11. C. H. H. Van Deurzen, J. G. Conway, and S. P. Davis, J. Opt. Soc. Am. 63,158 (1973).

12. C. E. Moore, Ale Ultraviolet Multiplel Table, 1, Natl. Bur. Std. (U.S.) circ. No. 488 (U.S. Government Printing Office, Washington, D.C., 1950).

13. C. E. Moore, A Multiplet Table of Astrophysical Interest, Natl. Stand. Ref. Data Ser., Natl. Bur. Std. (U.S.) 40, (1972).

14. L. Iglesias, An. l{eal Soc. Esp. Fis. y Quire (Madrid) [A] 58, 191 (1962).

15. L. Iglesias and R. Velasco, An. Real Soc. Esp. Fis. y Quim (Madrid) [A] 54, 83 (1958).

16. L. Iglesias, J. Res. Natl. Bur. Std. 72A, 295 (1968). 17. C. H. H. Van Deurzen, Ph.D. thesis, University of Cali-

fornia, LBL-1657 (1973). 18. J. P. Walters, Appl. Spectrosc. 26, 17 (1972). 19. T. P. Schreiber and D. L. Fry, Spectrochim. Acta 13, 104

(195S). 20. S. Mandelstam, Spectrochim. Acta 15,255 (1959). 21. R. D. Sacks and J. P. Walters, Anal. Chem. 42, 61 (1970).

Determination of Microgram Quantities of Thallium in Aluminum-, Iron-, and Nickel-Base Alloys*

K e i t h E. B u r k e

Paul D. Merica Research Laboratory, I~dernational Nickel Company, Sterling Forest, Suffer~, New York 10901

(Rece ived 2 Oc tober 1973; revis ion r(~ceived 15 October 1973)

Chemical techniques are required for the determination of trace constituents in commercial nickel and its alloys. An adequate method does not exist for the determination of microgram quantities of thallium; therefore, an extraction-atomic absorption method has been developed. Trace quantities of thallium in nickel are quantitatively extracted from a 10% hydrochloric acid solution containing 2% ascorbic acid and 9% potassium iodide by a 5% solution of tri-,it- octylphosphine oxide in 4-methylpentane-2-one (MIBK). The MIBK extract containing the thallium is then nebulized directly into the atomic absorption flame. The concentration range for the determination of thallium in MIBK is an order of magnitude lower than fnr aqueous media. The proposed analytical technique is accurate and rapid and has a precision (o) of ±0.15 ppm at the 2.00 ppm level. The limit of detection is 0.1 ppm of thallium. The method is also applicable to aluminum- and iron-base alloys. The solvent extraction procedure used to isolate thallium has also been validated for antimony, bismuth, lead, and tin and can also be applied to the determination of cadmium, copper, indium, and zinc. INDEX HEADINGS: Flame atomic absorption spectroscopy; Trace analysis; Solvent extraction; Aluminum-, iron-, and nickel-base alloys.

INTRODUCTION

Produce r s of h igh t e m p e r a t u r e a l loys are of ten re- qu i red to " c e r t i f y " the level of ce r ta in i m p u r i t y ele- m e n t s which m a y affect the i r p roper t ies . 1,2 T h a l l i u m is one e l emen t r e p o r t e d to cause dele ter ious effects in con- cen t r a t i ons as low as 1 to 25 p p m / A d e q u a t e chemi- cal m e t h o d s are no t ava i l ab le for th is de t e rmina t i on .

Severa l i n s t r u m e n t a l m e t h o d s have been r e p o r t e d for t he d e t e r m i n a t i o n of t h a l l i u m in complex a l loy sys tems . T h e mass s p e c t r o g r a p h measures 0.1 ppm, 4 and the hol low ca thode emission m e t h o d de tec t s 0.1 p p m ~, while op t ica l emission p lus concen t r a t i on techniques r epo r t ope ra t i ng ranges of 1 to 100 p p m 6 and 0.5 to 25 p p m 7 of tha l l ium. A s imple t echn ique is useful in o rder to ca l ib ra t e i n s t r u m e n t s or ver i fy an ana lys i s even when these expens ive i n s t r u m e n t s are avai lab le .

