Neutron Activation Analysis for Iridium in Platinum LLOYD S. JOWANOVITZ, FRED B. McNATT, ROBERT E. McCARLEY, and D O N S. MARTIN, Jr. Instifule for Atomic Research and Department of Chemistry, Iowa Stote University, Ames, Iowa
b A procedure is given for a high sensitivity neutron activation analysis for Ir in Pt which permits the use of Ptlg5"as a tracer. The standard deviation of seven analyses of a 0.04p.p.m. sample was 10%. Analyses were not influenced b y differences in the neutron spectrum and no interference from (n, p ) processes was evident a t the 0.04-p.p.m. level. The activation of Ptlg5" as an internal standard was unsatisfactory and an external standard is recommended. The effectiveness of separation procedures of Ir from Pt has been evaluated with the high sensitivity of the radiochemical procedure. The fractional crystallization of KzPtBr6 is proposed as the best means for the purification of platinum from an iridium impurity.
P t 1 9 5 m tracer which 1% ab h111g utilized for exchange studies. Consequently an activation analysis procedure was developed to demonstrate a practical Pensitivity for the analysis, to evaluate chemical separation and purification procedures for the separation of iridium from platinum, and to eitablish the iridium content of the current platinum stock. ii y-ray scintillation s1)ectrometer used for this work, was greatly superior to a conventional Geiger-Muller or proportional counter for the detection of radiation. It was possible to count y-rays from the Wg2with only minor interference from other nuclide.. Also, the contribution of any Ir1g2 to the platinum counting rates could be accurately evaluated.
EXPERIMENTAL
T
H E CHEMICAL behavior of platinum in its compounds may be strongly influenced b y the presence of an iridium impurity. Especially important is the readily reversed one-electron oxidationreduction couple of Ir(II1) and Ir(IV), whirh platinum does not possess. For example, Klason (3) commented that the normally convenient oxalate reduction of PtC16-' to PtC14-2 will not proceed in the absence of a t least a trace of iridium. Rich and Taube (5) have resolved some apparently contradictory exchange observations by noting that previously unsuspected iridium impurities strikingly inhibited a photoinduced exchange of the ligands of PtC16-2. It is, therefore, advisable to determine the iridium content of any platinum which is used for fundamental studies. Some iridium may remain with the platinum fraction in the refining of platinum metal ores. I n addition, iridium is frequently alloyed with the metal to enhance its physical properties. Therefore any platinum from a "recovered" source must be viewed with suspicion. IrIg1, abundance 38.5% ( 6 ) , has a cross section of 1000 x 10+4 sq. cm. for thermal neutrons t o yield Ir192 with a half life of 74 days ( 7 ) . With such a high cross section the neutron activation analysis for iridium is capable of providing a high sensitivity. This feature was emphasized by the repeated appearance of iridium activity in
1270
ANALYTICAL CHEMISTRY
Materials. Platinum was largely obtained from samples which had been recovered from laboratory experiments and originally was from several sources. Iridium was purchased as t h e trichloride from the Fairmount Chemical Co. Chemical reagents were normally of reagent analytical grade, meeting ACS specifications. Water was drawn from the distilled water tap and redistilled from alkaline permanganate. Equipment. Radioactive samples were counted b y means of thalliumactivated sodium iodide crystals. Packaged crystals from the Harshaw Chemical Co. were used. .4 crystal, inch long and 1'/' inches in diameter, mounted against the face of a Dumont 6292 photomultiplier, was satisfactory, although larger crystals have been utilized to give higher resolution also. The scintillation crystals and phototubes were operated with a single-channel recording spectrometer, Model 1810, of the Suclear-Chicago Corp. For spectrophotometric analyses a Beckman bIodel DU was usrd, and p H determinations weie made with Beckman Model G p H metw m-ith electrodes Model 1170 or 1190 E. Radioactivities. T h e neutron irradiations for analyses were performed in the Argonne Xational Laboratory C P 5 reactor and in the Oak Ridge National Laboratory graphite reactor. Most irradiations in the C P 5 reactor were performed in an isotope hole position for which the fast to thermal neutron flux ratio is a minimum. Nominal values for the fluxes were: epithermal
1 x 1010 n/sq. cm. aec., thermal, 2 X 10l2n/sq. cm. sec. However, to evaluate effects of fast neutrons, a few iriadiations were made in the vertical thimble positions with nominal fluxes of: epithermal 1.6 X l O I 3 n/sq. cm. see., thermal 2 X loL3n,'sq. cm. sec. Irradiated samples were available in our laboratory from 2 t o 7 days following irradiations. R i t h the additional processing time required, the 19-hour Pt197 and the 19-hour Irlg4 had suhstantially decayed away before the measurements. The principal coniponents of the radioactivity were Ir192(54 days), PtIg5" together with PtIg1 and Pt193 (with an over-all decay period of 3.8 days) and Au199 (3.15 days), t h r daughter from the ,%decay of PtIqY, formed by neutron capture. The photon spcctra obtained for the Ir, Pt, and -4u fractions are shoan i n Figure 1, in which the energies of gamma radiations with significant intensities are indicated by the lines rhose lengths are proportional to t l , c s intenqitie-. The usefulness of the s c ~ n j
f
Energy
(Iraq)
Figure 1. Photon spectra of AuIYb, Ptlg5,and lrlo2 lengths of vertical lines on the base lines indicate y - r a y intensities
tillatioii spectrometer for this study .1 demonstrated from these spectra, for the precence of significant -4u or 11 activities in a Pt fraction will be inimediately evident from the spectrum of the sample. Sinre the principal radiation counted for Pt is the K x-ra! TT hich follows internal conversion, it i. important to have a narrow window so the x-ray peak can be clearly distinguished from the noise.
