Microchemistry 1
Determination of the Size of Fine Abrasive Powders A Comparative Study of Microprojection and Sedimentation Methods FR4NK L. JONES, llellon Institute, Pittsburgh, Pa.
piece, a comparatively flat field is obtained. The distance t o the screen for the desired magnification is adjusted by focusing the microscope on a stage micrometer, a glass slide ruled with lines 10 microns apart. The projection screen is ruled at intervals of 10 mm. and, when the projected lines fall on the rulings of the screen, a total magnification of X 1000 is assured.
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HE determination of the size of particles finer than 326mesh is usually carried out with a microscope or by calculating the size from the rate at which the particles settle in a liquid or air. This report considers some of the differences in the size-distribution curves that were prepared for aluminum oxide abrasive powders using a conventional microprojection method and a common sedimentation method. The abrasive grain manufacturers and the users of fine abrasive powders have a standard method of checking and controlling the size grades, according to 13-hich the size-distribution curve is computed from sedimentation tube data. According to this procedure, methanol of special purity is the liquid through which the carefully dispersed abrasive settles, and the rate of accumulation of the grain a t the base of the tube is observed. The glass sedimentation tube is 2 em. in internal diameter and 94 em. long. -4collecting tube 11 mm. in internal diameter and tapering to a graduated neck 4 mm. in diameter is mounted a t the bottom of the sedimentation tube. The sedimentation tube is water-jacketed for temperature control. I n setting up standard specifications for the natural corundum abrasives produced for use in the optical indust'ry, however, i t was considered advisable to employ a microscopic method of determining grain size. The microscope is the standard instrument in this industrial field and the specialists responsible for the separation of the various size grades of corundum are accustomed to rely upon it' as a control instrument. Then, too, as the finest grades of corundum used in precision grinding of glass or metal surfaces are composed of grains with a n average diameter less than 10 microns, the dispersion of such material in a liquid is difficult and incomplete. The time required for the settling of the portion of the sample less than 5 microns in diameter is excessive.
Preparation of Slides .A method of slide preparation developed by the abrasive industry is suitable for use with this method. The dry powder is sampled carefully, the sample is reduced in size by quartering, and a small metal cup is used to pick up enough material to form a layer on the slide with few particles touching or overlapping. A clean glass slide is placed on a horizontal surface and covered with a brass tube 8.75 cm. (3.5 inches) in diameter and 50 cm. (20 inches) long. The t'op of the tube is closed with a rubber stopper through which projects the lower end of a glass tube, whose upper portion is bent to the shape of a sink t,rap. The sample of abrasive is placed in the bend of the trap and a sharp puff of air from a rubber bulb forces the h e powder into the upper portion of the brass tube. The grains settle through the air in the tube, forming an evenly distributed layer on the glass microscope slide. The amount of sample is chosen so that particles do not touch or overlap to any great extent. Two slides are prepared and protected under a watch glass until they are placed on the mechanical stagqof the microscope. From 10 to 20 fields are selected at random on each slide and the diameter of the grains is measured. The distribution of t,he different sizes of grain on the slide has been found to be very uniform. Drying the original sample in a crucible heated over a flame will overcome the tendency of very fine powders to collect in clumps and aggregates.
Determination of Average Diameter The particle diameter is measured on the projection screen with a millimeter rule. If the grain is not symmetrical, the average of the longest and shortest dimensions is taken as the diameter. It is convenient to hare the microscope fitted with an extension focusing rod, so that exceptionally large or small grains can be brought into sharp focus. Fine abrasive powders are made to be rather uniform in size and if the iris diaphragm of the substage condenser is part'ially closed the depth of focus of the system is such that many grains will be in focus simultaneously. The collection of data is facilitated if an observer measures and calls out grain diameters a t the screen as a helper puts a mark under the corresponding data sheet column. When
Microprojection Apparatus for Fine Abrasives A standard plant control procedure has been developed that is an adaptation of the method employed by Work (1) in studying fine powders. An ordinary microscope is used with a Bausch & Lomb model B microprojector support. A reflecting prism fits over the microscope eyepiece to direct the beam of light to the screen. A clock-feed arc lamp operated from an 8-ampere rheostat is the means of providing illumination. The model B microprojector carries a condensing lens by means of which the light from the arc is sharply focused through a fixed mirror onto the iris diaphragm of the microscope substage. A water cell absorbs heat from the light beam. When the microscope is fitted with a 16-mm. apochromatic objective and a X 7.5 compensating eye45
INDUSTRIAL AND ENGINEERING CHEMISTRY
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more than 100 grains of any single micron size have been measured, the count is stopped. The total grains measured for a determination vary from 200 to 2000, depending on whether the sample contains a few sizes or many sizes. Wellgraded abrasives do not require the measurement of a large number of grains. The number of each micron size counted is multiplied by the cube of the diameter to obtain the relative amount of that size b y volume or n-eight present in the sample and the percentage of the total amount is calculated for each micron size. To obtain the size data in the form of a n accumulation curve, such as the abrasive industry uses, the percentage of the total sample larger than each size is plotted against the diameter in microns.
