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Anal. Chem. 1985, 57,2268-2271
Separation of Traces and Large Amounts of Lead from Gram Amounts of Bismuth, Tin, Cadmium, and Indium by Cation Exchange Chromatography in Hydrochloric Acid-Methanol Using a Macroporous Resin F. W. E. Strelow National Chemical Research Laboratory, CSIR, P.O. Box 395, Pretoria 0001, Republic of South Africa
Traces and small amounts of lead can be separated from gram amounts of Bi, Sn, Cd, and I n by elutlng these elements wlth 0.5 M HCI In 50% methanol from a column contalnlng only 2.7 mL (1 g) of Bio-Rad AG MP-50 macroporous catlon exchange resin of 100-200 mesh particle slze in the H form. A 5.4 mL (2 g) resln column separates up to 100 mg of lead. Lead can be eluted effectively wlth 3.0 M aqueous HCI. Separations are sharp and quantitative. I n combination wlth flame atomlc absorptlon spectrometry, the method has been applled for the determination of lead In pure metals or chemicals of the above elements. Relative standard deviations were better than 1% for samples contalnlng 10 ppm of lead or more. The sensitivlty Is less than 0.1 ppm. Relevant eiution curves and results of analyses of binary synthetlc mlxtures and actual samples are presented.
Microgram amounts of bismuth can be separated from very large amounts of lead (up to 10 g) and gram amounts of most other elements by anion exchange chromatography in hydrobromic acid-nitric acid mixtures using quite small columns containing only 1 g or even 0.5 g of resin (1,2).The method also could be used for separating traces of lead from large amounts of bismuth, though considerably larger columns would be required. One would prefer to retain the trace element and elute the major element in such a situation, especially when gram amounts of the latter are present. Fritz et al. (3) have eluted bismuth with 0.2 or 0.3 M hydrobromic acid from a Dowex 50 H resin, while lead was retained. Only relatively small amounts of bismuth can be handled because of the low solubilities of bismuth oxybromide and oxychloride, unless unconveniently large solution volumes are used. When adsorption is carried out from nitric acid (3), serious overloading and saturation of the column will occur when gram amounts of bismuth are present. Because bismuth strongly competes with lead for exchange sites in nitric acid, lead will appear in the eluate prematurely. At concentrations of hydrobromic or hydrochloric acid large enough to increase the solubilities of the bismuth oxyhalides sufficiently to prevent precipitation, about 0.5 M, lead is not retained very strongly by the usual gel-type resins of 8% cross linkage (D = 40 in 0.5 M HBr; D = 70 in 0.5 M HC1). A recent paper on the distribution coefficients of elements with a macroporous resin in hydrochloric acid ( 4 ) shows that the distribution coefficient of lead increases to a value of 1270 in aqueous 0.5 M hydrochloric acid as compared with a value of about 70 with the Bio-Rad AG 50W-X8 gel-type strongly acid cation exchanger, while the value for bismuth (D= 7.4) is still quite low. The situation is even more favorable when working in partly organic solvents. In 0.5 M hydrochloric acid containing 50% methanol the distribution coefficients are 1840 and 2.7 for lead and bismuth, respectively (5). In addition,
the solubility of bismuth oxychloride increases significantly when the acid contains 50% methanol. This seemed to offer excellent prospects for developing a method which could separate traces of lead from gram amounts of bismuth using only small resin columns. Furthermore, other elements with relatively strong tendencies to chloride complex formation such as tin(1V) (D = 2.0), In (D = 4.6), and Cd (D = 9.7) should be separated as well. The quantitative aspects of this method therefore were investigated in detail and the method was applied to the determination of lead in pure bismuth and tin metals and pure compounds of the other two elements.
