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The determination of selenium by atomic absorption spectrophotometry
ANALYTICA
346
CHIMICA
ACTA
WC became intcrcstccl in tlw determination of selenium in the prcscncc of large amounts of sulphur during anr)thcr investigation in tlwsc laboratoricsl, that clcalt with cornpc~uncls and solid solutions of sulphur wit11 selenium. As the use of nhsorJ)tion sl)cctrol)liotomc!try for other clemcnts was under study at tlic atomic tin-w, this method sccmcd attractive, despite statcmcnts in ttw litcraturcz that it ‘I’hcrc is consiclcrablc intcrcst could nr)t be usccl for tile dctcrmination of selenium. in tllc clctcrmination of this c:lcmcnt in human and animal diets. Grain plants grown CJII sclcnifcrous soils, concentnttc tile olcment into the seed J2rotcin; animals fed tJn sucli gpin bccomc crnaciatccl, low hair and hoofs, ant1 CkVc!~O~~ other lXltllol~J~iCill Sclcnium poisoning affects large areas in U.S.A., symptoms of selenium toxicity:s. p.rticltl;wly in Nelwaslca ant1 tlw I>ilk(JtiLS, nncl also sonic areas in Northern Australia. Mwc rcwntly Sc~waw ANI) 1~01.1’~~ llavtr sllown tllat sclcnium is an essential trace clcnwnt in sfmw nnim;il diets, I,ow wticcntrations, of the orcicr of 0.1 p.p.m_, in tlw diet lw2vcnt ;I form of wtlcn~r in clliclcens, muscular dystrophy in calves and Ianbs and necrosis of the liver in rats. (:r)ticentratiotis of about 20 times this value give rise to symptoms of toxicity. It Ilns also been sufip5tccl tllat the sclcnium content of sulphidc arcs, in which ionic substitution is rcaclilg possible! with the i~lli(JtlS differing by only 80/0 in radius, an 1~: used to cliqp~se tlw origin of tlw dicposit. The clqncnt is usually isolntcd from sull~l~iclc ores of l~ydrotlicrni~il origin. Most methods for tlic cletcrmination of sclcniurn arc basccl 011 its separation from otlwr m;ttcrials by distilk~tion of tlw bromide from mixtures containing llydrogcn brctmiclc and broniine’~ followed by l~rwil~itntion of selenium with sulphur dictsiclc, l~ytlroXyli~tlli~lc, liydrwinc7, or ascorbic iiCiC17ir. Sclcnium is tlicn determined ioclimctricnlly after convwsion to selcnious n&l, cc~lorimetrically wit11 codeine SUll3ll~ltc
rcngcnt
crnl~loyccl
for
Or tiirl~idimctriciilly concentrations
greater
iLS tllc than
clcnicnt. I
or
2
‘TllCSC 1xp.m.
nicthocls M’hilc
CiLIl USUlllly our
work
was
bC in
.41~1~~~s~rclwrtcd that hc lint1 hccn able to ol)tain it sensitivity of 5 p.p.m. in tile analysis of simple sclcnium solutions by atomic ubsorption, wit11 8 liydrogcn ~~tltili~i~~nl ns a light sourc’c ilttC1 a nlonochrctn~i~tor set at 2040 A. Hc later informccl us (private communication) of considcrablc improvements in his method for detcrmining this clcmcnt. lx-ogress,
347
The units employccl in 8 start&d ~rr~il~~tn~llt for atomic alsorptiurr spectroptlotomctry in LLflame are shown in Fig. x. ‘The special features ~kssociated with these units, when used for the ckturmination of selenium, ;~rc discussedi in the following parapaphs. (ij Spectral s011rce. ‘The selenium atom has LL31) g-ound state and the lowest Cs;crgy transitions involving this strtte are shown in Fig. 2. ‘I’l~ose in which the 5s cxcitcd state is cfmo_s-necf, though forbidrlen under strict Russell-Saunders coupling ct~nciitions, arc prcst:nt at m0cferatc intensity in the emission spectrum of our sour4zts. ‘They did not show prq~tiblo absorption in our espcrimcnts, ‘The rulntive pq~ulations of the niemhers of the :#I’ state at the ternpslturc of a prop;knc!-nir flume! (say,
t-
I_
-IuL--~mR--L--------
1
i 1 t
tiomv rbield
nr
I
-VL
t
l--I
--e-q-
.Skifc
_._.
.._.
I
.*l+w/dittrt
-__.--._
,_..
wctvtdr~i~i~t
f Ai SP3 31’1
3r’o ..---
._ .__..._.__I -
I gfio ?O.lO 2011.3 --.-
._.--
-
_.
._
.-__
---,__
_...
. ..-....-..-_-INS”
-- ..__ “.._._ ...__. _.__ - _I_..___ So0
37 -...-.. -._..___ --.-._..-l_
__
f+~pfrftifinn tit
4 _..._. -...
