tines are sfigtitly ttiirtrrcr. The rnnt.erinI was dcfitiitely deficietrt iic ptdssium nod probably slightly too low in ninng~rne~e to producc

Tho strong' copper lines nre tlie result of bordeaux mixture yprnycd on the leaves to protect them froni blight nnd not removcd completely in the clenning process.

SESSITIVITI- ASD ACCURACY OF JIETIIOD SOIIIC idea of t1i.e sensitivity nnd nccurncy of tho uiethod may bc

gathered from the spectrogrnins No. 8 of spills impregtinted with ~1lougli of D s~rnclortl solution to sup~ily 04003 g. potnssium, 0.0003 g. cnlcium, 0.00006 g. mngncsiuni, O.ooOo6 g. sotlium, 0*000006 g. iron nnd 0.000001 g. ninngnncse, irnd by co~nparing spectrograms Nos. 8 nntl 9 hetween which there is n ten per cent. difference iu nmount of ench elcriierit coiiccriird. The mount of filter pnpcr burnt for spectrogrirnis contained nbout 0.000001 g. cnlciuni, m d ahont 0 . ~ ~ 1 g. sotliuin.

full ctop.

. '

' AORSOWLEDDIIEXTS ' The I~iiilding up o f this nicthod hCis been bnsed Irrrgely on thc

e'xpcncnrc of othcrs who used it in its rnrious stirges much more t lpn the writer.

It wis accelerated Iiy the provisioli of urlclitio~ial ivnrkcrs uiidcr :k,&l!t mode by thr Xgricuiturni Xesearch b I i U C i 1 to nugtnelrt i i joint team coniposcd of the Biuclicinistry Scckiori of thc Enst Xnfliit): Hfsenrcli Stotion wid wurlicrs, inentioned below, front the Itescarcl Instittrtc of Plant I'hysiology, Iniprrinl College of' Scietice and Technolo~:y.

A special debt is due to Major S. G. Thompson wlto uscd it i t 1 i ts cnrlieat nnd most troublesome stnges and who lind the iden of usiiig n coniniercinl oxy-ncetyletic blowpipe indepenclently of t.he writer. He 3130 invcstignted the possibilities nnd showed the value of thc method ng n inenns of dingnosis of mincrnl deficiencies in plnnts.

Mrs. W. 0. Roberts t.rninec1 i~ number of nssistntits in its use :itid iupervised their routine work. Dr. 13. D. Bolas contributed the iden of wnxing the pnpers to innkc tlic folds of the spills stick together. Dr. D. tV. Goodrrll tins used the metliod xnninly for quantitative work. klr. niches usetl i t for ib short tiinc. bliss 15. Ford Inns rfoirc routine rrnnlyses continuously for 34 years irtitl lies lrrtterly becri responsihle for the routine spcctrogrirphic ;iiitrly&, and mutine niinlysca hnvc been ilo~ic for n shorter le~igth of time by the hfisvcs D. Xlnrtiu, B. Kidwcli, D. Hook, J. Onndon nnd E. Pnrker.

Mr. -1. H. 1,nrciitlcr ~iiatle the wrcell (Fig. 28) tint1 took the pltoto- grnphh from which I'lntc 1 nntl the thwings for Yips. 28 iind 29 were prepared.

X i s hl. B. l31;iiji1rd did d l the drirwitkgs fit! the fi#ures. T'he nppunttiiu useif w i s p ~ - h m ~ d tliroo~rgl~ n grzint rrinJe l q the

Govcrnntcot Grirrit Cotiiniittce of the Royal Society. Hioclieiuibtry Scct.ion,

Enst h[alling Rc~eurcli Station, Nr. i~luidstonc.,

KClIt. llscelved October 17, 103:

INTERNAL STANDARD hUZTHOD OF SPECTROGRAPHIC ANALYSIS AS APPLIED TO THE DETERMINATION OF

LEAD IN HIGH PURITY ZINC* .. By LAURENCE CRIFFlTHt and JOHN N. KIRKBRIDES

This ruimrt i H nn ncrouiit of ni i iirvtwtigitioii into tliu fiindnnic~~t~rl fcntcirt-n uf thu intt!ntnl st~iridiird nirthod of wptvtrugrnpliiu nnulg~ix, a i f h qwrinl NFVWIIW to thu 11rterr~ii111rti~11i vf lmrtl in high piiritg zinc ri&g tlir tliivct ciirrunt nw.

111 thi. ~~n~l i i i - t iu i i of zinu Iiy thu clrctnilgtiv ~ir i~~ ' t *w* . lei111 is tliu ~ I I I ~ I , U I ~ I ~

'l'liih p1wr i n i a writtun h r . cwi*ii(miticin by ttir 1I.S.I. 1'1111~11 whidi pm- parti1 tliu l t ~ ~ r ~ i i i i i i u i i ~ l i ~ ~ l J l u t l i o ~ l ~ fur l'~iliirugrn~tliic nnil S~w~~rnjirupli i l : AirtrIpi* of Hiyli I'iiritv Xitic urtd Zinu A l l u p for I ~ i u C'wting. It hn* I r u n coiwiitultil tIiu 1 P i i i i d iind is refurreell to in tho Iiitroitnction ti) tho I I ~ L - I I I I I I I

of t11u I'I~IIPI'U work J~~bli*hL*ll IIV H i d fihjWtftI Stntiunarr miw. t AwLlnnt Su wintwiil~mt nf'l~twnrcli, Hntlroii Uiiy h i i x iind Skiiullirrg

Co., t t 4 ~ Flizi hou, 31aiJittjh- t 1n C$irge, .Sp&nzpc&iu L.&ur&xj,, 1LB.M. k S. i!a.

#$'-

rvbirh ruqubs tho clove~t cotitrd. It curnei from tlro load oncdw w d in thc clcctmlyeia nnd since thcir cundition. with mspcct to tho tmwfcr of lend from n n d u to cothodc, chnngcv ccintinunlly througliout Clie cyrln fwnr ona clcnning period to tho ncxt, tho ~ M C I C of zinc from ony pnrtirulnr row of trinks in tlio tnnkhoupo must h deturnnned dnilv. if tha cathodc sheet8 IIFD to lm sent to thc cormct molting furnm. 'For zinc of diu.. casting quiility, tho Britidi StunilnhLq Sl~ccifiartion No. 1003 olloas only i).003% cnch of Icnd. cnclniii~in und iron. Tho lilttar two iicqniritiorc nm elraily kept Icm tlinn 04JOl~o in tho cloctrolytic zinc pmww. but lcntl ruquires control. Obviondv to keep tliu tonnngo of thid gmdo of i n n ~ d iw high na powiblu, tho l c d is run 08 closo to the limit as is snfo. The morgiri of rnfcty ~ C ~ L . I I ~ Y i ipn tho ncciimey of tho wnilnblo mothodd

'of nnnlysis.

