group in the ortho position, which increases the acid strength and the stability of the benzoate ion. Another exception can also be found in the fact that o-methylbenzoic acid is more adsorbed on the resin than benzoic acid is. A probable explanation of this observed fact is that the methyl group in the ortho position may affect the anion exchange sorption of benzoate ion. The large log D value of rn-aminobenzoic acid, as compared with log D values of other meta derivatives or its isomers, is probably due to the fact that this acid molecule has much more polar property such as zwitterion, which can enhance the molecular sorption. Consequently, it is thought that the amino group in the meta position may have a considerable effect on its molecular sorption. Comparing the log D-pKa relationships of ortho, meta, and para derivatives, a probable conclusion about the molecular sorption of benzoic acids can be induced as follows. The effect of the substituent on the molecular sorption was more serious in the case of ortho and meta derivatives than in para derivatives. In general, the substituents can increase or decrease the log D value, depending on the nature of the substituent and its structural position on the acid molecule. Another interesting conclusion could be inferred from the comparison of the relationships between log D and pKa values of the isomers of benzoic acid derivatives. The increasing order of log D values of the isomers is in agreement with the decreasing order of their pKa values in the case of nitro and hydroxy isomers which are relatively strong acids, while this relationship cannot be found in other isomers. On the basis of the log D-pK, relationship, p-aminobenzoic acid should have larger log D values than ortho isomer. However, the experimental result showed the reverse phenomenon. This can probably be explained by the fact that the amino group in the ortho position increases the stability of the benzoate ion and makes the acid molecule bulky through the hydrogen bonding of the amino group.
As expected, the effect of hydrogen bonding on the log D value was more serious in the case of monohydroxy isomers of benzoic acid than amino isomers. This explanation, based on the effect of hydrogen bonding, was also noted in the study on the relationship between elution position and chemical structure of amino and hydroxy isomers of benzoic acid (2, 7). In the case of the chloro isomer of benzoic acid, the increasing order of log D values was in agreement with the increasing order of pK, values. It is impossible to explain this exceptional phenomenon without other supplementary experiments, which will be studied more carefully in the next paper. In order to confirm the conclusion mentioned above and apply it to the study of the sorption of the related compounds, a study will be continued in our laboratory.
ACKNOWLEDGMENT The authors are much indebted to T. W. Kim and Y. H. Kim for their help in the laboratory work.
LITERATURE CITED J. S. Fritz and A. Tateda, Anal. Chem., 40, 2115 (1968). S. Katz and C. A. Burtis, J. Chromatogr., 40, 270 (1969). E. Martinsson and 0. Samuelson, Chromatographia, 3,405 (1970). H. W. Lange and K. Hempel, J. Chromatogr., 59, 53 (1971). (5) L. Lowendahl, 0. Samuelson, and D. Thornton, Chem. Scr., 1, 227 (1971). (6) 0. Samuelson, V. Soldatov, and D. Thornton, Chem. Scr.. 4, 89 (1973). (7) J. Rexfelt and 0. Samuelson, Anal. Chim. Acta, 70, 375 (1974). (8)K. S. Lee and D. W. Lee, Anal. Chem., 46, 1903 (1974). (9) P.Jandera and J. Churaeek, J. Chromatogr., 86, 351 (1973). (10) K. S. Lee, D. W. Lee, and E. K. Lee, Anal. Chem., 42, 554 (1970). (11) K. S. Lee, D. W. Lee, and Y. S. Chung, Anal. Chem., 45,396(1973). (12) "Lange's Handbook of Chemistry", J. A. Dean Ed., 11th ed., McGrawHill Book Co.. New York, N.Y.. 1973. (1) (2) (3) (4)
RECEIVEDfor review May 1, 1975. Accepted July 7, 1975. This work was financially supported in part by the grants from the United Board for Christian Higher Education in Asia.
Separation of Lead from Tin, Antimony, Niobium, Tantalum, Molybdenum, and Tungsten by Cation Exchange Chromatography in Tartaric-Nitric Acid Mixtures F. W. E. Strelow and T.
