Surface area, particle size analysis

Analysis. Excellent reviews of phosphate analysis are available (28). SoHds characterization methods such as x-ray powder diffraction (xrd) and thermal gravimetric analysis (tga) are used for the identification of individual crystalline phosphates, either alone or in mixtures. These techniques, along with elemental analysis and phosphate species deterrnination, are used to identify unknown phosphates and their mixtures. Particle size analysis, surface area, microscopy, and other standard soHds characterizations are useful in relating soHds properties to performance. SoHd-state nmr is used with increasing frequency. 

Two general methods have evolved. One uses a stirred tank system, while the other depends on the measurement of particle size or surface area change via particle counting or image analysis techniques. 

Calculations of average particle size, specific surface area, or particle population of a mixture may be based on either a differential or a cumulative analysis. In principle, methods based on the cumulative analysis are more precise than those based on the differential analysis, since when the cumulative analysis is used, the assumption that all particles in a single fraction are equal in size is not needed. The accuracy of particle-size measurements, however, is rarely great enough to warrant the use of the cumulative analysis, and calculations are nearly always based on the differential analysis. 

Particle morphology Crystallographic Properties Thermal methods of analysis Particle size distribution Surface area Density... 

Carbon-black types are differentiated on particle size and surface area measurements, using techniques such as TEM, PCS (photon correlation spectroscopy), iodine or nitrogen adsorption and mercury porosimetry. The best method for qualitative carbon-black determination is based on accurate measurement of the CB particle size using TEM [222,223], Pyrolysis at 800 to 900 C followed by TEM analysis according to ASTM D 1765 allows identifying carbon-blacks used in rubber products [224]. TEM was used for identiflca-tion of carbon-blacks in vulcanisates [225], TGA has been used for quantitative determination of carbon-black [226,227]. Palla [51] used TEM for the characterisation of rubber components. Ultra-thin sections of rubbers suitable for TEM studies can be prepared by microtoming at low temperatures. 

Another important area of analytical chemistry, which receives some attention in this text, is the development of new methods for characterizing physical and chemical properties. Determinations of chemical structure, equilibrium constants, particle size, and surface structure are examples of a characterization analysis. 

Surface Area and Permeability or Porosity. Gas or solute adsorption is typicaUy used to evaluate surface area (74,75), and mercury porosimetry is used, ia coajuactioa with at least oae other particle-size analysis, eg, electron microscopy, to assess permeabUity (76). Experimental techniques and theoretical models have been developed to elucidate the nature and quantity of pores (74,77). These iaclude the kinetic approach to gas adsorptioa of Bmaauer, Emmett, and TeUer (78), known as the BET method and which is based on Langmuir s adsorption model (79), the potential theory of Polanyi (25,80) for gas adsorption, the experimental aspects of solute adsorption (25,81), and the principles of mercury porosimetry, based on the Young-Duprn expression (24,25). 

Particle-Size Analysis Methods for particle-size analysis are shown in Fig. 17-34, and examples of size-analysis methods are given in Table 17-1. More detailed information may be found in Lapple, Chem. Eng., 75( 11), 140 (1968) Lapple, Particle-Size Analysis, in Encyclopedia of Science and Technology, 5th ed., McGraw-Hill, New York, 1982 Cadle, The Measurement of Airborne Particles, Wiley, New York, 1975 Lowell, Introduction to Powder Surface Area, 2d ed., Wiley, New York, 1993 and Allen, Particle Size Measurement, 4th ed, Chapman and Hall, London, 1990. Particle-size distribution may be presented on either a frequency or a cumulative basis the various methods are discussed in... 

Materials. Synthetic hematite was obtained from J. T. Baker Chemical Company, Phillipsburg, NJ. Particle size analysis using a HIAC instrument (Montclair, CA) indicated the particles to be 80 percent (number) finer than 2 microns. Using nitrogen as the adsorbate, the B.E.T. specific surface area was found to be 9 square meters per gram. The point of zero charge, as obtained from electrophoretic measurements in the presence of indifferent electrolytes, occurred at pH 8.3. 

The self-emulsifying behaviour of a binary nonlonlc surfactant vegetable oil mixture has been shown to be dependant on both temperature and surfactant concentration. The quality of the resulting emulsions as assessed by particle size analysis showed that manipulation of these parameters can result In emulsion formulations of controlled droplet size and hence surface area. Such considerations are Important when the partition of lipophilic drugs Into aqueous phases and drug release rates are considered. 

Triplicate aliquots were taken for particle size analysis and two of those aliquots were mixed for BET surface area analysis results are in Table III. The nine samples were individually sieved for size distribution. A chi-squared test was performed on each triplicate set in order to check the apparent efficiency of composite mixing. For all three composite samples, there was a 90 percent probability that each of the three replicates from each composite sample came from the same population. The A and C samples were combined and evaluated for surface area by nitrogen adsorption (BET). The B samples were then subjected to scanning electron microscopy (SEM) analysis. 