Table I.
Data and Results of Activation Analyses
Ct Rate Irlg? A4ctivityIr19* .Ictivit>- Irl92 from Pt, in Pt in Std. (Infinite (Infinite (Infinite c.1'. I f . Irradiation) (Time after End Irradiation), Irradiation), Ir Content of Pt. C.P.M./ of Irradiation. C.P.M. / C.P.M./ Grams Pt Grams I r F g , 1rICh:inis Pt Grama Pt Days) -4ctivity
Reactor (Position). Sample Thermal Flux \Yt , l l g , N./Sq.Cm. Sec. 13-1
100.1 PtB 50.5
PtB
49,s PtH *TO , 7
PtB
TIL' :3
PtB 02.2
PtR 109 s l't B I OR 4
Weight of Length of I r in Irradiation, IrOp z H20 Hr . Std., M g .
CP5 ( I S 0 1)
x 10'E CP5 (IS0 2A) 2 x 10'2 CP5 (IS0 2B) 2 x 10'2 CP5 (VT YB) 'L x i o 1 3 Oak Ridge 5 x 10" Oak Ridge 5 x 10" Oak Ridges 5 x 10'1 CP5 ( V T ) 2.5 x 10'3
of
Pt195m
5.79
2
Procedure. X iveighed sample of poiv-drred plat,iiium metal, not more than ( # a .100 me.,was sealed in a small quartz tube foi an irradiation. For standardization of the analysis, 0.5 to 10 nig. of hydrous iridium(IV) oxide \vas scaled in a similar tube and t'he two tubcs were irradiated together for 12 to 24 hours in the reactor. Freshly ptq)ared hydrous iridium oxide, with only inoderate drying! was used, rather than the metal or aged oxide to avoid difficulties in dissolving these standards. When samples were ret,urned from irradiation, the platinum was dissolved in aqua regia and t h r solution \vas boiled down repeatedly with HCl. A measr i i , c d amount (30 t.0 50 mg.) of Ir as 1i.Clg-* carrier was added t o the solution. The gold was extracted from B 6-Y HC1 solution into ethyl acetate \\-hivh was saturated with 6 S HC1 by tlw procedure of Wilkinson (8). The wtraction was repeated twice again. After gold actirit'y was i.emoved, thv i d i u m was precipitated as iridium(1V) liydroxide by the pi,ocedure of Gilchrist and Wichers (a). The iridium was wparated as a uniform deposit on a 19-nini. diameter S & S Red Ribbon filtrr paper by suction filtration, with the paper supportcd hy a glass sinter i n a "chimney"-type assembly. The filter paper and precipitate n-rrc taped onto a n aluminum disk. This sample was counted by the scintillation spect rometer to give t h c iritliuni activity induced in the plat,inum. Additional carrier, ca. 40 mg., was added t o the fikrate from the first separation. A wrond and sometimes a third precipitate of the iridium(1V) hydroxide was separated and mounted for counting to test the effectiveness of the separation. Finally, a n aliquot' of the platinum filtrate remaining was reduced t,o metal, filtered onto a tared paper, washed, dried, weighed, and mounted for counting. All of the operat'ions could be handled safely in a hood with a filtered exhaust behind a lead brick body shield. The removal of the gold eliminated the
0.97 2.8G 0,:33 2.72 5.61 3.00 0.i6
