Topics in..
. Chemical Instrumentation
Edited by
5. Z. LEWIN, N e w Y o r k University, New York 3, N. Y.
sizes thmogll n flukl medium may be r.mployed as t l r agency producing a. separatim am1 sogregittion of the particles. , l h e frartirmatim terhninues have the
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These articles, most of which are to be contributed bg guest aulhors, are intended to serve ihe readers of this JOURNALb y calling attention lo nev~developments i n thz theory, design, or availability of chemical laboratory inslrumenlalwn, oi by presenling useful insigh& and ezplanalions of topics h l are oj praUical importance to those who use, or teach the use of, modern inslnrmmtalion and instrumalal techniques.
counting, and t l r former may utilise qnitr lnrgr ssmplrs, so that the problem of the selection of a repr~sentativesample is not a limiting factor in the final results. Horn-rwr, fractionation tends to give less dptailrd and precise information about the distrihntim of particle sises than is passiblr with direct ahsrrvrrtion of individual particles. Also, there is often considerable dillicult,y in achieving an adequately stable disl~ersionof the sample without producing considerable attrition of the particles, thereby materiallv altering the particle size distrihution to be measured.
VIII.
lnstrumentation for Particle Size Analysis. Part One S. Z. Lewin,
Deportment of Chemistry, New York University
The counting and classification accarding tu size of tho particles in avalid sample of a powdered material represents one of the inost difficult of d l measurement techniqurs. When the particle sises extend down into the micron sndfraAonal micron rmges, the experimental difficulties resulting from the large number of individual particles that must be included in the smnplc in order to give s. statistically reli:il,le, ropresentativo result, and the tendency toward agglomeration of small psrt.icles into clusters, combine to make this t.ype of measurement tedious, timeconsuming, and expensive a t best, and imprecise and inacetm.te a t worst. Howw r r , many of the most important properties of solids are related to the distribution of pnrt,icle sizes in the system, and the nerd for effective, practical means of obtaining such information has spurred the dr~vrlapmentof a number of instrumental approaches to particle size analysis. The distribution of particle sises is of rritirnl itnpartance in tho behavior of mtnl?-sts, cements, ceramic compositions, pigments, insecticides, grains and food pndnrts, pharmaceuticals, plastics, erplosives, fuels, adsorbents, and numerous otllrr products of the greatest economic import,sncr. In addition, increasing attmtion is currently being paid to the mc:~surement and monitoring of the particlo concentrations and size distributions of nrn,sol suspensions in connection with tlre control of air pollution, as well as in tlw maintenance of the so-called "white areas" or "clean rooms" in industries whm- components of such precise dimensions are produced that air-borne psrticulntes must be scrupulously controlled. Finally, particle counting and sizing has long heen, and continues to be, of major importance in medical and clinical work.
Types of Particle Size Analyzers Tlarw fundamentally different appntnrlws are commonly rmployrd in the
determination of particle size distributions. One group of techniques is based upon the disc~etecozmting and mensuration of the individual pa~ticlcs in tho sample. I n this case, the sample must be dispersed sufficiently to permit the individual particles to be sensod separately, and each particle must be measured in turn. The problem of dispersinl: the spccimm so that all agglomerates arc broken down into the individual particles, usithout at the same time breaking up the individual particles themselves is a diHieult one, and in most cases it is not possible to say when the first process lras been compl~=tedor w t r n the second h a comrnencrd. In addition, a large number of particles must, he counted and measured in order to yield a. statistically significant result, and if this is done visually by an operator. the technique requires very great patienre, and not inconsiderable skill. Complex and expensive instrumentation is now available for replacing the human observer by an electronic scanner. However, if the majority of the particle sizes in the sample are of the order of ma&ude of a few microns in diameter, the count,ing and measuring of several million particle may correspond to a total sample weight of the order of only a few milligrams. The prohlcm of selecting such a small sample from a largo batch of material and insuring that it will bo truly representative of the t,ot,al batch is s. formidnhlr one indeed. A second nppronclr to particle sise analysis is based upon fradionofion into predetermined size ranges, and messorrment of tlre weight or volume of t,hesr fractions. Here, too, the sample must be thoroughly dispersed, and the physical propprties of tlre part,icles are employed to create x sepxrstirm according to size. Thus, sieves with controlled mrsh dimensions permit particles smaller tlmn t,lltl hole sise to pass through, while larger sizes are retained. Alternatively, t,hr diffcrrnt rat,es of fall of particles of difererent
Figure 1. lmpinger of Gelman lnrlrvment Co. far dust rompling. Entrance lube is constrided to increore the velocily of the sir stream and cause portirler l o be trapped ot the boltom of Ihe tube becovre of inertio. A m a l l amount of liquid at the bottom prevents the porIic1.s frcm being lost in the effluent stream.