A t o m i c abso rp t ion spec t roscopy is a s imple t echn ique for some de t e rmina t i ons ; however , i t canno t be app l i ed

* Paper presented at the 4th International Conference on Atomic Spectroscopy, Toronto, Ontario, Canada, 1 Novem- ber 1973.

d i rec t ly to the d e t e r m i n a t i o n of less t h a n 0 .005% of t h a l l i u m in the presence of mos t me ta l s . Ser ious in ter - ferences have been r e p o r t e d 8'9 which ind ica te t h a t t h a l l i u m m u s t be s e p a r a t e d before i t can be accu ra t e ly dete~znined.

A mixed so lvent was p roposed a n d found useful for the q u a n t i t a t i v e ex t r ac t i on of a n t i m o n y , b i smu th , lead, and t in. TM T h e e x t r a c t a n t is a 5 % so lu t ion of t r i -n - oc ty lphosph ine oxide ( (T O PO ) d isso lved in 4 -me thy l - pen tane -2 -one ( M I B K ) . Th is mixed oxon ium so lven t has been t e s t ed as an e x t r a c t a n t for p p m levels of t h a l l i u m in a luminum- , iron-, and n icke l -base al loys.

I. EXPERIMENTAL

A. Apparatus

T h e e q u i p m e n t used for th is work was the Pe rk in - E l m e r mode l 303 a tomic a bso rp t i on s p e c t r o p h o t o m e t e r equ ipped wi th a p remix c h a m b e r and t r ip le s lot Boi l ing b u r n e r head . A P e r k i n - E l m e r t h a l l i u m hol low ca thode was used as a source l amp, a n d a Sa rgen t m o d e l S R G recorde r was used as a r e a d - o u t device . T h e t h a l l i u m resonance l ine a t 276.8 n m was used.

234 Volume 28, Number 3, 1974 APPLIED SPECTROSCOPY

B. R e a g e n t s

1. Standard Stock Solution of Thallium, 1,000 izg m1-1. 2. Working Standard Solution of Thallium, 10 Izg ml -~. 3. 5 % TOPO in M I B K

Dissolve 12.5 g of tri-n-octylphosphine oxide, (CsHi7)3PO, Aldrich 15,339-7, in 250 ml of methyl iso- butylketone, CH3COCH~CH(CHs)2.

4. Iodine Reagent

Prepare daily a solution of 30% (w/v) potassium iodide and 10% (w/v) ascorbic acid in 10% (v/v) hydrochloric acid.

All chemicals used were reagent grade. The acids used were found to contain less than 0.5 ppb thallium. Class A volumetric glassware was used.

C. Proposed Procedure

1. Calibration Curve

Transfer 0-, 0.5-, 1-, 5-, and 10-ml aliquots from the working standard thallium solution to six acid-washed separatory funnels. To each of the solutions add 15 ml of the iodide reagent and make the volume 50 ± 5 ml with 10 % hydrochloric acid. Pipet 10 ml of the 5 % TOPO-MIBK reagent into the separatory funnel, shake the funnel for 30 sec, and allow the phases to separate. Drain off the lower aqueous layer and discard. Transfer the organic phase to a stopped-glass vial for measure- ment.

2. Analysis

Weigh a sample to the nearest 1 mg which contains 2 to 100 tzg of thallium, prepare a reagent blank. Dis- solve a 1-g sample in a covered acid-washed 150-ml beaker with appropriate acids. Use a 600-ml beaker for a 10-g sample. Evaporate the solution to dryness.

Remove the beaker from the hot plate, cool, and moisten the salts with 5 ml of water per g of sample. Add 10 ml of hydrochloric acid per g of sample and heat to boiling. Add 88 % formic acid dropwise until no more oxides of nitrogen are evolved and evaporate to incipient dryness. Dissolve the salts in 20 ml of 1:1 hydrochloric acid and heat gently to complete the dissolution. Add ascorbic acid to those samples contain- ing iron. The disappearance of the dark yellow color due to iron(III) chloride indicates the complete reduction to iron(II). Transfer the solution to a 125-ml separatory funnel and extract thallium according to the procedure given under the calibration curve paragraph.

After its extraction, thallium is determined using atomic absorption by direct aspiration of the organic phase into the acetylene-air flame. Obtain the requisite data for the calibration curve and samples and calculate the amount of thallium.

