Anal. Chem. 1981, 5 3 , 1637-1640
1637
Quantitative Separation of Bismuth from Lead, Cadmium, and Other Elements by Anion Exchange Chromatography with Hydrobromic Acid-Nitric Acid Elution Franz W. E. Strelow" and Tjaart N. van der Walt National Chemical Research Laboratoty, P.O. Box 395, Pretorla, 0001, Republlc of South Afrlca
Trace amounts and up to 20 mg of bismuth can be separated from up to several grams of Cd, Pb(II), Zn, In, Ga, Fe(III), Ai, Mn(II), NI(II), Cu(II), U(VI), and many other elements by eluting these elements with 2 M nltric acid-0.03 M hydrobromic acid from a coluimn containing only 1 g (2.9 mL) of AGl-X4 anion exchange resln of 200-400 mesh particle slze. The retalned blsmuth is effectlveiy eluted with 0.05 M dlethylenetriamlnepentaactrtlc acld (DPTA) in 0.1 M ammonium nitrate of pH 4.5. Separrition factors are very large (>1000) and separatlons sharp and quantltatlve. About 0.8 pg of cadmium and 4 pg of lead are found in the blsmuth fraction when approxlmateiy 600 mg and 4 g, respectively, are present orlglnaiiy. Only Hg( [ I ) and Au(II1) and some platlnum metals accompany blsmuth. Sllver preclpRates as Insoluble bromide.
The separation of bismuth from other elements by ion exchange chromatography has received considerable attention in the past. The first method published probably was that by Lurje et al. ( I ) who showed that bismuth is eluted by 0.05 M sulfuric acid containing 1%of potassium iodide from a cation exchange column while copper and lead are retained. Cadmium, mercury, and ieome rarer elements will accompany bismuth, but most of the common elements should be separated. The method has a disadvantage in that all the common elements are retained. A large column therefore is required. Furthermore, iodide interferes with many methods of determination and is rather nnessy to remove. Probably one of the most selective and generally applicable ion exchange methods for the separation of bismuth is based on the work of Nelson et al. (2)who showed that bismuth is retained by strongly basic anion exchange resins from hydrochloric acid over the whole concentration range. Lead can be eluted with 8 M hydrochloric acid followed by elution of iron(II1) and most other elements with 0.5 M hydrochloric acid. Bismuth is retained and can be eluted with 1M sulfuric acid. Numerous applied and adapted versions of this method have appeared in the litlerature. In hydrochloric acid-ammonium fluoride mixtures, elements such as tin, germanium, arsenic, and antimony can be separated from bismuth (3). Again the eluting agent tends to interfere with further work and for this reason often is not very attractive. It also has been shown that bismuth is selectively adsorbed from ammonium nitrate solutions by anion exchange resins (2), but the distribution coefficient for bismuth with a value of about 90 in 8 M ammlonium nitrate is not very high and the high salt concentration often presents problems for further work. One of the most selective separations and a large distribution coefficient for bismuth can be obtained when nitric acid-methanol mixtures are used instead of ammonium nitrate, and coabsorbed thorium, lanthanides, and lead are eluted with 0.6 M hydrochloric acid in 90% methanol followed by 1M hydrochloric acid before the final elution of bismuth 0003-2700/81/0353-1637$0 1.25/0