A typical feed composition was 1000 g capsul, 2334 g deionized water and 200 g orange oil. The finished powders were stored in amber bottles at -25prior to accelerated storage study and relevant analyses. Particle Size Analysis. To ascertain the effect of atomizer voltage on the particle size, the particle size distributions of three powders were first determined. The Microtrac laser light particle size analyzer (Medallion Laboratories, Minneapolis, MN) was used in this study. The volume percent data over particle diameter ranging 2.8 p. to 176 jii was recorded. Mean value of the volume percent distribution and calculated surface area were also obtained. 

In essence, the test battery should include XRPD to characterize crystallinity of excipients, moisture analysis to confirm crystallinity and hydration state of excipients, bulk density to ensure reproducibility in the blending process, and particle size distribution to ensure consistent mixing and compaction of powder blends. Often three-point PSD limits are needed for excipients. Also, morphic forms of excipients should be clearly specified and controlled as changes may impact powder flow and compactibility of blends. XRPD, DSC, SEM, and FTIR spectroscopy techniques may often be applied to characterize and control polymorphic and hydrate composition critical to the function of the excipients. Additionally, moisture sorption studies, Raman mapping, surface area analysis, particle size analysis, and KF analysis may show whether excipients possess the desired polymorphic state and whether significant amounts of amorphous components are present. Together, these studies will ensure lotto-lot consistency in the physical properties that assure flow, compaction, minimal segregation, and compunction ability of excipients used in low-dose formulations. 

Further problems can arise because of uncertainties concerning the stoichiometry of the adsorption reaction. For most metals it is assumed that the surface stoichiometry with H2 is H/M = 1. However, there is evidence especially for very small metal particles (of the order of 1 -5 nm) that the stoichiometry can exceed H/M = 1. For quantitative measurements of surface area it is necessary to establish the chemisorption stoichiometry and structure. In practice it is usually possible to achieve approximate estimate of the surface area by some other independent method (for example, from particle size analysis by X-ray line broadening or by TEM). In the case of CO, the CO/M ratio is generally taken as 1.0, but the true value may depend on the particle size and on the particle morphology. With N2O the N2O/M ratio at monolayer coverage is usually assumed to be 0.5, but once again there is no certainty about the validity of this particular assumption. 

There are several reasons why nitrogen (at 77 K) is generally accepted as the most suitable adsorptive for mesopore size analysis. First, the thickness of the N2 multilayer is largely insensitive to differences in adsorbent particle size or surface structure (Carrott and Sing, 1989). Second, the same isotherm can be used for the evaluation of both the surface area and the mesopore size distribution (Sing et al. 1985). However, in spite of these considerations, there is an emerging view that ideally more than one adsorptive should be used for the characterization of meso-porous solids (e.g. see Machin and Murdey 1997 Llewellyn et al., 1997). 

An examination of the literature,10,21-24 authoritative guidances,6-9 and current industrial best practices, suggests that the analytical techniques in Table 1 be considered for the characterization of reference standards. Other techniques are occasionally employed but are not discussed here. These may include particle size analysis,25 nephelometry, heavy metals analysis,26 surface area,27 bulk density,28 pH,29 dissociation constants, microbiological testing30 and other spectroscopic measurements (e.g., NIR, fluorescence, CD, etc.). 

Since PCBs are generally adsorbed on the particle surface, the concentration in sediment and soil samples is much more likely to be related to the particle surface area per volume unit than to the mass unit (16). For this reason, the concentration of each sample, expressed in pg g (dry weight), was normalized by dividing it by the relevant CS, expressed in square meters of surface per cubic centimetre of dry sample (m cm ), as obtained by particle size analysis (5). Table 9.9 shows the normalized mean concentration of the seven selected congeners along with the total and the normalized total mean PCB concentration. In this case, the calculated value of the total PCB content was very close to the experimental one (the difference was always lower than 10%). Table 9.10 shows the total mean PCB concentration and the normalized mean PCB concentration for each matrix analysed. In four stations where marine sediments were collected at different depths, a concentration of about 100-200 (pg g )/(m cm ) was generally observed in a surface layer of about 10-15 cm, while in deeper layers PCBs were below the LoDs. These results show that the normalized total mean PCB content in marine sediment samples was 150 (pg g )/(m cm ) and did not show any significant difference from open sea to the coastal line. Lake sediment and soil samples showed a normalised total mean PCB content of 240 and 130 (pg g )/(m cm ) which did... 

CS is the specific calculated surface area of the sample as obtained by particle size analysis and is expressed in cm ... 

The characteristics of disperse systems (see Section 1.1.2) are determined by geometrical parameters, i.e. linear dimensions, projection areas, surfaces, volumes, and, sometimes, angular dimensions. In addition, other physical characteristics, which do not directly represent particle size, may be used for the determination of these parameters. In such cases, a mathematical conversion into the desired geometrical dimension takes place. The term particle size analysis defines the experimental determination of particle characteristics and the statistical treatment of results. 

Sing KSW. Adsorption methods for surface area determination. In Stanley-Wood NG, Lines RW, eds. Particle size analysis. Proceedings of the 25th Anniversary Conference (1991), The Royal Society of Chemistry, 1992 13-32. 

---