22.56~” _ ___ 100
10 5 -_
rgGg”) and at the temperature of the acetylene-air flame (2qjcP) ;kre given in Table I. If it is a!ssumecI that true equilibrium exists ;tt the: fiarnu tcmpurature, it is obvious that the greatest sensitivity will be given at xq60 A. The source usually rccommen&xl for atomic absorption measurements is a large discharge tube, filled with neon or argon at low pressure, and with a hollow-
348
C. S. ItANN, A. N. HAMl3LY
and low melting point of selenium cathode ma& of the cloment(‘*l~~. The volatility lamp were made. The first had pose special clifficultics. ‘l’wo types of hollow-cathode a copper cathode into the central hollow of which was packed some copper selcnide. This form gave trouble lxcausc of the VdatiliSatkJII of selenium from the active positions in the cathode. The second form had as cathode a rod of spectrographic and the whole W;IS soaked in molten carbt with a lirtlc &+llccl into the ccntre,
sclcniurn in a vacuum. This gave a much Inngcr lift in that selenium lost by cvaporatictn was rcplacccl by ~1iffusi~~~lfrom the s~lrr~~~lnclill~graphite. ‘fhcre was considcrahlc &position of reel sclcnium on the walls ol the tube and the silica end-window of the 1iLtllp had to lx clcarecl by careful iL1qIlication of a pointccl flame. 1%>th forms, particularly tlic scconcl, showed a rapid “clean up” of the inert ~;Ls~~), and continuity of operation coulcl only lx ol~tainccl h_v pet’ioclically nclmitting further argon from a reservoir attachctl to the Iamp. As iLIl nltcrnatisc source, an clcxtt-oclc-less tlischtrrg:c in a vapour containing sclcnium was used. Small silica tubes with a central capillary about 2 cm long and cncl-pieces that were witlcr ancl about 1 cm long were ~ViLCW~tCCl and ;I small amount of selenium, or a sul1)hur-sclcniun1 mixtiirc, was clistillccl in from a sicle tube. The tulle was scalccl ilIlC1 the cncl-picccs wcrc held in clips to which were attached the slticlclcd lcacls from a 15 MC/S raclio-frequency transmitter. On heating the tuhc with a burner a clischnrgc commcncccl which, if the correct amotlnt of chalccgen was prcscnt, settled clown to a steady state wl~n the lxx-ncr was removccl. Careful adjustment of the filling of tlic tube allcl of tlic power supply was necessary to maintain this clischargc at a low sclcniuni prcssurc so that there was not too great a self-reversal of the xc)(io A line. (+i) Corttvol of i?tcifle?it bea~z. The output from tlte hollow-cnthrtcte, or from the central part of the clectroclc-less discharge, was at the focus of n lens 2.5 cm in diameter so that a piu%l!el beam passed thr(JUgh the flame. A similar lens focussed
DBTERMINATION
OF
SC
I3Y ATOMIC
AI3SORI’TIOX
JIEASUREMEST
the ‘beam on to the slit of the monochromator the
same
from
aperture
the source
in cliameter when
could
before
this
incident
as the
narrow
light
collimating
be isolated
the
second
beam
was
used
modulated
and
at
lenses
a diaphragm
Measurements are
the
were
of the monoclLrdmator.
by inserting lens.
was
mirror
349
referred
a frequency
with
of optical to
of 300
a central
densities
;Ls “small c/s
chosen
by
area
to have
A narrow
beam
hole
I mm
in the
flame
densities”.
a rot:Lting
disc
The placcct
the source and the first lens, (iii) Fk~ze. A major disadvantage in working at wavelengths as short as r#o A is the absorption of light 1,. the flntnc gases (‘l’ublc II). This absorption not only
between
.-_--_.-
I_.
(/II _-..
.
.._
.
.- . . . . ____-
Y’rccnsmission __. -- -. .--. -.__
Il’rwrlrngfll
._._ . _ ._
2rfi4
.-I cctvlow” - _
.._. _._
(‘;:‘,) --.-
--.
__._.____
Cb8
2070
88
s:
2dJ2
00
50
2O.)O
H.1
44
I C)T,O
34
tllc
.__
__..-.- . . . ..____..__. _____ Glf:jrair ffw~~c
_ .__- _.._._....
100
reduced
,..
_.... -.
IX
~LWLiliLble signal
to less than
zoCyO of its initial
value
at the
dcsirecl
~CLVC-
turbulcncc in the flame, wlklt has Fourier components which wrnild be ucceptecl by the uniplifier, into “noise”. The absorpticm vnricd considerably according to the operating ccmclitions. ‘L’llc transmission of iL fuel-rich acctylcnc-air flnmc coulcl. climinish by 20’%, wlkcn clistillcd water was sprayed into it but the effect WLS much less in ‘lcan’ flames. ‘I’wo forms of burner (Fig. I) wcrc cml~loycd both of whicll used the spray chamber of an ‘EEL flrkmc photometer (Evans Elcctrosclcnium Ltd., London). The carlicr work WLS carried out with a ro-cm slit burner similar to tllat described by CVfI~LIsfl (Fig. 113). With this type of burner, operation with acetylcnc was more satisfactory than with propane, and absorption due to selenium atoms was stronger for fuel-rich niixturcs than for those of low acetylene content. 13est results were obtained when the acctylcne- air flame shc~ccl a “feather”, clue to C3 molecules’“, for about 5 mm above the inner, pale blue cone. ‘I’he distribution of selenium atoms when a solution of selenium in mixed nitric and sulphuric acids was sprayed into this flame is sliown in Figs. 3n and 0. The contours of CClUill optical density at Xc@0 A length,
were placed
but
obtained
it iLlso converted
by scanning
in the OpticiLl path.
ztny
the flame For
lean
horizontally acetylene-air
ancl vertically mixtures
past
a x-mm
the sensitivity
aperture would
be
higher if small arca densities were used in the central region of high absorption ; advantagrs of small arca densities were not so pronounced for the fuel-rich flames. A sensitivity of I p.p.m. of selenium was obtainccl in the acetylene flame. much the
A~nl. Chim.
flcln,
32 (rgG5) 346-354
C. S. RANX,
354
A. N. HAMBLY
The second form _qf hur_n.g~~wasmade by adding a T-shaped silica combustion chamber as a flame shield above the standar~l burner unit of the ‘EEL’ flame photsmeter (Fig. IA). This was made from silics tubing of z.fi-cm internal diameter and had a zg-cm horizontal section ccntred along the optical axis, with a vertical section, 3 cm long, inserted at the mid-point. The tube was mounted with the opening of the
-1
+ Cmr
Rich
Pig, 3. Distribution of sclcniurn ototrls in acetyfcnc-air flil?tlf2 wing st:wd:trd (a) Fuel-rich flilnlc; (tif IWlll IlillSlCwith tl0 “fcathcr”. -f- = lbsition al burner are spacccl at 0.x unit of OptiCiti tletlsity with the highcut tlcnvity in ccntrc.