with 1 M nitric acid (4). Unfortunately the kinetics of the separation in nitric acid-methanol mixtures are rather bad with the 8% cross-linked resin used. In addition some of the lanthanides and also uranium may appear in more than one of the separated fractions. A selective cation exchange procedure for the separation of bismuth utilizes the fact that bismuth forms a very stable complex with EDTA and therefore is not absorbed at low pH values (5) or even from 0.5 M perchloric acid containing 0.008 M EDTA (6). Most other elements are adsorbed, but elements such as In, Ga, Th, Sc, Zr, Hg(II), and Fe(III), when not reduced, also form very stable EDTA complexes and tend to accompany bismuth. Fritz et al. (7)have shown that bismuth can be eluted with 0.2 M hydrobromic acid from a cation exchange column and separated from Cd, Pb(II), Cu(II), and numerous other elements which are retained. Cadmium has only a distribution coefficient of about 35 under these conditions, and the separation factor Bi-Cd is rather small. Only limited amounts of cadmium can be separated, even when fairly large columns are used. Furthermore, the low solubility of bismuth oxybromide in dilute hydrobromic acid limits the amount of bismuth which can be separated. Cation exchange in aqueous dilute hydrobromic acid has also been used for the separation of bismuth using forced flow liquid chromatography (8). Because the column used has only a small capacity and most elements are more strongly retained than bismuth, the amounts which can be separated are limited. Better separation of bismuth from cadmium and numerous other elements can be obtained by eluting bismuth with 0.1 M hydrochloric acid containing 60% acetone (9). The distribution coefficient for cadmium is about 70 in this case, while that for bismuth is around 1. Selective separation of bismuth from other elements is also possible with a macroreticular catiog exchange resin and hydrochloric acid-acetone mixtures of somewhat higher acetone concentration (78%)for elution (IO). The separation factor for the Bi-Cd pair is about 6 in this case, and the separation was developed only for trace amounts. Other eluting agents which have been proposed for the ion exchange separation of bismuth include potassium thiocyanate ( I ) , tiron in 3 M ammonia solution ( I I ) , tartaric-perchloric acid mixtures (12),sodium nitrite (13), and malonic acid (14). Only one or a few other elements were investigated in each case and none of the methods seems to be particularily selective for bismuth. In addition, most of these eluting agents are rather tedious to remove. Furthermore, a special resin which is claimed to be highly selective for BiI, has also been prepared (15) but has not been made commercially available. Recently it has been pointed out that bismuth is adsorbed very strongly by anion exchange resins from solutions containing a mixture of 2 M nitric and 0.03 M hydrobromic acid, while most other elements are not adsorbed at all (16). Even cadmium the next strongly adsorbed element has a distribu0 1981 American Chemical Society
1638
ANALYTICAL CHEMISTRY, VOL. 53, NO. 11, SEPTEMBER 1981 ADSORPTION -2
I M HN03 :[*0025M pH 4 5
O M H N 0 3 - 0 0 3 M HBr
O T P A - O I O M "N , OI
'-t
Pb
t
( I mmol I
Bi I O I rnrnoll
J
E
K
Y w
d
I-'
0
0
< - , -100
200
300
400
~
^ , ^
^. &
I
500
mL ELUATE
Flgure 1. Elutlon curve Cd-Bi with 2.0 M HNO,-0.03 M HBr: 2.9 mL (1 g) AGl-X4 resin, 200-400 mesh; column length 36 mm, 4 10 mm; flow rate 1.6 f 0.3 mL/mln.
tion coefficient of only 2.7 as compared with that of 4800 for bismuth. This indicates that it should be possible to separate bismuth from very large amounts of other elements using only very small columns with this reagent. Furthermore, because the distribution coefficient for bismuth is very high it should be feasable to use a 4% cross-linked resin in order to improve the rather slow kinetics of the 8% cross-linked resin in nitric acid systems. Results of an investigation of the quantitative aspects of this separation are presented in this paper.
