Powders: Standard

Many commercially available particle size distribution measuring instruments may be considered to be sub-standard in that they need to be calibrated at regular intervals using standards powders of known size 

Early calibration materials were often pollen since these have a very narrow size range. Arguments persist as to whether these swell in liquids or change size over time. Polymer latices are also available as calibration material. When instruments are used regularly to test a limited range of products the product itself can be used for calibration purposes. Thus a manufacturer of titanium dioxide may set aside an easily dispersed, stable sample for sample splitting and subsequent use for instrument evaluation. 

A method of preparing narrowly classifies powder has been described by Muta et. al. [129]. A fine copper powder of approximately the required size is mixed with twenty times as much calcium carbonate and heated for 10 minutes at 1100°C in a hydrogen atmosphere. The calcium carbonate is then dissolved out using a solution of 10 parts water to 1 part concentrated nitric acid by volume. The copper spheres so produced had a mean diameter of 6 pm and 80% by weight lay in the size range 1.4 to 10.5 pm. This method was also used by Colon et. al. [130] for the production of glass spheres. 

Narrowly classified latices are available from Dow Chemicals [134] but doubt has been expressed on the accuracy of their sizing. A 3.49 pm polystyrene latex was independently sized by electron microscopy and found to have a mean diameter of 3.40 pm [135]. This standard was used by Coulter Electronics as a standard to size other Dow latices [136] that are available from them as standard suspensions. 

The Community Bureau of Reference has five quartz samples prepared as reference materials for the calibration of apparatus, these are described in chapter 11. These powders were analyzed by gravity sedimentation except for BCR 68 which was analyzed by sieving. Analyses were carried out at five laboratories and the results compared to give a measure of the quality of the standards. 

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Joint Committee on Powder Diffraction Standards (JCPDS), Standard Powder Diffraction File, International Center for Diffraction Data, Swarthmore, PA. 

Joint Comlttee on Powder Diffraction Standards Powder Diffraction Files, ASTM, New York, 1967, Set 6-10, p 399. 

The last method for producing standard patterns for phases not in the PDF is more involved. In many instances single crystals of unknown phases can be removed from reaction mixtures. If this is the case, a full three dimensional crystal structure analysis will yield the positions of all atoms in the structure. Once the crystal structure is known, it can be used to calculate the X-ray powder diffraction pattern for the phase. This powder diffraction information can then be used with confidence as a standard powder pattern. 

Standardize a recording spectrophotometer to 100% reflectance from 420 to 700 nm, using the white standard (powdered barium sulfate) in both the sample and standard ports of the reflectance attachment. 

Powder X-ray diffraction (XRD) patterns of the catalysts were obtained using a Philips APD X-ray diffraction spectrometer equipped with a Cu anode and Ni filter operated at 40 kV and 20 mA (CuKa = 0.15418 nm). Iron phases were identified by comparing diffraction patterns of the catalyst samples with those in the standard powder XRD file compiled by the Joint Committee on Powder Diffraction Standards published by the International Center for Diffraction Data. 

It was then established that the new processes would permit the ballistic effect of black powder to be secured with the same pressure and with the charge reduced to about a third, and that the power of the arms could be increased notably, with a slight reduction of the charge, while still keeping to the ordinary pressures. The standard powder for the model 1886 rifle was determined in the early months of the year... 

The standard powder made possible an increase of velocity of 100 meters per second for the same pressures. 

X-ray diffraction (XRD) investigations were performed by means of a standard powder diffractometer using Cu Ka radiation. Electron microscopy investigations were conducted using the Hitachi H-800 microscope operated at voltage of 200 kV. The magnetic properties were measured by means of the ballistic method in the magnetic field up to 800 kA/m in the temperature interval of 77 to 673 K. 

Sodium analyses were carried out by flame photometric methods after dissolution of the sample in HF. X-ray diffraction was carried out on standard powder diffraction equipment. 

Powder Patterns. Powder patterns on titanium dioxide were obtained using a North American Philips x-ray machine and a standard powder camera. This work was done by A. Caron. 

Small Volume Parenterals (SVP) Appearance, color, clarity (particulates), pH, and sterility checks at reasonable intervals are minimum standards. Powders for reconstitution should also include residual moisture and stability checks after reconstitution. Except for ampules, upright and inverted storage of final product should also be evaluated. 

Qualitative and quantitative analytical applications of X-ray diffraction both require reference diffraction patterns to identify and quantify the different polymorphic modifications. Experimental powder patterns may be suspect for their use as standards as a result of experimentally induced errors or aberrations or the lack of polymorphic purity in the sample itself (which may even result from the sample preparation). The availability of full crystal structure determinations for any or all of the polymorphic modifications can considerably facilitate generation of standard powder patterns. A variety of public domain software is now available for calculating powder diffraction patterns from single crystal data (ICDD 2001, lUCr 2001)." ... 

NIST produce two standard powders for gravity sedimentation. SRM 659 is a silicon nitride powder that has equiaxed particles in the size range 0.2 to 10 pm. SRM 1978 is a zirconium oxide powder of granular irregulary shaped particles in the size range 0.33 pm to 2.19 pm. 

Although it is common practice to calibrate the Coulter Counter using a standard powder, it is possible to calibrate the instrument with the powder being examined. This is the preferred British Standard method [17]. It is reiterated that this procedure cannot be carried out with some instruments due to count loss. Essentially one balances the volume of particles passing through the measuring aperture with the known volume in the measurement sample. This serves a multiple purpose in that ... 

Some manufacturers of instruments for characterizing fine particles claim that their instruments do not need calibration. Such claims should be treated with skepticism. In practice, many sizing instruments have to be calibrated using standard powders available from several vendors.Because the various methods of exploring the size distribution of a powder evaluate different physical parameters, the size distributions of a powder generated by different methods do not always agree. The relationship between distribution fimctions, as evaluated by different methods, should be explored experimentally. ... 

Standard powders are available from Duke Scientific Corporation, 135D San Antonio Rd. Palo Alto, CA 94303. 

Powdered cellulose has acceptable compression properties, although its flow properties are poor. However, low-crystallinity powdered cellulose has exhibited properties that are different from standard powdered cellulose materials, and has shown potential as a direct-compression excipient. ... 

It can be concluded that the Couette-type viscometer may well find its place in standard powder testing because it can measure the stress-strain relationships for aerated powders and thus provide a basis for design of equipment for handling of such powders. 

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