II. D I S C U S S I O N A N D R E S U L T S

The extraction of thallium by oxonium-type solvents has been described, n Aliphatic ethers, ketones, and phosphates have been used for the complete extraction of the halothallium species. Aliphatic phosphine oxides (TOPO) are the most efficient oxonium extractants, al-

though a 5 % solution of TOPO in toluene extracts less than 5 % of thallium(I) from 2 M hydrochloric acid. TM

Irving and Rossotti n showed both thallium(I) and thallium(III) to be completely extracted by diethyl ether from an iodide media. No data are available on the extraction of metal iodides by TOPO.

The chemical behavior of thallium must be considered in order to effect its separation from a variety of alloys. Dissolving an alloy with aqua regia oxidizes thallium to its trivalent state. Thallium(III) is reduced to thal- lium(I) by potassium iodide in hydrochloric acid solu- tion. Thallium(I) chloride and iodide are relatively in- soluble; however, they are sufficiently ionized in solution to allow the determination of microgram quantities of thallium. The solubilities at 25°C of thallium(I) chloride and iodide are: 3.8 and 0.083 g liter -1, respectively. Thallium will exist in solution with hydrohalic acids as HT1X4 or HT1X2. The chlorothallate(III) species has been reported 13, 14 and is the normal species reported in the organic phase for trivalent metals.

Specker and Pappert 15 were unable to determine the species of thallium (I) extracted from oxonium solvents, i.e., tri-n-butylphosphate (TBP) and cyclohexanone. They reported complete extraction of thallium(I) in the presence of at least a 1000-fold excess of iodide. They predicted tetraiodothallate(III) because of the reported H[T1C14].3 TBP species. 14

The data in Table I substantiate the proposed method for the extraction of thallium by a 5 % solution of TOPO in MIBK. A 100-~g quantity of thallium in 50 ml of the designated aqueous phase was equilibrated with 10 ml of the specified extractant. The level of thallium remaining in the aqueous phase was ascertained by re- extracting the aqueous phase, always in the presence of iodide, with 10 ml of the TOPO reagent. Examination of the extraction data for tests 1, 3, and 5 show low D values when the extraction is performed in the presence of only hydrochloric acid, regardless of the extractant. Thallium(I) is formed in the presence of an excess of

TABLE I. Variation of extraction parameters for the separation of 100 ~g of thallium.

Thai- Aqueous Test lium a phase 10% No. valance Extractant HCI plus D b %E c

1 + 3 5% TOPO in cyelo . . . . 2.7 35 hexane

2 + 3 5% TOPO in cyclo- 9% KI, 1.5% 12 70 hexane ascorbic

3 + 3 100% MIBK . . . 21 81 4 "-1-3 100% MIBK 9% KI, 1.5% n 0 96

ascorbie 5 --t-3 5% TOPO in MIBK . . . 19 79 6 --I-3 5% TOPO in MIBK 9% KI, 1.5% >199 >97

ascorbic 7 -{-I 5% TOPO in cyclo . . . . 0.23 4.5

hexane 8 + l 100% MIBK . . . 0.89 15 9 +1 5% TOPO in MIBK . . . 1.6 24

a The oxidation state of tha l l ium added to the aqueous phase [thallium (III) obtained with Br2 oxidation and tha l l ium(I ) by dissolution of tha l l ium in H NO 31.

b A single 60-see extraction wa,~ used to obtain the distribution coefficient, [Tl]o

D - [Tl]aq "

e Percentage extraction = 100 D/(D -F 5).

APPLIED SPECTROSCOPY 235

g

SO

40

30

20

° t 0 0

PE Model 303

= 276.8 nm

3-Slot Burner Head

C,H~-Ai r

MiBK

2 4 6 8

FIG. 1. Comparison of atomic absorption calibration curves for

p o t a s s i u m iodide. 16, ,7 I n the presence of iodide, M I B K gives essent ia l ly comple te ex t rac t ion , and 5 % T O I ' O - M I B K gives q u a n t i t a t i v e ex t r ac t ion wi th a single ex- t r ac t ion . T h a l l i u m ( I ) is no t comple t e ly ex t r ac t ed f rom a chlor ide media . W h e n t h a l l i u m ( l ) b romide was dis- solved in d i lu te hydroch lo r ic acid solut ion, no ex t r ac t ion was obse rved in the absence of iodide. T h e degree of ex t r ac t ion found for tes t s 7, 8, and 9 m a y ind ica te the presence of some t h a l l i u m ( I I I ) .