EXPERIMENTAL SECTION Reagents. The resin was AG MP-50 macroporous sulfonate cation exchanger of 100-200 mesh particle size marketed by Bio-Rad Laboratories of Richmond, CA. Analytical reagent grade chemicals were used, and standard solutions for quantitative separations of synthetic mixtures were further purified by passing the metal chloride solutions containing 0.5 M free hydrochloric acid through a larger ion exchange column (20 g of resin). Bismuth and tin were made up to volume in 2 M hydrochloric acid containing about 20 mg of the elements per milliliter. Indium and cadmium were made up in 1 M acid containing about 40 mg/mL. Suitable dilutions were applied when smaller amounts of bismuth were required. Water was distilled and then passed through an Elgastat deionizer. Apparatus. Borosilicate glass tubes of 11 mm i.d. and 140 mm length, fitted with a no. 1 porosity glass sinter and a buret tap at the bottom were used as columns. At the upper end the tubes were joined to a wider part of 20 cm i.d. and 120 mm length, fitted with a B19 female ground glass joint at the top to receive a separatory funnel as reservoir. The columns were filled with a slurry of the resin until the settled resin reached a 5.4 mL mark in the lower part. The resulting resin columns were 54 mm long and contained 2 g of resin in the dry state. For some separations of trace amounts of lead, 1-g resin columns (2.7 mL) also were prepared. These columns were somewhat smaller with a lower part of 9 mm i.d. and 80 mm length and an upper part of 90 mm length. The resulting resin columns were about 40 mm long in this case. The columns were purified by passing through 60 mL of 5 M nitric acid, followed by 10 mL of deionized water, and then equilibrated by passing through 20 mL of 0.50 M hydrochloric acid containing 50% methanol. Atomic absorption measurements were carried out with a Varian-Techtron AA-5 instrument using the air-acetylene or the nitrous oxide-acetylene flame (tin). Elution Curves. Elution of Lead with 3.0 M HCI. Strongly retained elements such as calcium, barium, lanthanides, etc. tend to show considerable tailing when eluted from AG MP-50 resin with hydrochloric acid (4-6).Because lead also is retained quite strongly, its elution behavior was studied in more detail. About 25 mL of 0.2 M nitric acid containing 50 mg of lead (as nitrate) was passed through a 2-g resin column and the lead washed onto the resin with a few small portions of 0.2 M nitric acid. Then 500 mL of 0.5 M hydrochloric acid containing 50% methanol was passed through the column to simulate the elution of bismuth and the other elements. Finally lead was eluted with 3.0 M hydrochloric acid using a flow rate of 2.5 A 0.3 mL/min.
0003-2700/85/0357-2268$01.50/00 1985 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 12, OCTOBER 1985
0 . 5 0 ~n c i I N 50% cn30n
+
+ADSORPTION
~ , O YH C I
mi IN sox cn30n
-o.soy
mL
mL
ELUATE
ELUATE
Figure 3. Elution curve for Pb(I1)-Sn(1V). Column and resin are given in Flgure 1. Flow rates are given in Figure 2. ADSORPIION -3- 0 5 0 M HCI IN 50% METHANOL+30
MCI IN 50% CH30M+-30M
ADSORPTION +OSOH
36.7
nci
20.3
4 18
Figure 1. Elution curves for various amounts of lead with 3.0 M HCI: 5.4 mL (2 g) AG MP-50 resin; 100-200 mesh; column length, 54 mm; diameter, 11 mrn; flow rate 2.5 f 0.3 mL/mln for 3.0 M HCI.
-+-~.oY
2269
M MCl
HCl
22.5
:I
11
I
IO t ENLARGED
a , ‘ ,w i ...
00
mL E L U A T E
100
m
600
Too
rnL E L U A T E
Figure 2. Elution curve for Pb(II)-Bl(III). Column and resin are the same as those given in Figure 1. Flow rate was 2.0 f 0.3 mL/min for HCI-CH,OH and 2.5 f 0.3 mL/mln for 3.0 M HCI.
Figure 4. Elution curve Pb(I1)-Cd: 2.7 mL (1 g) AG MP-50 resin; 100-200 mesh; column length, 40 mm; diameter, 9.2 mm; flow rate, 1.7 mL/min.