N. van der Walt
National Chemical Research Laboratory, Pretoria, Republic of South Africa
Probably the best known ion exchange separation of lead from other elements including tin and antimony is based on the fact that lead is eluted by fairly concentrated HCl from strongly basic anion exchange resins ( 1 ) while Sn(1V) and Sb(II1) are retained together with numerous other elements (2, 3 ) . At low concentrations of HCl (0.35M),Sn(1V) is not adsorbed by Dowex 1-X8 resin while Pb(I1) is; but, once adsorbed, Sn(1V) is extremely difficult to elute ( 4 ) . Sb(II1) is retained together with Pb(II), but Sb(V) is not. Another even more selective separation of lead from other elements is possible by anion exchange in HBr and HBrHNO3 mixtures ( 5 ) . The quantitative recovery of Sn(1V) and Sb(V) in the above procedures sometimes is not quite satisfactory for work of high accuracy because of the tendency of these elements to hydrolyze in dilute solutions of 2272
*
HC1 or HBr. Furthermore, elements such as Ta(V) and Nb(V), which have very strong tendencies to hydrolysis, cannot be separated from Pb(I1) at all. For a satisfactory separation these elements should be present as soluble complexes. Nelson et al. (6) have used HC1-HF mixtures for the elution of Pb(I1 ) and Sn(1V) from an anion exchange resin. This approach is suitable only for trace amounts because of the limited solubility of lead fluoride in the eluting agent. Ariel et al. (7) have absorbed Pb(I1) from and eluted it with 7M HCl, while 1M H F was used for the elution of Sb(II1) and 6M NaOH for the elution of Sn(IV), also using an anion exchange resin. Only up to 0.4 mg amounts of the elements were separated on very small columns, and the published results for alloys seem to have a tendency to be slightly low. An alternative organic com-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
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Figure 1. Elution curve Sn(lV)-Pb(ll). Column of 60 ml [ 19 X 2.0 cm] AG 50W-X8 resin, 200 to 400 mesh. Flow rate 3.0 f 0.3 ml per minute
plexing agent which gives stable soluble complexes with Sn(IV), Sb(III), Nb(V), and Ta(V) and, to a lesser extent, also with Pb(I1) is tartaric acid. Khorasani et al. (8) have separated Sb(V) from Fe(II1) and several other elements by eluting Sb(V) with tartaric acid from Zeo-Carb 225 cation exchange resin. Pb(I1) and Sn(1V) were not investigated quantitatively, but it has been remarked that Pb(I1) is also retained while Sn(1V) is not. Kimura et al. (9), on the other hand, have described a separation of Sb(II1) or Sb(V) from Sn(I1) eluting Sb(II1) or Sb(V) from an Amberlite IR-120 cation exchanger with 0.4% tartaric acid while Sn(I1) is retained. No information seems to be available on the quantitative aspects of the separation of Pb(I1) from Sn(IV), Sb(III), Nb(V), and Ta(V) in tartaric acid media. A systematic study of distribution coefficients of elements, which was undertaken in this laboratory, indicated that a separation of lead from the above elements in tartaric-nitric acid should be possible with very favorable separation factors. The quantitative aspects of this separation were investigated in detail, and W(V1) and Mo(VI), which form insoluble precipitates with lead, were included in the investigation.
EXPERIMENTAL Apparatus. Borosilicate glass tubes of 20-mm i.d. with fused-in glass sinters of No. 2 porosity, a buret tap a t the bottom, and a B19 ground glass joint a t the top were used as columns. A PerkinElmer 303 and a Varian-Techtron AA-5 instrument were used for atomic absorption and a Zeiss P M &I1 for spectrophotometric measurements. Reagents. The resin used was the AG 50W-X8 sulfonated polystyrene cation exchanger of 200- to 400-mesh particle size, marketed by Bio-Rad Laboratories of Richmond, Calif. Niobium(V) and tantalum(V) chloride were obtained from Fluka AG, Buchs, Switzerland. Standard stock solutions of Nb(V), Ta(V), Sb(III), and Sn(1V) were prepared by dissolving the chlorides of these elements in tartaric acid of suitable concentration. Solutions of Mo(V1) and W(V1) were prepared by dissolving the ammonium salts in tartaric acid and those of Pb(I1) by dissolving Pb(I1) nitrate in 0.1M " 0 3 . Water was distilled, passed through an Elgastat deionizer, and stored in plastic containers before use. Elution Curves. Sn(IV)-Pb(IZ). A solution containing about 1 mmol of Sn(1V) and 1 mmol of Pb(I1) in 80 ml of 0.25M tartaric and 0.05M HC1 was prepared and acid containing 0.05M "03 passed onto a column containing 60 ml of AG 5OW-XB resin of 200 to 400 mesh particle size. The column was 19 cm in length and 2.0 cm in diameter and had been equilibrated by passing through 100 ml of 0.10M tartaric acid in 0.01M "OB. The elements were washed onto the resin and then eluted with the same reagent a t a flow rate of 3.0 f 0.3 ml per minute. Fractions of 25-ml volume were taken from the beginning of the adsorption step using an automatic fractionator and the amounts of Sn(1V) and Pb(I1) in the fractions were determined using suitable analytical procedures. No lead was found in the first 1250 ml of eluate. In another experiment, the elution of Sn(IV) was stopped after 400 ml of eluting
