Mercury-helium method

A helium-mercury displacement method is sometimes used to check the pore volume obtained from the adsorption-desorption isotherm at the saturation pressure (Ries, Van Nordstrand, Johnson, and Bauer-meister, 50). The helium measurement which gives the solid volume and solid density is obtained by means of a miniature isotherm type apparatus. This part of the determination is essentially the same as that described by Smith and Howard (58) and Schumb and Rittner (55). Mercury displacement yields the pellet volume and pellet density and is measured volumetrically by means of a mercury buret attached directly to the same sample bulb in which the helium measurement is performed. The difference between the pellet volume and the solid volume is obviously the pore volume. 

It is clear that the permeability of material to water depends mainly on the volume of capillary pores. It is not linked with total porosity but depends rather on the pore size distribution. However, Marsh [133] gives attention to a fairly good correlation between the total porosity, determined by means of helium method, and permeability. This relation is shown inFig. 5.60 [133]. The diying of samples by substitution of water with propan-2-ol and -pentane was applied. Porosity and pore size distribution determined with mercuiy porosimeter do not correspond to the real stmcture, because the intmsion of mercury under a high pressure, causes the destmction of hydrates plugging the pores, particularly in the pastes of cements with fly ash. 

A much more accurate method of determining the pore volume of a catalyst sample is the helium-mercury method. One places a known weight of catalyst (IT) in a chamber of known volume. After the chamber has been evacuated, a known quantity of helium is admitted. From the gas laws and measurements of the temperature and pressure, one may then proceed to... 

Helium and mercury densities were determined on the 6-8 mesh fraction. The larger mesh size was used to avoid the possibility that mercury would not penetrate the space between particles in the mercury density measurements. The coal was placed in a calibrated density tube, evacuated at room temperature for one hour, and then heated at 100°C. in vacuo for 2 hours. The weight of the coal after this treatment was used to compute the densities. Helium densities were determined at 30°C. by the method of Rossman and Smith (11). Mercury densities were determined by admitting mercury at an absolute pressure of 1140 torr to the coal sample after evacuation, following the helium density measurement. 

The true density of coal is usually determined by helium displacement and therefore is often referred to as the helium density. Helium is used because it has the ability to penetrate all the pores of a given sample of coal without (presumably) any chemical interaction. In the direct-pressure method, a known quantity of helium and a weighed sample of coal are introduced into an apparatus of known volume, whereupon the pressure of the helium at a given temperature allows calculation of the volume of the coal. In the indirect method, mercury is used to compensate for the helium displaced by the introduction of the coal. 

Methods of measurement of coal density include use of a gas pycnometer and particle density by mercury porosimetry. However, the difference in density values using different gases must be recognized since, for example, density values measured by nitrogen may be greater than those obtained when helium is used. Density measurement depends on adsorption of gas molecules, and differences (between nitrogen and helium) may be due to nitrogen adsorption on the coal surface. 

Another method of determining porosity involves measuring the density of coal by helium displacement and by mercury displacement (see Section 6.1). Thus, the porosity of coal is calculated from the relationship... 

For porous materials pp < Pabs and cannot be measured with such methods. A mercury porosimeter can be used to measure the density of coarse porous solids but is not reliable for fine materials, since the mercury cannot penetrate the voids between small particles. In this case, helium is used to obtain a more accurate value of the particle density. Methods to measure the particle density of porous solids can be found in Refs. 2 and 5. 

Calibration methods. Wavelength calibration is achieved with spectral lamps, e.g. mercury (Fig. 2) or helium. The separation of mercury lines 2 nm apart is readily achievable. Calibration in intensity (relative) is done using an incandescent source of known temperature. 

Surface area Porosity (pore size, volume, and distribution) BET method (Brunauer, Emmett, and Teller method) Physical gas sorption Chemical gas sorption Helium picnometry Mercury intrusion porometry (MIP)... 

Nitrogen physisorption methods for total surface area (BET), and more recently macropore surface area determination (t-plot) are used to quantify relationships of the amount and type (zeolite, matrix) of surface present. Nitrogen and mercury pore size distribution (NPSD HGPSD) are used to determine sizes of pores within the catalyst. Bulk, particle, and skeletal densities can be measured with standard volumetric apparatus or more recently with sophisticated pychnometers using helium as a fill gas. 

