Thermometric enthalpy titration

Kelly and Leyden10 studied the interaction of APTS with silica gel by thermometric enthalpy titration. This technique provides information regarding kinetic and thermodynamic parameters, which govern the reactivity of immobilized functional groups. They found that 26% of the APTS molecules were irreversibly bonded to the silica surface and attributed this stability to ionic interactions. Both values are equal within experimental error. 

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

Figure 6. Thermometric enthalpy titration of HMB with sulfide 27 mL of 3.60mM HMB titrated with 0.123M Sz EP, endpoint. Note a second poorly defined endpoint (not shown) is apparent at an approximate stoichiometry of 1 HMB per sulfide.

Figure 7 shows pH dependent eleetrokinetic profiles in 7.5 mM NaCI for APS-coated quartz. The APS surface is best described by a two acid-single base model of the surface. The acid sites appear to be silanols of the same pK l as for clean glass, and the basic site an APS amine of pKb I0, which is consistent with a value obtained from thermometric enthalpy titration 24 ... 

Jespersen, N.D., and Jordan, J. Thermometric enthalpy titration of proteins. Anal Lett. 3, 323-334 (1970). 

Table 17.4. Reaction Types Amenable to Thermometric Enthalpy 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 zinc-cyanide complexes are on the interface of organic and inorganic chemistry. We include them here because of their Zn—C bond. (Perhaps this will encourage thermo-chemical investigations on the isoelectronic alkynylzinc species, much as there has been interest in both cyano and alkynyl silver-containing species. ) The species Zn(CN)2, Zn(CN)3 and Zn(CN)4 have all been thermochemically characterized in aqneons soln-tion by thermometric titration. The enthalpy of reaction values kJmoL are shown for reactions 9-11. 

Psing a method of enthalpy dependent thermometric titrations , Jordan and Dumbaugh have measured the heats of ionization, at 26 C, of a number of acids. The heats and derived entropies of ionization of the halo-acetic acids are shown in Table 5. 

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