. Chembl lnstrumentution
ropics in..
1
--.am feature
Edited by GALEN W . EWING, Seton Hall University, So. Orange, N . J. 0 7 0 7 9
These articles, most of .which are lo be contributed by guest authms, are intended to serue the readers of this JOURNAL by calling attention to new developments i n the theoy, design, or availability of chemical laboratmj instrumentation, or by presenting useful insights and explanations oJ topics that are of pracEical importance to those who use, or teach the use of, modern instrumenlation and instrumental techniques.
XXXV. Modern Laboratory Balances Part One-lever-arm
Balances
R O L A N D F. HIRSCH, Deportment of Chemistry, Seton Hall University, South Orange, New Jersey 0 7 0 7 9 INTRODUCTION The determination of mass is e..sential to experimental chemistry. I t is one of the mast precise inst,rumental techniques used in analysis. Several t,ypes of instruments have been developed. Each is suitable for certain tasks and not for others, and economy and speed of opers, tion vary greatly among the different types. It is hoped that this article will help clarify the factors affecting the choice of one instrument over another for a. psrticdar application. All weighing t,echniqnes determine the force W (the weight) produced h y a mass M in a gravitational field 8:
This force depends on g, and t o convert a weight into the corresponding mass one mnst either know the value of g (which varies with altitude and latitude on the Earth), or use s. set of known mmses t o sttmdmdize the measuring device. The second alternative is almost always chosen. A primary standard of m a s is stored a t the International Bureau of Weights and Meaxw.9 in Paris, snd a. replica of this standard is a t the United States National Bureau of Standards in Washington, D.C. Secondary standards are issued far the cslibratian of working sets of masses or weights. Most lahornt,ory supply houses sell wwking sets of weights elasvified according to their accuracy ( 1 ) . Sometimes i t is desirable t o oalihrate a new set of weights, hnt usually one can select s. set whose tolerances are s a t i o factory for the particular application. Recslihration of weights after frequent handling is necessary, however, since wear or corrosion may cause significant ' T h e Ohmart Corp., 4241 Allendorf Drive, Cincinnati, Ohio, mam~iacture.; devices using this principle. * I t is interesting to speculate about the mass-measuring devices most, snited t o zero-gravily condibions, such as in an Esrt,h-orbiting laboratory.
changes i ~ r the mass. This eslil,l.xlirr~~ may he perfwmed against ,z secondavy standard, 01. by the method of T. \V. Richards (S, 3). The force pt.oducetl by n mas* in ;I gravitational field can he measured in several different ways. A spring "my he stretched, a fiher bent, or twisted, or a lever deflected from the ho~.isonlslposition. A second force acts lo eountec helance that due to the utrknrmn mass, and brings the device to a rest, position. If one can memure the second force, the rmknown force (the weight) may he calmlated. I n most halwnaes (he eaunl.erforl:c returns the balance to its iuitial (empty) position-this is rallod lho ndl-halanro condit,ion. I n some hnlnuees the deflection from tho null point is mcnswed (the stretched spring ix n good example). Actually even most null halances a1.e not. returned exsoily lo their. inilia1 position, as this can be a ve1.y iedio~wprocedure. Rather a eomhi~mlicmof n~dlingand me* swement of ihe r e s i d d deflection ih used in the weighing opcrnlinn. The deflection is often measwed directly ns an angle formed with ihe hot~isantnl, or a- the length on n scale, but sometimes a transducer converts it lo nn electlmic signal (examples w e i.he lmad cell, eapneitor, and differential t,rnrrsfotmer, which are descrihed in Pwt 11of l.his article). A few inst~wnenlsdl, nol, weigh ohjoels directly, hnt mensure lheil. masses throtlgh different properties, such as the transmission of high-energy radialion.' There are many chemical m d physical l,erhniq\~es which allow indired deteminstiun of masses. This article will be limited, however, to those inxlln~mentswing the halanceof-iarres prirleiplos dcscl.it,ed in thc previous p a r a g r a p l ~ s . ~ A general discossiou of (.he evalmlion of halanee gel.formitnr:e has heon published by I,. B. blnetirdy (4). A committee of ( h e Ameriem Chemical Society has defined cerlain parameters t o describe balances. The qnotslions in the following paragraphs ;we fl.om t,he committee's report (6).