Organic so lvents have been widely used in a tomic abso rp t ion because they efficiently in t roduce molecules in to the f lame? s One s l ight d i s a d v a n t a g e wi th 5 I I B K , as a so lven t for TOPO, is i ts so lub i l i ty in wa te r and d i lu te hydroch lo r i c acid solut ions. T h e so lub i l i ty of M I B K in wa te r is 2 % ? 9 A n a l y t i c a l resul ts arc no t affected b y this so lubi l i ty because a ca l ib ra t ion curve is p r e p a r e d in the sam(=, m a n n e r as the samples . So lven ts such as 2 , 6 - d i m e t h y l - 4 - h e p t a n o n e ( D M H ) or cyclo- hexane have solubi l i t ies in wa te r of less t h a n 0.06 %. Ne i the r so lvent forms emuls ions or t h i rd phases , and b o t h b u r n in the ace ty lene-a i r flame. The ca l ib ra t ion curves shown in Fig. 1 i l lus t ra te the va r i a t i on in sensi- t i v i t y as a func t ion of the solvent . The sens i t iv i ty for t ha l l i um in D M H and cyc lohcxane is less t h a n for M I B K . T h e bu rn ing charac te r i s t i cs a n d / o r nebu l i za t ion p roper t i e s are no t as ideal as those of M I B K . Tha l l i um is q u a n t i t a t i v e l y ex t r ac t ed b y the 5 % T O P O - D M H sys tem. T h e no rma l work ing range for t ha l l i um in aqueous m e d i a is f rom 2 to 20 ug m 1-~ and in M I B K 0.5 to 8 ug ml -L

The y - in te rcep t s for the ca l ib ra t ion curves shown in Fig . 1 give an ind ica t ion of the b l a n k as well as back- ground. T h e level of t ha l l i um found in the acids and reagen ts is negligible; therefore , the b l a n k due to con- t a m i n a t i o n b y t ha l l i um can n o r m a l l y be ignorcd. A t -

10

IT ~,~.Lg m{ -1

DMH

I0% HCI

12 14 16 18 20

10% HC1 and three nonaqueous solvents containing 5% TOPO.

TABLE II. Determinat ion of thall ium in various standard al loys by the T O P O - M I B K method.

Source and Tl designation Material Found (ppm)

Inco-CC Electronickel <0.1 Into-CO Nickel pellets <0.1 Inco-CO Nickel 123 <0.1 UNKN. Ferro-nickel (19%Ni) 0.8 NBS l l l b Ni-Mo steel <0.1 NBS 126a Ni36 steel <0.1 NBS 348 Ni25-Cr15 steel <0.1 NBS 838 Mo high speed tool steel <0.1 NBS 55e Ingot iron <0.1 NBS 82a Nickel-chromium cast iron <0.1 NBS 341 Ductile iron <0.1 NBS 64b Ferrochromium <0.1 NBS 57 Refined silicon 0.8 NBS 87a Aluminum-silicon alloy 0.4 NBS 168 Cobalt-base <0.1 NBS 169 Ni77-Cr20 alloy <0.1 NBS 349 Waspaloy ~ 0.6 NBS 671 Nickel oxide <0.1 NBS 1194 A-286 <0.1 NBS 11(.)5 Discalh)y 24" 0.1 NBS 1 2 0 5 ]NCONEL alloy 713-C • 0.2 NBS 365 Electrolytic iron <0.1 BSC 310/1 NIMONIC 90" <0.1 BCS 371 Commercial nickel (Inco) <0.1 BCS 387 NIMONIC 901 alloy ~ <0.1 BCS 149/3 High purity iron <0.1 BCS 242/1 Ferro-tungsten 0.4 BCS 243/1 Ferro-ti tanium 0.8 BCS 312 Permanent magnet alloy 0.3 BCS 365 Alcomax ] I I • <0.1

a Trademarked products (trademark, proprietor): Was- paloy, United Aircraft Corporation; Discalloy, Westinghouse Electric Corporation; ]NCONEL and NIMONIC, Interna- tional Nickel Company; Alcomax, Aluminum Company of America.

2 3 6 Volume 28, Number 3, 1974

TABLE III. Application of the T O P O - M I B K method for the determination of 2 ~g of thal l ium added to various materials .

