1224
ANALYTICAL CHEMISTRY
ati;titicd. A n i o i ~sii( ~ f'd :tppro:wh appr;ircd t o l w (,liniinatcd elt r:wtion of fprric c-hlorido froni tion of iroii(II1) i)y r 5.\- h\.droc-hloric: arid into ethrr. For thi? method it is csseiiti:d that the volunit. of th,c sample tic small and the hisiiiiith cottux{ration Ilc ahovc0.001 molr per litcr, thns pcrniittirig fiirth(,r dilrition iiftcr mtrac-tion. \Yithoiit final dillition x f t r r thc estritvtioii procws, t hP salt concxntration arising from t h o iieiii rdiz:tt ion o f hydrochloric wid becomes too high t o allon. i t i i ;tcac,ilr:ttca tlcic:rniiii;tt ion of Iiismiith. If thew requirc.mcnt s :trc i n r t . oil(! rn;t>~r(!inow it tcnfold czrcss of iron(II1) \\-it houi :ti1 :q)piwi:ii)lc
coscLusIos
rl)c~c~tro~)tiotonirt ric prwcdure for the determination of bisnirith hy nicimR of Yersenc is proposed. As to its practical sipnifiraricc as a inct hod of iiltraviolet spectrophotometry of bieniuth, this nic'thod may compare with the determination of the clcnicnt :ts the chloro roniplcs ( 6 ) ; the latter method, hon-ever, h a p thc adv:tritagc of inc;isui~iiigthe a h o r h a n e e a t a considerahly Iiiglicr w : i w letigt h n-hirh reduces the possihilitiw of interference.?. To : ~ p l ) l ~ail. malogous procedure for the determination of I c ~ does l iiot appear t o he justified from a practical standpoint cii'or, (\srcpt undcr spwi:d ciiciiniatnnccs. R.it 11 t hc exwpiion of nitrat(>>wmnion anions do not iiit(~r~crc, If I)i,sinnth is determined at a p H of 1, thc presence of equal Sitcttc, :ts is thc gciieral rasc in iiltrHviolct spcc.trophotoiiic,tr?., :tniorints of :tntiiiiony(III) or tin(I1) causes only slight interfernirict INS alwrit. Sulfatr, prrrhlorate, w c t a t r , and c*hloridc VI^*(^, itltliough hritvy intcrfcrciicw from iron(II1 1 and copper(I1) h a v ~n o dfert, provided the h t t c r i p not prcsrnt its it Iitrgc' cs( still exist. In the prccriice of moderate escesseF of lead, or in of tiytlrorhloric: arid which niity givt riw to t h r forniatioii of t h c t hv presenw of harium mid strontium, a peichlorate medium is to I~isniiithc*hloritlcconiplcx. (Coinpar(>plots I and I1 in Figiirc 1.) IIC prcp:ircd ovcr zulfatr for controlling ths pH. An escess of I r d 1111 to 50 tinics the eoncentration of bismuth may be elimiI'ROCP:UllRE riatcd by prec4pit:ttion :is the sulfate; higher co!iccntrations c~)prccipit:itc hismiith. A11 nppropriatc aliquot of the unknotvn is tratisfrrrcd t o :t 50nil. flask. The final conc.entratioii of bismuth should not e s c ~ ~ ~ l LITERITUHE CITXD 25 p.p.m. Two and a half niillilitcrp of 0.02.Cf disodium Versciiate arc added. Because bismuth solutions generally rontain largo ( I ) Uroiikvist, K. E., Fai.vi. Rtau, 52, 305 ( 1 9 5 3 ) . amounts of actid, it is necessary to ncutralize c s ( w s :rc*idh!. addi( 2 ) Laiidgrcn, 0.. Sivnsk Farm. T i d s k r . , 56, 241 ( 1 9 5 2 ) . tion of ammonia, adjiistiiig t h c final pII by means (3) hlerritt, C., Hershenson, H. AI., aiid Rogers, L. B.. .%SAL. CHEM., tate. The solution is then diluted to the mark and 25, 573 ( 1 9 5 3 ) . is measured against a blank caontaining a n e q u d c~oiic~ntrxtion (4) I'iihil, It., aiid Cuta, J., Collcctior~Czcchoslo~.C'henl. Conamuas., of Versenc. The blank should contain approxiniatelJ. the siinic 16, 391 (1951). amount of arid, ammonia, arid sodium acetate as thc u n k t i o i v n if ( 3 ) l'filjil, R., and Matyska, E.. I b i d . , 16, 80 ( 1 9 5 1 ) . the concentration of thew reagents is high. T h r ahsorhanrieP ((i) I h i d , , p. 139. should he measured a t 263.5 mp and the bismuth concentration (7) Piihil, It., aiid JIatyslia. 5.. C h e m L i s t y , 44, 305 ( 1 9 5 0 ) . he determined from thc calibrat,ion curvc. If it is dcairahlc to (8) Underwood, -4. L., .&NAL. CHEX., 26, 132%( 1 9 6 4 ) . rrduce the effect of intcrfcririg rations by adjusting the aciditv R L C I ; I ~ Efor D review 1:ehruary 2 1 , l!l54. .4ccegted April 7, 1!1.35, Presented of the solut,ion t,o pH 1, 25 1111. of sulfate huffer fihould hc added hefore tile Divibion of Inalytical Cheiiiistry a t the 127th lleeting of the instead of sodium acct,ate. The final pH of the mmplc should Ahi~nic.4 C~H L I I I C SOCIETY, ~L Cincinnati, Ohio. lio in the range of pH 0.8 to 1.2.