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Effect of Irregular Grain Shape For grinding optical glass surfaces a symmetrical abrasive grain is used. For some types of abrasive paper, however, the grains are purposely made irregular in shape, including both thin flakes and elongated particles. All agreement between the standard sedimentation test and the photomicrographic method of determining grain size described above disappears when the sample is composed of such long, narrow, irregularly shaped grains. The size number obtained from the accumulation curve is much larger when the curve is drawn from microprojector data than when the curve is computed from the sedimentation tube data. Even when the smallest visible dimension of the particle under the microscope is substituted for the average of the longest and shortest dimensions as the size of the particle, the size-distribution curves obtained by the two methods do not agree. Figures 1 and 2 indicate the agreement in size c u r ~ e swith symmetrically shaped abrasive powders and the difference in size curves with irregularly shaped particles when the same sample is measured by sedimentation tube and by the microscope. There is little point in saying that one method is more accurate than the other for irregular grains.
Advantages of the Microprojection Method
FIGURE 1
2
C V R V E S FOR FINEA 4 L c M I N L M 1, SIZE-DISTRIBUTION OXIDE Solld curxe calculated from sedimentatlon t u b e d a t a Broken curve calculated f r o m mioropro~ector d a t a
There are many ways of calculating and expressing average diameter and the term thus has no uniform meaning. The buyers of abrasive powder use the term “average diameter” to designate the particle diameter corresponding to the point a t which the accumulation curve crosses the 50 per cent line.
Comparison of Methods T o ascertain if the size as measured by microscopic projection is similar to that determined by the standard sedimentation method, various abrasive companies were asked to run a sample of graded aluminum oxide by the sedimentation method and send it to the author as an unknown for microscopic measurement. For samples consisting of grains approximately symmetrical in shape, such as would be obtained by grinding wet corundum in a ball mill, there was good agreement in the figures for arerage diameter as determined by the two methods. This agreement was within the limit of error of the methods. The shape of the sizedistribution curve was characteristically different, however, depending on the method of measurement. The percentage of fine particles was greater when measured microscopically. The difficulties in dispersing fine particles individually in a liquid and the tendency of large grains to carry fine material with them in settling would explain the failure of the sedimentation method to reveal all the fine particles present in the sample. Xotwithstanding the great care used in selecting the settling medium and the care taken to clean the grain by ignition, which make this particular sedimentation method especially free from coagulation troubles, the percentage of fine material indicated by it is low. I n the coarse portion of the distribution curve the sedimentation data show more large particles than do the microscopic data.
All the abrasives used in a n optical plant are of the more symmetrical grain shape. The fact that many of the samples to be examined are below 10 microns in average particle size has led to the adoption of a microprojection method for determining the size-distribution curves used in controlling plant production of corundum for optical grinding. I n this size range the grading is close, only a few particle sizes are found in a sample, and the number of particles to be measured to obtain a smooth size-distribution curve is low. Results are reproducible within 1micron for plant samples. The time that is needed to prepare the sample, measure the grains, and compute the accumulation curve is less than 2 hours. There is no uncertainty about the quality of a reagent (such as the methanol used in the sedimentation method) nor about the dispersion of the grains in the liquid. For coarse abrasives the number to be counted is larger and the time required may be longer unless a lower magnification than x 1000 is used. The shape and structure of the abrasive grain can be observed during the size determination and lots containing poorly
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70 65
60
50 4 5 40 35 30 25 20 IS PARTICL€ DIAMETER IN MICRONS
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10
5
0
FIGURE 2. SIZE-DISTRIBCTION CURVES F O R ABR.4sIVE GRAINC O S T A I S I X G M A N Y LONGOR NARROW SHAPES 1.
2.
3.
