Zeolite Microstandards Gerald A. Sleater and David H. Freeman Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234 David H. Olson and Howard S. Sherry Central Research Division Laboratory, Mobil Research and Deuelopment Corporation,Post Ofice Box 1025, Princeton, N.J. 08540
INOROANIC Z E O L m CRYSTALS, because of their ion exchange properties, show the same capabilities to perform as chemical microstandards as already established with organic ion exchange resin beads (I, 2). Key to this performance is the optical determination of mass. This is feasible using conventional microscopy to determine the length of the edge of a regular and well-formed crystalline particle. The other essential requirement that the crystalline material examined in the present study meets is a high degree of interparticle homogeneity. The present experiments were carried out on well formed cubic crystals of zeolite NaA selected from a mixture of zeolite NaA and NaX, as shown in Figure 1, that was crystallized using the technique developed by J. F. Chamell (3). Approximately 15 per cent of the mixture is wellformed cubic crystals of zeolite NaA; the remainder are octahedral crystals of zeolite NaX. Verification that the cubic crystals were zeolite NaA was obtained by mounting a single crystal and taking a precession photograph of the zero layer using M O L radiation. Thus, the cubic crystals are consistent with the NaA ideal cell (4): Naso [(A102),,(Si02),6]. 216 H,O. A preliminary indication of sample homogeneity was provided by density gradient experiments. The sample was stored for over a week at 20 "C in a closed chamber at ca. 52% relative humidity (over saturated aqueous sodium dichromate solution). A density tube was prepared using mixtures of ethylene dibromide and carbon tetrachloride to give a gradient near 0.08 g/cm4. The particles were briefly agitated ultrasonically and, after coming to rest, occupied 1.932 to 1.944 g/cm8 as the density range. The standard deviation can be taken as one-third of the range of variation. This corresponds to an estimated 0.2 % relative heterogeneity with respect to the interparticle density. After giving this sample the usual conditioning washes, sodium counterions remained along with less than 0.01 %each of Mg, K, Ca, and Cu as indicated by emission spectroscopy. Neutron irradiation was carried out and individual cubic crystals ranging in size from 30 to 47 pm were then selected, photomicrographed, and measured for activity with a gamma ray spectrometer. The length of the edge (d) was determined by microscopic measurements using a movable hairline ocular. Copper foil was employed as the calibration standard to monitor the neutron flux. The results are shown in Figure 2 where the sodium content of the zeolite particles is plotted against the microscopically
Figure 1. Mixed zeolite cyrstals in the above photomicrograph include those with cubic habit. Inset of one of the cubes 45 pm in size is shown at 10-fold higher magnification
I W
determined particle volume. T h e observations are in accordance with the expression: mN. =
(1) D. H. Freeman, L. A. Currie, E. C. Kuehner, H. D. Dixon, and R. A. Paulson, ANAL.CHEM., 42,203-209 (1970). (2) P. Hahn and B. Schleien, ibid., pp 1608-1612. (3) J. F. C h a r n e l J. Crystal Growrh,~S,291 (1971). (4)D. W. Breck, W. G. Eversole, R. M. Milton, T. B. Reed, and T. L. Thomas, J . Amer. Chem. Soc., 78, 5963 (1956). 1898
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where the partial density for sodium pNLis determined as 0.253 + 0.007 g Na/cma, which corresponds to 0.03 relative,standard deviation for 23 measurements, excluding one rejected measurement. The deviations did not appear to vary with particle size.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
We conclude that the present type of cubic zeolitic particles offers the matrix geometry and homogeneity needed for preparing inorganic or geologic standards. For preparing standard multi-element mixtures, the achievement Of Uniform cation distribution would need to be shown, but that should be the case if the ion exchange processes occur with sufficient speed.
ACKNOWLEDGMENT
Spectrochemical analysis of the zeolites was done by V. Stewart of the Spectrochemical Section, NBS. Assistance in zeolite irradiation was given by D. Becker of the Activation Analysis Section, NBS. RECEIVED for review May 18, 1971. Accepted July 23, 1971.
