Pycnometric technique

The volume of the solid phase Vp is usually measured by a pycnometric technique, which measures the excluded volume of a pycnometric fluid, whose molecules cannot penetrate the solid phase of PS. A simple example of a pycnometric fluid is helium [55], The pycnometric fluid fill in all void space (pores) accessible to it, and presumably do not adsorb on the surface of PS. In the case of microporous PSs, measurement of the volume accessible for guests with various sizes allows the determination of a distribution of micropores volume vs. the characteristic size of guest molecules. This approach lays the basis of the method of molecular probes. The essence of this method is in the following we have a series of probe molecules with different mean sizes (dl>d2>d3>---). The pycnometric measurements of the excluded volume will give a series The difference A V=Vpi-Vpi(i>j) corresponds to the volume of micropores with pycnometric sizes of d in a range dt[Pg.283]

Thus, there are two limitations of the pycnometric technique mentioned possible adsorption of guest molecules and a molecular sieving effect. It is noteworthy that some PSs, e.g., with a core-shell structure, can include some void volume that can be inaccessible to the guest molecules. In this case, the measured excluded volume will be the sum of the true volume of the solid phase and the volume of inaccessible pores. One should not absolutely equalize the true density and the density measured by a pycnometric technique (the pycnometric density) because of the three factors mentioned earlier. Conventionally, presenting the results of measurements one should define the conditions of a pycnometric experiment (at least the type of guest and temperature). For example, the definition p shows that the density was measured at 298 K using helium as a probe gas. Unfortunately, use of He as a pycnometric fluid is not a panacea since adsorption of He cannot be absolutely excluded by some PSs (e.g., carbons) even at 293 K (see van der Plas in Ref. [2]). Nevertheless, in most practically important cases the values of the true and pycnometric densities are very close [2,7],... 

The diameter and volume of the micropores were also determined by the measurement of the density using as displacement molecules with different sizes, e g., helium, water, benzene, decaline. It was found that in the case of CMS-Kl and CMS-K2 sieves, the micropores with the pore size within the range 0.255-0.528 are dominated and that the used measurements enable characterisation of the structure of carbon molecular sieves. For equilibrium sieve the analysis of the micropores volume with the use of the pycnometric technique does not give proper results. 

For description of textural properties of carbonaceous adsorbents, adsorption/desorption isotherms of vapours and gases in static conditions as well as mercury porosimetry are used. The latter method often leads to destruction of porous structure of investigated materials while the usage of the former one is affected by the specific properties of molecular sieves described above. Taking into account these limitations, in this work the authors have made an attempt of determination of porous structure of carbon molecular sieves with the used of the pycnometric technique. 

The pycnometric technique makes possible the calculation of the pore volume with any size (Tab. 2), but remarkable values were obtained only for kinetic sieves. Hence, this analysis does not give proper results for equilibrium carbon molecular sieves. 

The two methods, most commonly used to measure gravimetric density, are pycnometric and flotation. In a pycnometric technique, the volume of the material is determined from the volume of a fluid displaced by the known mass of a material. In a flotation approach, a small particle, whieh can be a single crystal, is placed in a low-density fluid, where it... 

Because the excess volume for this system was expected to be small (10), and any effect is second order (15), only a few determinations were deemed necessary. A standard pycnometric technique was used (16). The densities at 29°C of three samples of known mole fraction, in addition to the pure components, were found. The excess volumes calculated from these results are shown as Table I. For the accuracy required by the calculations, a value for VE of 1.0 cm3 mol 1, independent of composition over the range of x considered, was sufficient. The effect of error in VE is discussed later. Differentiation of Equation 12 then leads to... 

Feldman and Beaudoin correlated strength and modulus of elasticity for several systems over a wide range of porosities. The systems included pastes hydrated at room temperature, autoclaved cement paste with and without additions of fly ash, and those obtained by other workers. Porosity was obtained by measurement of solid volume by a helium pycnometric technique and apparent volume, through the application of Archimedes principle. 

The crystal structure of cadmium rhenium(V) oxide, as determined by single-crystal technique,1 is of the face-centered cubic pyrochlore type (a = 10.219 A.). The only positional parameter for the 48 (/) oxygens is x = 0.309 0.007 when rhenium is at the origin. The density, determined pycnometrically, is 8.82 0.03 g./cc., compared with the theoretical value of 8.83 g./cc. for Z = 8. The resistivity between 4.2 K and room temperature is very low (10-3-10-4 J2-cm.) and has a positive temperature coefficient. Over the same temperature range the magnetic susceptibility is low and temperature-independent. These properties indicate that cadmium rhenium(V) oxide exhibits metallic conductivity. 

With X-ray fluorescence stoichiometry can be determined to 0.01-0.1 % when suitable standards are available. Precise lattice parameters and pycnometric density determinations have been used to determine deviations from stoichiometry or nonstoichiometry in Bertholide-type compounds. Other instrumental techniques such as nuclear magnetic resonance and comparison of ferroelectric Curie temperatures have also found applications. ... 

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