334
1385
05 _I
E 0 4 QY
y
03
0
8
-
02
s 01
100
EXPERIMENTAL SECTION Reagents and Apparatus. Analytical reagent grade chemicals were used. Water was distilled and then passed through an Elgatat deionizer. The resin was the AGl-X4 quaternary amine anion exchanger on polystyrene base marketed by Bio-Rad Laboratories (Richmond, CA). Resin of 200-400 mesh particle size was used for column work. Borosilicate glass tubes of 10 mm i.d. and about 70 mm length, joined to an upper part of 20 mm id. and 100 mm length were used as columns. At the bottom the columns were fitted with a fused-in No. 1porosity glass sinter and a buret tap and at the top with a B19 ground glass sleeve to accommodate a separating funnel. Eluting Agent for Bismuth. A 500-mmol sample of diethylenetriaminepentaaceticacid was dissolved in 150 mL of water plus 100 mL of 25% ammonia solution and made up to 1 L. A 100-ml portion of this solution and 100 mL of 1 M ammonium nitrate were diluted to 1 L eluting agent. Atomic absorption measurements were carried out with a Varian-Techtron AA-5 instrument. A slotted tube in the beam path over the burner head as described by Watling (17) was employed to obtain better sensitivity for bismuth, lead, and cadmium. A quartz tube instead of a stainless steel tube was used. Without scale spreading the slotted tube provided a fourfold increase in sensitivity, and, because it gave steadier readings, it also allowed more scale spreading when required. Elution Curves. The ion exchange column was marked at a volume of 2.9 mL (=1g of dry resin in the chloride form) and filled to the mark with a slurry of AGl-X4 resin. The resin was converted to the nitrate form by passing through about 50 mL of 1M nitric acid, and then equilibrated by passing through about 25 mL of a 2 M nitric acid403 M hydrobromic acid mixture. A solution containing about 1mmol of cadmium and 0.1 mmol of bismuth as the nitrates in about 50 d of a 2 M nitric acid403 M hydrobromic acid mixture was prepared and passed onto the resin column. The elementa were washed into the resin with small portions of the eluting agent, and cadmium was then eluted at a flow rate of 1.6 f 0.3 mL/min using 240 mL in total. The bismuth was eluted from the column with 10 mL of 1M nitric acid followed by 0.025 M diethylenetriaminepentaacetic acid (DTPA) of pH 4.5 containing 0.10 M ammonium nitrate, using the same flow rate. Samples of 25 mL volume were taken from the beginning of the adsorption step, using an automatic fractionator. With the beginning of the elution of bismuth the fraction cutter was changed and fractions of 10 mL volume were taken
200
300
400
SO0
mLELUATE
Flgure 3. Elution curve Pb(I1)-BI with 2.0 M HN03-0.03 M HBr: 2.9 mL (1 g) AGbX4 resin, 200-400 mesh; column length 36 mm, 4 10 mm; flow rate 1.6 f 0.3 mL/min; 4.6 g Pb -t 500 p g BI. 0 . 5 M H N 0 3 - 0 . 1 M ICH2)2CS
I M HNOj -0.03M
"Os-
THIOUREA MIXTURES
HBr
0.5 r
EI o. 0.3 0'4
d g
0.2
0 01
400
500
600 700 m L ELUATE
800
0
Figure 4. Elution curves Pb(I1)-Bi with HN03-Thlourea: AGl-X4 and AG1-X8 resin, 100-200 mesh; columns of 4 10 mm, 1 g resin; flow rate 2.0 f 0.3 mL/min.
instead. Excess acid was removed by evaporation when required, and the amounts of the elementa in the fractions were determined by atomic absorption spectrometry after suitable dilution, using the air-acetylene flame and the 228.8- and 2.23.1-nm lines for cadmium and bismuth, respectively. The experimental elution curve is shown on Figure 1. An experimental elution curve for the Pb(I1)-Bi pair is shown on Figure 2, but adsorption took place from 40 mL of solution, and 200 mL of 2 M nitric acid-0.03 M hydrobromic acid was used for the elution of lead. Figure 3 shows the separation of a very large amound of lead (4.6 g) from 500 pg of bismuth. Adsorption was carried out from a volume of 100 mL containing 0.05 M hydrobromic acid in this case and the elution of lead was continued until 300 mL of the eluting agent had been passed. Furthermore, fractions of only 5 mL volume were taken for the elution of bismuth. For comparison two elution curves for bismuth using a 1-g column of either Agl-X4 or AGl-X8 resin of 100-200mesh particle size and nitric acid-thiourea mixtures for elution are also included
ANALYTICAL CHEMISTRY, VOL. 53, NO. 11, SEPTEMBER I981
1639
Table I. Results of Quantitative Separations other element Cd Cd Cd Cd Pb Pb Pb Zn Ga In Al
a
amt taken, mg Bi other element 21.02 21.02 0.0993 0.0098 21.02 21.02 0.0993 21.57 21.57 21.57 21.34 21.34 21.34 21.34 21.34 21.34
125.7 0.00519 633.7 1259 205.5 0.00967 4110 103.3 82.6 104.6 29.07 58.28 65.72 61.56 72.44 238.7
Fe(111) Cu(I1) Ni(11) Mn(I1) U(V1) Results are the mean,s of at least triplicate runs.