Flame Spectrophotometric Determination of Copper in Nonferrous Alloys JOHN A. DEAN Deportment o f Chemistry, University o f Tennessee, Knoxville, Tenn.
'Ihe Brcltniari \lode1 D C flame sprctrophotomcter has been applied to the determination of copper in aluminum-, tin-, and Line-base allo?S . The copper arc emission lines in an oxjacetj lenc flame at 324.7 and 327.4 nip were employed. The copper 324.7 mp line is more sensit i \ e and is recommended for copper concentrations less than 100 p.p.ni. IIoweter, this line suffers more severe13 from self-absorption than does the coppcr 327.4 line, which is useful for concentrations of coppcr exceeding 100 p.p.ni. Of all the elemrnts conirnonlj encountered in the aboxe alloys, onlj appreciahle amounts of nickel offered interference. The sensitivitj of the flame spectrophotometric method is apprmimatelj 1 p.p.rn. per instriinient scale dirision. l h c standard deviations from the mean of replicate Bureau of Standards samples and from thr certificate values are both within 3%. The flame analjsis method coniparr5 fa\orahlj in precision and accurarj with con\entional colorimetric methods in the concentration range 0.0 t o 5 . 0 7 ~copper present. The time of analysis is shortenrd drasticallj to a few minictrs following dissolirtion of thr sample, as no prcliminarj separations arc recliiirccl prior to the flanie analysis.
T
€118 invcstigation continues a series of reports from this laboratory on the application of flame spectrophotomet,ry to ihc rapid determination of some of the components commonly present in alloys and ores. -4lthongh many papers have described incthods for the determination of the alkalies and alkaline earths, relatively few discuss flame methods for any of the other elements. Spcctroscopic methods employing an arc or spark, or combination thrrcof, have been depcribed for most of the elements, but thcrc are occasions when a lcss expensive array of equipment, such as a flame spectrophotometer, would suffice for analyses. I:iirtherniore, the flame offers certain desirable attributes lacked Iiy an arc or a spark. Compared with an arc struck between carbon electrodes, the temperature of a flame is not particularly high. As a result, only ii few of the metals which may be present in the sample will be c.:trlscd to emit their characteristic radiation. This simplicity of flainc emissions is one of the definite advantages of flame spect rophotornetry, particularly when high concentrations of linerirh elements are present. Furthermore, in the flame where excitation occurp mainly as a result of collisions behveen atoms iind moleculcs, some syst enis may be relatively st,rong compared to their emission in an arc or a spark. This is certainly true for < w i l > , cscitcd clcmcntp. It is ICQS difficult in the flame than in
V O L U M E 2 7 , NO. 8, A U G U S T 1 9 5 5
1225
oftrn :I.* tliri sulfide, which is very tinic~-c~oii~iiiniii~ wlicAii mmp:tretl to tlie rapidity of fl:irne an:ilj+q, C E N E R A L EXPERIXIENTAL WORK
0
324
Figure 1.
325 326 327 WAVE LENGTH, mu
329
General emission pattern of ox>acetylene flanie containing copper and silber Present.