Roland F. Husch is an Assislm,l Pmfexcm uf Chemistry at. Set,on Hall Universi1.v. H e graduated with n H.A. from Oherlil~ College i l l 1!16l, and rereived hi6 1'h.D. (in analylir:tl rhemist,ry) from the University of Alirhigan in 1!165. His researrh inlec ests are in the tarens of aualytival separations (partic:~~larly ion exchauge) and applicatmls of radioisotopes to nnwlysis. He is a member i d ihe American Chemical Society, American Society for Testing xnd Materials, and Sigma Xi. The sensiliuils istmetimes called thc
Volume 44, Number 12, December 7 9 6 7
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A1023
Chemical Instrumentation hal;uwes, tol.aion hrrlmrei, lonrl cells, nml ~rerordingl~nlarweswill l,r r,c,verd.
Theory of the Lever Arm Balance (6,s).
Fb
Fa Figure 1.
Lever-Arm Balance of Forces.
At the f d e m m K ,the forces d ~ l ct,,, i h e masses each supply a torq~le?' = PI/ F i g . 1 The beam itself, and the snmplr hr,lding devires, r i l l also cw~trilntt,e torqnes, and these ran he neglected ml,v if the nxlslr.ortion is symrn~t~.ir~nl nlmul. the fulcrum. If the beam is hcwimul:d (n~dl-balmre condilinn) the two I~IITIIIP~ mist jtwl ntnrel each alhrr:
Fda
=
Fd,,
Sinre F = 1M.y, and y is rat~slnulwithin the spwe of the instrument, Afdm =
a d if d,, =
.Vd,,
db,
L
'11" = .if,,
Therefwe, if an equd-:wm lml:tncr is p~.uperly constructed, the mexsrtrernwl, of all ~wkrmwnmass (say M , ) is m : d r by adding known mnises t o the other sample holder tutti1 the hesm is i n l h r hw.izo~itwl pasitiun, m d lolaling the masses dls. The single-pan, s u l ~ s l i t . u l i ~ ~ ~ I , a l a ~ ~ does ~ . c not have erlual wms, su the t ~ q n and ~ ,not the masses are lmlanred \Iwe will he said aborlt this type nf I d a w e shortly. As mentioned earlier, rearhing the exact I - b e is letlio,~, and f~.equenlly one determines the degwe of im1~ala1~r.r by meast~ringthe angle ( 8 ) of deflrctiou of the hesm from the ho~.inoulal (Fiy. 2 ) . A vertical pointer ,nay be used 1.1, irnpmve the prerisinu of t.hia m r a w v c
wa Figure 2.
Reriduol Deflection of Lever Arm.
men(; i t may be magnified s t its lmvw end. The angle of defldiott depelds not ouly on the diil'erenr.e (1TV) Ilelwem TV, nnd IT'&, b ~ t~, l a non the sensitivity (s) of the bnlaoee to lhis weight d i l l e w ~ ~ w Therefore, lo mlcnlate TV., we mltsl m?asll1.e 8 :L!K~ Wb,and also delel.minc s:
IT',
A 1024
/
=
TI',
+ +/a
journal of Chemical Education
Chemical Instrumentutien The rensilivity depeuda on several iaclors: tho combined weight ( W , ) of the beam, sample holders tmd loads, the lengt,h of t,he arm (d), and the distance he1,ween !he knife edge supporting the beam and t,he centel. of g~.avit,yof the nssemhly ( h ) . The lomtion of the center of gravity is ndjuslnble on many balances by means of n weight whore position can be varied npwards or downwards (it is ntlnrhnd lo the poinler in Fig. 3, and denoted ns n ~ m ~ h6n in . Fig. 6). Overall, 8 = Alird/Wth, SO
=
slam
=
,l/rrr,n
The seusitivily llrl~sdecreases with inweesing load R,. A significant advantage of eonslanGload ( s ~ ~ h s t i t ~ ~ t i a n ) techniqne is its conslnnl se~~sil.ivily.
(Courier" Arnerivan Rnlnnce Corp.) Figure 3. Knife edge Suspension. Also shown ore o rider and a chain device.