S11cc!r~~rup!c An EKE Hilgcr Antotiintic large quartz ~pectrogrnpli s*ns etir-

pl0y~t1. With tlic plnte tliiiptirngin witlo open, 11s is required for step sector esposurcs, reflections froin the collininting lens ciruscil hcnvy fog on pnrts of tlie plnte. The colliiiintor llinplirngrn which vovcrs the centrol portion of tlic lcns, WIS thercrore widened until 110 rcflected light could he seen with the eyc pliiccd ot the position t ior~~inl ly occupied by tltr plirte Lolder. Tlic slit din~il~rngrti hos beeii rcplnccrl by one hnriiig ir slot long cnougli to occo~~nno~l;rta the fu l l Iriigtli of t.Jw step SPC~QT; mi11 with six 0,090 i ~ . 1~01~3 en& ottpsitc one step of the step sxtor. 133' dieting tire ycot'er l d c iii lroiit of tlie slit, csliosircs iiii~y he cut to zt suitiiblc value in tlie iiinnner usritrlly carried out by 11 rotating sector of variiiblc opcihrg. The step sector h:is sis stee3, each' 0- 100 iu. high : the stqi]"d openings give retirtive tritnsmissioii~ of 2, 4, 8, 16, 32 irnd 64. The shaft for the sector is ciirried by Imrriiigs in 16 steel post, 1vhic11 is inoiintetl on the opticiil bench so tliirt thc scctor rotates, art 953 r.p.ni., i r n eighth of nil inch from the dit. A Iielt drive leiids to the motor diicli is rnouritctl on the 8~lcctrogrrlph lieitch tu one side ; two earlier set-ups with tlic motor on the opticnl Iieiicli giive II little trouble due to rilitntion. Tho colidenscr leiis, wbicli is inonntctl n few iiiillitneters iieck of the sector, focuses nn image of the irrc on the colliniiit.ing lens, tlie itppcr clectroile being iiiiiiged on the colliniritor diiiphriIgin, tlic lower on the triotlifietl diupIir:igia covering the centre of the icns ; thus only the lo\ver tltirti or t i i t t colliinntor is used. Arc Sltriitl

The iirc stunt1 was origiiinlly of the striniliirll Hilger type, Iiiit t h insiili~tors coiiltl riot sttiitti, the Iiigli ioltngefi from the igtiitor circuit. 'I'Iie irrnis wcrc thcrbforr reinrircil u d strips of rcd fibre 4 in. x 1 in. x f ill. were firstencil Di i 1)s hDks tlrrouKfi the oriFiiiol irisultrtors. lieirvy spring el&tnilo holrlcrs, sindlur to the B~iuucli arid turirlr type, are hltccl'to the Irw citds of tirest. s t r i p , wiiig ieicvy knurled ,nuts so thirt. ticw c l i p cnti be i r rwctc t l quiddy. UIqicr clcctrodcs (6 in. ~rnplti tc rocls) itre iitoiuitcd directly in tlio upiicc eliring clips liut the lower (& x I i i i . ) clcctrodcs nrc pliiced in I\ ainiilt cup 2 in. tliiuni. x $ in . Iiigli, with i t 41 in. hole .f i l l . deep 44iill with steins 3 in. x :f in.; no set si*tc\vs to l~old tlw electrnrics tight irre uscd.

spring c l i p i r n d Iii\vcr elc~ctrotlc holders irre cli~ingerl wheii IL

ct~ntngc is niirde froin the iinirlysis of zino to thiit of c.irilmiuni or copper ; they are wiislicd with irciil eitcli night. Ercitol ioir

The direct currcllt, iirc \pus used with i in ignitor circuit ; it sclicini~tic diiigrnni of the nrc control circuit, whicli was built by the Cnnutlinn &lienil Electric Conipiny, is S I I U I Y I ~ i n Fig. 1 illid the ignitor circuit is sltowii in F i i . 2. \Vhilc the roltirge in the latter is high (50-70 ICv.), it requires iio protection us the power is wry low. When it WUY

originidly put into opcrution, it ,wiis Ito~id' thtrt it would servc to dccrcrise the btrckgrountl ; this hope I i i ~ riot bwii rcttliscil Iiut tho griwt ly inilirovcd Btoadiiiass of the nrc tliorou~lily jw4fics its use.

Tile uro is very stcirtly ; the horizontal motion or tlic nrc st~arid i~ tiever used aticl, if cum is tnkc irr sottiiig tho ol~ctroilcs, tbo wrticel iiioti021s do not linvc to I )o toiie)ieti nfter the stnrtiijg hatton is psuvr i .

A I*P.IIL\T US

Titie Htcuiiiriwu is ciup b xlf,veraI ftictorx : ..

u) G R l m M A P V ~ K R I U D ~ - - ~ h T E B N A L s T A S D A R S blEPNOb Or k'UTBWJMPE1CJ AN&L>WS AS APPLIED TO DETERYIYATIOY OY LWD [ F S ~ W . lm

111 m e short lower electrodve. (2) The typo of loner hoIder used. (3) Prchcating Of the lower holder rind the ulipet elect~odes

(1) The heavy arupernge (16 amps). (5) The ignitor. (6) The make of electrotle. (7) The zinc base. (8) Tbc dryness of the electrodes.

Densi&meter The densitonleter was the Applied Rewnrclt Laboratories I942

mdel described by Diekrt anti Schuch1, uwd throughout the research described here, the response cuvc was not a straight line but wns as shown in Table I.

TADLE I

before starting R series of nrcings,

I(eaponae Curve o/ Dmuilolncler Ileading True Treosmisaion

6 3-9 10 8.8

some years: 20 18-7 30 28.0 40 3B-2 50 JB-4 w) a - 7 i 0 89-8 80 7B-9 no Qo.0

The eflcct of these fnctors hns been studied over n period of [T:$J The figures in LOO this table were obtained hy 100-0 making two sets of

readings on a plate calibration pattern, e filter having a tranemieaion of about 70% being in the projection lnmp beam during the epcond sct. The response curve was found by determining, by trial and enor, a curw which satie6ad the ttvoaefsof readings. hngemente have since been ma& ta stdghten the W8pOnSe curve by inwr-

t prating iuverse feed back into the trmpliticr circuit of the densito- H

* W O V W F W Gm * rnekr. Attention is dmwn to the deiisitometfr reeaponsc primarily to

indicate that, even in an instrument as well built and 88 well designed as the A.R.L., non-linear response can occur. In certain of the ehpler methods of spectrographic analyeis, notably those in wbich plate calibration is not employed, the photmnctric ratios am a function of densitometer response : under such condition9 it should not be expected that the analytical curve will be a ~traight line.

Two testa were made on the reproducibility of the deneitometer. A single line was reed 40 times ; the average rading was 12-03, the standard tleviotion 0.142. In the second tag0 the lead line

125 V CXCrmL

CRLCr2,CIW ... kklays C . . . . . . . . . Clutch Colt TIt .._ ,.. TlmerX&)n M . . . . . . . . Tlmer Motor A . . . . . . . To Am 11 . . . . . . . . . Ammeter O - U I a m p Y ......... Yottmeter 0-3MI tnIU 8 YrnL 8riteh R 15 obm W l d l O I

. . . . . . . . . . . . . . . . . . FlQ. 1

CRS... ... XeLy TI ... 1 1 0 - 1 0 ~ volt tnmformtr u.SO E.V.A. CI . . . . . . Coldenkr 0.005 mld WUY, V UDPU worou L? . . . . . . LJ C o d c o w s ~ ~ * - m f d "S& v C o d o u r 0.w mld" w'wo v: . . . . . . . . . . . . ...

