitation � Titrations titrations with silver nitrate � Titration of

MORE APPLICATION (EXAMPLES) � Precipitation � Titrations titrations with silver nitrate � Titration of sulfate � Titration of aluminium with fluoride � Titration of total orthophosphate � Titration of nickel � Titration of anionic and cationic surfactants � Titration of non-ionic surfactants

MORE APPLICATION (EXAMPLES) � Miscellaneous � Titration aqueous titrations of fluoride with boric acid

MORE APPLICATION (EXAMPLES) � Miscellaneous � Titration aqueous titrations of fluoride with boric acid � Determination of formaldehyde

THANKS

THANKS