A comparison of true particle density, apparent particle density, and bulk density can provide information on total porosity, interparticle porosity, and intraparticle porosity. Methods include true particle density measurements via helium pycnometry, mercury intrusion porosimetry, and poured and tapped bulk density. 

A more accurate procedure is the helium-mercury method. The volume of helium displaced by a sample of catalyst is measured then the helium is removed, and the volume of mercury displaced is measured. Since mercury will not fill the pores of most catalysts at atmospheric pressure, the difference in volumes gives the pore volume of the catalyst sample. The volume of helium displaced is a measure of the volume occupied by the solid material. From this and the weight of the sample, the density of the solid phase, P5, can be obtained. Then the void fraction, or porosity, of the particle, p, may be calculated from the equation... 

The first procedure for the separation of As(lll), As (V). MMA and DMA in naturai waters and urine depended on pH-selective reduction reactions with sodium borohydride and separation of the voiatiie arsines produced by selective voiatilization from a cold trap. Quantification was performed by atomic emission spectrometry (Braman and Foreback, 1973). This method was iater modified on the basis of another approach for arsenic speciation in natural waters (Andreae, 1977) 0.5 to 5 mL of unpretreated urine is diluted with 50 mL of deionized water in the reaction chamber of a mercury-hydride system. The pH is then adjusted to 1-1.5 by addition of 5 mL of oxalic acid (10% w/v). Helium is subsequently passed through the system for about 1 min to remove the oxygen. After this 6mL of sodium borohydride (4% in 0.05 M NaOH) is added. The generated arsines are trapped in a U-tube immersed in liquid nitrogen. When the U-tube is removed from the liquid nitrogen, arsines of inorganic arsenic, MMA and DMA are volatilized successively due to their different boiling points and travel via the carrier gas into the AAS detector (Norin and Vahter, 1981). 

A much more accurate method of determining the pore volnme of a catalyst sample is the helium-mercury method. One places a known weight of catalyst (W) in a chamber of known volume. After the chamber has been evacnated, a known quantity of helium is admitted. From the gas laws and measurements of the temperature and pressure, one may then proceed to determine the volume occupied by the helium (Vne)- This volume is equal to the sum of the volume exterior to the pellets proper and the void volume within the pellets (Vv,jj<,). The helium is then pumped out and the chamber is filled with mercury at atmospheric pressure. Since the mercury will not penetrate the pores of most catalysts at atmospheric pressure, the mercury will occupy only the volume exterior to the pellets proper (Eng)- Hence,... 

The porosity and pore stmcture can be also determined with mercury porosim-etry, as well as with helium flow method proposed by Feldman [48]. However, a more adequate method of the total porosity measmement is helium pycnometry [57]. The applicability of the mercury intmsion porosimetry covers the pore diameters from 3 nm to about 10 nm, while the capillary condensation— from 4 to 50 nm respectively. 

Although more labor-intensive and less efficient, information on the densification kinetics and densification can also be obtained from density measurements on different individual samples as a function of time for otherwise identical sintering conditions. Bulk density measurements on <92% dense sintered samples containing open porosity can be determined from the measured mass and dimensions of the compact, while Archimedes method works well for closed pore, >92% dense bodies. The density of closed pore samples can also be determined by pycnome-try (e.g., helium pycnometry),, 37 mercury porosimetry, and by the sink-float method (i.e., whereby the buoyancy of the sample is assessed and compared in different density liquids). Density can also be estimated from micrographs using quantitative stereology. 

Another useful technique is mercury pycnometry which can be used to determine the geometrical volume of the dry gel (pores and solid), hence giving complementary data to helium pycnometry which measures only the pore volume. In this characterization method, a volume-calibrated chamber containing the sample is filled with mercury the mercury volume (or weight) is measured to compute the sample volume. Unlike mercury porosimetry, no pressure is exerted so that mercury neither enters the pores nor crushes the sample. 

Two standard methods (mercury porosimetry and helium pycnometry) together with liquid expulsion permporometry (that takes into account only flow-through pores) were used for determination of textural properties. Pore structure characteristics relevant to transport processes were evaluated fiom multicomponent gas counter-current difhision and gas permeation. For data analysis the Mean Transport-Pore Model (MTPM) based on Maxwell-Stefan diffusion equation and a simplified form of the Weber permeation equation was used. 

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