YuIrrlul : mllil r t a l C b k a (60 turns, 10 c'ol 'dla M 16) fi B.F. Inruramo. ~i& i turm 10 cnr. W. FIQ, ?A

Dinyrant OJ Elrctratr I l d d i r Hrmnd.ry 260 tm 6 cm. dln.

FlO. 2.

i71riior circvit aDjst6rt. H. W. and Sb'hSdI, J. J., Opt. he. hmar. I fu t . at. S.

2833.&? and tire bismuth iinc ~897.98 were read euccesiair-ely 40 tinics. The average ratio wns 1.0893 and the stnndard deviation 040556 or 0.61%. Since the B I O ~ R of the plnte cnlibrntioa cutvo niid nlao of the analytical curve nre nbout 46", the error contributed to an nrinlyais will nlso be nbout 0.5%. These results are nffected to eonie extent by the plnte grain error.

THE PUNDAMEXTAL PROBLEM OF SPECTROQRAPHIC ANALYRIS The bnsic nssuniptioli of spectrogrnpbic nnnlysis is thnt, when rrn

element is excited by some meniis or other, the intensity of any of the nionochronntic rndintions eniitted by it is in some way relnted to the qunntity of the element which is excited. This relntionsbip is lint n siniplc one, since the intensity of the light emitted depends up011 mnny fnctors such ns the excitation con- ditioris mid up011 other elements which nre excited nt the Bnme time.

If the high rolbge spnrk is used ns the source, sag betweer1 ainc elcctrodes contnining 1% lend, after 11 short time the intensity of the lend rodiotion, e.g. nt wavelength 2633.07 A., beconres quite stcndy, nnd rcninins so, siircc t.herc is n relntively inexllnuatible supply of rnctol to be vnporised. This intensity may be relntcd to the qunntity of lend in t,Iic zinc and so umd for catiinnting its pereenfage. The URC of the ekctric arc lretwecn p p h i t e ekecctrudes giws quite differcat conditions however ; in the method of ntin!ysis describcd in tbie rcport only 33.3 milligrams of tlie zinc to bc nunlysed is put iiitq the fiource. This qunntity of inetnl is rapidly rnporisecl nud dieappears a i d nny lend in the zinc show its clinrnct- cristic rndiotioir only for nbout 30 seconds nftcr the nrc hnR been struck. Morcorcr, the intensity of the lend radiation vnries througli- out this period.

In the arc two distinct plicnonicna occur : these nre the transfer of innterinl from tlic grnphito electrode into the gns spnce betwctir tlie electrodes, nnd tlie excitation of the innterinl in this gns so tlint i t emits nioiiochromntic rndintions. The trnnsfer of uinterial into the gns spncc is very similnr to n rnpid distillntion : the grnphito electrode ~ ~ c t i n p ns the distillntion vessel, the vnrious eleiuehts mid cornpourids come off roughty in the order of their boiling p i n k . Supcrimposod on this there is n certain nniount of uiechnnicn! cnrry-wer. especially during the first few monieiita of arcing. Once in the gas spnce the temperature of the arc, which rnnCrr from 3,500" to 9,oOo". causes the emission of nionochroiirntic rndis- tion8 from nny rlcnicnt. approxiriintely according to the follo\viii~ cquntion :

AE i = NP e- - 1T ..................... .. -

where i is the irrternjity. N is the P U ~ J C C of utoriiv per uuit voluiiie ; L' tttc probehiiity o r

traiisitioii fmni nnc to the other of the two electronic levele involved ; AE the energy difference between the initinl uiicl finnl levela ; k the Roltzninnii cunetnnt ; nnd T the nhsolutc tenipernture. Sill<-,. the equetion npplics truly only to m systeni in thermal equilibriurti, T in the case of an nrc isbettertmiued the '' tenipcrnturc 1inrnnieter."3 N and 1' depend upon the conditions in t,he urc. N is controlletl by the rnte of trniisfer of ntonis from the electrode into the gns spnce untl by their truiisfer into the surrounding cool nir. 1' depeiitls up011 niirny hietors such as the coinposition of the gw, the power input t o tlic iirc, ete.

Both of the nbove fuctors, h' ailti T, niuy Huctuitte greatly frutll onq nrcing to niiotlier and, while much cnn be clone to obtain stearly irrcing cotiditions, t!ie chief method of ottnck hns been of a co11i- pcnwtory iintiire, P I Z . : the interiinl stundnrd proposed by Ger1ach.b The URC of the iiitcrnal stendard is bnwd on the following cousidrrt~- tioiib : if tbc ninterinl to lie nnelywd contains n fixed qunntity of sonie cleiiicnt other thirn that, to be determined, or hns sonie sncll eleiirrirt aildrtl in fixed quantity, tlieii the rntio of the iiitemity of n ccrtain spctral line o f the unknown element to the iirterreity of II

WuWnJnrk. 0. S, utril 1.11 Rur. J. 31., J. Opt. bc.. Airm., )MI, 81. 146. 6Ce?krh. W. and Svbwilxr, E., " Fouurllrt ioirs niai Jlc.tha& of C1w!nh~rl

Anrilyria iiy tlw Exnisxiun Siwtrum.': Lm11ui1, Aikm Hifger f.&. I@%.

AE N, Pa e - -$

- - Na Pa ......... (2)

wliere the subscripts 1 end 2 refer to the eleineiit to be determined nnd the intcrnnl stniidnrd respectively. Now if the iiiteriinl stnudnrd element is so chosen thnt i t distils from the electrode nnd diffuses from the gns spncc into the surrounding nir at the snme rnte ns the element to be cletcrmincd, nrid if the 1ight-einitt.iiig atoms nre in the 8nnie stnte of ionisntion, then the rntio N, to N, will be a eonstnnt. Furtlicrmore, if the two spcctrnl lines chosen nre such that El = E,, the exponentin1 term becomes eqnrl to unity nud the

tenlycrnturc of the gns censcs to hnvc influcncr. The rntio 3, La

if i t can LF relntcd to the .coiicentrntion of the unknown clement jji the mntcrid being nnnlysed, a i l 1 thus pcrinit nu improved pre- cisiojl to be o b t n h d , nstho mtufyais xi11 he indep~siiknt ofcxcitntion cuizdition.r. Perfect i~iternnl atnixintiis nttd perrect pairs of iirrea are inipoasible to find, but very satiafnctory pnrtiol cornpusation coii be obtained. It shouid nlso lie noted thnt the rotio i, to 4 i3 dijnensionles ; the rntio is unchnrigccl 110 lnattcr whut units of nioosureiiie~~t mny be employed.

