Density pycnometry

Density. Measurement of the density of water by pycnometry is the classical method (30) for estabHshing deuterium concentrations in heavy water. Very precise measurements can be made by this method, provided the sample is prepared free of suspended or dissolved impurities and the concentration of oxygen-18 in water is about 0.2 mol %. However, in nearly all heavy water manufactured since 1950 in the United States, the... 

The three most common ways of obtaining true density measurements are gas pycnometry (gas displacement), liquid displacement, and flotation in a liquid. These three techniques have been compared based on accuracy, ease of use, and instrumentation [63], and the results are summarized in Table 4. Gas pycnometry will be discussed in this section because of its wide use and ease of operation. 

Flotation in liquid Simple operation Inexpensive Demonstrates density distribution Accurate to 4 significant figures More time consuming than gas pycnometry Density vs. temperature curve suggested for each liquid Sample should be insoluble in liquid... 

In pycnometry, the sample is weighted in a calibrated pycnometer before and after this is filled with a liquid of known density. The volume of the sample and thus its mass is determined. Powders may be used, and it is often advantageous to... 

Plrue true density of the particle (usually determined by helium pycnometry)... 

A plot of density versus pore radius, from the data in Table 21.2, is shown in Fig. 21.3. The horizontal line indicates the true density obtained by helium pycnometry. This higher density by gas displacement reflects the volume of pores smaller than about 18 A. 

Powders are porous materials and their bulk and relative densities can change with consolidation (6). However, a powder s true density is the density of its solid phase only and thus is independent of the state of consolidation. The true density of organic excipients typically ranges from 1.0 to 1.6g/cm3 while inorganic excipients (e.g., calcium phosphate) show values greater than 2g/cm3. True density is used to determine powder or compact solid fraction (SF) (see below) and it may be a consideration when selecting excipients if segregation is a concern. True density is often determined by gas pycnometry. 

In addition to lactometry (determination of the extent to which a hydrometer sinks), the density of milk can be measured by pycnometry (determination of the mass of a given volume of milk), by hydrostatic weighing of an immersed bulb (e.g. Westphal balance), by dialatometry (measurement of the volume of a known mass of milk) or by measuring the distance that a drop of milk falls through a density gradient column. 

Pigment Density. Density is determined by pycnometry at a standard temperature of 25°C. For standards, see Table 1 ( Density ). Apparatus pycnometer, vacuum pump, or centrifuge. 

Sample Sample code Apparent density (Buoyancy method) (gem 3) Apparent porosity (%) Theoretical porosity (%) True density (Helium pycnometry) (gem 3)... 

The indirect determination of the buoyancy is obtained by the assessment of the sample volume from its density or by pycnometry - as in the previous section and with the same implications for the location of the Gibbs dividing surface. 

Helium is often used in adsorption manometry for the determination of the dead space volume (see Chapter 3), but this procedure is based on the presupposition that the gas is not adsorbed at ambient temperature and that it does not penetrate into regions of the adsorbent structure which are inaccessible to the adsorptive molecules. In fact, with some microporous adsorbents, significant amounts of helium adsorption can be detected at temperatures well above the normal boiling point (4.2 K). For this reason, the apparent density (or so-called true density ) determined by helium pycnometry (Rouquerol et al., 1994) is sometimes dependent on the operational temperature and pressure (Fulconis, 1996). 

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. 

In this model, the buoyant mass is then the sum of the buoyant mass of the three components, assuming that these are independent of the mass of solvent occupied in the solvation shell. Thus, the mass of the adsorbed shell can be calculated if information about the mass and density of the core particle and the density of the macromolecule and solvent are known. Photon correlation spectroscopy, electron microscopy, flow FFF, or other sizing techniques can readily provide some independent information on the physical or hydrodynamic particle size, and pycnometry can be used to measure the densities of the colloidal suspension, polymer solution, and pure liquid. 

The bulk densities were calculated from weight and volume measurements. Skeletal densities were measured by He pycnometry N2 adsorption-desorption isotherms were determined at 77 K on a Carlo Erba Sorptomatic 1900 and their analysis was done using a set of well-known techniques [5], Mercury porosimetry up to a pressure of 200 MPa is performed on a Carlo Erba Porosimeter 2000. Samples were examined using a transmission electron microscope to obtain particle and aggregate sizes [2]. 