40 p.p.m. of copper 100 p.p.m. of sil\er
Apparatus. A Beckman 1Iodel Dl* qu:Lrtz sprc~troptiotomrter with 1Iodel 9220 flame attac.hment and photomultiplier unit wad employrd. A metal burner. cwnstrucsted for use with osygeii :ind acetylene gases, was used in all esperimrntal work. To study emission intensities iri variorw regions of the flame mantle, the bririier was removed from its usual mouiititig ant1 rigidly attached to the rack-and-pinion movemerit from :I 1111boscq c-olorinieter. arid tlie a.hole assembly \vas inouiitrd on :I. sturdy tripod. Provision \vas matle for simiilt:meously raising or l n ~ ~ r i t :iI gc.ontaiiier \vhicli Iiel(1 the solution ~iiideririvtbstigation. Reagents. A standard solution of copper, 1 .OO nil. equivalent mg., \vas prepared by dissolving :3.942 grams of fresh of reagent grade cupric wlfate pentnhydratr in d r m i r i e d er, and diluting to 1 liter. i\ staridaid solution of ailver, 1.00 ml. equivalent to 2.00 mg., was prepared by dissolving 3.138 grams of silver nitrate. nieetirig ACS sprcificatioris. in deniirieralized water, nntl diliitirig to I liter. Demineralized water, iisrd escliisively in preparing :dl solutions arid samples. was prepared by passing ortlinai,y distilled water through a bed of Ambeidite 1IR-Y resin. Flame Spectrophotometer Settings. The instrument srt tingx usrd for measuring the farnr emissions W P ~ Pas follo\vs: Sensitivity control Seleca tor switch
most other ~ o i i r w cto e.stal)lish ri~producil)lecondirioris anti so t o obtain accurate qiiantitative determinations. The element studied in this investigation is copper. It 1’shibits two rather ,sensitive emission lines in the ultraviolet portioii of the spectrum, :it 321.7 arid 327.1 nip, and pow of weal; emission h n d r in the visible region (8). sttidy has heen reported which tieall with the flame spectrophotometric determination of copper, although several investigators have emplo!-ed the two lines mentioned to deterniiiir. copper in biological materials ( 5 ) and iii othw siitist:incw (4). Lundeg%ldh( 7 ) observed that the emirsion intensit>-or luniincwity of copper \v:is strongly dependent iipon operating conditions, and that both liner T T P ~ Pnnl)je(.t to Ptronp .wlf-ai)sorption. The l:Lttfli, ir not surriri.4iig in view of the low escitotioii potetiti:tls of thcw lines, the value? tieing reported :is 3 . i ; voltr for ( q p v 324.T and as 3.80 volts for copper 32T.I ( I ) . Thtw conditions woiiltl seem to indicate that the flame spectrophotonietri~~ tlrJtermin:ttion of copper might he something l e ~ ath:in desimhle, TIonever, i n the present study a gener:tl procedure is dricribecl wliirli c s i i , cumvents man>-of the previous difficulties. For some type. of samples, the simple measiirenwnt of thr. luniiimitv of :i copper line minus tlie background luniiiioqity suffices. On the other hand, many sample components enhance or s ~ i p p r e the ~ s h i e emissions of copper and the flame 1)ac~kground to a different degree. When this situation esists, the well known spectroscopic principk of internal standard c:tIihr:~tion offers :I solution ( 2 ) . The silver line a t 328.0 mp provitles a satisfnctory internal .;tandud reference line. A%ltlioughthe spectrophotometer employed in this work was a single-beam type instrument, the flame conditions with an ordinary integral, metal aspirator-burner are sufficiently reproducitile and constant for short periods of time t o eriahle an operator to scan either or both of the copper line$-,the silver linr, and the background radiation immediately adjawnt to the time of each of the respective lines. Results ohtaintd upon applying the flume spectrophotometric. method for copper in various nonferrous alloys indicate that the method poswweq a prevision and accuracy within approsimately 3Y0. T h w , the fiame analy9es compare favorably with conventional colorimetric methods in the lower concentration ranges, while drastically shortening the time of analysis to a fen minutes following dissolution of the sample. For larger amounts of copper the time saved is even more significant, :is conventional procrdnres usually require the preliminary sepnrnt ion of v o p p r ~ ,
Phototube resistor Slit .-\cetyletic Oxygen
I’liototubc voltage
5 or C, t u r n s from clockaiw 0.1 22 megohms 0 ,030 inin. 5 pounds per square invli ii pouritis per square iiicli 00 volts per tlyiiode
liiiiit
Characteristics of Copper in Flame. Tlir general emission pattern of an osyaretylene flanir coritainirig copper over the wave-length region 323 to 330 nip is shonn 31;Figure I . Copper cshil,itr t w o arc lines a t 334.7 and 327.4 mp, .Uwnotice:tble i r i Figure 1 :ire sonie band striica ‘es diie to the flame itself. \\’enk .tern : i w olwcarved ai.oiiiid :323.:( renin:mtr of tlir, 0 1 1 band nip, and some weak CO txind heads appear at 325.3 and :120.:3 m p . Thr. i,eni:iinder of the I)ac*kgroiindr:diation in the vickiity of thrx cvppei’ line.+ is essentidly t~oiitini~oiis and is :ittrii~iitaI)le t o thc roriliiiuour spertrum of the carlmii monoxide flame. This continuoris I)ac*kgroiirid prove.: usef~ilin analyses, 3 s it c u i l w employed :is :I form of ititeriul st:~iid:irdor measure CJIfl:tmr m i i Pt %tic!.. Rotli of the copper linci :ire components of the wnir doul)let, They w e Ion energy h i e s involving :iii electi,on whicli has I)ceii rlrvated from the l o w s t energy lwel, or ground state, to the nest higher energy levrl. \\-hen the rwited copper atom returns to its ground state, the t\vo lines appear iii emission. They are represcwted t,y these spec~trowopir term rymliols: 3*Sl/24*/’3,? for the 323.7 line and 1?S, - 42P1$,for the 32T.4 line. Transitions from ground state to the most easily rsc,ited upper energy ltxvel, the 3P-Irvt~lfor copper, are known :is r ~ s o i i : i n ( ~ ~ line.+. R e s o n m r e linea are prone t o suffer self-alisorptiori niid c4oppt.r lines are no wception. Self-:~l)sorption occurs during the passage of emitted radiation through the outer fringes of the flame mantle. Unescited atom? of copper present in the outel, portion of the flanie mantle tend t o al)sorl) some of tlie copper radiation through interaction with the emitted light quant:t. The atiaorhed copper light cannot therefore contribute to the ohserved luminosity. h t relatively Ion. concentrat ions selfahsorption is not serious b e m u s e the vapor tfemity of rinesc~itrtl copper atoms \Till be loa-. Howrver, a s the colicelitration of copper atoms injected into the flame increases, an increasing portion of the copper emiasion is a1)sorl)e.d before it rearher thr. periphery of the flame, arid hence i? not registered by the photownsitive detector of the rpectrophotometer. The over-all effecat on the luminosity of the two (topper lines is shown in Figure ?, i r i wliicdi t h r luniinesc-enw of the two emission linea of copper i.i