It is po9siblc to mensure the intensity of n spectrnl line directly by use of n bolometer, photo-cell or other such device ; if the illtensity is con8tnlit, ns is the case with well-dcsigued spnrk methods of excitation, i t Cali be correlnted to concentrntioii. In the nrc with graphite electrodes, however, the intensity of nn element plnced iri the clcctrodc chnnges continuouslp with time nnd the correlation of iutensity with concentrntion becomes nlmost hopeless. This difficulty nioy be overconie by noting that, if i is IL function of rnticent,rntioit, YO nleo is

.................................... I i d t (3) J

where t rciircscntcl t h e , the liinits of the iiitegrnl beittg arbitrnrily fix~d.30 This integrntion cnn be cnrricd out by rnrious photo- srilsitive chemicnl renctions, t,he iiioit convenient o f which i e tho photogrnphic plntc.

I%e cfrarctcterisiirn QJ Phkgrqdiie Plaits

1'9

PHDTOMETH\'

The trniimikiOI1 of n developed photogrnphie ittiup iu defiriccl

(4) Trniisinission =- ........................ I I,

wilere I is the light trunsinitted by the irnngo nml I, iu the light traiisiiiittetl by n clenr uliexposetl portion of the plok. Density is tlefiticd lig the relutioii.

I) = logl, Truiisiiiissiori = loglo ............ (5)

1)eiisity is p p r t i o n n l to the nuinbcr of clevelqwtl silver grains. The relutioll betweeii density niid exposure E (which for the

Itiolllelrt mny be ~Ooee~y tlefillcd ns qiiuiitity of iightJ WUR investigated by Hurter and Driffield" in 1690. If deusity is platted ngninst the lagnrithiii of expojure, nil S-sha1ietl curve CCItUkk ; the '' toe '' nod m i ~boultler " nre curved Ijut between these is u long straight line prtioii. Ttic equntion of tlie stntight li~ie.portion of siic1i.n curce is givcn by

rliere j , the inertia, corresponds to tlie value of E rvhero the (ex- trl1llrtl) straight line CUtH the line 1) = 0, unil & is the R I O ~ of tbc

I,

Such 11 curve is shown in Figure 3.

........................... l ) = j + 8 I o g E (6)

4L ligllt'nll AXD L\IRKURIDE-I?ITPA?IAL STANDARU . \ l S T I l b l b SrWZU

atsight line. The vrrfuc of 6, which m y be termed the contwst of tke @ate, is depcndent upon the deveioping time, incrensing as the developiug time is increased up to n maximum hm, which is chnracteristic of the type of plate used.

r I

- I

Ro: 3 Log CrpoerrrP

If the exposure E is given hy the qunutity of light, i.t, the pliato- grephic plnte is ohriously n w r y sntisfnctory method of eviilunt~iiig the integrnl

i i (it Photocbemicnl sptcnis, tlic exteiit of w.hose cliniigc is proportioiial

to this intcgrnl, are mid to obey the Reciprocity Lnw of Buiisen and Roscoe. nnfortunntely, photographic mnterinls do trot, in genernl, obey this Inn-ir ftict which may cnusc much clifbcult~ in epcctrogruphic nnidysi~. The vnlue of i.t required to p~orlucc n certniii clcrisity is iiot n coilstant, indepeiideiit of tlic ititetuity used, but vnrks with the iritrirsity. If n plnte obeys the reciptocit~

......... (7) Thuv if t i pttcrn were put on n plnto by ii series of ligiih ( i f kriowii

intensities, nll nctiiig for the mine time, and nlso 11 pntterii liy bing n h e d light for H series of known tiiiies, then tlic two curves, density vcrgus intensity and density versus time, wheu lrlot tcd sido by side, would be pnrnllel in nll pnrts. On the other Iiaiid,

if the reciprocity law werc not obcyed, tlie curves woultl iiot be pnrnllel, but tliey would indicnte to wlint degree, riiid iii wliich density rnnges, reciprocity law failure occurred.

A second phenomenon which occura in photogrnpliic pliites liiitl

mittoucy effect. In general, n coiitiiiuous cxposurc of intensity i for n time t docs not produce thc same density ns n seco~id esposuro of equal energy given in a series of discreet iustnliiients. ititer- mittcnt exposures, produced by n rotnting sector, nrc very cori- veuieut iii epcctrogriipliic iinnlysis for obtaining i in expovurc of suitnble ~nognitude nnd nlso to olrtniii a series of kiioini reliit ire cxponurcs to determiiic tlic relntioii between density niid crposure. It hna been sliowii by \Vebb13 thiit, if the frequency of iiitcrruptiuii is nbove n certnin I c \ d which depcnds up011 i, tlic irrtertaittciit expoeuro produces the sniiic efrect 118 n coiitiiiuous cxposiire of t h sonif totd time.

Inw, then D = j + i l0gE = j + 8 log i -f- L( tog t

which is eonnectetl with reciprocity Inw fnilurc is termed t h L ' inter- '

~*$VPM>, G . A, J. 51~1. SLJT- Awr. , tW1, 23, I&?.

WUPiJlo AWAI.YBIH AS AI'rLiI$o TO J h r t i R m I N A t i n s DY 1 .~41 , IrsDrau).. z v m

A third phenorrr~nori which carney difficulty is termed the Eber- hard effect. It occurs when hrge differences in dcnsity occur over solnll distances on the plate. The effect is due to locnl differenccv in the concentration of the developer, which ia exhausted in the regions of high density. Owing to it, n density-ititcnsity curve for a spcctrnl line hns n grentcr slope thau one for continuous light. The Eberhnrd effect is controlled to some exteiit by the ngitirtioii used during development ; it tlisnppeiirs if I& plutc is developetl tn innximum contriist.

A fourth chnracteristic of photogrnphic plntev which cnn cnuw tlifficulty is termed " Intensity Retnrtlntioii of Developinent " ; this lins been well discussed by Strock.I2 The Piloior*ie(ric Method Employed

Each pfntc is ciilibrntcd by impressing a step sector pntterii upon it, nrcing iui elcetrode impregnirtetl with a zinc sample (coii- toining bismuth) RS tlie source. This piitteru is usunlly plnretl iit the bottom of the plntc followiiig tlic rpectrn of the snniples. The bismuth liue 2897.98 is rend in thc ci~librntioii p t t c n i aiitl the mndings lire plotted ngninst the relntivc exposure tiincs on log-log pnper. The lend line 2833.07 in thc first snnrple spectrum is then mud, wing the lowest rending obtnincrl during the trnvcrse ; the background is rend nl)out 0-5 n ~ a . to the high ansdcrigth d v , using titr highst reading, tben the Itistnuth litic 2897-98 arid its background. The second spcctrutii ia theii rend, but in revcrw order ; this procedure is used to miuimise the effect or uny deriaito- incter drift.. As ench rending is mnde, the correapondiiig '' is rend from the mlibrntion p p h . Elicli line rncrgy is cur- rcctcd by cletlucting the correct biickgroiiritl eiiergy rind the riitio - Corrected Iaird Energy dirided Iiy Correctccl Bisiiiutli Energy -is cnlculntetl. By refemnee to nn '' Aiinlyticd Curve " built up by putting snmplcs of known lend coiiteiit throiigll tlic procetlurr!, the photonictric rutio is t.rn~iscril)erl iiito Ii*iitl coiiteiit. The P l i t a Used

Enstmun 33 pliitc.~, with untihiilntioir bnaking:, liirve been user1 throughout the work. These plntcv show ttinrked devintion from the reciprocity lnw ut spectroscopic levels, the chnage in exposure necesenry to produce n clcnaity of 0.6 chnnging from 0.4 uietrr- cnndle-seconds n t 0.5 seconds exposure to 0-fi rtieter-cniidle-secoiirls nt 30 seconds exposure under stnndnrd duyliglit i~lunii~intioti iintl stnndnrd clcvclopi~ig cniitlitions.' I t wng funred tlint this ftiiliirr~ aoultl ciiusc errors iii iiiinlytici~l work i b l ~ l l . 11% showr) bclow, t . l irw crrors hnre been reiilised.

