Several months’ operation of the torch tip assembly, during which time hundreds of samples in hexane and acetone were run, were concluded by an inspection of the tip and glass envelope. Only a slight carbonizing of the shielding nut was visible. The envelope remained clear. Each night the assembly was kept hot with the hydrogen-air flame shut off. No water condensation in the photomultiplier tube housing ever resulted using this technique.
With the sulfur filter in place the response to parathion and methyl trithion was about one tenth that obtained when using the phosphorous filter. This is in agreement with the results obtained with the Micro-Tek.
RECEIVED for review May 23, 1969. Accepted July 23, 1969. This was sponsored in part by public Health ServiceGrant #ES 00208 from the National Institute of Health.
New Rapid Method for Average Particle Size Me!asurement Fred 0. Cartan and George J. Curtis Idaho Nuclear Corp., Idaho Falls, Idaho 83401
T m BULK volume of the particles needed to form a single layer on an area of fixed size is proportional to their average particle size. This observation provides the basis for a simple and rapid method for the measurement of the average particle size of particulate solids in the sieve range. A measurement requires less than three minutes. The apparatus consists of a modified glass funnel attached to a graduated tube and a supply of adhesivecoated paper. This method was developed for the analysis of the radioactive particulate solids produced by the Waste Calciner Facility at the Idaho Chemical F’rocessing Plant. A method for these particles, which are roughly spherical in shape and 100- to 1OOO-fi diameter must be adaptable to remote us&and use a small sample. The method developed meets these requirements. I t also is useful for nonradioactive and nonspherical particles. EXPERIMENTAL.
Apparatus. The apparatus is a 3-in. diameter glass funnel joined at its apex to a graduated tube (Figure 1). A 4-in. by 4-in. sheet of adhesive coated paper placed over the funnel provides the defined area on which the solids are deposited as a single layer. Automobile bumper stickers, such as those used in political campaigns, have proved satisfactory. Bumper sticker stock is available from most print shops. The calibrated tube provides the measure of the volume of solids on the surface of the adhesive paper. The volume of the calibrated tube is selected dependent on the size range of the particles. For example, a 4-ml tube is suitable for particle sues between 200 and IO00 p. Procedure. Mix the sample of solids and pour a representative fraction into the funnel until the level is near the zero mark of the calibrated tube. Tap the funnel lightly to settle the particles and record their level. Remove the
5-ml mohr oioelle tcut lo 4-mi and sealed off square)
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Figme 1. The apparatus
I$guipure 2. Distribution of the adhering solids on the Flaper surface
protective cover from the adhesive coated paper, place the paper on the funnel, and seal it around the rim of the funnel by finger or remote tong pressure. Invert the funnel with a swirling motion such that the particles are rapidly and evenly distributed over the surface of the paper. Return the funnel to its upright position, tap it lightly to return all unattached particles to the calibrated tube, and record their level. The difference between the initial and final levels is the volume of the particles adhering to the paper. Determine the average particle diameter using a calibration curve or equation based on standards similar in size distribution and particle shape to the particles being measured. Standards can be sieved fractions of typical samples, or better, a series of typical samples previously measured by sieve analysis or another reliable method. RESULTS AND DISCUSSION
The accuracy of this method is dependent on the validity of two related, basic assumptions. First, both the fraction of the bulk volume actually occupied by particles (the packing density) and the fraction of the adhesive surface actually covered by particles are independent of particle size. Second, VOL. 41, NO. 12, OCTOBER 1969
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Figure 3. Calibration curves for various solids
the ratio of the bulk volume to the area covered on the adhesive paper is a constant for any given average particle size. If these two assumptions were completely valid, one calibration curve could be used for all types of particles. Actually, the assumptions are only approximately correct because the particle shape and particle size distribution affect the arrangement of solids in the bulk volume and on the adhesive surface. Different curves are therefore needed for different kinds of solids. With nonspherical particles, some nonrandom alignment of particles occurs in the bulk volume and on the paper. In such cases, the volume of solids that form the single layer can be larger or smaller than the volume for completely spherical particles. Figure 2 shows the actual arrangement formed by three types of materials with different shapes on the adhesive paper. These materials are glass beads (the transparent spheres), calciner product from the Waste Calciner Facility (the white flattened spheres and egg-shaped particles), and broken Devarda's alloy (the angular fragments). The differences in the calibration curves for these three materials (Figure 3) are attributed to the different amounts of nonrandom alignment of the three differently shaped particles. The presence of a broad range of particle sizes in a sample affects the measurement because of an effect on the packing density. For such samples, the packing density increases because the smaller particles occupy the spaces between the larger particles. However, this effect of increased packing density on the measurement is partly compensated by an increased coverage of the surface of the adhesive paper. To show the effect of a range of particle sizes on the measurement, two series of mixtures were prepared. One series consisted of glass beads with mean sizes of 139 and 775 p, the other of beads with mean sizes of 230 and 385 p. The results of the measurements are shown in Figure 4 in a normalized form to facilitate comparison. The mixtures from the first series, with a nominal size ratio of 5.5, show a considerably greater deviation from linearity than the mixtures from the second series with a ratio of 1.67. As expected, the greatest deviation appears when the larger particles predominate. This demonstrates that calibrations should be obtained with standards similar to the samples. The method is less applicable to powders and very small nonspherical particles. These materials do not pack reproducibly under the mild rapping used in the procedure and do not attain the same packing density as do larger particles 1720
ANALYTICAL CHEMISTRY
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Figure 5. Comparison of funnel and screen results
with the same shape. A related method ( I ) should be more precise for such samples. In this method, the particles that form the layer are weighed. This method is longer and requires a knowledge of the density of the particles. With careful attention to technique, the method is capable of excellent precision. Relative standard deviations of 2.5 and less have been obtained from repeated measurements of the same samples. The accuracy of the method compared to a sieve analysis is shown by a correlation plot (Figure 5 ) for a series of particulate solids produced in a pilot plant for the Waste Calciner Facility. This comparison is considered as a fairly severe test of the method because the samples differed in their average particle size, particle size distribution, and density. RECEIVED for review June 20, 1969. Accepted July 8, 1969. Presented at the 24th Northwest Regional Meeting of the American Chemical Society in Salt Lake City, Utah, June 12-13,1969. (1) Moon Wha Hong, Seoul Univ.J. 8, 325 (1959); Chem. Abstr. 54,2666e, (1960).