The application of eq. (1) makes only sense if V p has been completely detected. To check the measured values, the expected pore volumes caic were calculated with the aid of the macroscopic density p of each sample and the skeletal density determined by He-pycnometry according to... 

He-pycnometry yields an average skeletal density of = (2020 10)kgm for Vacac-doped silica gels with no dependence of the skeletal density on V/Si ratio or hydrolysis conditions under consideration [13]. From SAXS, an average value of... 

Vanadia-doped sihca aerogels with densities between 200 kg m and 400 kg m were investigated via helium pycnometry, nitrogen adsorption and small angle X-ray scattering. While evaluating the scattered intensity with the two-phase-media model, the... 

The bulk density of materials was measured by Hg pycnometry from independent measurements of the mass and the volume of monolithic samples. The geometrical volume of the sample is determined fi om the weight difiference between a flask (calibrated volume) filled up with mercmy and the same flask filled up with the sample and mercury. As mercury is a non-wetting liquid and as no pressure is exerted, mercury does not enter in the porosity of the sample or crush it. 

Bulk density is easily measured from the volume occupied by the bulk solid and is a strong function of sample preparation. True density is measured by standard techniques using liquid or gas pycnometry. Apparent (agglomerate) density is difficult to measure directly. Hink-ley et al. [Int. J. Min. Proc., 41, 53-69 (1994)] describe a method for measuring the apparent density of wet granules by kerosene displacement. Agglomerate density may also be inferred from direct measurement of true density and porosity by using Eq. (21-96). 

The true density of the talc samples was measured using helium pycnometry, and the data is presented in Table IV. The true densities were similar for the three lots and the minor differences can be attributed to die impurities in the lots. 

Some credence was given the latter of these alternatives by the observation that densities measured by displacement of helium in a gas pycnometry cell were essentially identical for the highest and lowest surface area products. The difference was less than 2Z. It is suggested that an intrinsically amorphous structure should have a range of void spaces due to inefficient filling of its volume, the net result of which should be a significantly diminished density compared to the highly crystalline form. Thus, we favor the view that the more amorphous products are composed of smaller, but essentially ordered, particles. 

The true density of a solid is the average mass per unit volume, exclusive of all voids that are not a fundamental part of the molecular packing arrangement. This density parameter is normally measured by helium pycnometry, where the volume occupied by a known mass of powder is determined by measuring the volume of gas displaced by the powder. The true density of a solid is an intrinsic property characteristic of the analyte and is determined by the composition of the unit cell. 

Gas pycnometry is probably the most commonly used method in the pharmaceutical industry for measuring true density. Gas pycnometers rely on the measurement of pressure changes, as a reference volume of gas, typically helium, added to, or deleted from, the test cell. 

The porous texture of the dried gels and the pyrolyzed gels was characterised by the analysis of nitrogen adsorption-desorption isotherms, performed at 77 K. The analysis of the isotherms was performed according to the methodology proposed by Lecloux [19]. Samples bulk density was obtained by mercury pycnometry. Infrared and X-ray spectra analysis allowed to obtain data about the elementary composition of the samples and the aggregation state of the metals. 

Using simple, classical pycnometry, Etzler and Fagundus (1983) have measured the density of water and some other liquids inside the pores of a silica gel with an average pore diameter of 140 A. With acetone and methanol, the observed densities were close to the values reported for bulk liquids however, for water a value of 0.966 g/cm was obtained (compared to a bulk value of 0.997 g/cm at 25°C), i.e. 3% lower than the bulk value. 

For liquids at ambient conditions, one can obtain a reasonably accurate density simply by weighing a known volume of the liquid. For rough work, one can use something as simple as a volumetric flask for more precise work, calibrated volumes (known as pycnometers) are used. One can easily obtain 1% accuracy, and careful pycnometry can obtain 0.1%. 

The true density is thus a property of the material and is independent of the method of determination. In this respect, the determination of the true density can be determined using three methods displacement of a liquid, displacement of a gas (pycnometry) or floatation in a liquid. These methods of measuring true density have been evaluated by Duncan-Hewitt and Grant (1986). They concluded that, whereas liquid displacement was tedious and tended to underestimate the true density, displacement of a gas was accurate but needed relatively expensive instrumentation. As an alternative, the floatation method was found to be simple to use and inexpensive. Although more time consuming than gas pycnometry, it was accurate using relatively simple instrumentation. 

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