The plates are developetl iii Btrstiiiliti Knolhk UGIP i t t 18" tu it

cbntrost of 1 70 jubuiit 7 i,iinutea), f i w ~ d i,, F5 for 15 jninutcs, IVYOSIRII i5 rrrircutrs wid dried i i i II stmartr ol wmti air. 'Pwfw-M.rc rtrlt)ar- lined tiiiiks ininiersed ill ir constliiit tciiilienitiirc birth are risetl foi. rlcvclopiiig ant1 fixing. A friiiue, tiiiigecl tn oiie side of the cuiistiitit tciupcwtiire l~irth, ciirries tlie developing holdcr~i ; it rises wid rlrups shout 5 c m . 100 tiiiies per minute. A series oC clcirsitomrter rcadiiigl; oic II plirte uiiiforiiily fogged to II dcrisity of 0.3 iind tleve- loped nR tibove is sliowii iii Tiible 11.

TAUIX 11 Uiivvriiiity if I)etelopiiiuiil rcilh Urivlopiiig . l lc ,di i~e

lmft .\litltllu lliglit

.\llIkllO . . . . ! 6 * 8 114.5 us*;> lW.0 Ub41 I%ottoln .. w.0 97.8 93.2 118.0 08.5

For coriiiinrisori, iiriothcr plutu froiii tlic ~ t i t i i ~ box, fogpd iri the siitiie wily, wits riirefully tle\~elopcd iiRiiig the brusir tecliiiiqiw ; tlie results lire sliowii in Tnl~lc 111.

Othrr siiiiilnrly tcstctl plutes tlerelo~ierl i n tliu IiincIiiiic H 1 1 0 \ \ .

'Euntiiiun Ko$k Coriipniiy " PliutograpLio l ' l ~ h a for 116" irt Spuctrowopy

"Strork, 1.. W.," SIuctrum Bnalys~s with tliu Curbon Aro U~tbDdu kyrr."

I'rm, Somnth Suinrncr Confmtiw on S p t i r n n c q ~ ~ aid itr Apjdiru~ionn,"

T y .. .. !)7*2 1W*O 100.0 100.0 100.1)

urid htmrioniy, 1037.

lo,i~:fuu. Aduu Iiilger Ltd.. IO3LI.

Kuw Vwk, ,loliii Wiuy nnil f?;ana, ID$%

similar vnristiona in deiisity but with the gmrletioti iit otlier directious. I t hee thcrcfore been concluded that clirectiotial devclop- ment is hot occurring. The virrintions found im similar to thosc dcncribcd by Dobson, Griffith and Hnrrison2 11s occurring 011 plntes derelnpcd in thcir specinl t,niik.

TAIILX TI1 ti?iiforiiiify of Developtiiciit i t~ imj Brush II'cchitique

Loft Middle Right Top _ _ .. 04.0 02.S 93.0 !18-0 08.5 M1ddlo _. .. I84.0 92.1) 92.2 90.0 . 07.8 Bottom .. 03.8 01.5 !)8*8 II7.B 100.0

INVEETIGATIONS IXTO THE ~'IIOTOIIIETJIIG I\IETHOD

Thc photometric method shich has been described woultl be exact, provided that the plrrtcs cuiployed showed 110 reciprocity law failure, nor Eberhard effect, uor intensity rctardntioii of davclop- mcirt, nor other such uiiplenssnt chnrncteristics. Proridcrl ccrhin rigid conditions, which I inw heen specified hy Dobeon, Griffith and Hnrrisonz ore rnet, it is indecd possililc to obviate thc errors introduced by thew cliaructeristics, hut unfortunately thew COII- Ilitiona arc too difficult to sntisfy in routine nna1yRi.s.

AS pill bc %bowl hter, ibtometric ermrs h ~ v e nn inilixxice on the dcterxrriiiution of kad k xiirc ; tsc therefare did rriueh work investignting the [rhotoirictric inethod in the hope of eradicating these errors, either by cliaiigiiig to a now type of plate, ur chniiging the proccdurc. This work hiis not been Iirought to R successful eonchsio~i, but in the followiiig sections ir few disquieting cxpcri- mcnts nrc described to cnll nttcnPioii to the problem. (a) I'alidity af the Step-Sector

The step sector has been used for plirte cirli1)rntiou in ell the aidyticcrl work. The rnlidity of it.s use has been tested by corn- pnring it ngniust n step dit which wns built for the purpose. The step Slit wn8 illuminated by throwing the iinagc of B pre-burnt high-purity lower electrode through i t on to the colli~iintor : n zinc sninple carrying the bismuth interuul stuudarcl wns used with tho strp seetor : both cnl iht ion jiattenin vicrc inudc 011 the saniu plitte. The results of sue11 ii test nre shown in TiiliI~ IV rind itre illitstrirr~d i n Figure 3.

T A R L E IV Coatpariaon 01 Stpp Slit trrrd Step S d o r

Sfrp Slil (0.5 uec. u p . ) Sfep 8cdor (:XI dec. txp.) R o l n ~ n o Dcnsitolnctor I l O h t i ~ O LMnsikoiuotcI- LiBhL A Rending ESptlUN3 Itcniiiiig Bi

Admil id o t ?.W Ang. Time L ~ O 2,811;. nh 1 -1 03-0 2 98.4 ] a i l 88.4 4 88. I 3-0 7.171 K .xi-c) 0.9 4iJ.D 1u :15.4

1 3 0 1'8.9 a2 11.1 28.d 17.6 01 fl.0

As will bc shown Inter, tho coiitnist for contiiiuous light is nut 38 grent PB thut for n spectrul line owing to tlic Ebcrhard eflect. Tho abovo test wns thcreforo rcpentetl, using the coutiiiuaull light from L g10Wing electrode 11s t,he.sourc:e for 110th fitepperl slit mid stoppcd sector, uutl the results lire slio~rii in Tiiblc 1' iintl irrc illus- t.rnted in Figurc 4.