Solid curve from sedimentation t u b e d a t a Broken curve from microproJector data. Average of long a n d short dimensions taken as average diameter of particle D o t t e d curve f r o m microprojector d a t a . Smallest visible dimension taken as average diameter of particle
JANUARY 15, 1938
ANALYTICAL EDITION
shaped particles rejected. A microscopical determination of the size-distribution curve of a fine abrasive powder requires no more skill and care in manipulation than does a determination of equal accuracy by the sedimentation method. Size numbers obtained b y the two methods are in close agreement for symmetrically shaped abrasives. For abrasive flours coarser than those shown in Figures 1
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and 2 the sedimentation tube method is relatively rapid and easy to carry out.
Literature Cited (1)
Work, L. T.,Proc. Am. Soc. Testin0 ;Materiels, 28, 771 (1928).
RECKIVED
June 4, 1937.
Microchemical Analysis of Colored Specks and Crystalline Occlusions in Soap Bars HERBERT K. ALBER1 AND CLEMENT J. RODDEN2 Washington Square College, 4 e w York University, New York, N . Y.
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HE emphasis in the microchemical literature is usually
placed on the description of new methods or on the development of special apparatus. The practical application of these methods to the solution of analytical problems which cannot be solved by the usual methods is seldom mentioned, except in the analysis of valuable objects of art, etc. The present paper describes two cases which illustrate the practical value of the so-called “classical” micromethods. Considerable time and effort spent in the application of ordinary methods of analysis to these two problems failed to yield a solution. Since the difficulties entailed the loss of a large amount of merchandise, a discovery of the cause was of considerable importance to the manufacturer. Rlicromethods of analysis were therefore used and in a comparatively short time yielded the necessary data for locating the source and preventing a further recurrence of the trouble. T h e final solution may seem surprisingly simple. I n the following discussion, only deviations from known microprocedures are described in detail.
the elements, which will be described in a future publication. The ; hydrogen, analysis showed the following results: carbon, +; halogen, -; nitrogen, -;,phosphorus, -; sulfur, (the strength in each test is indicated by over 25 per 1 to 10 per cent; -, negative cent; ++, 10 t o 25 per cent; reaction). Emich’s sulfur test consists in oxidizing the sulfur of the organic compound to sulfate by heating with nitric acid in a sealed capillary; since the temperature should not exceed 300’ C., the test can be carried out just as accurately in less expensive Pyrex glass capillaries of about 1-mm. wall thickness and 1- to 2-mm. bore, with the usual precautions. B y comparing the length of the barium sulfate column with those produced from known amounts of sulfur-containing materials, an estimation of the sulfur content within the given limits is easily made. The supernatant liquid is transferred to another capillary and the phosphate is precipitated and estimated as ammonium phosphomolybdate by comparison with k n o m amounts of phosphoruscontaining substances.
+,
++ +++,
++
Qualitative Microchemical Analysis of Colored Specks in Soap Bars SAMPLE.Colored specks were distributed on and in a single soap bar, from which they were isolated by cutting the soap in such a way that at least one surface of the particle was exposed. Under the binocular microscope the specks were lifted off with fine needles and collected on a microculture slide with a small concavity. About 35 such specks, a few of them pure soap only, were submitted for microanalysis. Investigation under the microscope showed several types of particles which are illustrated in Figure 1. With the microchemical manipulator designed by one of the authors (1) all b, d, and e particles were separated, and then the major part of the adherent white material was removed under the binocular microscope (magnification, X 30). I n order to obtain preliminary information on the ingredients and technic best suited to the problem, the white particles were analyzed first. ANALYSIS OF W H r T E
PARTICLES. About 15 white particles,
a, and separated white portions d and e collected in a platinum dish 10 mm. in diameter, weighed 48 micrograms. With 20
micrograms of this sample a qualitative organic elementary analysis was carried out, using Emich’s procedures (6, f f ) and some modifications by Alber for estimating the percentages of 1 Present address, Biochemical Research Foundation of the Franblin Institute. Philadelphia, P a . 2 Pie-eut address, National Bureau of Standards, Washington, D. C ,
(x
FIGURE 1. COLORED SPECKS ISOLATED FROM S O A P 20) T a k e n with a Reichert photomicrographic a p p a r a t u s i n reflected light u . P u r e n h i t e particles n-ithout a n y special structure b. Reddish brown a n d dark-brown particles, uniformly colored throughout c . Yery slightly yellow particles d. White particles with very small dark spots, which appear black. sometimes d a r k green i n reflected light. T h e d a r k specks have distinct boundaries, h u t are apparently isotropic in polarized light e . Different combinations, of t h e above types. such as white or yellow uarticles u i t h brownish Dortions. the color of which shades off gradually