fo-cm
burner,
port. Contours
vertical section cft. I cm above the burner of the “EEL” unit to admit some secondary air, but this was insufficient for total combustion and there were small flames at the cncls of the horizontal 8x-m.The noise clue to turbulence in the flame was much less in this arrangcmcnt . Propane - irir flames wcrc used with this burner and an effective sensitivity of 1 p.p.m, was again achieved. This ‘arrangement was employed for analysis since its adjustment was less critical. (iv) Monoclzro~~tor nnd dctcctor. A Zeiss reflecting monochromator SPMr, with an aperture of 3 : 6.7, and fitted with a sodium chloride prism, was used. The effective half-width of the q&o A line on the recorder chart was I A, The radiation passing the exit slit of the monochromator was received by an E.M.I. 625% photomultiplier tube, fitted with a ‘LSpectrosi10 window. This tube had a particularly low background current, The output was amplified by a Hewlett-Packard, Model 302A, wave analyser which has a selectivity of & 3 c/s at the 3-db points and can be locked to the chopper frequency hy an automatic frecjuency control circuit. The output of the amplifier was simultaneously presented on a meter and on a Varian Model Gxo recorder. And.
Cl&a
Acta,
32
(196s)
346-353
DETERMINATION
OF
SC
BY
ATOMIC
ABSORPTION
JIEAStJRlSIENT
351
Catibration and test sohrtions Stock solutions were prepared by dissolving I g of grey selenium in each of the following reagents: (i) mixed concentrated nitric and sulphuric acids which converted the element to selenious acid; (ii) concentrated aqueous sodium sulphite solution with which it formed a selenosulphate; (iii) concentrated aqueous sodium cyanide sofution which converted it into sefenocyanate, These solutions were then diluted to r 1 and, from these stocks, standards containing IO p.p,m. and IOO p.p.m. of selenium were prepared for test. The absorption due to the selenium sprayed into the flame was expressed as an optical density, log JO/I, where 10 is the intensity of the radiation reaching the detector when distilled water is sprayed into the flame, and 1 is the comparable intensity when the solution under test is sprayed. There were losses in intensity due to scattering of the incident radiation by solid particles produced from the reagents in the sprayed solution, and in some cases due to absorption at x9Go A by other materials formed from the solution, The density of the flame for the above 3 solutions is shown in Table III; a correction, calculated to be about 0.01 could be applied to these figures for lossec., in intensity not due to absorption by selenium, It is apparent from these values, which each represent the mean of 3 or more determinations, that there was no significant difference for the 3 radically different methods of preparing the calibrating solution, and that there was no significant absorption from transition to the 6s level of selenium.
..“_I___
___......
~~Ye~t~ej~cy
fni
.
.
2O”~O zaG3 207tJ 2rQ ___I.__._-_.._-
.
.
.
----__._-._I_l-_
_l_-____-_._______.__
of Se
i-P.P.t@*l ..a
-----a--V.-e
xgGo* xgGo*
.
Cortcm. -.-.
----
NC-&&O3
1iacrv
O.OSU
o.oGg
’
a.037
0.25
Ck2.# 0.125
100 IO0 X00 100 _-..._.-I._.----
0.24
HzSO4~FlfNO3 ---
-_--_-_I_
I Cl IO0
Rcage,tl trsed _fovstock sottiliotz
0. x34 0.030 O.Ott) O.OO@
0.03n O.OIn, 0.013 __.- ___l___-_.f._-_
0.x32 0.0.55 o.otg O*OIQ
* These two sets of measurements were ofttainctI with the utectrodc-tcsa under diffcrcnt conditions from tbovc Rppfici~bl~ to the uthcr ‘1 sets.
tfischar~e operating
With the electrode-less discharge tube the relative sensitivity at 19Go A was less than the calculated value. While the monochromator accepted the full radiation at r9Go A, the selenium atoms in the flame only absorb& an extremely narrow range (about 0.01 A) in the centre of the emission line. It is just this central region which suffers self-absorption by selenium atoms in the outer part of the discharge, In this case the sensitivity was reduced and the optical density of the flame was much less than proportional to its selenium content and varied with the operating conditions of the discharge tube. It was, therefore, necessary to obtain the absorptions due to the calibrating solution and the unknown concentration during a minimum time &&erval. A typical calibration curve is shown in Fig. 4. With the first form of burner Axial. Cltim. Acta,
32 (rgG5) 346-354
352
C. S. RANN,
A.
N.
HAMBLY
one could increase the sensitivity by a factor of two by using small-area densities. This was not effective with the flame confined in the cylindrical shield (Pig. IA), and was not employed in the analyses below. Flames must always be operated under a suitable hood because of the toxicity of selenium.