TIN: third approach involves the application uf a twhnique based upon the
rrrm.wrrwmnt qf a pavticle-size-depadent integral propert!, of the powder. Certain over-all prrlperties of a sample can be related to the numhw and sises of the particles present in a given volume or weight, and tho measurement of these properties can, in principle, be interpreted in terms of the particle size distribution. Thus, r.g., the resistance to the flow of air
Volume 40, Number 4, April 1963
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A257
Chemical Instrumentation tlrrnuglr a standard il>irknmsi d ;t s:tnrl,lr: r c m p a r t d in :L st,md:tnl w:ty is a iunrtim of thr lmrtiele siar distrihot,im in t l r s a n q h . I Itll Casclla, Ltd , London, England
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Journal o f Chemical Education
(available in the U.S. through Cook.. T R , u ~ L and ~ , , ~Simms, Inc., lvsiden 18. Jlxssachusrtt~; single rhnnnel instrw
mmt 86'236, 6ve channel model, $12,000). The design principle of this inetrument is
(r'ontznued on page A266)
Chemical Instrumentation fashion that the stagc is r:iusrd to advance s distauee of 0.05 mm :is it i,srilliltes from left t o right thruugh n distnncc of 10 mm. The stag? nrlvnnrrn n total distance of 5 rnm while making 1il0 c n i tltcse sidr-tclside trawrsrs: tlwn itn dirrrtim of advance rrvrrsrs, ma1 it returns tr, the stsrting position d r i k making :tnthrr100 traverses. Tliu8. :an :awa 01 5 111111 X 10 mm is scnnnrrl in L'lllf psssps during o variahk slit with opmcomplete run. .i ings as s n x d :m 0.3 microns passea s portion of tlrr f i ~ l duf v i ~ wof the mieroscope ta a plwt~multipliw tube. The pulses seen I,y the latter an. amplifid. elsliaified scrnrding t o amplitude, and totalized in the elurtronie circuitry. The count rworderi in ~ : t c lelrannel ~ is n iunetion of the slit width and the nurnbt.r
Figure 6. Design principle of the Covlter Counter. Mercury Rowing dong !he gias tube at lefl activates the ,tort ond stop circuit,. Fluctuations :in electrolytic candudirity caused by parlicles polring lhrough the micro orifice ore fed to the main ompl~fier.
Figure 7. Complete ret-up Coulter Counter.
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,' journal o f Chemical Education
of
She Model A
Chemical Instrumentation not eount,ed. Thus, in n given scan, the total count will correspond to thr total nomher of particles lnryrr than :I snlertrd size. By mrnns ai surcessivr scans with different refrrencp pulse amplitudes, thr size distribution iunetiun can be dwivrtl. Particle hetwren 1 and ID0 microns in dimmuions ran he siwd and r o n n t ~ din this way. Resolution of the vimnl pn,scntation on thc 8 X X-inell inonitr,r scrvcn is 0.1 micron.
Electrolytic Conductance Pulse Counting 4 n instrument is availnblc that counts and sizes individual pnrticla dispersed and suspended in a liquid medium, rathw than rm a glass slide :la in the instances Figure 8. Design principle of !he optical system of just drecrihed. It is the Conlter Counter, monitors. made by Cuult,er Electronics Inc., Chicago 14, Illinois (Mc~IelA, 14Rl100 I. The sample is put into suspensim in :m clrctn~lytic;~lly ~wlativetc, thr arrx of the orifice, and pulsc Iwigllt sdeetirm circuitry is employed to ronductive liquid, and is caused hy tlw permit registratirm or the numbers of movement of a mcrrurv uistcm to Rwx at n pulses hwing amplitudes in cxcess oi various selected threshold values. Apertures from 30 t,o 200 microns in d i a m ~ t e r are used to count particles ranging from 0 . i to 150 micrnns. A given aperturc permits particle sizes spanning a 20 to 1 stop it. I h r i n g the counting cycle, the range t o he d ~ t e c t e d . The start and stop passage of a suspended pnrtirle int,, the cont,arts are sroaratcd hv a distancc tlmt orifice causes tlla eKcctive resistance eorrcsponds to 0.5 ec, m d this volumc between t,he sensing clcrtrrdes tc, increase, and sn ~leetricalpulse is fed t o the an~pli- passes thl.ouglr the aperture in 10 t o 15 sreonds. T l r purtirlc conrcntrztinn m a y fier. The magnitudes of the pulses arc a iunetion of the v d u m r 111 the p x r t i r l ~ s hr as high xs 1 millinn per millilitw:
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Journal o f Chemical Education
the
Royca lnstrumenlr lane of
air-borne
porttie
the suspension liquid must disperev the smnple eumplett~ly, and sgplcmpration should be prevented during tlw timc nvedcd for rompletion of a nm. I t may I,r ncezssarp to add suriacp-sctivr agmtr to produce a sufliciently stahle d i ~ p ~ m i m . The complete instrumentation us read-out {lf up to 12 dilkrent cirp l~wela.