majoy radiation hazard, antl thc rcmaining sample could he handlrd much more conveniently. 'I'he iridium standard \vas too hot, for thc preparat'ion of a wrighed counting sample. Thc standard samplr of hydi~ousiriclium oxide to be ii,i,atliatc'd lras 15-cighrd out, and a t the same time a sample of the material was dissolvcd i n hot HGr and analyzed hy t h r colorimetric procedure of hlacSeyin and Kriege (5). When the snmplcs werc returncd Ei,oni the irradiation, thc s t m a n d a d!vas completely dissolved in hot, HBi. and a measured aliquot of thi? solution was spotted over a 19-mm. filtcbr paper antl allowed to cl-aporatc. T h r filtcr paper was mount,ed f o r counting. and it had the sainr phj-siral sizc as other counting samples. Sclf-absorption and self-scattering w r e not. serious, as would be the case for p-ray counting, because only x-rays Equnl and y-rays n-ere counted. amounts of an iridium radioactirity. 1r.s than 1 mg. in weight, n-ere mixed with several different amounts of carrier up to 100 mg. Hon-ever, there was no difference in the counting rate of the samples greater than reasonable statist'ical fluctuations. It was necessary, of course, t o control carefully the geometry factors such as the extent of the sample and its distance from the rounting cryst'al. For a given irradiation: (micrograms I r in Pt sample) equals (micrograms I r in standard) X (count rate under the Ir peak a t 300 kev. from sample)/(efficiency of Ir recovery from sample) x (count rate under I r peak at 300 kev. from the standard). RESULTS AND DISCUSSION
The data and results for analyses of two platinum samples are in Table I. Sample A was a typical laboratory sample. Originally a Gilchriqt and
Wichers ( 2 ) separat'ioii had been performed to remove iridium. Thcn over a 1-year period i t had been crystallized in a number of different compounds. Sample B n-as purified for the prep:iratiori of Pt-tracer when tlic Ir activit'y was noted in early n-ork. The piirification consisted of a fractional crystallization of the K2PtHre, rind it represents the purest material we have obtained. It wa3 important to use t,he bromide system rather than rhloride. It is bclieved that in bromide, iridiuni is maintained in a + 3 oxidation statme ivhich is necessary for :I rlcan wparation hy crystallization of :i platinum(1V) cwnpound. 1able I includes the results of s e w n :ictimtions of Sample B. The average of these s e w n analyses is 0.043 pg, of I r per gram of 1%. The standard deviation of the analyses is 0.004, or cn. 10% of t h r average. The accuraq. of the malyses was not limited by cwunting rat,e statistics but rathrr 1))- weighing errors and uncertainties in the iridium analysis of the standard. I n the early work, difficulties were encountered from either contamination of the very pure samples, or possibly from inhomogcncity of the pori-dered platinum metal; and Ypecial precautions were taken to prel-cnt such possibilities with Sample Ti. While our experiments were uiider \ray, .iiroldi and Germagnoli (1) published results of a single activation of a 10-p.p.ni. sample. They compared the 300-k.e.v. y-ray peak of Ir1g2 with the 15Sk.e.v. 7-ray peak of h 1 9 9 without chemical separation. Because of uncertainties in cross section,?, decay schemes, and detection efficiencies they have claimed a n accuracy of not better than 307,. These uncertainties are not inrludctl in thc present work. since identical I r acltivity was utilized for the stnntlard and f,hc unkimwn. 7 .
