Titration thermometric

Chemical reactions take place spontaneously if accompanied by negative AG but, depending on the sign of AS , the reaction may be exothermic i.e. accompanied by negative AH or endothermic. In this case AH is positive, since 

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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). 

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... 

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. 

Smith, R. L. Popham, R. E. The Quantitative Resolution of a Mixture of Group II Metal Ions by Thermometric Titration with EDTA, /. Chem. Educ. 1983, 60, 1076-1077. 

The sodium hydroxide is titrated with HCl. In a thermometric titration (92), the sibcate solution is treated first with hydrochloric acid to measure Na20 and then with hydrofluoric acid to determine precipitated Si02. Lower sibca concentrations are measured with the sibcomolybdate colorimetric method or instmmental techniques. X-ray fluorescence, atomic absorption and plasma emission spectroscopies, ion-selective electrodes, and ion chromatography are utilized to detect principal components as weU as trace cationic and anionic impurities. Eourier transform infrared, ft-nmr, laser Raman, and x-ray... 

Thermometric titrations based on the measurement of the heat evolved from the reaction with the titrant have also been used [239,240]. 

Twenty weakly acidic drugs, including niclosamide, were determined by a nonaqueous catalytic thermometric titration method. Catalysis of the anionic polymerization of acetonitrile was used for endpoint indication. The solvent used was a mixture of acetonitrile and dimethylformamide or pyridine, and the titrant was sodium methoxide, potassium hydroxide, tertiary butanol, or tertiary butanol-sodium nitrite. Recoveries, limits of detection and relative standard deviations were tabulated [31]. 

Barthel, J. "Thermometric Titrations" Wiley New York, 1975 Chap. 2. 

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. 

As the dielectric constant of a solvent exerts little effect on the thermometric titrations, the latter may be employed effectively in most non-aqueous media. 

A standard thermometric titration assembly essentially consists of the following four vital components, namely ... 

Fiaure 11.5, represents the schematic thermometric titration assembly complete with a bridge-circuit. To minimise heat transfer losses from the solution by its immediate surroundings, the thermometric titrations are usually carried out in an isolated-beaker tightly closed with a stopper having provision for a burette-tip, a motorized-glass stirrer, and a temperature-monitoring arrangement. 

Figure 11.5 Schematic Thermometric Titration Assembly Complete with a Bridge-Circuit (After Linde et aL, Anal Chem. 25, 494,1953).

J) In the course of a thermometric titration, the thermister attached to the insulated-beaker is connected to one arm of the Wheatstone Bridge as displayed in Figure 11.5. The values of the circuit component listed are for a thermister having an approximate resistance of 2 KI2 and a sensitivity of 0.04 Q/Q/°C in the 25°C temperature range. Hence, an observed change of 1°C = an unbalanced potential of 15.7 mV, and... 

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. 

In usual practice it has been observed that thermometric titrations are mostly feasible with such systems that provide rates of temperature change more than 0.01°C/second. 

Various important applications of thermometric titrations are enumerated below ... 

Materials Required Thermometric titration assembly as per Figure 11.5, minus the burette a stopwatch or timer standard nitrating acid mixture [mix 2 volumes of 70% HN03 (d = 1.41) with 1 volume of 95% H2S04 (<7= 1.82)] Bakelite screw-cap bottle (4 oz. capacity) 2. 

How would your differentiate thermometric titrations from TGA and DTA Explain. 

Describe a standard thermometric titration assembly comprising of the following FOUR important components and explain its working ... 

The design and operation of solution calorimeters is an extensive topic. Reference (125) reviews modem calorimetry and identifies earlier discussions. The thermometric titration type of calorimeter has been perfected during the past fifteen or twenty years. It is especially useful for measuring heats of reaction that take place in several steps. The availability of advances in thermometry has had a major effect on calorimetry. 

Concerning the nature of the acid sites on dehydrated alumina, the chemical evidence is more in favor of Lewis-type than of Bronsted-type acids. Trambouze and Perrin (343) estimated the content of Lewis acid sites by thermometric titration of a slurry in benzene with dioxane. The authors stated that Lew is acidity was not observed in boehmite and its dehydration products, only in the products obtained from hydrargillite. As mentioned earlier, Webb (339) found no indication of Bronsted acidity ammonia did not form ammonium salts. The quantity of ammonia chemisoi-bed per unit area in the range from 175° to 500° was not a function of the hydrogen content. After exten.sive dehydration, more ammonia was chemisorbed at 100 mm pressure, even at 500°, than corresponded to the hydrogen present (341b). 

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