ANALYTICAL CHEMISTRY
1226 plotted as a function of the concentration of copper injected into the flame. The copper 324.7 mp line is the more sensitive of the two lines for low concentrations of copper. Above approximately i 5 to 90 p.p.m. of copper, the luminosity curve for this line flattens out rapidly until the slope of the copper 324.7 mp line becomes less than the slope of the copper 327.4 mp line. The onset of serious self-absorption does not occur with the latter line until the copper concentration exceeds 300 p.p.m. Consequently, t h e copper 327.4 mp line is actually more sensitive and better suited for usc when the concentration of copper present lies in the interval from 00 to 300 p.p m.
IOC
0, PRESSURE,
6
7
9
lb< /sq.in.
IO
si ;
0
Figure 3.
I 0
Figure 2.
I
I
I
I
20 40 60 C 0 N C ENT RAT I0 N,P.F!M.
I
I
I
80
100
Emission intensity of copper lines
Luminescence given as instrument scale divisions
SLITWIDTH. On the basis of Figure 1 it seemed likely that a slit width of 0.030 mm. was satisfactory. .4t this slit opening the effective band width of the emission lines was sufficiently narrow to enable a background reading t o be obtained within 0.3 mp of the wave length corIesponding to thc peak of the emission line. If the slit width were any wider, difficulty would be evperienced in the region of the copper 324.7 nip line due t o the weak flame band centered around 324.3 mp, and in the vicinity of the copper 327.4 mp line and the silver 328.0 mp line due to their overlap. A photomultiplier attachment or equivalent amplification of the photocurrent is necessary in order to achieve reasonable response to luminescence a t slit openings :is small as 0.030 mm. OPTIVLNFUELT ' R L ~ ~ ~ R I : ~ . The relatlve intensities of flame emission lines are influenced by the ratio of oxygen and fuel pressures. Not always reali~edis that with an integral atoniizerburner, the oxygen pressure also influences tlie spray rate, and therefore, the flame temperature indirectly through variation in the amount of liquid aspirated into the flame per scrond. A study of the copper luminescence as a function of the t n o parameters, pressure OF oxygen and of acetllene, ici~ealedthat the optimum operating conditions were pressures ot G pounds per square inch of oxygen and 5 pounds prr square incli of met>lcne when a relatively wide orifice burner W,LS emplored (rated a t a pressure of 10 pounds per square inch oxxgen by the manufacturer). Relevant data are shown as Flgure 3. The portion of the flame mantle focused upon the entrance slit of the monochromator should also be considered (6) Figure 4 shows the luminescence as a function of distance from the tip for different oxygen-acetylene ratios. From these luminosity
12
4
CzH, PRESSURE, Ib.
60
II
70F--7IO
80
8
; /rq.in.
Optimum fuel and oxygen pressures
~ u I ' w - :it ib seeii that thc sensitivity of the copper lines depends to a Inrgc extent upon the position of the flame mantle in relation to the optical axis of the monochromator. None of the present commercial instruments provide means for altering the position of the fl:tnic, after initial optical alignmcnt during assembly of the equipment, except in so far as different fuel flow or gas presslire may vary the position of the tip of the inner cone of the flame. The need for proper alignment is strikingly shown in Fignrc 4, which indicates that a considerable increase in copper luminorities reaching the photosensitive receiver mould be accompliPhed through choice of optimum position of the flame mantle. Fortunately, the optimum position of the flame mantle, with respect to the entrance mirror on the monochromator, almost coincides wit,h the normal position of the flame mantle when the burner is mounted in the usual manner in the l)\irncr housing of the Beckman instrument. The traces in Figure 4 confirmed tlie original choice of fuel pressures, which were then maintained invariant throughout the remainder of the work. Thesc fuel pressures gave a very hot flame. and although the data would seem to indicate that even a hotter flanie is desirable in terms of emission sensitivities, burner maintenance then becomes a serious prolilem. The brass burner Iiecomrs suficicntly hot itself t o volatilize the collodion coating covering the screw heads used to position the aspirator tube. Oxygen is then able to escape into the acetj-lene orifice and causes erratic flame conditions. As ii general guide to selecting optimum fuel pressures with different burners, one should use those pressnres which 1)rovideas hot n flame as possible. yet with a relatively ICJK an1)irntion rats. PREPARATION OF C4LIBRATION CURVES
Two types of standard CUTVL'S were prepared. One consisted simply of the plot of the concentration of copper present against the 1uminosit)- observed a t the pcak 01 the respective copper line, L, from which value was subtrxted the background reading, H . Typical of this type of curve is Figure 2. The choice of wave length from \\ hich to obtain thc Ixdcground reading, H , depends upon which other elements might IIC present. If none are present that emit lines a t 325.0 and 327 0 inp, then these two wave lcngths are suitable for use with copper 324.7 and copper 327.4, respectively. Because of increasing eelf-absorption a t higher concentrations, t h e calibration curve, although initially a straight line, will gradually bend toward the concentration axis.