T.ulL*; v Sccoi~d Cuiiipariwl of Slcy Slil r i r r t l S t e p Sretor

Continuons light at alwnt '1,UOO A Strji Slit ( I *ii SCC. crp.) Sfrp Stit (5 we. my. wide alit)

ltolari\ o J)rnuitor~iotcr Expnilm Ikmritoiwtur Light RC~tIdiIl~ Tinio Hcariinn

llclut is0

"

The curve6 are by no means parattet, and in&& thet serious reoipwcitp arrora CDU ocour.

' ' O I

Fro. 4

Log c x p a c r c

The eflect of qiecd of rotation of the sector w i i ~ irlso iuvestigiitrtl. Results are shown in Tublo VI.

TABLE VI J:ffed of S& OJ Rdolios OJ Step S d o r

JIeuniimrnont~~ on Bismuth Lino 2,847-08 S p d

Tiino O S r.p.in. 61 7.0 13-D 32 14-1 30,2 16 30- 1 01 .G 8 55.8 86.8 4 . 82.8 07.3 ? M.7 33.4

The wiitrwt lies not \mil dituiged by the cfirrugc iu spcd , but tlicre is rliffercnce of shiipe in the law cxposurc portion of thc CIITVCS.

(I) The Eberhurd Effkcl The vnrintion of plate coittrust over the w~veleiigth rirugc iiecdcd

in the nnulysis for leiid is shown in Tnblc VII :tnd i R illustrirtcd i l l Figure 5.

TABLE VII

30 r.p.ru. Rolntivo Exposum speal

Coiiiprriauii of C'urrtrrrele of Lead am1 Bimsttth Liiice otld tlieir Backgroirrrda

1'LItO Expouulu Lino 1% I.lll0 Ui

"5.1

If1 38.3 74.2 '8-4 U B . 0 n:i. u iJ(I.2

8 48.7 8o.n

07.u 4 76.4 07.8 :! 02.8 00.2 111 -2

I t nil1 I IC bCCll tliirt tho s lop of tho bimutlr liric is eliglitly bat Ilsfi~iitcly ICSR than tliut of tho l e d liiio ; the bnokgoui~tl eoutrust i s dcfiritcly loss, dthough it uppcim to ba comiug to tho s~tuio slop6 08 the baokgrourid beconlea Itwvier. To test this. 8 plate \ v a ~ girric cigitt. tititvs +Ate iiurmel cxpeaurc r i d &he cnIibr&ou pnttrru rend,

llolat ivo Yb Bi

Tiine 2,833.07 Background %8B7 88 Bat4;grooud

8-o .I?.:, RBI' w 7.1) 3 . 6 I/loi4:i 3? 15.0 Go4 16.2

UU

The results i s r e ahowu in Figure 6. The Ebarbanf effect is this exccrtiog an iofIr(euce, and may MUSO eerioue emra in the bnck- ground correction when the photometric method which Iins becii outlined is used ; the m o m should become smaller ns the hackprmld

Ro. ;I Lop rxpoaiirc

density increases. A pnir of plates developed, oiic with thc dCVel6p- ing machine, the otlicr by the brush, showed thnt thc slop0 of a bnckground cnlibration wns closer nt all densities to thnt of thc bismuth line when hush devolopmant wnR used. Tho oliief dis-

advantage of hush developing ia in the cost which would entd an additional $600 to $700 per genr in labour alone ; thia ia equivalent to 1 to 2 ceiits per determination. (c) Other Factor8 Affding the Photottrefric dlclhod

Thc validity of using for cnlibrntion bismuth liuo from nn arced snmple, which is supcrimposcd on n background, wna investigated. Two calibrntion patterns were placed on a plate, oue of nn arced 04032% lend stnndnrd, the other of n spnrk wing load electrodes : the lead lines 2,833.07 were read. The r ~ ? S d b , which could be predicted from the datn that have nlrenrly beerr given, are showu in Figure 7. These curvcs nrc very dilferent from thosc given

,

by Pierce nnd Knchtriobe ; the curve they s h o ~ for a cnlibratiou asing a linc with bnckground is no which wo have not been able to rcyroducc experiincntally; it nppcnrs to be lev~4l i~g off a t a ileiisity of 0.16 which is tho effect of crtrnncoos !ight, that je, liglrt not contrnllcd by tho 8kp seetor. Figure 4 which is thrr wlibnitim z:uwu For n wry tienvy exposure, strows sigrrs of ti& cffcct, the fog being clue to stray light in tha spectrograph.

Tlrc spnrk cnlibrution shown in Figure 7 possibly ahowe e.oInc hint of stroboscopic effect, nlthough tho sliced of the sector (962 r.11.n~) is not in phase with the high-voltage tranaformer supply (60 cycle). Howcvcr these experiments have beoii repented bc;til with the IVeNtbury A.C. arc oil silver electrodes and with a D.C. iron arc, both ruii so as to obtniii light backgrounds. The long sweeping " toe " iqqieurs to be chnrncbristic of the D.C. tire wit], impn?giiatCd gruihitz ClCCtrOdCS.

The plates arc uniform in their chnrircteristics niid it is quite ensy to hold the contraet within 3% of 1 *O ; the dcvelopiiig time is cliauged slightly ns tho doveloper ages. For routine work i t would be possiblc to dispense with calibrnting each pinto. We have not found that liue densities chauga with nga, BB wns reported by Nalpica7 to occur with Hamnicr Dry plntca : orla plate which wns rend deily for three days ant1 then each weak fvr four weeks showecl 110 progrcssivc chnnge in any of the dolieitice nud tlic asmy values were the siiiiic until the plute becrriire hopclcns to rest1 because of scratches.

The e f b t of the stray light, which occur& iiinido tltc spctrograplr "ilpiw, J. T. N., Qon. Eleo. Rov., I N , 48, 2k8. VPiercv, 1%'. C. end Neokkiel, N. H., Id. gag. Chonn., An& a., LNI,

t8. 774.

when the Littrow mount is used, can c a w trouble in plate calibration in that the exposure value assigned to ench step of the calibrntion pattern should bc increased by the strny light intensity ; the stray light may or may not be uniform over thc pnttern. At the hegiuning of our work on the internal standard method this etrny light gmc much trouble and strenuous efforts were made to get rid of it Our present belief, that the smnll amount that still remains i s negligible, may be wishful thinking.

The effect of plnte grnin hoe not been investigated ns this involves changing the densitometer slit. The lntter is only 0.012 Inn. (0.0005 in.) widc by 1.1 mm. (0.045 in,) long, nccording to the mennfneturer, nnd there is no doubt that the precision of the inetbod of nnnlysis could be improved by r d i n g 8 greeter nren of the plate. Any such slit must, howcver, be n compromise between resolution and heccurnciee due to p i n . For some of our work 8 wider R l i t wodd be unsatisfactory.