VOL. 32,
NO. IO, SEPTEMBER 1960
1271
?-ields of the (n,p) procestz. The f n c t that activations in the vertical thimbles of the CP-5 reactor agreed n i t h the thermal flux activations of the iqotope % of holes indicates that this alternative Activity Total activation was not serious. I n the (End of h a d . ) , Iridium vertical thimbles the epithermal-toFraction C.P.M. .Ictivity thermal flux ratios are 1000 times .lu extract 160 0 28 greater than in the isotope hole.. 1 s t IrOpppt. 55410 98 8 The possibility vias investigated that 2nd IrOa ppt. 368 0 66 3rd IrOl ppt. 24 0 0.1 tlie P t 1 9 5 m activity might serve as an Remaining internal standard to monitor the reactor PPt. 132 0 23 flux. Kith a satisfactory internal >tandard the auxiliary external I r standard nould not be needed. Therefore. for each analysis 3, sample of the platinum The cross section of iridium Eo1 target Ivas separated from the I r and neutron capture is so high that there was l u activities. The K x-ray a t ca. some concern about possible eelf-absorp60 k.e.v. in thew samples T m s counted tion in the standard samples. Some n ith the scintillation spectrometer and of the powder m s weighed and packed the results are included in Table I. into a capillary with a known diameter. There 11as a TT ide variation in the ratio, The indicated density of the packed (activity Ir192,'grams of Pt) : (activity pon-der was ca. 1.3 grams per cc. A Pt'90m grams of Pt) and certainly a w r v rough calculation of the self-absorption drict control of the neutron flus di indicated that although the margin of tribution would be required. The charsafety was not high, self-absorption acter of the flux can be changed by the should be less than a tolerable limit of prc..ence of other samples in the irradia1 t o 5%, a t least for a strictly therilial tion post. The use of the internal flux. The weights of the standard Gtandard, therefore, is not reconisamples were therefore intentionally mended. varied over a six-fold range to test this It has been posible to evaluate conclusion. The constancy of the critically the Gilchrist and JF'ichers analyses indicates that self-absorption (2) separation of I r from Pt by radioin the standard is not serious so long chemical methods rrhich are ideal heas the Ir-content is less than 6 nig. cause of their high sensitivity. -1note Ir1g2 can be formed b y the process: of caution is appropriate. The efPt192(n,p)Ir192. Fortunately, the Pt1Q2 fectiveness of this separation is criticontent of normal platinum is low c ally dependent upon the exact condi(0.68%) ; and high energy neutrons are tions and timing n i t h n-hich operations required to provide the energy to expel are performed. Some practice and exthe proton across the high coulomb perience b y the analyst are required barrier. This process may limit the before even moderate reproducibility sensitivity of the activation analysi.. c a n be achieved. -4fter the initial Therefore irradiations in the high iepnrntion of the counting sample, ca. energy flux are expected to give higher 40 mg. of I r carrier as IrC16-2 n-as added, Table 11. Effectiveness of Separations in Analysis of Sample A
and a m o n d separation n a s effected by the qame procedure. Still a third -eparation r a s made. Results of this exprriinent are given in Table 11. Jt can be seen that less than 0.3% of I r activity was lost with the separation of gold. The initial separation of 40 mg. of I r carrier from 100 mg. of Pt carried 99.10% of the iridium activity colitained in the sample a t that time. The iridium activity of the second and third -aniples fell off rapidly and totaled another 0.7%. The efficiency of the -Libsequent separations fell off rapidl: and ca. 0.27, was not removed from the platinum. It is believed that this m a l l residue exists as an inert coinplex which does not exchange with the I r carrier. However, a t this very l o r Ici el the possibility of a radiochemical impurity can not be excluded. LITERATURE CITED
.iiroldi, G., Gerniagnoli, E , En~tgirc ?curleare (Xalan)4, 301 (1957). ( 2 ) Gilchrist, R., Kichers, E , J . .-1?11. Chem. SOC.57. 2565 (1935). ( 3 ) Klason, P., her. deut. c h e n ~ .Ges. 37, 1360 (1904). '4) NacSevin, IT. hl., Kriegc, 0. H., ASAL. CHEM.28, 16 (1956). ( 5 ) Rich. R. L.. Taube. H., J . d v 2 . Cheiii. SOC.76.2608 i1954). ( 6 ) SamGson, 11. G., Bleakney, Phys. Rev. 50, 732 (1936). ( 7) Seren, L., Friedlander, H. N., Turkel, S.H., Ibid., 72,888 (1947). ( 8 ) Wilkinson, G., Ibid., 75, 1019 (1949). I
1)
RECEIVEDfor review March 17, 1960. Accepted June 30, 1960. Division of .lndytical Chemistry, 133rd Meeting, -4'3, San Francisco, Calif., April 1958. Contribution S o . 825. Kork perfornied 111 The Ames Laboratory, U.S. Atomic Cnrrpy Commission.
Use of Neutron Activation Analysis for Determining Effectiveness of Zone- Refining Techniques in the Purification of Aluminum W. D.
MACKINTOSH
Atomic Energy o f Canada, Itd., Chalk River, Ontario, Canada
b When aluminum i s zone-refined, each impurity behaves differently. It is therefore desirable, when assessing the effectiveness of a particular zone-refining process, to determine the changes in concentration of as many impurities as possible. Neutron activation analysis can b e advantageously employed to determine a con-
1272
ANALYTICAL CHEMISTRY
siderable number of the impurities. In the present study of a bar of zonerefined aluminum, 3 impurity elewere determined' The results indicate the degree to which the zonerefining process has reduced the concentrations of these impurities in the central region of the bar.
a bar of aluminum metal is zone-refined (6),the impurities t h a t it contains are in general displaced toward one end or the other of the bar. -4s the extent and direction of movement of an impurity depend on how it affects the melting point of the aluminum, each impurity will behave differently under a particular set of zoneHEN