1227
V O L U M E 2 7 , NO. 8, A U G U S T 1 9 5 5 For a given plot of emission readings against concentration, as in Figure 2, the accuracy attainable a t any given luminosity is directly proportiond to the slope, S , times the concentration a t the given point, G, a,nd inversely proportional to the minimum difference in concentration, AG, that can be detected a t that point. The concentration can be expressed as parts per million of copper present. T o determine the optimum value, or range of valuep, for the luminosity of each copper line, a series of luminosity intervals was studied using varying amounts of copper, and determining when the average value of ( S X G)/aG was a maximum. The flame emissions, after correction for the background radiation. for a typical series of standards are shown in Table I. hssuming that all plots are straight lines over the intervals covered, the slopes S 7 for the copper 324.7 and 327.4 are given in C O ~ U 3~ and nip lines, respectively. The slopes continually decrease. The products, 5' times G (for a median luminosity value) are given in columns 4 and 8, respectively. These rapidly increase to a maximum, then remain fairly constant. The results of the relative tttairiable for each line are shox-n in columns 5 and 9. For pui'poscs of calculation a difference of one scale division (1% 1iimino.sity)w a assumed ~ to be recognizable. The x w n d type of calibration curve was obtained 1 ) ~ . the nirthod of internal standardization. The silver :328.0 m p line wm chosen a s the most ideal emission line for w e as internal standard for ih(>scrcasonp: Silver itself is seldom present in the alloj s uiicier examination. Both the silver and the copper lines are ON energy lines which involve similar energy level transitions and which possess similaz excitation potentials, the values being 3.80 volts for Cu 324.7, 3.77 volts for Cu 327.4, and 3.75 volts for Ag 328.0. Both the silver and the copper lines are subject to self-absorption to approximately the same degree. The silver line is conveniently located relative to the two copper lines, so 1t8to reduce errors to differences in general radiant energy.
A
0
C
Silver salts can be obtaiiiecl in a verj' high state of purity with respect to copper and any other elements t h a t might offer serious interference to the flame analysis of copper. The ratio of the average relative luminosity ( L - H ) of the copper line t o that of the silver line (also L - H)-i.e.,
L of CU 324.7 - ti a t 325! L of Ag 328.0 - H at 328.3 for e x a m p l e i s plotted against concentration of copper on log-log paper to give the calibration curves. The plot of log luminosity ratio us. log concentration gives a straight line over limited concentration intervals. As rnight be expected from the variance of the copper luminosities with concentration, a fixed amount"'of silver serves ideally as internal standard over only a limited range of copper concentrations. The following ranges were found suitable for quantitative work: 50.0 p.p.m. of silver for the interval 10.0 to LOO p.p.m. of copprr, and 100 p.p.m. of silver for the interval 7 5 to 400 p.p,m, of copper. .
Table I.
For C u 324.7 Line
50
41.0 51.0 59.9
60
67.0
70
7:1 0
80
78 3
!IO
8:1 3
88
100
.i!l 0
130
73 0
200
81 0
300
t1r.e
ai'cu-
,5
S X G
1.03
26 3
33
1 00
35.0
33
0 85
38 2
32
(17.5
41.2
32
39.0
23
87 .i 93 0
racy
~~
~
~
Rela-
ICiiiis-
Yiun, hcalr di\-ision? 18.0 2 ,i.j
32
s .5
0 75
x
ti1.e HCCII-
U
racy
18.;
14 4
70
24 3
17 5
065
29:3
193
U GO
33 0
IC1 3
0 55
35.7
19.8
18.7
IJ
.j
39 0
41 2
23
0 . 5 0 37 6
42 3
21
0 45
38.0
17 3
42 7
10
0 40
38 0
Id 2
0 . 2 8 0 33 0
0 29
35.2
10 0
0 lU0
28 0
0.23
43 7
10 H
0 130
29 2
0 . 19
12 8
8 2
0 110
30
x
0 13
41 R
(i 2
u
100
2.30
~~~
For Cu 327.4 I h c
~~~~
Rela-
hion,
Copper, scale P.P.M. diL-ision, 20 29.3
40
~
Selection of Optimum Copper Concentration
En&-
30
~
.-
30 2 0 IO
0 30 20 IO 30 20 DISTANCE FROM BURNER TIP, m m .