PREPARATION OF SANPLES Preparation of eamples is very similar to the method eirrploycd

i i r tbe A.B.T.Y. Tentntivc Nethod of Test E 26-35T (Quantitative Spectrochemical Aodysis of Zinc). A snmplc of suitable size i s di~~olved in an c q u d mixture of Iiydiochbrie and nitric actda, hoifed down Somewhat, niid co&l ; the biarnath intacnd stant!urd if3 added and the solution is then diluted with the same 1nixcc1 acids so thnt the zinc couccntration of the final solution is 0.333 g. per ml. The lower grnpbite electrodes are drilled 6-35 mm. (0.25 iu.) deep with II 4-0 mm. ($ in.) drill in n jig which also trims the tops of the electrodes to n very flat cone. Using n micmburet of the tppe described by Standen and Fuller," 0.10 d. of the sample is introduced into an electrodc which is then dried a t 93' for 30-60 minutes. Four electrodes nre mnde up from ench sample nlid, in arcing, the spectre of them four electrodes tire superimposed

TEE AVALYTIGAL CURVE Standards for lead in zinc were first mnde up, using n purchamd

zinc of high purity. Two solutions were mnde as dcecribed nbovc, one of the pure zinc, the other of 99.90/, pure zinc and O~looO./, t a t lead ; from these solutions, standards were made by aucccsrivc dihtioir. The truc vnlucs of the standnrds differ from their norni~lal values by the quantity of lead in the pure zinc, a vnlue which i s unkiiowi.

lf the photometric ratios of standards nre plotted against no~uit~ul v~lurson log-log paper, a curve results, the curnitwe being geiiernl$~ eonaidered to be due tc the background affect and ta res?aidtm~ im- purity it1 tire pure etendurd baa@. Since il hnckground correctiort hnn becii applied, it wna expected that by usnuming a suitable value for reaidusl leud a straight line would result. However, n number of attempt5 to obtniu the t.ruc vtilue of standtrrds by thie method p v c very doubtful results ; nnd a residual impurity correction of nbout O.O004% was iiccen~nry to bring the lowest stnndord (O*ooO195% noniinnl wlue) into line with higher results. This cnuscd an unwelcome waviness in the curvc in the region of nominal vnlues O.o0039% and 0.00078%. Morcover a year or so previously tho lend content of the purclinsed p m zinc hud h e n entininted, using the A.S.T.M. Conipurison Standard Method and ehrc mode in the Rracnrch Laboratories, which contained no visible lend, for the proprntion of sta~idsrds ; the residual lead was ostiinntcd

The Burenu of Standards zinc sample No. 109, the l e d content of which is given ns O . ~ Z O % , was therefore purchased. I t BW nssuiacd to contain O~OOZoO./, lead und the nnnlytical c w c \CUB

ndjubted no that unnlyses of this samplc gevu thie vnluo ; this inenlit ddirtg 0~00016% lead to tho iiomiuul value of cnclr stnndurd, i.e. the pure zinu for stnudurds contained 0*00016% lend.

Tho resultiug analytiml curve is shown in Fignro 8; uaiug t b curve tlin results on thu Bureau of Standards sninple are OAObl890/,

8, !MI.

011 the plate.

ns O*C!#2%.

"Clt.ndan. U. W. -4 L'uikr, X. L., Ind. Eng. Cbm.. A& Ed.. 1934,

average deviation O*OOOfMl% (15 results). Extropokting this curve into the " toe," direct analysts of the pure titic gave the value O-O0019%, which is only fair agreemelit with the O.@J(l16% given nbovc. It will be noticed that the general shape of the nnalyticnl curve is in accord with the work of Strock.12 Our atbmpts to investigate reciprocity fnilurc hnve already been dcecribed. In so far as retarded development is Concerned we hnve fnilcd tu get direct evidence ; clnta having snmc hearing 011 this nre described below.

I 0 1-l 500

Ftfl. 8

Leod in zinc. p.p.in.

This nunlytical curve wae ckgsed 89 quite unsatisfactory and therefore a etock of zinc containlng less than OGJW#l% l a d wns pwpared. Since the bismuth wed as internal standard contained a little lend, the nominal vdue of ench standard prepared from this zinc wne increased by O . W % . A plot of the photometrio ratios yielded by these new standards gave a straight line on logerith- d o paper from tho lowest standard, 0~000195% up to 0.0125% lead. Purthor, as thin line was at 45 degrees, a plot on ordinery paper was nleo D straight he.

THE ?&TERXAL 8TAXnAXD

Bismuth h a frcquelitly k e n uiwd an itituriial standard for Icad, as it i s posaible to find p i r e of blsmuth and lend Lines which fornr g00d hOmOl0gOllE pairs. The rastes st whicb energy is emittad from the arc in the radiations bismuth 2897.98 mid lead 2833.07 arc shown in Figure 9. Thc p b b was moved eaoL five seconde aud wns hol ly calibrated no that the demitameter redmgs could be couvcrtcd to energies; bookgroyd correctiow were applied. The curves nre not identiml, the difference sliawing more clearly if the differential forms of the curves nre dmwii. The sample employed contained 0.00325% l a d ; 411d the photometric rntio, as taken from the curve nt 30 seconde, 18 1-06, which corresponde to 0-003,31% Iced. Tho justification for the use of n 30 second arcing tinlo is well 5hOWRn by the C v ' +

The internat standard stook eolutlorl IS mnde up tb cuiitatnin 0.833 g. per litre of bismuth. Of thie, 10.0 ml. is used for each 1 0 * 0 ml. of final volume ofthe sample solutipn. A ~111ple coutainingO*OKW% lead then givce a photometric rntlo of approxhutely unity, Sinoe tho grcntest pccc&n is munlly considetall to bn obbincd at thin mtio, the rc'(308011 for the particular quantity of bkinutb w d may be wen by ~fQrriIlg to the B.S. Spe~lication mentioned iii the Introduction. EWrarfes

Tho n w i i w of prepwillg thc I w c r doctrades taw beuu dewcibud. -

Tlic depth of thc holc is hpor t an t ; nkhongh we hnd biowo this for same time, it was forcibly brought to our attention while Bcobie.9 WAR ctrrryiug out some work on traces of tungaten.

.\ I .

source containing lines of known relat,ive intensity. A relntivc intensity patteru, ratlmr than relative time pattern, could .be employed to eliminnte reciprocity error, provided the cxclhtion method is such thnt the radiation is given out a t uniform mtc, in which intensity, rnther thnn energy, can be relnted to concen- tration. With the arc method using graphite electrodes, .Such a solution is iiot possible nnd the only soIution lies in the usc of plsteu which show very little reciprocity failure.

The second error is the Eberhirrd effect, for which there nre serernl remedies. The background correction cm be d e + ~ ~ l l e t l , using a bnckgrouiid calibration curve rnther than n line cnhbralion curve, iiccording to the method described by Strock. Tl!e h w - back in this method lies in the fact that the mirnbcr of de~rsltomter rending3 pcr deternhntion woultl be increased from fonr to nt least tae l re , which would entoil the use of another two op'rntors and nnother densitometer, while the plnte cost per determnntioii would fiw fire-fold. The second solution would be to dcrelOP thc plate fully : with 33 plates (uap 5: 1.9) this would be inconretiicnt in our work, in thnt the densitometer readings are Its3 yrrcise below 10% orid above 70% transmission; nnd tho plnk confmst has bcen stirndordised at 1.0 so that most of the work ha?dled d l fall into this range. There art: however, visunl densrtpmekrs on the tnarkct which tnaintaiii their precision up to n densttypf 4, and it is poeeiblc to convtruct photoelectric densiton~ctc~ havfllg a resporiae which is npprorimately logarithmic. The third solution, and prohalily the best, is brush cleselopment, the disntlralltage of which has already been discussed. It does not, honew, appear hopeless to \relieve thnt a developing mnchine could be designed which a u l d cqunl brush develoyrnent.