A sarnplc of selcniferous wheat (T~iti~tr~~sp.) was obtained from South Dakota by courtesy of the Division of Plant Industry, C.S.X.K,O., and a specimen of gnlcna from the take George Mine, Captains Flat, N.S.W. ‘The major advantages in the use of atomic absorption methods arc their spcccl and their freedom from interference by other elements. ‘I”hcse uclvantngcs are only rcaliscd if the prcparatiun of the test solution does not involve ~~~~ori(~us separation procedures. Mc~~~o~~s which involve the c~~~cs~~~s~ of the sample followed by distillation of s&nium as bromide by boiling with hydrogen bromide and bromine as rccommundcd tscffor standard clctcrminrrtions of the elcmcnt, arc to be avoided if possible. The sample of South Dakota wheat was supplied with a certificate that an of two 10-g samples had shown an average analysis, by the method of lICr4erXQo, selenium content of x6.7 p,p.m. Such wheat is toxic. The solution fur spraying was prepared by burning zo g of the roughly ground wheat in the bomb of a Parr catorimeter unit, f3ccnusc of the small size of the bomb, which was dcsigncd for other purposes, 5 successive 2-g lots had to be hurncd in oxygen at a pressure of 30 atm. Samples of Australian wheat, checked by this methocl to be free of selenium, were ground, and prepared ‘as calibration standards by appropriate additions of standard selenium solutions. fn the first experiments a small quantity of aqueous alkali was present in the bottom of the bomb to absorb the acidic products of combustion, including selenium dioxide. Later a test was made by bubbling the gases from the
D&TERhlINATlON
01: Se
BY
ATOMIC
ABSORPTION
MEASUREMENT
353
bomb slowly through water at the conclusion of the combustion, and it was shown that there was no appreciable loss of selenium from the cold bomb even when no alkali had been added. After the 5 combustion stages, the bomb was washed out with water and the extract filtered to remove small amounts of solid. These washings (ca. 50 ml), which were acidic with nitric and sulphuric acids formed during the combustion, were evaporated to less than IO ml and then made up to this volume. The lower part (to IO p.p.m.) of the calibration curve obtained from the wheat sample sto which selenium had been added, corresponded to that obtained with the solution described on p. 351, but at higher concentrations the curve for the treated wheat samples fell below that for simple solutions. Small amounts of incompletely oxidized organic matter in the wCashings made these appreciably more viscous than the standard solutions, and thus modified the spraying conditions. With the calibration curve from the wheat samples a value of x6.3 p.p.m. was obtained from 4 detcrminations on the selcniferous wheat with a standard deviation of & 0.75 p.p.m. Assay
of sclenircnt $8 gatena A 3-g sample of finely ground galena was carefully mixed with about 20 g of dry sodium peroxide in a nickel crucible. T11c surface of the charge was covered with a layer of anhydrous sodium carbonate and the oxidation started by careful heating with a burner. After the first vigorous reaction had subsided the crucible was kept at a red heat for 3 min and then tllc soluble contents were leached out with boiling water. The total extract was evaporated to less than IOO ml, filtered to remove precipitated lead dioxide and then made up to roe ml. This solution was sprayed directly into the flame. A so-ml aliquot of this solution was used for determination of selenium by a gravimetric method. The aliquot was made strongly acidic with
ANALVSifS _--_.-.
OF
GALENA,
LhKfC
GEOHGIS
.__.I __._ - --_.._ -. _-..-.___--
.-- ---.
Sclcr~irctrr
Suttrplc
co)ilcnl
- _..-.__._-
..
cl
lotnic
MINE,
(I%,)
_.- ._,-_..__.._
IfLAT,
N.S.W.
I
0.1 IJ
o.t1q
2
0.15:
3
0.175
'3.159 O,i7&
tfcviation
_._
Grtrvimcfric
fibsorption
-_ _.- __^--_--_-
Statltlard
CAPTAINS
___..-....-.--
f
o.ocJ.~~;/~
hydrochloric acid and an cxccss of hyclroxyammonium chloride was added. The solution was heated on a water bath for 5 11and cooled overnight, and the red precipitate of selenium was collected in a sintercd glass crucible (porosity 4) and weighed. There was excellent agreement (Table IV) between the results of atomic absorption and gravimctric determinations on 3 samples which were known to contain somewhat different amounts of pyrite and chalcopyrite as impurities. Previous analyses of ore samples from this mints had indicated a much lower selenium content. With solutions containing such large quantities of inorganic materials as these mineral extracts, there was a considerable loss of signal by scattering and absorption
C.
354
S.
ItANN,
A. N. HAMBLY
in the flame due tu constituents other than selenium. The correction for this (which could be ils high as 0.08 density units) was obtained by measuring the optical density of the flame during the spraying period usin& a continuum at ctz. Igso A which was together with the difficulties emitted by the clectrocle-less discharge. ‘This correction, of spraying such concentrated solutions, would make sodium peroxide fusion unsuitable for concentrations less than 50 p.pm, of selenium. WC gratcfuliy acknowledge an Australian National University
the support postgraduate
received by one of uw (C.S.R.) scholarship.
from
The problems involved in the cleterminatiort of selenium by atomic absorption spcctrophotomctry arc cliscussccl. A practical method is described for which the sensitivity is alq~roximatcly x p,p.m. Such concentrations can he dotcrmincd in trcatrnent rcntlcrs the organic samples, hut with mineral samples the preliminary in a sulphidc ore. procedure unsuitable for conccntrtrtions lower than 50 p-pm. IZIISUNI? Lcs autcurt; ont examine les diffQents s&nium, par spcctruphotomdtric par absorption praticluc, pr&entant une sensibilit6 d’environ pcuvcnt Ctre &tcrmin&s dans cles substances substances min&alcs, It: traitemcnt pr&iminairc p.13.m. de s~l~ni~irn, dans un ~~liner~li sulfur&
Probleme, die bci der spcktrophotomctric auftrctcn, bci der die Eml~findtichkeit ~cstei!lsl~r~~bc~l becinflusst @nstig und crlaubt i5.R. die
probl&mes clue poac lc closa@2 du ._ _ atomicluc. 11s decrivcnt uric m6tllode I p.13.m. De tcllcs cunccntrations organiqucs; ntais clat~s Ie cas de ne pertnct pas de doser mains de go
13cstimmunl,: van S&n mit dcr atomaren Absorptionswcrden cliskutiert. Es wit-d cinc Mcthode beschriebcn, in organischen Probcn un~efiihr I p.p.m, betriigt. Bci die vorhcr~ehcnde ~~cllan~llun~ ‘das Verfahrcn unRestimmun~ van nur 50 p,p+m. in Schwefeleriren.