203
300
400 SO0 ml ELUATE
600
700
BOO
Figure 2. Elution curve Sn(lV)-Pb(ll). Adsorption from 0.2M HCI. Otherwise conditions as for Figure 1 0.2SM Torlorlc acid
+ 0.01M H W 3
adsorption
/-'$cO.lOM
larlorlc o d d
+ 0.WY
HN03
Figure 3. Elution curve Nb(V)-Pb(ll). Column of 60 ml [ 19 X 2.0 cm] AG 50W-X8 resin, 200 to 400 mesh. Flow rate 3.0 f 0.3 ml per minute
agent (including adsorption step) had been passed through. LeadThe experimental curve is (11) was then eluted with 3M "03. shown in Figure 1. In a third experiment, 1 mmol of Sn(1V) was adsorbed from 60 ml of 0.2M HC1; washed onto the resin with 40 ml of the same reagent and then eluted with 0.10M tartaric acid in The experimental curve is shown in Figure 2. 0.01M "03. Nb(V)-Pb(II). Standard solutions containing 1 mmol Nb(V) in 10 ml of 1M tartaric acid and 1 mmol Pb(I1) in 10 ml of 0.1M "03 were added to 15 ml of 1M tartaric acid which had been diluted to about 100 ml with deionized water. Precipitation of lead niobate took place. After addition of about 5 g of wet ion exchange resin (f50% water), the precipitate dissolved completely with stirring. The solution plus resin were then transferred quantitatively to the top of an ion exchange column containing 60 ml of AG 50W-X8 resin as described above, using O.1M tartaric acid in 0.01M "03 for transfer. The solution was allowed to drain to the level of the resin bed, and Nb(V) and Pb(I1) were then eluted as described for Sn(1V) and Pb(I1) above. The elution curve is presented in Figure 3. Ta(V), Mo(VI), and W(V1) also form precipitates with lead similar to Nb(V). The precipitates dissolve on addition of resin to the tartrate solution, and their elution curves are very similar to that shown for Nb(V) in Figure 3. The elution curve of Sb(II1) is similar to that shown for Sn(1V) in Figure 1. Quantitative Separations. Amounts of standard solutions of lead and one other element were measured out and mixed, adding enough water, tartaric acid, and nitric acid to give about 100 ml of and 0.25M in tartaric acid. Prea solution about 0.01M in "03 cipitation took place in mixtures of lead with tantalate, niobate, molybdate, and tungstate. About 5 g of wet AG 5OW-XS ion exchange resin of 200- to 400-mesh particle size was added to these mixtures with stirring. This completely dissolved the precipitates. Precipitates forming after mixing the lead solutions with tin(1V) and antimony(II1) solutions (probably lead chloride) dissolved after addition of tartaric acid, dilution, and warming. The solution plus resin (when present) were quantitatively transferred to the top of a column containing 60 ml of AG 50W-X8 resin of 200- to 400-mesh particle size as described, and the "other elements" were eluted with 300 ml of 0.01M "03 containing 0.10M tartaric acid
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
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Table I. Results of Quantitative Separationsa Taken Pb, mg
Found
Element
mg
209.1 sn(W 117.3 418.2 Sn(IV) 23.36 21.32 Sn(IV) 234.5 413.3 Sn (IV) 0.209 0.0687b Sn(W 234.5 0.0687 Sn(IV) 0.209 207.0 Sb(II1) 133.1 0.0743 Sb(II1) 266.2 414.2 Sb (111) 6.53 207.2 NbW 83.5 207.2 MO(VI) 96.4 207.2 W(V1) 184.6 207.2 Ta(V) 142.7 a The results are averages of triplicate analyses. Average of six analyses. Table 11. Analytical Methods Used Element
Method
Complexometric titration with EDTA at pH 5.5; Xylenol Orange indicator. Atomic absorption f o r small amounts and elution curves. Sn(1V) Gravimetrically as SnO,, after precipitation with tannic acid from neutral tartrate medium. Spectrophotometrically a s complex with Pyrocatechol Violet and N-cetyl-N, A T , N-trimethylammonium bromide in O.4N H,S04 containing tartaric acid for small amounts and elution curves. Sb(II1) Volumetrically by titration with iodine in presence of tartrate: starch a s indicator. Gravimetrically as Nb,O, after precipitation Nb(V) with tannic acid. Spectrophotometrically a s complex with 4 - (pyr idyl- (2)-azo) -resor cinol and tartrate for elution curves. Ta(V) Gravimetrically as Ta,O, after precipitation with tannic acid. .Spectrophotometrically a s addition complex between TaF,- and Nile Blue A after extraction into chlorobenzene for elution curves. Mo(V1) Gravimetrically as complex with 8-hydroxyquinoline. W(V1) Gravimetrically a s W 0 3 after precipitation with tannic acid and chinchonine. Tartaric acid destroyed by fuming with H,S04 and "0,. Pb(I1)
at a flow rate of 3.0 f 0.3 ml per minute. The eluate was taken from the beginning of the adsorption step and used for the determination of the "other element". The tartaric acid was eluted from the column by passing through about 100 ml of 0.1M " 0 3 , discarding this eluate. Lead was then eluted with 300 ml of 3.OM " 0 3 using the same flow rate as above. The bulk of the nitric acid was removed from the eluate by evaporation and the lead determined by complexometric titration. When trace amounts were present atomic absorption spectrometry was used. The results are presented in Table I and the analytical methods used are summed up in Table 11.