10 0
Figure 4. Luminescence of copper 324.7 m y line as a function of distance from burner tip and for various fuel and oxygen pressures 0 x 1 gen pressure, 8 pounds per square inch. icetl lcne pres.sure, 5 pounds per square inch B . Oxtgen pressure, 6 pounds per square inch. 4cetylene pressure, 5 pounds per square inch C . O \ \ gen prwqure, 6 pound3 per square inch. icrt: lene pressure,
A.
4 pounds per square inch
In tlie iiitcrnal standard method t h e czlJt~riirieiita1coriciitious under wliich the luminosity ratio is measured cannot usually be perfectly duplicated. If this were not so! L: permanent standardization could lie made and no further rwdirigs of luminosities from stmdarti solutions would be requii,cd in a routine seyies of app1ic:ttions. Urifortunatcly, as pointed out by Churchill ( 3 ) ) many variables in spectroscopic proctdurrs other than concentration affect the measured luminositj. ratios. -4s a result, working curves show a slight irregular drift and it is necessary to run frequent standard samples to correct for this drift. While all tho causes of drift are not understood, it is known that a part of the drift is causcd by :in actual change in the luminosity ratio and a part is caused by errors in measuring the luminosity ratio. The principal factor affecting the actual luniinosity ratio produced by a flame excitation source is the gradual clogging of the burner orifices by carbon deposits, particularly the very constricted oxygen orifice on the metal burners imed with the Beckrnan instrument. I n tiirn this results in n diff(arent fuel ratio and a change in the aspiration rate. Both of the latter mill affect the flame temperature, and unless the copper and silver are present i n Psnctly ccIuivnlent amounts viith respect to their luminescence, :t v u k t i o n in the luminosity ratio must I)rt rupected.
1228
ANALYTICAL CHEMISTRY Table J l . Acid Concentration Moles 'Liter HCIOI 0 5
Influence of Acids - - _ _ _Copper _ _ _ _Founda ___~
324.7
327.4 nip
rnp
40 40 41 41 45
40 39 42
40 40 39.: 43
40 40 40 38
1 .i
41 41 41
2 3 5 0
47
40 40 42 42 43
1 0 1.5
2 5 5.0
HSOs 0 5 1 0
40 40
..
H~SOI 0 3
40 39 37 36 35
40
40 38 38 36 40.0 p.p.m. of copper present in each solution. 1 0 1 5 2.3 5 0
from tin-base alloys. Consequently, tin, along with any antimony and arsenic, waq removed from all samples by volatilization of the bromides. Thus, of all the elements enumerated in Table 111, only appreciable amounts of nickel, s s might be encountered in nickel alloys, could be conqtrued i i q offering any qerious interference.
Table 111. Element Tested Aluminum
Influence of Diverse Elements
Cadmium C Iiromiu in
Cobalt Iron Lead
Present 20 20 20 40 40 20 20 20 40 40 40 40 40 40 40 20 20 40 40 40
10,000
I oon 2,000 io0
1Iagnesium ISFLUENCE O F ACIDS
Copper, P.P.M. _______-
Concentration. P.P.M. 1,000 2,000 5,000 2,000 4,000 1,000 2,000 1.000 2,000 1,000 2,000 4,000
1Ianganese
J00
The influence of hydrogen ion concentrstion and the effect of various anions commonly associated with the former in acids wed for the dissolution of the alloys were considered first. The results are tabulated in Table 11. Any of the common acidsperchloric, sulfuric? hydrochloric. or nitric acid-can be tolerated a t concentrations 1.011 or less. Obviously, the use of hydrochloric acid is precluded when silver ions are incorporated in the soliltions as an internal standard. Arid concentrations esceeding 1.0.1P generally cawed the flame hackground to increase slightly without any appreciahle increase in copper luminosity. Of the two copper lines, the copper 327.4 line proved less susceptible to alteration in intenpit>- when larger amounts of acid were present than did the copper 321.7 line. This was equally true both for " L - H" measwenlent.- and for internal standard measurements, IXFLCESCE OF OTHER ELEJZEVTS
Xichel
1,000 2 , 000 4,000 2,000 5,000 2,000 5,000
Potassiri in Sodium
Zinr
Found 20 20 20 40 40 20 21 18
34 40 42 40 40 41 42 20 20
40 42 45 40 40 40 40 20 20 100
4n
40 40 40 20 20 100
1 ,000 3 , oon 9,000
METHOD OF PROCESSIhG SAMPLES