346
CHIMICA
ACTA
WC became intcrcstccl in tlw determination of selenium in the prcscncc of large amounts of sulphur during anr)thcr investigation in tlwsc laboratoricsl, that clcalt with cornpc~uncls and solid solutions of sulphur wit11 selenium. As the use of nhsorJ)tion sl)cctrol)liotomc!try for other clemcnts was under study at tlic atomic tin-w, this method sccmcd attractive, despite statcmcnts in ttw litcraturcz that it ‘I’hcrc is consiclcrablc intcrcst could nr)t be usccl for tile dctcrmination of selenium. in tllc clctcrmination of this c:lcmcnt in human and animal diets. Grain plants grown CJII sclcnifcrous soils, concentnttc tile olcment into the seed J2rotcin; animals fed tJn sucli gpin bccomc crnaciatccl, low hair and hoofs, ant1 CkVc!~O~~ other lXltllol~J~iCill Sclcnium poisoning affects large areas in U.S.A., symptoms of selenium toxicity:s. p.rticltl;wly in Nelwaslca ant1 tlw I>ilk(JtiLS, nncl also sonic areas in Northern Australia. Mwc rcwntly Sc~waw ANI) 1~01.1’~~ llavtr sllown tllat sclcnium is an essential trace clcnwnt in sfmw nnim;il diets, I,ow wticcntrations, of the orcicr of 0.1 p.p.m_, in tlw diet lw2vcnt ;I form of wtlcn~r in clliclcens, muscular dystrophy in calves and Ianbs and necrosis of the liver in rats. (:r)ticentratiotis of about 20 times this value give rise to symptoms of toxicity. It Ilns also been sufip5tccl tllat the sclcnium content of sulphidc arcs, in which ionic substitution is rcaclilg possible! with the i~lli(JtlS differing by only 80/0 in radius, an 1~: used to cliqp~se tlw origin of tlw dicposit. The clqncnt is usually isolntcd from sull~l~iclc ores of l~ydrotlicrni~il origin. Most methods for tlic cletcrmination of sclcniurn arc basccl 011 its separation from otlwr m;ttcrials by distilk~tion of tlw bromide from mixtures containing llydrogcn brctmiclc and broniine’~ followed by l~rwil~itntion of selenium with sulphur dictsiclc, l~ytlroXyli~tlli~lc, liydrwinc7, or ascorbic iiCiC17ir. Sclcnium is tlicn determined ioclimctricnlly after convwsion to selcnious n&l, cc~lorimetrically wit11 codeine SUll3ll~ltc
rcngcnt
crnl~loyccl
for
Or tiirl~idimctriciilly concentrations
greater
iLS tllc than
clcnicnt. I
or
2
‘TllCSC 1xp.m.
nicthocls M’hilc
CiLIl USUlllly our
work
was
bC in
.41~1~~~s~rclwrtcd that hc lint1 hccn able to ol)tain it sensitivity of 5 p.p.m. in tile analysis of simple sclcnium solutions by atomic ubsorption, wit11 8 liydrogcn ~~tltili~i~~nl ns a light sourc’c ilttC1 a nlonochrctn~i~tor set at 2040 A. Hc later informccl us (private communication) of considcrablc improvements in his method for detcrmining this clcmcnt. lx-ogress,
347
The units employccl in 8 start&d ~rr~il~~tn~llt for atomic alsorptiurr spectroptlotomctry in LLflame are shown in Fig. x. ‘The special features ~kssociated with these units, when used for the ckturmination of selenium, ;~rc discussedi in the following parapaphs. (ij Spectral s011rce. ‘The selenium atom has LL31) g-ound state and the lowest Cs;crgy transitions involving this strtte are shown in Fig. 2. ‘I’l~ose in which the 5s cxcitcd state is cfmo_s-necf, though forbidrlen under strict Russell-Saunders coupling ct~nciitions, arc prcst:nt at m0cferatc intensity in the emission spectrum of our sour4zts. ‘They did not show prq~tiblo absorption in our espcrimcnts, ‘The rulntive pq~ulations of the niemhers of the :#I’ state at the ternpslturc of a prop;knc!-nir flume! (say,
t-
I_
-IuL--~mR--L--------
1
i 1 t
tiomv rbield
nr
I
-VL
t
l--I
--e-q-
.Skifc
_._.
.._.
I
.*l+w/dittrt
-__.--._
,_..
wctvtdr~i~i~t
f Ai SP3 31’1
3r’o ..---
._ .__..._.__I -
I gfio ?O.lO 2011.3 --.-
._.--
-
_.
._
.-__
---,__
_...
. ..-....-..-_-INS”
-- ..__ “.._._ ...__. _.__ - _I_..___ So0
37 -...-.. -._..___ --.-._..-l_
__
f+~pfrftifinn tit
4 _..._. -...