Atomic Absorption Spectrophotometry Applied to the Determination of Breakup Statistics for Fractured Stressed MateriaIs Ben K. Seelyl and Anthony F. Veneruso2 Sandia Laboratories, Albuquerque, N . M . 87115 ATOMICABSORPTION is recognized as a valuable and versatile procedure for quantitative determinations of submicrogram concentrations of many elements. This paper describes the adaption of the technique to the determination of the distribution of initial surface area remaining on particles procured from the residue of fractured metal coated plates (4 in, X 4 in. X 0.06 in.) of stressed ceramic material. The term “initial surface area” specificially refers to that face on the particles which, before fracture, was part of one of the 4 in. X 4 in. surfaces on the stressed plates. The problem resulted from a need to assess quantitatively the fracture characteristics of various types of chemically-strengthened glass and ceramic structures. This application of atomic absorption is unique and provides a method for obtaining the probability density function, f(x) of initial area, x, remaining on fractured particles of stressed materials. The technique greatly simplifies the determination of breakup statistics such as the mean, variance, and probability of obtaining particles containing a greater than specified amount of initial surface area. Computer selected samples resulting from the residue of fractured, chemically-strengthened ceramic and/or glass materials provided particles for chemical analyses. The initial surface area of the metal coated particles ranged from about 5.5 X 10-5 sq. in. to 0.03 sq. in. Interest was focused on the initial surface area remaining on the particle for the following reasons: First, to evaluate the destruction of the surface integrity of the stressed material. Second, stressed glass or ceramic materials of constant thickness have been observed to fracture into particles having a uniformly rectangular cross section. Hence, estimates of the mass and total surface area statistics for the particulate residue can be obtained using this technique. An atomic absorption method was chosen because it offered greater simplicity and efficiency in data collection for the total of 15,000 fractured particles involved in this study of stressed materials. Our paper presents the results for 1124 particles from one of the sample materials tested. Measurement data were automatically recorded in %Ton computer punch cards in a format suitable for input to a digital computer used in data reduction. Chemical Analysis Division 5521. Exploratory Systems Division 1213.
Several metals were considered for deposition on the surface of the stressed ceramic materials. Silver was selected because of the following advantages: immediate and complete removal of the film by 1:1 nitric acid, ease of vapor deposition of a uniform film thickness, good adherence to the substrate, silver can be determined with high analytical sensitivity in the air-acetylene flame (under the working conditions used, 0.04 produced 1 % absorption or 0.0044 absorbance), low noise background permitting the amplifier to be operated at the minimum gain setting, low noise-to-signal ratio, improbable chance of sample contamination through the various manipulative procedures, analytical wavelength in the region of low absorption in an optical air path, and minimum drift of the Ag lamp source during the period of analysis. EXPERIMENTAL
Apparatus. A Techtron AA4 Atomic Absorption Spectrophotometer (Techtron Pty. Ltd., Melbourne, Australia) was used to collect all transmittance data. A Techtron Premix AB51 Burner; designed for air/acetylene and having a 10-cm-long slot, 0.5 mm wide was adjusted in the optical path for maximum response. A silver hollow-cathode lamp (Atomic Spectral Lamps, Melbourne, Australia), operated at 5 mA and at the lowest gain setting on the photomultiplier and amplifier was used. A Hamamatsu R213 Photomultiwas used to detect the 3080.7-A plier, range of 1850-8000 analytical wavelength for silver. A Dymec Integrating Digital Voltmeter was modified to automatically average two separate transmittance displays of 10 counts/sec. The electronics were by Sandia. This data logging system (Sandia No. SE160) automatically cycled and recorded the atomic absorption transmittance measurements on an IBM card. A Honeywell 19, 6-in. strip chart instrument recorded peak plateaus. ProFedure. Silver was uniformly deposited at a thickness of 4000A over one surface of the 4 in. X 4 in. x 0.06 in. flat ceramic material. By means of a Talysurf Profilometer it was determined that the silver layer was deposited with a uniformity of better than 5%. The silver-coated ceramic sample was placed in a closed holding fixture with a removable collection tray. This tray was ruled with 16 equal-area squares for sampling purposes, and plate fracture was initiated by means of a small diamond saw penetrating to a depth greater than the compressive stress layer of the strengthened material. Release of the potential energy, stored in the stressed plate,
A,
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