The ~ensil.ivityis raul.inely determined by o b s e t ~ i n gthe angle of deflection (or number of scale divisions 1,raversed by the pointer) pl.oducetl when a. known small weight is added t,o one of t,he sample holders. If the balance has nn optical scale for determining the last significant figures of the weight., bhen this scale is calibrated by sdjmling l.he sensitivity until a knowu weight produces the corred, deflect,ion on the scale.
The Suspension System A horisonbnl heem is mpported a t or near its cenl,et.hy a knife edge or a torsion hand. The beam mrtsl be light, to place e minimmn load on the slmpension point, and strong, so that i t does not bend under heavy loads. Alumimun alloys are commonly used in aonsl.rnct,ing beams. Sample holders are hung or supporled from one or hoth ends of l.he beam. The purpose of the suspension device is to hold t,he components of the meehs, nism t,ogether while contributing a minimum of frictional resistnrrce t o deflection of the beam. The less resistance, the more sensil.ive (.he balance will be t o slight departures from the null condition. Kuife edges are t,he most popular devices for suspetding beam and sample holders iu lever-arm hslances. Torsion bands m e also often used. Ot,her lypes of suspensions are seleded where law sensit,ivity is srttisfactary, 01. very high sensitivity is (Continued a page A1088)
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Journal of Chemical Education
Chemical instrumentation -~requil.etl (the simpk fixed pivol of n trip hala~lce,and the clun~.trfibel. uf an ,dIt.;+ microbalnnee are examples). Flexure bearings are sometimes wed i u highraparity balances.' Tho knife edge system cm,si.;ti r,f a set, of triangolar prisms fixed to the Iwnm (Fig. 3 ) and resting on polirhetl heal.il~g planes. I t has the fallowing reqni~wneuls for good performance: ( I ) the mnlwinl ft.om which the knife blades are nmstmct,ed must he hard, nnd mlwl mtt deform w d e r any likely load-all the edges must lie in the same plane; ( 2 ) the material must not wear significn~rtlyfrirtion hetween the edge and hcuiltg plane increases if the h w p edge is wwtt or chipped: ( 3 ) the poinls where edges and planes meet must he kept clean. Synlhetir sapphire is the I,e4 material f w construction of knife edges. It id mtlrh hanler and more re&ant to went. than agate, which is p o p r h hera\we of its Iowa. price. Steel h i f e edges :ve snitable only for tr.ip and l~.iple-lxam halanres. Pmvided f h e edges are linear, a 10119 knife blade is prefewed Iu a shml. one, sillre the fowe per unit le~cglhwill be less, and wear will be reduced. Tu prevenf damage to the edgec nod planes, t,hey n1.e kept out of C O I I I R P ~ (iw~.~sled) while weighbs m e added l r , C I I . ~.emovrrl fl.om I he paus or holdws. Torsion snspenrions (do no1 r m h a e with tol&u fwce bslanees, whirh will he disriwsrd later) have both xdvxulages and disarlvantages cornpaved 1.0 knife edges. Two parallel beams are held in posit,ion by three metal bands, which are pulled h u t around t,rrwses (Fig. 4).
Figure 4. Pans.
Torsion Bond Surpen5ion With Sample
The pan(*) and weight holders are snpported by the appropriate hands. The center t r t w is fixed to a. block whirh mpports the entire nssernhly. Dirt cannot, interfere with the movement of t,he beams and wear is not significant,, sinre theve is no friction between parts. Sensitivity is therefore constant over & longer time period than with knife edge systems. Eventudly, however, the sensitivity dow change, and the bands m w t be ret,ensioned. The Ormond Corp., 11969 Enst Iliviera Rd., Santa Fe Springs, Calif., makes flexure joints for use as pivots in bnlnnces and force measuring devices with rapitcities from 500 to 6,000,000 lh. The Transmetrics Model 5301 halance uses flexure pivots to snpport beam and sample pan. (Continued on page A1OSO)
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Journul of Chemical Education
Chemical Instrumentation 111 praclice, the torsion s~lspension balances, and microbalances, offer afurther advantage--the beam is never srrested, eliminat,irlg two or more steps in the weighing operation. Only the less-sensitive precisian knife edge balances do not require a t least partial arestment when adding or removing samples or weights. Torsion suspensions are also much less affected by exterior VLhl.atiolls than knife edges.
(Cou,i*-,, S~rrleiri-Kol,lliusch,In