Other element, mg
117.2 * 0.3 23.29 * 0.12 234.5 * 0.4 0.208 0.003 234.6 * 0.2 0.207 * 0.003 133.0 f 0.1 266.1 * 0.2 6.53 i 0.01 83.4 * 0.1 96.3 * 0.3 184.4 * 0.2 142.8 * 0.4
ion exchange resin is added as described. Separations are sharp and quantitative for amounts of lead ranging from micrograms to several hundred milligrams, and results for determinations in synthetic mixtures are very accurate. The distribution coefficients of lead are larger than 1000 between 0 and 0.1M HN03, and 0.1 and 0.25M tartaric acid while those for the other elements are about 1 or lower. Small amounts of lead therefore can be retained on much smaller columns than those used for obtaining the results presented in Table I. The large columns were used intentionally to demonstrate that, even on a column of the described size, quantitative recoveries of microgram amounts of lead and/or the other elements can be obtained, in case a large column should be required for a further chromatographic separation of lead from those elements which are also retained. According to distribution coefficients determined in this laboratory (unpublished), almost all divalent transition metals and elements such as Mg, Ca, Fe(III), Al, U(VI), the lanthanides, and T h should accompany lead. Lead can be separated from most of these elements by selective elution with 0.6M HBr, the other elements still being retained (10, 11). For an even more selective separation, anion exchange in HBr-"03 mixtures ( 5 )can be applied when required. Figure 2 shows that it is essential that tin(1V) is put onto the column in tartrate solution in which it does not exist as a cation. Any tin(1V) adsorbed from dilute HC1 (0.2 or 0.3M) shows very strong and prolonged tailing on elution with tartaric acid, probably because of hydrolysis reactions and maybe partial precipitation inside the resin particles. This is only very slowly reversed. For Ta(V) and Nb(V), the tendency to hydrolysis in dilute HC1 is very much stronger and precipitation can take place in the external solution even in the absence of lead when a complexing agent is not present. LITERATURE CITED (1) (2) (3) (4)
(5)
DISCUSSION
(6)
The described method presents a useful means for the separation of lead from Sn(IV), Sb(V), Mo(VI), W(VI), Nb(V), and Ta(V). These elements either tend to hydrolyze or form precipitates with lead in noncomplexing solutions. Even in tartaric acid, precipitates with lead are formed when large amounts of some of the above elements are present, but the precipitates dissolve smoothly when some
(7) (8)
2274
Pb, mg
209.1 i 0.1 418.3 f 0.2 21.31 i 0.02 413.5 i 0.4 0.0689 * 0.0002 0.0687 i 0.0003 207.0 * 0.2 0.0741 f 0.0003 414.3 * 0.2 207.2 * 0.1 207.2 * 0.1 207.2 0.1 207.2 * 0.1
(9) (10) (11)
F. Nelson and K. A. Kraus, J. Am. Chem. SOC.,76, 5916 (1954). Y. Yoshino and M. Kojima, Jpn Analyst, 6, 160 (1957). M. Ariel and E. Kirowa. Talanta, 8, 214 (1961). W. H. Gerdes and W. Rieman, Anal. Chim. Acta, 27, 113 (1962). F. W. E. Strelow and F. von S. Toerien. Anal. Chem., 38, 545 (1966). F. Nelson, R. M. Rush, and K. A. Kraus, J. Am. Chem. SOC., 82, 339 (1960). M. Ariel and E. Kirowa. Taknta, 6, 214 (1961). S. S. M. A. Khorasani and M. H. Khundkar, Anal. Chim. Acta. 21, 406 (1959). K. Kimura, N. Saito. H. Kakihana, and Ishimori. J. Chem. Soc. Jpn, Pure Chem. Sect., 74, 305 (1953); Chem. Abstr., 47, 9850 (1953). J. S. Fritzand R. G. Greene, Anal. Chem., 35, 811 (1963). F. W. E. Strelow and C. R. van Zyl, J. S. Afr. Chem. lnst., 20, 1 (1967).
RECEIVEDfor review May 19, 1975. Accepted July 9, 1975.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
