Experimental procedure dissolution measurements

Thermal properties are measured by some form of calorimetry, an exacting experimental procedure in which some kind of reaction is carried out, such as dissolution of a solid phase, and the heat (q) released or absorbed is measured. If the reaction occurred at constant pressure, the measured is a AH, and if not, it is fairly easily converted into a AH. Entropy can also be measured by calorimetry, though of a different type, and combining the enthalpy and entropy measurements gives AG numbers. Values of AG° can also be obtained by other methods, to be discussed in later chapters. All these quantities are related to the heat capacity, which turns out to be a very fundamental and important parameter. If pressure changes are important, then the volume or density is also required. 

The double-barrel SECCM platform allowed conductance measurements to be confined to microsized areas. The experimental procedure was to approach the pipette toward the crystal surface at a low speed until the meniscus at the end of the pipette wetted the substrate surface, without the pipette itself making contact. This event was readily detected as an abrupt change in conductance current (jump-to-contact) (Figure 19.15a), used to automatically stop the motion of the pipette and maintain meniscus contact with the crystal surface for a predetermined etch time. Dissolution of the crystal was promoted by the use of a greatly undersaturated solution (5 mM NaCl) in the pipette. The resulting ion dissolution flux from the crystal surface into the solution in the meniscus and pipette decreased the resistance between the two QRCEs and hence increased the current on a longer timescale (Figure 19.15b). 

FIGURE 19.15 Crystal dissolution studies with SECCM. (a) Schematic of the experimental procedure for microscale dissolution (i) pipette held in air (ii) dissolution occurs upon contact of the meniscus with the crystal for a defined short period (iii) tip is retracted, breaking meniscus contact with the surface, (b, left) Plot of barrel conductance current-time above a schematic of the corresponding vertical pipette position during a dissolution experiment, as described in (a), (b, right) Current-time plots of multiple repeat measurements of dissolution pits at different spots on an NaCl crystal, for different meniscus contact times (as short as 3 ms), showing consistency in the transients. (Adapted from Kiimear, S.L. et al., Langmuir, 29,15565,2014.)... 

Use of the enthalpy of solution experimentally measured by Chauvenet (assumed to be valid at 298.15 K), of Af// (Br , ca. 4OOOH2O, 298.15 K) = - (121.20 + 0.15) kJ-mof, and of the same calculation procedure as for the dissolution of the tetrachloride, leads to Af//° (ThBr4, cr, 298.15 K) = -(932.6 + 4.7) kJ-mol, without consideration for the uncertainty in the experimental value reported by Chauvenet. This value is very distinctly less negative than that resulting from better documented resnlts and is given for information only. Consequently, this review selects ... 

The mass transfer coefficients in the crossflow cell were estimated from independent measurements of dissolution of a plate of benzoic acid into water at two different crossflow rates 50 L h and 120 L h , at 30 °C. Mass transfer coefficients for docosane and TOABr were estimated based on the experimentally measured benzoic add mass transfer coefficients values and the Chilton-Colbum mass transfer coeffident correlation. Details of the procedure applied are described elsewhere [32]. 

The experimental development of a radiochemical separation can be supported by the use of radiotracers. The chemical separation is simulated step by step with addition of radiotracers (and inactive tracers also) for the element to be separated (B) and for the interfering element(s) (D), in order to check quantitativeness and selectivity, respectively. The tracer has to be brought to the same chemical form as the radionuclide to be separated. This is often not possible for the dissolution step preceding the actual separation procedure. Therefore, the dissolution procedure selected should guarantee quantitative recovery of the radionuclide to be measured. 

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