Procedure for Aluminum- and Zinc-Base Alloys. Xeigh samples containing 1 to 15 mg. of copper into 150-ml. beakers. Dissolve in the minimum amount of 6 S perchloric acid or 8 5 nitric acid. Heat t o action and simmer until all the sample is dissolved. Raise the cover glasses n i t h hooks and evaporate nearlv to dryness to remove ewess acid. Add 25 ml. of demineralized water and transfer t o a 50-ml. volumetric flask. Add 2.50 ml. of standard silver solution, and dilute to the mark with demineralized water. Mix the solution meell. Atomize the solution into the flame, and read the luminosities on the instrument. Bracket the unknowns u i t h a series of standards. Obtain the luminosities a t the folloLving o a v e lengths: copper a t 324.7 mp. background a t 328.0 and 327.0 mp, copper a t 327.4 mp. silver a t 328.0 mp. and background at 328.3 mp, Two series of readings, or more if necessary to obtain reasonable duplication, are taken for both standards arid unknoivns. The appropriate background
Of the other elements investigatcd, only those were ronsidered which constitute the matrix of the copper-containing alloys or were present in these alloys in appreciable quantities. For each siibstance tested for interference, a series of solutions was piepared containing several k n o m concentrations of the test s i i b stance and 40 p.p.m. of copper. Table I11 shows the effect of various elements on the measurement of copper. The noninterferenrc of large qnantitiw of aluminum, cadmium, Icnd, and zinc is noteworth>-. This enables the flame 5pectrophotonietric .method for copper to be applied to Table IV. Analysis of Bureau of Standards Samples b y Flame aluminum-, lenti-. a n d zinc-haw alloys Spectrophotometric Method without prior separations ii' no other Values Found,- %--_ Concn. of Certified elements are present in concentrations Cii Value. L - H Internal Sample, which offer interference. The smal! Sample P.P.M. n /c method standard amounts of nickel, chromium, iron, and Aluiiiinum alloy 85a 1,800 2 48 =t 0 . 0 1 2 52 & 0.04 2 . 4 8 3z0.04 94 AI, 2 .Mg magnesium normally encountered in Aluminum alloy 86c 8,000 7 9 2 r 0 0 3 8 1 1 ~ 0 1 3 8 1 9 zk0.17 8 00 * 0.18 8 00 =t 0 . 2 0 1,250 90 Al, 1 Zn, 1 Fe these alloys offer no difficulties. Largr Aluminum allov 87 20.000 0 . 3 0 3z 0 01 0 297 =t 0 006 0.293 k 0.006 89 A I , 2 Zn amounts of the alkalies can be toler3 . 1 9 + 0 02 3.24 1 0.06 3 18 i 0.10 Tin-base alloy 54b 2,500 ~
ated, should it be necessary t o introduce them. Brief tests with tin indicated the impossiblity of preventing estensive hydrolvsis with the amounts of tin normally present in samples composited
87 Sn, 7 Sb, 2 Pb Tin-base alloy d4c 86 Sn, 7 Sh, 2 P h Zinc-base alloy 94a 95 Zn, 4 . i l
2,000
10,000 .5,000
4 30 1 0 . 0 2
4 33
1.08
1 10 i. 0 . 0 2 1 10 1 0 . 0 3
0 00:
'0.08
Values given are mean of a series of resiilts. with a-socinted standard deviation, -
4.34 1 08
~
= t o 10
k 0.03 1 . 0 9 1 0 04
u =
V O L U M E 27, N O . 8, A U G U S T 1 9 5 5 readings are subtracted from the unknovm and standard readings to obtain net relative luminosities, which are averaged for each sample. The average net relative luminosities of the standards are plotted against concentration to give L - H calibration curves for both copper lines to which tlie average relative luminosities of the unknowns are referred. Or, if the internal standard method of calibration is employed. the ratio of the average net relative luminosities of the copper line to that of the silver line is plotted against concent,ration of copper in the standards on log-log graph paper to give the respective calibration curves. Procedure for Tin-Base Alloys. Weigh samples containing 1 t o 15 mg. of copper into 125-nil. Erlenmeyer flasks. Dissolve in 20 ml. of 487, hydrobromic acid containing 2 ml. of bromine. Cover and heat gently until dissolution of the sample is compiete. Add 10 ml. of 12%’ perchloric acid and heat in a well-ventilated hood, while swirling, over an open flame, until white fumes first appear. Then heat moderately and intermittently to decompose any lead bromide and t o expel all hydrobromic acid. A stream of compressed air passed into the flask materially hastens the removal of stannic bromide and antimony bromide. For large amounts of tin, it ma>- be necessar>- to repeat the volatilization step with an additional 5 ml. of hydrobromic acid. Treat the entire sample residue, or an aliquot portion, iri the same manner as the alloys.