Description Material Found (ug)

NBS 126a Ni36 steel 2.2 NBS 168 Cobalt-base 1.7 NBS 169 Ni77-Cr20 alloy 2.2 NBS 349 Waspaloy ~ 1.9 BCS 310/1 NIMONIC 90 ~ 2.2 BCS 371 Commercial nickel (Inco) 2.0 BCS 149/3 High purity iron 2.5

See Table I I for iderltification of trademarks.

t empts to eliminate the background by purification of the reagent failed to eliminate the background.

Aspiration of M I B K equilibrated with aqueous solu- tions gave percent absorption values for the blank of 4 ± 2, at the 276.8 nm thallium resonance line. This effect is not as serious at the higher wavelengths, such as used for silver and other elements. The background for silver, at the 328.1 nm resonance line, is around 1% absorption. When M I B K is used as a solvent for the extraction of thallium, it also extracts some water, hydrochloric acid, and inorganic salts. The presence of these materials in the M I B K extract produces light scatter or molecular absorption. A correction of this effect is especially important for measurements made at wavelengths less than 300 rim. The level of the back- ground is not appreciably reduced by filtration or drying of the organic phase.

Either M I B K or TOPO in M I B K could be used to concentrate thallium; however, the versatil i ty and general scope of the TOPO method make it superior. The TOPO method is capable of determining thallium under a wide var iety of conditions. Minor operator errors will not affect the accuracy. Quanti tat ive extrac- tion, for 50 ~g of thallium, was obtained when the con- centration of hydrochloric acid was varied from 0.2 to 20% (v/v) , the level of potassium iodide from 0.1 to 15% (w/v), or the amount of ascorbic acid from 0.1 to 10 g. A 30-sec extraction period has been recommended, but complete extraction is obtained for any equilibration period greater than 5 scc. The effect of these four variables was determined when all other factors were constant. Nitric acid remaining after the dissolution step causes the formation of free iodine in the TOPO phase which gives low results. Evapora t ion of the sample to dryness and heating the resulting hydro- chloric acid solution with formic acid effectively elimi- nate nitric acid.

Standard materials containing a certified level of thallium are not available. A synthetic s tandard was prepared with a gram of 99.999 % pure nickel spiked with 2 ug of thallium. Twenty synthetic standards were similarly prepared and carried through the proposed procedure. The mean and standard deviation (~) of the set is 2.00 ± 0.15 ppm. The relative s tandard deviation at the 95 % confidence level is 15 %. These data were obtained for sets of 10 on separate days.

D a t a given in Table I I show the amount of thallium found in a number of typical materials. The first three materials are various types of commercial nickel. They

contain less than 0.1 ppm of thallium and are suited for m a n y uses including high tempera ture alloys. The fourth sample listed is a ferro-nickel; it contains nearly 1 ppm of thallium. I t produced an absorbance of about 0.025 unit (5 % A) above background. The detection of less than 5 ~g of thallium is easily at tained because complete recovery was obtained from between 1 and 10 g of nickel spiked with 1 ug of thallium. A limit of detection of 0.1 ppm has been obtained with a 10-g sample.

Thall ium was determined in a var iety of steels, irons, addition alloys, and high temperature alloys. Several representative materials were spiked with 2 ug of thallium and carried through the procedure with a gram of the alloys. The determination of thallium in the presence of nickel, iron, a steel, a cobalt-, and several nickel-base alloys is shown in Table I I I . All . f the data indicate tha t thallium can be determined by the pro- posed method without interference. I t cannot be deter- mined in the presence of i ron(I I I ) , bu t i ron(II) does not interfere nor does ascorbic acid used to reduce i ron(II I ) . The method is not applicable for the de- termination of thallium in alloy systems which are predominantly: antimony, bismuth, cadmium and copper, 2° lead, tin, zinc, or in general, the metals of groups IB, I IB , I I IA , IVA, and VA, except aluminum. These metals form strong anionic comolexes which are extracted, e.g., HMX~ for a tr ivalent metal.

A C K N O W L E D G M E N T

The author wishes to acknowledge Emil S tumpp for his assistance with the experimental work.

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Chem. 25, 730 (1963). 15. H. Specker and W. Pappert, Z. Anorg. Allgem. Chem.

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APPLIED SPECTROSCOPY 2 3 7