on Figure 4. The flow raters were 2.0 f 0.3 mL/min and the other experimental conditions are indicated on the figure. Quantitative Separations of Synthetic Mixtures. A series of columns containing 2.9 ,mL of AGl-X4 resin was prepared as described under Elution Curves, Appropriate volumes of standard solutions of bismuth and one other element as the nitrates in dilute nitric acid were measured o u t accurately in triplicate, mixed, and adjusted to a volume of about 50 mL containing 2 M nitric acid-O.03M hydrobromic acid. When 20 mmoX of lead or 10 mmol of cadmium was present, a volume of 100 mL and 0.05 M hydrobromic acid was used. Three extra portions of each standard solution were measured out and kept separately for comparison. The mixed solutions were passed through the equilibrated columns and washed into the resin with a few small portions of 2.0 M nitric acid4.03 M hydrobromic acid. The other element (Cd, Pb, Zn, etc.) then was eluted with 2!.0 M nitric acid403 M hydrobromic acid using 120 mL including the washing portions. A total volume of 200 mL was used when ‘20mmol of lead and 10 or 5 mmol of cadmium were present. The eluates were collected from the beginning of the adsorptioin step, and, after the excess acid had been removed by evaporation, were made up to convenient volumes. Small amounts of bismuth (100 rg) were eluted with 10 mL of 1 M nitric acid followed by 30 mL, of 0.05 M DTPA containing 0.1 M ammonium nitrate. For large amounts the volume of the DTPA solution was increased to 40 mL. Flow rates were kept at 1.6 f 0.3 mL per minute. Eluates containing small amounts of bismuth were collected in 50-mL,volumetric flasks directly and made up to volume for determination. Suitable dilutions were applied for larger amounts. Solutions containing very small amounts of bismuth (10 pg) were evaporated and made up to 10 mL volume. The amounts of the elements in the eluates were then determined by uppropriate analytical methods.
RESULTS d4ND DISCUSSION The results obtained for the quantitative separations are presented in Table I and the methods for the determinations are summed up in Table 11. The described method provides an excellent means for the quantitative separation of bismuth from Cd, Pb(II), Zn, In, Ga, Fe(III), Cu(II), Mn(I1), Ni(II), U(VI), and A1 and for its highly accurate determination. Many other elements such as Co(II), Ti(IV), Zr, Hf, ‘rh, Cr(III), Sc, Y.the lanthanides, alkaline earths, and alkali metals have not been investigated in detail but should be separated very easily because thay have much less tendency to form bromide complexes than some of the investigated elements. Elements such as Mo(VI), W(VI), V(V), and Nb(V), which tend to form oxy anions, are also not adsorbed provided hydrogen peroxide is present to prevent polymerization reactions. As(III), Sb(III), Ge(IV), Se(IV), and Te(1V) should not be adsorbed according to their known anion exchange behavior in 2 M nitric acid (18,19) and
Bi
amt found,a mg other element
21.02 f 0.05 20.99 f 0.08 0.0993 f 0.0003 0.0099 f 0.0001 20.94 i: 0.06 21.04 f 0.05 0.0992 f 0.0003 21.59 f 0.04 21.57 f 0.06 21.59 f 0.10 21.37 k 0.04 21.36 ?: 0.03 21.32 f 0.04 21.34 f 0.03 21.37 f 0.06 21.35 f 0.10
125.7 f 0.1 0.00520 f 0.00002 633.7 f 0 . 3 1259 f 1 205.5 * 0.1 0.00969 i: 0.00002 4110 f 2 103.4 f 0.1 82.6 f 0.1 104.6 f 0 . 1 29.07 i: 0.01 58.27 f 0.03 65.72 f 0.03 61.55 f 0.04 72.45 f 0.07 238.7 f 0.1
Table 11. Analytical Methods Used element Bi Cd
Pb, Zn
Mn(11) Ga, Fe(II1) In Al
Cu(11) Ni( 11)
U(W