1229
fianie rpectrophotonirtric procwlures should particularly :tpptal to anaa 1-vsts. Iiesults obtained for the samples enumerated in Table IV are apparently unaffected by the measurement method eniploycd. Those obtained by L - H measurements and by the iritc,rrial standard method show no significant differences. This is intrrpreted to mean that the measurement of the flame luminosity a t the base of the emission linea of copper will adequately c~ompcnsate for variations in the flame background, and that furthrrniorc, the copper emission lines are essentially unaffected by the prcsencc of relatively large amounts of diversc clement,i in the flame. Apparently, whenever the flame background is enhanced or depressed by other elements, the copper line emission remains rirtually unchanged, and is simply an additive factor onto thc flwme background. Consequently, a t least for the ci reported in this investigation, it is not necrswr trouble of incorporating an internal standard element in thr sitiiiple and observing the additional luminosity readings required. ACKNOW LEDGM E S T
D1 SCU SSION
Table IV summarizes the results obtaiiied on Bureau of Standards aluminum-, tin-, and zinc-base alloys. The reproducibility of the flame analyses wits very good. On replicate sample? the standard deviation from the mean was approximately 3%. I n many cases the results obtained irere within adequate agreement with the certifirate values. Because of the rapidity with which flame analyses can be accomplished, the procedure for copper offers a competitive method for the determination of copper i,ivaling the conventional colorinirtric or gravimetric niethodpl. Dissolution of the mmple is the only preliminary step required prior to the actual flame measurements, which themselves require only a fwr niiiiittcs of the operator’s time. The prwiiion of flame analyses is not quite a s high as for some c~olothietricmethods and for iiiost gravimetric methods, hut for many piirposeP the higher degree of prerision is not required. I t is in thip latter type of analyses that
The author wishes to thank Clarice L. Thompson for her vooperation in performing some of the cxperinient~in the early phase of this work. LITERATURE CITED
(1) rlhrens, L. H., “Spec.trocheinica1 Anal Wesley Press, Cambridge, 1950. (2) Cholak, J., and Huhbard, D. AI., ISD. Esc. C H E v . , AXIL. ED., 16, 728 (1944). (3) Churchill, J. R., Ibid., 16, 668 (1044). (4) Ells, V. R.,J . O p t . SOC.4 m e r . . 31,534 (1941 ( 5 ) Griggs, A I . A, Johnstin. R., and Elledge, B. E.. ISD. E r c . CHEM., Api.4~.ED., 13, 99 (1941). (6) Lundegbrdh, H., Lanthruks-Hogskol. A4nn.,3 , 49 (1936). (7) Lundeghrdh. H., ”Quantitatire Spektralanalyse der Elemeiite.” Part I, Gustav Fischer, Jena. 1929. (8) Singh, N. L., Proc. Indian Acad. Sci.. 25A, 1 (1947). ) .
RECEIVED for review January 21, 19.53. .Iccei)ted May 2 , 193.5, I’rcsentcd \SociEry. a t the Southeastern Regional hieeting of the A M E R I C ACHEIIICL Birmingham, 41a.. 1954. Contribution S o . 140 from the Departiricnt of Chemistry, Cniversity of Tennessee, Knoxrille.
Flame Photometric Determination of lithium in Silicate Rocks R. B. ELLESTAD, Lithium
Corp. o f America, Inc., Minneapolis, M i n n .
E. L. HORSTMAN, Rock
Analysis Laboratory, University o f Minnesota, Minneapolis, M i n n .
Wet chemical methods are unsatisfactory for the determination of small amounts of lithium in rocks, so that geochemical studies hate depended on spectrographic methods. Trace amounts of lithium in silicate rocks and minerals are rapidly determined by flame photome t r y . The alkali metals and magnesium are separated from other rock constituents by an acid decomposition followed by a single precipitation with basic lead carbonate. A Beckman DU spectrophotometer is used and the test solution is burned in an inexpensive flame attachment using natural gas and compressed air. Sodium and potassium interferences are compensated for by the use of appropriate additions to the lithium standards. The method is sensitive to 5 p.p.m. of lithia in the original silicate sample, with a maximum deviation of 0.0005% of lithia in the range 0.001 to 0.03%. The method can he applied to a wide variety of silicate materials.
.
F
L4ME photoriiet,ric methods are graduallj- displacing [vet methods for the determination of the alkali metals.
The
J . Lawrence Smith method, although uscful for the deterinination of sodium and potassium in silicate rocks and niincrals, is not suitable for the determination of lithium. Killiams and hdaiiir (10) arid Broderick and Zack ( 3 )have applied the flame photometer to the determination of lithium in gl Fanus (4)and Beer ( 2 ) to the analysis of spodumene; and .\IrCoy and Chrietiansen ( 7 )to portland cement. Sone of these mctliod?, or the recent fluoroniet1,ic method of Khite. Fletcher, and I’arks ( { I ) , has been applied to tlie determination of trace amounts of lithium in silicate rocks. The determination of trace amounts of lithium in silic*;ttch rocks and minerals hits been confined to spectrographic nietlioli+. In this field the procedures of Strock (81 and Lundeghrdh ( p i ) have heen most widely used. The flame spertrophotonietcr gives results for trace aniounts of lithium that comp:are favor:tl)ly with determinations made with the spectrograph.