22.56~” _ ___ 100
10 5 -_
rgGg”) and at the temperature of the acetylene-air flame (2qjcP) ;kre given in Table I. If it is a!ssumecI that true equilibrium exists ;tt the: fiarnu tcmpurature, it is obvious that the greatest sensitivity will be given at xq60 A. The source usually rccommen&xl for atomic absorption measurements is a large discharge tube, filled with neon or argon at low pressure, and with a hollow-
348
C. S. ItANN, A. N. HAMl3LY
and low melting point of selenium cathode ma& of the cloment(‘*l~~. The volatility lamp were made. The first had pose special clifficultics. ‘l’wo types of hollow-cathode a copper cathode into the central hollow of which was packed some copper selcnide. This form gave trouble lxcausc of the VdatiliSatkJII of selenium from the active positions in the cathode. The second form had as cathode a rod of spectrographic and the whole W;IS soaked in molten carbt with a lirtlc &+llccl into the ccntre,
sclcniurn in a vacuum. This gave a much Inngcr lift in that selenium lost by cvaporatictn was rcplacccl by ~1iffusi~~~lfrom the s~lrr~~~lnclill~graphite. ‘fhcre was considcrahlc &position of reel sclcnium on the walls ol the tube and the silica end-window of the 1iLtllp had to lx clcarecl by careful iL1qIlication of a pointccl flame. 1%>th forms, particularly tlic scconcl, showed a rapid “clean up” of the inert ~;Ls~~), and continuity of operation coulcl only lx ol~tainccl h_v pet’ioclically nclmitting further argon from a reservoir attachctl to the Iamp. As iLIl nltcrnatisc source, an clcxtt-oclc-less tlischtrrg:c in a vapour containing sclcnium was used. Small silica tubes with a central capillary about 2 cm long and cncl-pieces that were witlcr ancl about 1 cm long were ~ViLCW~tCCl and ;I small amount of selenium, or a sul1)hur-sclcniun1 mixtiirc, was clistillccl in from a sicle tube. The tulle was scalccl ilIlC1 the cncl-picccs wcrc held in clips to which were attached the slticlclcd lcacls from a 15 MC/S raclio-frequency transmitter. On heating the tuhc with a burner a clischnrgc commcncccl which, if the correct amotlnt of chalccgen was prcscnt, settled clown to a steady state wl~n the lxx-ncr was removccl. Careful adjustment of the filling of tlic tube allcl of tlic power supply was necessary to maintain this clischargc at a low sclcniuni prcssurc so that there was not too great a self-reversal of the xc)(io A line. (+i) Corttvol of i?tcifle?it bea~z. The output from tlte hollow-cnthrtcte, or from the central part of the clectroclc-less discharge, was at the focus of n lens 2.5 cm in diameter so that a piu%l!el beam passed thr(JUgh the flame. A similar lens focussed
DBTERMINATION
OF
SC
I3Y ATOMIC
AI3SORI’TIOX
JIEASUREMEST
the ‘beam on to the slit of the monochromator the
same
from
aperture
the source
in cliameter when
could
before
this
incident
as the
narrow
light
collimating
be isolated
the
second
beam
was
used
modulated
and
at
lenses
a diaphragm
Measurements are
the
were
of the monoclLrdmator.
by inserting lens.
was
mirror
349
referred
a frequency
with
of optical to
of 300
a central
densities
;Ls “small c/s
chosen
by
area
to have
A narrow
beam
hole
I mm
in the
flame
densities”.
a rot:Lting
disc
The placcct
the source and the first lens, (iii) Fk~ze. A major disadvantage in working at wavelengths as short as r#o A is the absorption of light 1,. the flntnc gases (‘l’ublc II). This absorption not only
between
.-_--_.-
I_.
(/II _-..
.
.._
.
.- . . . . ____-
Y’rccnsmission __. -- -. .--. -.__
Il’rwrlrngfll
._._ . _ ._
2rfi4
.-I cctvlow” - _
.._. _._
(‘;:‘,) --.-
--.
__._.____
Cb8
2070
88
s:
2dJ2
00
50
2O.)O
H.1
44
I C)T,O
34
tllc
.__
__..-.- . . . ..____..__. _____ Glf:jrair ffw~~c
_ .__- _.._._....
100
reduced
,..
_.... -.
IX
~LWLiliLble signal
to less than
zoCyO of its initial
value
at the
dcsirecl
~CLVC-
turbulcncc in the flame, wlklt has Fourier components which wrnild be ucceptecl by the uniplifier, into “noise”. The absorpticm vnricd considerably according to the operating ccmclitions. ‘L’llc transmission of iL fuel-rich acctylcnc-air flnmc coulcl. climinish by 20’%, wlkcn clistillcd water was sprayed into it but the effect WLS much less in ‘lcan’ flames. ‘I’wo forms of burner (Fig. I) wcrc cml~loycd both of whicll used the spray chamber of an ‘EEL flrkmc photometer (Evans Elcctrosclcnium Ltd., London). The carlicr work WLS carried out with a ro-cm slit burner similar to tllat described by CVfI~LIsfl (Fig. 113). With this type of burner, operation with acetylcnc was more satisfactory than with propane, and absorption due to selenium atoms was stronger for fuel-rich niixturcs than for those of low acetylene content. 13est results were obtained when the acctylcne- air flame shc~ccl a “feather”, clue to C3 molecules’“, for about 5 mm above the inner, pale blue cone. ‘I’he distribution of selenium atoms when a solution of selenium in mixed nitric and sulphuric acids was sprayed into this flame is sliown in Figs. 3n and 0. The contours of CClUill optical density at Xc@0 A length,
were placed
but
obtained
it iLlso converted
by scanning
in the OpticiLl path.
ztny
the flame For
lean
horizontally acetylene-air
ancl vertically mixtures
past
a x-mm
the sensitivity
aperture would
be
higher if small arca densities were used in the central region of high absorption ; advantagrs of small arca densities were not so pronounced for the fuel-rich flames. A sensitivity of I p.p.m. of selenium was obtainccl in the acetylene flame. much the
A~nl. Chim.
flcln,
32 (rgG5) 346-354
C. S. RANX,
354
A. N. HAMBLY
The second form _qf hur_n.g~~wasmade by adding a T-shaped silica combustion chamber as a flame shield above the standar~l burner unit of the ‘EEL’ flame photsmeter (Fig. IA). This was made from silics tubing of z.fi-cm internal diameter and had a zg-cm horizontal section ccntred along the optical axis, with a vertical section, 3 cm long, inserted at the mid-point. The tube was mounted with the opening of the
-1
+ Cmr
Rich
Pig, 3. Distribution of sclcniurn ototrls in acetyfcnc-air flil?tlf2 wing st:wd:trd (a) Fuel-rich flilnlc; (tif IWlll IlillSlCwith tl0 “fcathcr”. -f- = lbsition al burner are spacccl at 0.x unit of OptiCiti tletlsity with the highcut tlcnvity in ccntrc.