Fractions of 10 mL volume were taken from the beginning of the elution with the hydrochloric acid-methanol mixture and the lead in the fractions was determined by atomic absorption spectrometry using an air-acetylene flame, the 217.0-nm line, and suitable dilution in 0.5 M nitric acid. The experiment was repeated with 10-mg and with 1-mg amounts of lead. The experimental elution curves are shown in Figure 1. Elution of Binary Mixtures. About 1g of bismuth and 50 mg of lead in 150 mL of 0.50 M hydrochloric acid containing 50% methanol were passed through a 2-g resin column; the elements were washed into the resin with small portions of 0.50 M hydrochloric acid in 50% methanol and then eluted with the same reagent using 450 mL altogether. The large elution volume was used intentionallyto establish how strong relatively large amounts of lead could be retained on such a small column. Finally lead was eluted with 100 mL of 3.0 M hydrochloric acid. A flow rate of 2.0 mL/min was used. Fractions of 10 mL volume were taken from the beginning of the absorption step and, after evaporation of the eluent, the bismuth and lead in the fractions were determined in 0.5 M nitric acid by atomic absorption spectrometry, using standard conditions, suitable dilution, or scale expansion when required. The experimental elution curve is shown in Figure 2. Figure 3 shows an elution curve for 1 g of tin(1V) and 10 mg of lead also using a 2-g resin column. Absorption took place from 200 mL of 0.50 M hydrochloric acid containing 50% methanol in this case, only 100 mL of the same reagent was used to elute the tin and 100 mL of 3.0 M hydrochloric acid to elute lead. Atomic absorption spectrometry in hydrochloric acid with the nitrous oxide-acetylene flame, the 224.6-nm line, and scale expansion were used for determining tin. An elution curve for 2 g of cadmium and 1mg of lead is shown in Figure 4. In this case a column containing only 1g of resin, as described under ”Apparatus” was used. Adsorption was carried out from 100 d of the eluting agent. Another 500 mL was passed
through to elute cadmium and to establish how strongly the lead was retained. No lead could be detected up to a volume of 600 mL (detection limit 0.02 ppm). The flow rate was 1.7 f 0.3 mL/min. Both elements were determined by atomic absorption spectrometry in 0.5 M nitric acid using the air-acetylene flame and standard conditions. The elution cullre for indium is very similar to those of bismuth, tin, and cadmium. Quantitative Separations of Binary Synthetic Mixtures. Appropriate volumes of standard solutions of lead and one other element as the chlorides in hydrochloric acid (see “Reagents”) were accurately measured out in triplicate, mixed, adjusted to an appropriate volume of 0.50 M hydrochloric acid containing 50% methanol, and passed through 2-g resin columns. Volumes of 200 mL were used when 1g of bismuth or tin(1V) was present and 100 mL in the other cases. Another triplicate set of standard solutions was measured at the same time and kept separated for standardization purposes. After some time was allowed for degassing, the mixed solutions were passed through the ion exchange columns and the elements washed into the resin bed with several small amounts of 0.50 M hydrochloric acid in 50% methanol. Bismuth, tin(IV), indium, and cadmium then were eluted with the same reagent using about 120 mL in total. The eluates were collected from the beginning of the adsorption step. Finally lead was eluted with 3.0 M hydrochloric acid using 120 mL for 100-mg amounts, 100 ml for 50 mg, and 60 mL for 10 mg or less. The flow rate was 2.0 A 0.3 mL/min. The eluates were evaporated to dryness on a water bath, redissolved in 0.5 M nitric acid, and made up to a convenient volume for determination. Large amounts of lead were determined by complexometric titration with EDTA at pH 5.5 using xylenol orange as indicator; small amounts were determined by atomic absorption spectrometry at 217.0 nm using the &acetylene flame. Large amounts of bismuth and cadmium were determined by complexometric titration with EDTA and indium by back titration with zinc sulfate, using
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 12, OCTOBER 1985
Table I. Results of Quantitative Separations of Synthetic Mixtures amt taken, mg
amt of other element found in Pb fraction,
amt found: mg
other element
Pb
other element
Pb
other element
fig
Bi(II1) Bi(II1) Bi(II1) Bi(II1) Bi(II1) Sn(IV) Sn(1V) In Cd Cdb
113.2 56.62 2.064 0.206 0.0103 0.1031 0.0103 0.1031 0.1031 1.031
210.3 1051.5 1051.5 1051.5 1051.5 1085 1085 2026 2256 2256
113.2 f 0.1 56.63 f 0.03 2.062 f 0.005 0.206 f 0.001 0.0104 f 0.0001
210.2 f 0.2 1051.3 f 0.7 1051.6 f 0.6 1051.4 f 0.6 1051.4 f 0.7 n.d. n.d. 2025 f 2 2256 f 1 2257 f 2
n.d. n.d. n.d. 7.3-13.1 6.6-12.9 1.8-7.4 2.1-6.7 0.6-2.2 1.4-2.7 n.d.
0.1030 f 0.0003
0.0103 f 0.0001 0.1031 f 0.0003 0.1031 f 0.0002 1.032 f 0.003
“Averages of triplicate runs. * 1 g resin column. Table 11. Results for Determination of Lead in AR Grade Chemicals AR grade chemical
amt of sample taken per analysis, g
bismuth metal, 99.98%, Fisher Scientific Co. cadmium chloride E&A tested purity reagent, Fisher Scientific Co. tin metal pro analysi, Merck Darmstadt indium chloride tetrahydrate (purum), Fluka A.G.