Scattered Light particle counters .4 series af instruments for the moni(Continued on poyv :IL'i3)
Chemical Instrumentation toring of air-borne particulates by xutomatic counting and sizing is matic by Rqyco Instruments, Pala Alto, California. These are based upon the optical arrangement illustrated in Figure 8. Light from a tunmten filament lamp is focused by means of a condenser lens system onto a 1 X 2-mm slit, in order t o define the beam, and hlock out extraneous light. The light diverging from this slit is focused by means of a projection lens onto the air stream, whieh continuously flows through tho instrument. The light beam is finally caught in a light trap and absorbed. Light. scattered f n m part,iclrs in the- air stream is focused onto a photomultiplier detector, producing pulses the magnit,udes of which are proportional to the particle size. A copper-clad Lucite light-pipe is employed t o feed chopped pulses of the incident beam hack into the fivstem in a manner whieh enables the phototube t o detect them. Thesc pulses are farmed by holes in a rotating disc located directly behind the light-trap. The pulses are used to provide a. known amplitude and frequency of signals for cillihration of the optics and rlect,mnics of the instrument. The flow through the device of the air stream heing sampled is designed t o produce laminar flaw at a velocity t h a t causes particles t o enter t,hc viewing region one a t s, time. With a transit time of millisecond per particle, aerosol concentrations up t o 20 million particles per cubic foot can be admitted without significant errors occurring due t o coincidence losses. The elretnmic circuitry of the Modcl P C 2 0 0 A Particle Countor (87500) is shown in block diagram in Figure 9. Particle sise readings can be programmed aver any desired combination of 16 ranges from 0.3 t o 10.0 microns, for counting intervals from 0.3 to 10 minutes. A convenient instrument for the calibration of aerosol particle monitors is the Model 255/6 Aerosol Generator (Sago), the design of which is indicated in Figure 10. This instrument produces a stable aerosol of spherical polystyrene latex particles of known sise, over the range of 0.1 t o 5 microns.
Bibliography AND SMITH, M. L.,"Smdl Particle Statistics," Elsevier Publishing Company, New York, 19533. ORE, C., JR., AN) IIALTAVALLE, J. M., "Fine Particle Mensurcment,," The Maemillan Company, New Ywk, 1!)59. CADLE,R. D., ''Particle Size Detonnination," Interscience Puhlislr~rs, Inc., New York, 1956. I)ALLAVAI.LE,J. M., "Micrameritics," Second Edition, Pitman Publishing Corporation, Ncw York, 1948.
HERDAN,G . ,
GREEN, H. L.,
AND
LANE, W. R.,
"Particulate Clouds: Dusts, Smokes and Mists," F. N. Spon JAd., London. ROSE,H. . "The Measurement uf Particle Size in Very Vine Powders," j Constable and Co., Ltd., I.ondon, 3063.
(('ontinad on pnge 4 274)
,
Chemical Instrumentation PAYNE,R. E., "The Measurement of tlre Particle Size of Sub-Sieve Powders," Proceedings of t,he I.S.A., 2, 1948. O'KONSKI, C. T., BITEON, M. D., A X D H ~ o u c ~ W. r , I., "Ligh&Seattering Instrumentatian for Particle-Size Distribution Measurement," ASTM Publiext i m No. 234, 1958. HARNER,H. R., AND MUSGRAVE, J. H., "A Pl~otocleetrieSedimentation Method for Particle-Size Determination in the Suhsieve Range," ASTM Publication No. 234, 1958. MICHAELS, A. I., "Turbidimetrie ParticleSize Distribution Theory: Application to Refractory Metal and Oxide Powders," ASTM Publication No. 231, 1958. SAWYER,K. F., A N D WALTON,W. H., "The 'Conifuge9-A Size-S~pamtionI)?vice for Airborne Particles," J o w . 3%. Instru., 27, 272, (1950.) DAESCHNER, H. W., SEIRERT, E. E., A N D PETERS, E. I)., "Application of Eleetroformed Precision Micromesh Sieves to the Determination of ParticleSize Distribution," ASTM Publication
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Figure 9. Block diogrorn of the eleclronic circvilry of the Royco Model 2 0 0 A SUPPLY
Porticie Counter.
OUTPUT TO DILUTE SOLUTION OF LATEX PARTICLES AND DISTILLED WATER ATMOSPHERE
No. 234. 1958.
OBER, S. S., AND FREDERICK,K. J., "A Study of the Blaine Finenesrr Tester and a. Determination of Surface Srea from Air Permeability Data." BSTM Publication No. 234, 1958. SPILLANE,F. J., "Automatic DirectReading Apparatus for Determining the Surface Area of Powders," ;Inol~/st 82, pp. i12-715, 1959. Nezt: Conclusion of the s u m q of p a d d o size analysis instrumentation.
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Journal of Chemical Educofion
16-20 LITERS PER MINUTE (DRYING AIR)
INTAKE
VALVE Figure 10.
Design cf the Roym Aerosol
Generator. Model 25.516.