fo-cm
burner,
port. Contours
vertical section cft. I cm above the burner of the “EEL” unit to admit some secondary air, but this was insufficient for total combustion and there were small flames at the cncls of the horizontal 8x-m.The noise clue to turbulence in the flame was much less in this arrangcmcnt . Propane - irir flames wcrc used with this burner and an effective sensitivity of 1 p.p.m, was again achieved. This ‘arrangement was employed for analysis since its adjustment was less critical. (iv) Monoclzro~~tor nnd dctcctor. A Zeiss reflecting monochromator SPMr, with an aperture of 3 : 6.7, and fitted with a sodium chloride prism, was used. The effective half-width of the q&o A line on the recorder chart was I A, The radiation passing the exit slit of the monochromator was received by an E.M.I. 625% photomultiplier tube, fitted with a ‘LSpectrosi10 window. This tube had a particularly low background current, The output was amplified by a Hewlett-Packard, Model 302A, wave analyser which has a selectivity of & 3 c/s at the 3-db points and can be locked to the chopper frequency hy an automatic frecjuency control circuit. The output of the amplifier was simultaneously presented on a meter and on a Varian Model Gxo recorder. And.
Cl&a
Acta,
32
(196s)
346-353
DETERMINATION
OF
SC
BY
ATOMIC
ABSORPTION
JIEAStJRlSIENT
351
Catibration and test sohrtions Stock solutions were prepared by dissolving I g of grey selenium in each of the following reagents: (i) mixed concentrated nitric and sulphuric acids which converted the element to selenious acid; (ii) concentrated aqueous sodium sulphite solution with which it formed a selenosulphate; (iii) concentrated aqueous sodium cyanide sofution which converted it into sefenocyanate, These solutions were then diluted to r 1 and, from these stocks, standards containing IO p.p,m. and IOO p.p.m. of selenium were prepared for test. The absorption due to the selenium sprayed into the flame was expressed as an optical density, log JO/I, where 10 is the intensity of the radiation reaching the detector when distilled water is sprayed into the flame, and 1 is the comparable intensity when the solution under test is sprayed. There were losses in intensity due to scattering of the incident radiation by solid particles produced from the reagents in the sprayed solution, and in some cases due to absorption at x9Go A by other materials formed from the solution, The density of the flame for the above 3 solutions is shown in Table III; a correction, calculated to be about 0.01 could be applied to these figures for lossec., in intensity not due to absorption by selenium, It is apparent from these values, which each represent the mean of 3 or more determinations, that there was no significant difference for the 3 radically different methods of preparing the calibrating solution, and that there was no significant absorption from transition to the 6s level of selenium.
..“_I___
___......
~~Ye~t~ej~cy
fni
.
.
2O”~O zaG3 207tJ 2rQ ___I.__._-_.._-
.
.
.
----__._-._I_l-_
_l_-____-_._______.__
of Se
i-P.P.t@*l ..a
-----a--V.-e
xgGo* xgGo*
.
Cortcm. -.-.
----
NC-&&O3
1iacrv
O.OSU
o.oGg
’
a.037
0.25
Ck2.# 0.125
100 IO0 X00 100 _-..._.-I._.----
0.24
HzSO4~FlfNO3 ---
-_--_-_I_
I Cl IO0
Rcage,tl trsed _fovstock sottiliotz
0. x34 0.030 O.Ott) O.OO@
0.03n O.OIn, 0.013 __.- ___l___-_.f._-_
0.x32 0.0.55 o.otg O*OIQ
* These two sets of measurements were ofttainctI with the utectrodc-tcsa under diffcrcnt conditions from tbovc Rppfici~bl~ to the uthcr ‘1 sets.
tfischar~e operating
With the electrode-less discharge tube the relative sensitivity at 19Go A was less than the calculated value. While the monochromator accepted the full radiation at r9Go A, the selenium atoms in the flame only absorb& an extremely narrow range (about 0.01 A) in the centre of the emission line. It is just this central region which suffers self-absorption by selenium atoms in the outer part of the discharge, In this case the sensitivity was reduced and the optical density of the flame was much less than proportional to its selenium content and varied with the operating conditions of the discharge tube. It was, therefore, necessary to obtain the absorptions due to the calibrating solution and the unknown concentration during a minimum time &&erval. A typical calibration curve is shown in Fig. 4. With the first form of burner Axial. Cltim. Acta,
32 (rgG5) 346-354
352
C. S. RANN,
A.
N.
HAMBLY
one could increase the sensitivity by a factor of two by using small-area densities. This was not effective with the flame confined in the cylindrical shield (Pig. IA), and was not employed in the analyses below. Flames must always be operated under a suitable hood because of the toxicity of selenium.