amt of Pb found,” ppm 181.4 f 0.4
1.212
4.000
0.15 f 0.01
pg of major element remaining in Pb fraction
4.8-9.0 0.8-1.6
1.0375
12.8 f 0.1
1.8-5.4
3.000
22.50 f 0.05
0.5-1.3
” Results are means of quadruplicate runs. standard procedures (7) and taking suitable fractions. Tin(1V) was not determined because it is partially volatile when the eluate is evaporated. Furthermore a separation of about 2 g of cadmium from 1mg of lead was carried out in triplicate on 1-g resin columns. Adsorption again took place from 100 mL of 0.50 M hydrochloric acid in 50% methanol. A further 120 mL was used to elute cadmium completely and 30 mL of 3.0 M hydrochloric acid was used to elute lead. The flow rate was about 1.7 0.3 mL/min in this case. Blank runs were taken through the whole procedure and correctionswere applied when minor or trace amounts of lead were determined. When trace amounts of lead were separated, the amounts of the “other element” remaining in the lead phase also were determined using atomic absorption spectrometry and scale expansion. The results are presented in Table I. Determination of Lead in AR Grade Chemicals. About 5 g of bismuth metal, 99.98% pure, was weighed out accurately, dissolved in a minimum amount of nitric acid, taken to dryness, converted to the chloride, and finally made up to 250 mL in 2 M hydrochloric acid using a volumetric flask. About 5 g of tin metal was dissolved in a calculated volume of 5 M hydrochloric acid containing hydrogen peroxide and also made up to 250 mL volume. In the case of cadmium and indium 20 and 15 g of the AR grade chlorides were weighed out, dissolved in 1 M hydrochloric acid, and made up to 250 mL volume with the same acid. Aliquots of 50 mL volume of the above solutions were measured out in quadruplicate, and adjusted to contain 0.5 M hydrochloric acid and 50% methanol. The lead was then separated on 1-g resin columns as indicated above. Lead was finally eluted with 30 mL of 3.0 M hydrochloric acid using a flow rate of 1.7 h 0.3 mL/min throughout, except when gravity flow was slowed down by small elution heads. Blank runs were taken through the whole procedure together with the samples. The lead-containing fractions were evaporated to dryness and made to 10 mL volume with 0.5 M nitric acid (25 mL for bismuth metal). Lead then was determined by atomic absorption spectrometry as indicated above, using scale expansion for very low amounts and blank runs. The amounts of the other elements remaining in the lead fraction also were determined by atomic absorption spectrometry using standard methods as described above.
*
RESULTS AND DISCUSSION The results for the analysis of synthetic mixtures are shown in Table I and those for some AR grade chemicals in Table
11. The described method provides an excellent means for the separation of traces and also larger amounts of lead from gram amounts of bismuth, tin, cadmium, and indium and its highly accurate determination, even at trace amount levels. Separations are sharp and recoveries quantitative as indicated by the elution curves and the results presented in Table I. A column containing 1g of resin is sufficiently large to separate 1 mg of lead from as much as 2 g of cadmium, the most strongly retained of the separated elements (Figure 4). The large element-free elution volume between the peaks (elution was prolonged intentionally) indicates that such a column could handle even considerably larger amounts of cadmium and also somewhat larger amounts of lead. Up to at least 100 mg of lead are retained quantitatively by a 2-g resin column. The amounts of the other elements found in the lead fractions are only a few micrograms when gram amounts are present originally (Tables I and 11). Lead is retained very strongly by the resin from 0.5 M hydrochloric acid in 50% methanol (D = 1840). The separation factors for the lead-bismuth and lead-tin pairs with values of 680 and 920, respectively, are very large and apparently the largest known for these pairs under conditions where lead is retained preferentially. Those for the lead-indium and lead-cadmium pairs with values of 400 and 190, respectively, are somewhat smaller, but again seem to be the largest known for these pairs when lead has to be retained. How good the separation is, even from cadmium which has the lowest separation factor, is shown on Figure 4. Other elements such as gold(III), thallium(III), mercury(II), and the platinum metals, which also form stable chloride complexes, have not been investigated but have quite low distribution coefficients in 0.50 M hydrochloric acid containing 50% methanol (5) and should also be separated.
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Anal. Chem. 1965, 57, 2271-2275
Though the solubility of bismuth and tin(1V) in the eluting agent is higher than that in aqueous 0.5 M hydrochloric acid and considerably higher than that in acid of lower concentrations, it is nevertheless still limited and presents a problem which needs to be watched. Tests showed that 100 mL of the eluting agent will dissolve about 1g of bismuth and tin(1V); but to be on the safe side and to avoid side effects due to hydrolysis, adsorption of 1 g amounts of these elements was carried out from 200 mL of solution. Another problem is due to possible background contamination when very low amounts of lead have to be determined. Glassware has to be cleaned carefully and good AR purity reagents have to be used in measured amounts. A typical series of blank runs (triplicate) gave a value of 0.74 f 0.03 pg of lead. This relatively good reproducibility of the blank runs contributed significantly to the reproducibility and sensitivity of the method. The results obtained for actual samples (Table 11) show an excellent reproducibility, while results obtained for synthetic
mixtures (Table I) seem to indicate that the reproducibility is matched by an absolute accuracy almost as good. When relatively large samples are taken as for cadmium chloride (4 g), the sensitivity of the method is less than 0.05 pg of P b per gram of sample. Registry No. Pb, 7439-92-1; Bi, 7440-69-9; Sn, 7440-31-5; Cd, 7440-43-9; In, 7440-74-6.