method atomic absorption spectroscopy; airacetylene flame at 223.1 nm using a slotted quartz tube as described (1 7) complexometric titration with DCTA in slight excess ammonia, methylthymol blue as indicator; small amounts by atomic absorption spectroscopy, airacetylene flame at 228.8 nm complexometric titration with DCTA at pH 5.5, xylenol orange as indicator; small amounts of lead with atomic absorption spectroscopy, air-acetylene flame at 217.0 nm using a slotted quartz tube as for bismuth complexometric titration with DTPA at pH 10; solochrome 6B as indicator complexometric titration, excess EDTA and back-titration with thorium at pH 2.5-3.0, xylenol orange as indicator complexometric titration with EDTA at pH 2.5-3.0, xylenol orange plus 1,lO-phenanthroline as indicator complexometric titration with DCTA; excess DCTA and back-titration with zinc sulfate at pH 5.5, xylenol orange as indicator complexometric titration with DCTA at pH 5.5, methylthymol blue plus 1,lO-phenanthroline as indicator complexometric titration with DCTA in slight excess ammonia; murexide as indicator gravimetrically as U,O, after precipitation with C0,-free ammonia
in dilute hydrobromic acid (20). Tantalum(V) and tin(1V) interfere because of their strong tendency to hydrolyze. The presence of a larger concentration of hydrobromic acid in the adsorption step could suppress the hydrolysis of tin and the presence of tartaric acid should suppress the hydrolysis of both elements sufficiently, but this will have to be investigated in detail. Au(III), Hg(II), and some platinum metals are the only elements to accompany bismuth. Silver interferes by forming an insoluble bromide. The separation factors for the described separation are very large. Even the factor for the Bi-Cd pair, the most critical one in the separation, has a value of larger than 1000. The factor for the Bi-Pb pair is even larger than 3000. Separation of bismuth from both cadmium and lead is considerably better
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 11, SEPTEMBER 1981
than by anion exchange in hydrochloric acid (2), in which two steps are required and a considerably larger column has to be used to retain bismuth when gram amounts of lead are present, because the distribution coefficient of bismuth has only a value of about 90 in 8 M hydrochloric acid, the eluting agent used for lead. Separation of large amounts of cadmium from bismuth is impractical by anion exchange in hydrochloric acid because cadmium is retained comparatively strongly and tails very heavily at the very low concentration of hydrochloric acid required. During anion exchange in nitric acid-methanol (4) lead is strongly retained together with bismuth in the absorption step and eluted selectively with 1 M hydrochloric acid only a t a later stage. For this reason very large amounts cannot be separated. In addition, uranium(V1) has a distribution coefficient of about 35 (21) and shows very heavy tailing. Furthermore, the anion exchange kinetics of 8% cross-linked resins generally are quite slow with nitric acid-organic solvent mixtures and tailing occurs even at quite slow flow rates. Because of the large distribution coefficient for bismuth (D= 4800 for the 8% cross-linked resin) and the very large separation factors for separations even from the most critical of the other elements (cadmium and lead) a small column containing only 1g (2.9 mL) of resin is sufficient to separate up to at least 20 mg of bismuth from gram amounts of lead and other elements. Furthermore, the presence of 2 M nitric acid dramatically suppresses the formation of insoluble oxyhalides of bismuth and halides of lead which occurs in dilute halide solution and makes separation of larger amounts of these elements in such solutions possible. Up to 5 g of lead and several grams of bismuth become soluble as nitrates in 100 mL of 0.03 M hydrobromic acid containing 2 M nitric acid. Separations are sharp and quantitative and the amount of tailing is insignificant as is shown on Figures 1-3. The completeness of the separation is demonstrated by the fact that only 0.8 f 0.3 pg of cadmium and 4 f 2 pg of lead were found in the bismuth fraction when 633.7 mg and 4110 mg, respectively, were present originally (Table I). It also may be pointed out here that by increasing the column size to 3 g of resin (8.7 mL) as much as 200 mg of