Titration curve thermometric

The end point is indicated by the intersection of the titration branch and the excess titrant branch. In the idealized thermometric titration curve (see Figure 9.14a) the end point is easily located. When the intersection between the two branches is curved, the end point can be found by extrapolation (Figure 9.14b). 

A quantitative analysis for NH3 in several household cleaning products is carried out by titrating with a standard solution of HGl. The titration s progress is followed thermometrically by monitoring the temperature of the titration mixture as a function of the volume of added titrant. Household cleaning products may contain other basic components, such as sodium citrate or sodium carbonate, that will also be titrated by HGl. By comparing titration curves for prepared samples of NH3 to titration curves for the samples, it is possible to determine that portion of the thermometric titration curve due to the neutralization of NH3. 

Thermometric titration curves usually represent both the entropy and the free energy involved. The titrant is added to the solution at a constant rate in order that the voltage output of the thermister-temperature-transducer changes linearly with time upto the equivalence point. 

Figure 2. Thermometric titration curves for neutralization of (%) Williamson River humic substances and (O) Satilla River humic substances with NaOH...

Figure 17.13 represents an idealized thermometric titration curve. Region 1 is the baseline. Ideally horizontal, in practice is has a finite slope as a result of frictional heat added by stirring, resistive heat added by the thermistor (Eqn. 17.12), and the transfer of heat from the cell to the thermostat (Eqn. 17.14). If frictional and resistance heating are constant and equal to W, then the slope in Region 1 is... 

It is also possible to titrate biochemical species. Antibodies have been titrated with antigen, and enzyme-substrate mixtures have been titrated with appropriate coenzymes. Proteins are readily titrated with acid or base, or precipitated with phosphotungstic acid, yielding very informative thermometric titration curves [7, 8]. 

The heat of ionization of tr/i-(hydroxy-methyl)aminomethane (TRIS) is +11.45 kcal/mole. Sketch the thermometric titration curve of 50.0 ml of 0.01 M TRIS with 1 M HCl. Sketch the titration curve of the resulting cation with 1 M NaOH. Estimate the temperature change for each titration. 

Although not commonly used, thermometric titrations have one distinct advantage over methods based on the direct or indirect monitoring of plT. As discussed earlier, visual indicators and potentiometric titration curves are limited by the magnitude of the relevant equilibrium constants. For example, the titration of boric acid, ITaBOa, for which is 5.8 X 10 °, yields a poorly defined equivalence point (Figure 9.15a). The enthalpy of neutralization for boric acid with NaOlT, however, is only 23% less than that for a strong acid (-42.7 kj/mol... 

The thermometric titrations (TT) make use of heats of reaction to obtain titration curves. In usual practice, the temperature of solution is plotted against the volume of titrant. TT is performed by allowing the titrant to flow from a thermostated-burette directly into a solution contained in a thermally-insulated vessel, and subsequently the observed change in temperature of the solution is recorded precisely either during continuous addition of titrant or after every successive incremental addition. The end-point is aptly indicated by a sharp break in the curve. 

TITRATION (Thermometric). This technique consists of the detection and measurement of the change in temperature of a solution as the titrant is added to it, under as near adiabatic conditions as possible. Experimentally, the titrant is added from a constant-delivery burette into die titrate (solution to be titrated) which is contained in an insulated container such as a Dewar flask. The resultant temperature-volume (or time) curve thus obtained is similar to odier titration curves, e.g., acid-base, in that the end point of the reaction can be readily ascertained. Since all reactions involve a detectable... 

A typical thermometric enthalpy titration curve is shown in Figure 6. The well defined endpoint provided a convenient method for quantitating sulfidic sulfur. 

Thermometric methods (not to be confused with thermometric titrations) in which temperature-time curves are obtained at various intervals. Heat evolution or absorption occurs continuously and preferably at a constant rate. The amount of heat supplied per unit time is not measured directly but may be calculated as a fraction of the total heat of melting of the substance. 

Figure 17.11. Characteristic curves obtained from thermometric titration and direct-injection enthalpimetry.

Figure 16.31 Representative theimometric titration plot. The endpoint is denoted by an abrupt change in the slope of the curve. The thermometric titration plot for HCl (a strong acid) with NaOH would look virtually identical to the thermometric titration plot for boric acid (a very weak acid) with NaOH because the AH of reaction is almost the same for both acids.

Unfortunately, the equivalence point of a titration reaction is the least quantitative one, as we demonstrated in the preceding section. This is why logarithmic titrations are the most sensitive to the extent of the titration reaction. This is not the case for linear titrations. Let s first recall that linear titrations are those in which the dependent variable (the registered one) is directly proportional to the fraction titrated or to the concentration of the independent variable (in no case to the logarithm of its concentration or of its activity). Of course, the independent variable may be the activity rather than the concentration. Examples of linear titrations are spectrophoto-metric, amperometric, thermometric titrations, and so forth. This is the reason why Gran s method is interesting. Indeed, its principle involves the linearization of the logarithmic titration curves. 

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