LITERATURE CITED (1) Strelow, Franz W. E.: van der Walt, Tjaart N. Anal. Chem. 1981, 53, 1637-1640. (2) van der Walt, TJaart N.; Strelow, Franz W. E.: Haasbroek, Floors, J. I n t . J. Appl. Radiat. Isot. 1982, 33, 301-303. (3) Fritz, James S.; Garralda, Barbara G. Anal. Chem. 1962, 34, 102-1 06. (4) Strelow, Franz W. E. Anal. Chem. 1984, 56, 1053-1056. (5) Strelow, Franz W. E. Anal. Chim. Acta 1984, 160, 31-45. (6) Strelow, Franz W. E. Anal. Chlm. Acta 1981, 127, 63-70. (7) Strelow, Franz W. E. Anal. Chem. 1978, 50, 1359-1361.
RECEIVED for review April 2, 1985. Accepted May 28, 1985.
Effect of Particle Conformation on Retention in Sedimentation Field Flow Fractionation J. J. Kirkland,* L. E. Schallinger,' and W. W. Yau E. I. d u Pont de Nemours and Company, Central Research and Development Department, Experimental Station, Wilmington, Delaware 19898
Basic concepts in sedimentatlon fleid flow fractlonatlon (SFFF) retention assume that particle shape or conformatlon Is not a factor and that retentlon Is a direct function only of particle mass. Unexpected results wlth blomacromoiecules of very hlgh aspect have led to a study on the effect of various partlcle shapes on SFFF retentlon. The effect of conformation on retentlon was tested for partlcles conslstlng of spheres, Irregulars, rods, and piates In a range of operating condltlons. Results show that particle shape has llttle or no effect on SFFF retention until the aspect ratio becomes qulte large (>50-100). Only wlth extremely hlgh aspect ratios can retention errors become large, and In these cases, very small moblle phase veloclties can be used io obtain correct retention and accurate calculated partlcle sires or molecular weights.
The utility of sedimentation field flow fractionation (SFFF) for characterizing the size distributions of a wide range of particulates has been clearly established (1-5). The latest SFFF equipment and techniques permit the handling of particles in the 0.005-2 pm range (6).Soluble macromolecules with molecular weights of about 5 X lo5 to lo8 also can be separated, isolated, and characterized (7-11). In SFFF the particulates or soluble macromolecules are introduced into an open channel formed by parallel plates that are shaped like a ribbon or belt and suspended in a centrifuge. The channel is then rotated without mobile phase flow for a relaxation or 'Present address: 3M Center, Analytical and Properties Research, St. Paul, MN 55144.
preequilibration step, causing particles to be forced toward the wall regions of the channel. Sample particles that have an effective mass greater than the mobile phase are forced toward the outer wall. Diffusional force in opposition to this external centrifugal force causes the particles to establish a specific layer thickness near the wall as a function of effective particle mass. Liquid mobile phase is then caused to flow continuously through the channel with a characteristic laminar flow profile. Smaller particles that are less influenced by the external force field are engaged by regions of faster flow and elute from the channel first, followed by particles of increasing mass that are closer to the wall and intercepted by slower flow streams. The resulting elution pattern or fractogram provides information on the masses of sample constituents as a result of the quantitative relationships describing SFFF retention (1,8). The basic concepts in SFFF retention assume that particle conformation is not a factor and that retention for particles of equal density is a direct function of particle mass. However, preliminary experiments with very long, linear biomacromolecules indicated a significant change in calculated particle mass with changes in mobile phase flow rate. Since this unexpected phenomenon was not anticipated in view of basic SFFF concepts, a study was carried out on the effect of various particle shapes on SFFF retention.
THEORY In SFFF when equilibrium is established with particles as a result of the force generated by the external force field and the normal particle diffusion, the species is distributed across the channel as an exponential concentration function (1)
c = coe-r/l
0003-2700/85/0357-227 1$01.50/0 0 1965 American Chemlcal Society
(1)