bismuth can be retained. DPTA half neutralized with ammonia was preferred for the elution of bismuth. The ammonium salt gave considerably less background signal for bismuth during the direct atomic absorption determination than the sodium salt. Furthermore the DPTA complex of bismuth is more stable than the EDTA complex and DPTA is much more soltible in acid. Though the effect of the eluting agent on the atomic absorption readings for bismuth was relatively small (99.9%) became questionable. Some improvements were obtained by using a higher concentration of thiourea, resin of smaller particle size, and slower flow rates. The most significant improvement occurred when the 8% was
replaced by a 4% cross-linked resin. Comparison of Figures 1-3 with Figure 4 shows that DTPA is considerably more effective as eluting agent than thiourea; 0.05 M DTPA is even slightly better than 0.025 M, especially when larger amounts of bismuth are present. Tailiig disappears almost completely. The described method also seems to provide a better separation of traces of bismuth from large amounts of lead with less cross contamination and more accurate recoveries than a method using extraction chromatography from dilute perchloric acid containing thiourea and “Voltalef 300 CHR’ (a chlorotrifluoropolyethylene) with tributylphosphate as stationary phase (22). In addition, tributylphosphate extraction under the conditions described (22) is not very selective, and some multivalent elements will tend to accompany bismuth. The method has been applied successfully to the determination of bismuth in “Merck” lead foil GR “pro analysi” for which an upper limit of 0.005% bismuth is claimed. An analysis carried out in sevenfold gave a result of 10.93 ppm bismuth with a standard deviation of 0.05 ppm. Samples weighing 10 g were taken for this work. After dissolution in nitric acid and evaporation to dryness, the salts were dissolved in 150 mL of 0.5 M nitric acid containing 0.05 M hydrobromic acid. Adsorption took place from this solution, while elution was carried out as described before. An average of 4.2 pg of lead was found in the bismuth fractions. The described method has also been applied successfully for the separation of carrier-free bismuth iostopes from cyclotron irradiated lead targets. Only about 0.0001% of the originally present was found with the bismuth.
LITERATURE CITED Lurje, J. J.; Filipova, N. A. Zavod. Lab. 1948, 14, 159; Chem. Absfr. 1948, 42, 8696. Nelson. Frederick: Kraus. Kurt A. J. Am. Chem. SOC. 1954. 76, 5916.
Nelson, Frederick: Rush, R. M.; Kraus, Kurt A. J. Am. Chem. Soc. 1980, 82, 339. Ahluwalla, S. S.; Korkisch, Johann Fresenius’ 2.Anal. Chem. 1985, 208, 414. Taketatsu, T. Nlppon Kagaku Zasshi 1957, 78, 48. Suicek, Zdenek; PovondrB, P.; Kratochvil, V. Collect. Czech. Chem. Commun. 1989, 34, 3711. Fritz, James S.; Garrakla, Barbara B. Anal. Chem. 1962, 34, 102. Willis, Raymond 8 . ; Fritz, James S. Talanfa 1974, 21, 347. Fritz, James S.; Rettig, Thomas A. Anal. Chem. 1962, 34, 1562. Kawazu, Kazuyoshi; Kakiyama, Hitoo J. Chromatogr. 1978, 751, 339. Golovatyl, R. N. Ukr. Khlm. Zh. (Ross. Ed.) 1981, 27, 261; Fresenlus’ 2. Anal. Chem. 1982, 186, 323. Sulcek, Zdenek; Boseova, M.; Dolezal, J. Collect. Czech. Chem. Commun. 1989, 34, 787. Bhatnagar, R. P.; Arora, R. C. Indhn J. Chem. 1965, 3 , 89. Chakravorty, M.; Khopkar, S. M. Chromatograph& 1979, 72, 459. Strelow, Franz W. E. Anal. Chem. 1978, 50, 1359. Lewandowskl, A,; Szczepaniak, W. Chem. Anal. (Warsaw) 1982, 7 , 593.
Watling, R. J. Anal. Chim. Acta 1978, 97, 395. Faris, J. P.; Buchana, R. F. Anal. Chem. 1964, 36, 1157. Ichikawa, Fujio; Uruno, Shinobu; Imal, Hisashi Bull. Chem. SOC.Jpn. 1961. 34. 952.
Andelsen; Torkikl; Knutsen, A. Bye Acta Chem. Scand. 1982, 76. 849.
Korklsch, Johan; Janauer. G. E. Talanta 1962, 9 , 957. Sulcek, Zdenek; Slxta, Vaclav Collect. Czech, Chem. Commun. 1971, 36, 1561.
RECEIVED for review March 12,1981. Accepted May 4,1981.