A sarnplc of selcniferous wheat (T~iti~tr~~sp.) was obtained from South Dakota by courtesy of the Division of Plant Industry, C.S.X.K,O., and a specimen of gnlcna from the take George Mine, Captains Flat, N.S.W. ‘The major advantages in the use of atomic absorption methods arc their spcccl and their freedom from interference by other elements. ‘I”hcse uclvantngcs are only rcaliscd if the prcparatiun of the test solution does not involve ~~~~ori(~us separation procedures. Mc~~~o~~s which involve the c~~~cs~~~s~ of the sample followed by distillation of s&nium as bromide by boiling with hydrogen bromide and bromine as rccommundcd tscffor standard clctcrminrrtions of the elcmcnt, arc to be avoided if possible. The sample of South Dakota wheat was supplied with a certificate that an of two 10-g samples had shown an average analysis, by the method of lICr4erXQo, selenium content of x6.7 p,p.m. Such wheat is toxic. The solution fur spraying was prepared by burning zo g of the roughly ground wheat in the bomb of a Parr catorimeter unit, f3ccnusc of the small size of the bomb, which was dcsigncd for other purposes, 5 successive 2-g lots had to be hurncd in oxygen at a pressure of 30 atm. Samples of Australian wheat, checked by this methocl to be free of selenium, were ground, and prepared ‘as calibration standards by appropriate additions of standard selenium solutions. fn the first experiments a small quantity of aqueous alkali was present in the bottom of the bomb to absorb the acidic products of combustion, including selenium dioxide. Later a test was made by bubbling the gases from the
D&TERhlINATlON
01: Se
BY
ATOMIC
ABSORPTION
MEASUREMENT
353
bomb slowly through water at the conclusion of the combustion, and it was shown that there was no appreciable loss of selenium from the cold bomb even when no alkali had been added. After the 5 combustion stages, the bomb was washed out with water and the extract filtered to remove small amounts of solid. These washings (ca. 50 ml), which were acidic with nitric and sulphuric acids formed during the combustion, were evaporated to less than IO ml and then made up to this volume. The lower part (to IO p.p.m.) of the calibration curve obtained from the wheat sample sto which selenium had been added, corresponded to that obtained with the solution described on p. 351, but at higher concentrations the curve for the treated wheat samples fell below that for simple solutions. Small amounts of incompletely oxidized organic matter in the wCashings made these appreciably more viscous than the standard solutions, and thus modified the spraying conditions. With the calibration curve from the wheat samples a value of x6.3 p.p.m. was obtained from 4 detcrminations on the selcniferous wheat with a standard deviation of & 0.75 p.p.m. Assay
of sclenircnt $8 gatena A 3-g sample of finely ground galena was carefully mixed with about 20 g of dry sodium peroxide in a nickel crucible. T11c surface of the charge was covered with a layer of anhydrous sodium carbonate and the oxidation started by careful heating with a burner. After the first vigorous reaction had subsided the crucible was kept at a red heat for 3 min and then tllc soluble contents were leached out with boiling water. The total extract was evaporated to less than IOO ml, filtered to remove precipitated lead dioxide and then made up to roe ml. This solution was sprayed directly into the flame. A so-ml aliquot of this solution was used for determination of selenium by a gravimetric method. The aliquot was made strongly acidic with
ANALVSifS _--_.-.
OF
GALENA,
LhKfC
GEOHGIS
.__.I __._ - --_.._ -. _-..-.___--
.-- ---.
Sclcr~irctrr
Suttrplc
co)ilcnl
- _..-.__._-
..
cl
lotnic
MINE,
(I%,)
_.- ._,-_..__.._
IfLAT,
N.S.W.
I
0.1 IJ
o.t1q
2
0.15:
3
0.175
'3.159 O,i7&
tfcviation
_._
Grtrvimcfric
fibsorption
-_ _.- __^--_--_-
Statltlard
CAPTAINS
___..-....-.--
f
o.ocJ.~~;/~
hydrochloric acid and an cxccss of hyclroxyammonium chloride was added. The solution was heated on a water bath for 5 11and cooled overnight, and the red precipitate of selenium was collected in a sintercd glass crucible (porosity 4) and weighed. There was excellent agreement (Table IV) between the results of atomic absorption and gravimctric determinations on 3 samples which were known to contain somewhat different amounts of pyrite and chalcopyrite as impurities. Previous analyses of ore samples from this mints had indicated a much lower selenium content. With solutions containing such large quantities of inorganic materials as these mineral extracts, there was a considerable loss of signal by scattering and absorption
C.
354
S.
ItANN,
A. N. HAMBLY
in the flame due tu constituents other than selenium. The correction for this (which could be ils high as 0.08 density units) was obtained by measuring the optical density of the flame during the spraying period usin& a continuum at ctz. Igso A which was together with the difficulties emitted by the clectrocle-less discharge. ‘This correction, of spraying such concentrated solutions, would make sodium peroxide fusion unsuitable for concentrations less than 50 p.pm, of selenium. WC gratcfuliy acknowledge an Australian National University
the support postgraduate
received by one of uw (C.S.R.) scholarship.
from
The problems involved in the cleterminatiort of selenium by atomic absorption spcctrophotomctry arc cliscussccl. A practical method is described for which the sensitivity is alq~roximatcly x p,p.m. Such concentrations can he dotcrmincd in trcatrnent rcntlcrs the organic samples, hut with mineral samples the preliminary in a sulphidc ore. procedure unsuitable for conccntrtrtions lower than 50 p-pm. IZIISUNI? Lcs autcurt; ont examine les diffQents s&nium, par spcctruphotomdtric par absorption praticluc, pr&entant une sensibilit6 d’environ pcuvcnt Ctre &tcrmin&s dans cles substances substances min&alcs, It: traitemcnt pr&iminairc p.13.m. de s~l~ni~irn, dans un ~~liner~li sulfur&
Probleme, die bci der spcktrophotomctric auftrctcn, bci der die Eml~findtichkeit ~cstei!lsl~r~~bc~l becinflusst @nstig und crlaubt i5.R. die
probl&mes clue poac lc closa@2 du ._ _ atomicluc. 11s decrivcnt uric m6tllode I p.13.m. De tcllcs cunccntrations organiqucs; ntais clat~s Ie cas de ne pertnct pas de doser mains de go
13cstimmunl,: van S&n mit dcr atomaren Absorptionswcrden cliskutiert. Es wit-d cinc Mcthode beschriebcn, in organischen Probcn un~efiihr I p.p.m, betriigt. Bci die vorhcr~ehcnde ~~cllan~llun~ ‘das Verfahrcn unRestimmun~ van nur 50 p,p+m. in Schwefeleriren.