The third error lies in the use of bismuth as a11 internal stolldard ; the lend line 2533.07 nnd the bismuth line 2897.98 are not a perfectly homologous pair. The two elements arc not volatilised from the elcctrodc nt idciiticnlly the snme rate, anrl thus errors due to m i - procity foilurc may occur. Fluctuations in arc temperature will affect the photometric ratio in a manner which hns nlrendy btten discussed. It is iiot very likely, however, that a better ~nkrmal standard for lead can be found.

The fourth error lies in the u s of the direct currei~t nrc \ y h L has achieved niuch notoriety because of its errntic bcbnvlour. After 11 struggle lasting for some years, wc achieved some nlensurc of control over i t ; and our conclusions 011 this point have been Dven. I s vicw of the nnalytical precision which we oltrrin and in view of the photometric errors, the densitometer error fin13 the plnte grain error, it icl certuin that the error in tho source is Rot very grent.

An iuvcritigntion bas been made into the intcrllal stondnrd iiiethod of spectroscopic arialyeis, using the determination of l e d i U high purity zinc as an example. The Eberhard effect, the failure of tho pbtes used to obey the reciprocity law, and the iniperfectioll of biamuth 8 6 an internal standard have an influence on the results ; aud the magnitude of the errom so introduced has beell 8hoan experimentally. Standardisation of the analytical conditiollrr $8

led ta a precision of a b u t 2%. It is believed that, by removing 8ome Of the above errors, greater precision is possible.

Received through the Zinc Development Association, oxford. Secretaries to the BritiRh Standards Institution Panel which drew up thc recommendations for the Polarogrnphic rind Spcetrogrnphic analysis of high purity zinc and zinc alloy8 for die cnsting.

Eeralvcd Hovrrnhr 48. lMS * - M€THOD FOR THE NIICRO-DETERMNATION OF TBE

I~SORPTION OF FUMIGANTS !.By F. P. W.

Brief reference ir urdo to tho importanoe of tho study of rorption in inuBntigeting problemo of fumigatiou. Thh npplirntiaa of tro-ohnmbsr ~oiptiun apyarotua to tha miom.&terminn~ion of the rorpthn of uk6micsUy mactive fudgantu is daaaribad in datd. iucccnfi of the mothod donen& lnrcelv on tho nmcirinn nf tha nnnlvticd

method of detarmining tho concentretion of furnignnt in the Sppmtur. For tho introduction of known weighre of fumigant into the nor tion apparatus a ~an~pl ing procedure hae been amployad. which sf~owa advantagcs ovor tho method of weighing the fumigant out in glass ampoulos. The application of the method af mnatIoa in olonwthanol- amine to the dotormination of methyl bromida concuntrationn in the two-chomkr upparatus is describcd. The annlytiaal wrom and the caIculation of tho averngo conwntrotion obtaining in tho free space of tho apparatus during nn oxprimcnt am discumd. In an earlier paper' ntteiition was drewn to the iuiportnnw of

studying the sorption of funlignnts. Such data hnve been used in cdculatiug the doses of ethylene dichloride required for fumignting wheat products in n gns-tight chamber, and in Studying the distri- bution of fumigant during fumigation. The dots were used to demonstrate that the firm retention of residual erhylene dichloride by fumigated wheat was due entirely to phyaicnl forces.% When D fumignrit such as methyl bromide is sorbed by wheat, however, it reacts chcniicnlly with one or more of thc wheat constituentt~~ Methyl bromide i s one of the more toxin halogenated hydrocarbons.' The effects of poisoning in man may be serioua and recovery may be slow. It is therefore important to detcrmhe if poesible what proportion of residunl methyl bromide in Funiigatcd products is chemically dcwznposed.

Lutmtti and Harrison* have cornpnred the sorption of methyl brornide with that of hydrogen cynnide, ethylene oxidc and tri- chloracctonitriIe. Their data related to whole wheat grain and pro- vided little information of quantitative iinturc or1 the reaction kinetics in wheat.

In a11 attempt to solve thew and other probleiirs iir relation to rnethyl bromide, its sorption by wheat and by certain wheat con- stituents has been studied.6 The experimental method used might usefully be extended to the study of similar heterogeneous systems invo~vhg physical equilibria and chemical reaction simultaneously, particularly when the total weight of available sorbent is small. The auccess of the method depends on the extent ta which the vapour phase lends itself to precise chemical analysis b3 n simplo method. It is the purpose of this pnper to describe the mathod. The data 80 obtained for methyl bromide and its interpretation are discuseed in a separate paper?

DEDCRIFTION OF THE N E T H O ~ PLtlciple. The two chamber appamtus applied to the study of

the sorption of ethylene dichloride' baa been used in a form slightly modified to permit the micro-analysis of tho gas phase. I t consists of two glass chambers comrnunicntiog by meam of e Jnrge bore glass cwk (D), Fig, 1 . O R 8 chamber (Ef contains ths weighed sorbent ; the other (C) is ueed for the quantitative introduction of the fumigant, aud for the subsequent aualysie of that mmainiog ia the apparatus after sorption has proceeded for tho allotted t i m .

When studying chemically reactive funugmts it is dcirable that the concontration of fumigant be kept as constant as possible during the experiment. By taking sufficiently sinnll weights of eorbent the fall in concentration has been limited in the present studies to about 16%. The apparatus is charged with fumigant by drawing a snmple in the evacuated chamber (C), from a reservoir containing u known conccntration of fumigant. This method has proved more accurate and convenient tlian weighing out the fumigant in ampoules, and i t enablcs a wholo wries of flasks to be charged with approximntely the snmo conccnbration. The limited fall in concentration due to sorptiou during an experiment permits the nvernge concentration obtaining in the " free space " of tho apparatus to be ealaulated to within about 1%. It also facilitates the interpretation of the date. The two-chainber apparatus permits the fall in concentretion to be determilied by the analysis of a rehtively large proportion of the

' Idem. ibid., 1944, 68, SBD. a Ghrsllvr, 9. A., Beshgetoor. A. W., Sbngbr, V. h.. I d . Eng. Ohsin..

liendemon, Y., Haggard, tf. W.. Noxious gases. Saw York. 1043,

Wintaringham, F. Y. W.. J.5.C.T.. 1044, 08, 144.

And. Ed.. l Y E , 14, 1.

p. 201. Lubutti, 0. F., Harrison, A., J.8.C.f.. 1911. U8. 353. Winbringham. IF P W U*rriann A 1hl.l i r * I n - au-mea