Thus we assume that the first-order dissociation of the PbNTA- is essentially negligible.
LITERATURE CITED 11) (2j (3) (4)
.
,
J. C. lmbeau and J. M. Saveant. J. Nectroanal. Chem.. 44. 169 11973). R. Joe Lawson and J. T. Maloy,'Anal. Chem. 46, 559 (1974). R. S. Nicholson and Irving Shain, Anal. Chem., 36, 706 (1964).
J. W. Ashley and C. N. Reilley. J. Nectroanal. Chem., 7, 253 (1964). (5)J. H. Carney and H. C. Miller, Anal. Chem., 45, 2175 (1973).
(6) K. B. Oldham, Anal. Chem., 44, 196 (1972). (7) Mark S.Shuman and Irving Shain, Anal. Chem., 41, 1818 (1969).
RECEIVEDfor review May 20, 1974. Resubmitted July 2, 1975. Accepted August 4, 1975. This work was supported in part bv The Research Foundation of The City Universitv bf New York under grant number 10754N. Part of this work was carried out at the Department of Chemistry, University of Alabama, University, Ala.
NOTES
Anion Exchange Sorption Behavior of Benzoic Acid and Its Derivatives Kil Sang Lee, Dai Woon Lee, and Won Lee Department of Chemistry and Natural Science Research Institute, Yonsei University, Seoul, Korea
The aliphatic and aromatic organic acids have often been chosen as the typical solutes to study the sorption and elution mechanism in anion exchange system (1-8). It is generally known that aromatic carboxylic acids are adsorbed by both ion-exchange and molecular sorption on the anion exchange resins. In the molecular sorption of organic acids, the most important factor seems to be the interaction between the nondissociated acid molecule and the resin matrix. However, there are many other complicated factors which influence the sorption behavior of the acids. An extensive review about the sorption and elution mechanism of carboxylic acids was given by Jandera and Chur6Eek (9). The purpose of the present work was to study the sorption behavior of benzoic acid derivatives from the relationship between their distribution coefficients and their acid strengths which are closely related to their chemical structures. Therefore, this sorption behavior will be useful in predicting the elution position and the sorption degree of the related compounds.
EXPERIMENTAL Materials. The chloride form of a strongly basic anion exchange resin, Dowex 1-X8(100-200 mesh) was used throughout this work. The procedure for purifying the resin was described previously (IO, 11). In the water-methanol media, the concentration of methanol indicates the percentage by volume. Benzoic acid, its derivatives, potassium chloride, and methanol which were used in this work were all of analytical reagent grade. Weight Distribution Coefficients ( D ) . The weight distribution coefficients of the benzoic acid derivatives were measured by a batch method. Fifteen milliliters of water-methanol medium containing 0.025 mmol of the acid were added to 0.5 gram of resin in a 50-ml glass-stoppered shaking flask. The flask was mechanically shaken for an hour and allowed to stand for several hours. An aliquot taken from the supernatant liquid was then analyzed spectrophotometrically a t the absorption maxima of the acids. The D values were calculated from the relation described in the previous paper (8). All experiments were carried out a t 20 & 1 "C.
RESULTS AND DISCUSSION Weight Distribution Coefficients (log D ) of Benzoic Acid Derivatives on the Resin. In order to investigate the 2270
sorption behavior and the relationship between chemical structure and the distribution data, log D values of the acids were measured in the water-methanol media. The results are given in Table I. The dissociation constants given in Table I are taken from a handbook (12) as a rough guide to the interpretation of the results since we presumed a correlation between pK, and log D . Because the log D values were measured a t the natural pH of the solution, each acid would be present partly in the molecular and partly in the anionic form. Therefore, this sorption is likely to proceed by an anion exchange mechanism and the molecular sorption. The molecular soprtion mechanism, in turn, may be due to van der Waals' interaction between the aromatic matrix of the resin and the benzene ring of the acid, and the ion-dipole interaction between the resin and the acid molecule. On the other hand, the distribution coefficients measured in alkaline medium were extremely large as may be observed from the comparison of log D values at two media: H20-50% MeOH and alkaline H20-50% MeOH (pH 10). This result seems resonable from the viewpoint of the dissociation of the acid in alkaline medium. It is in agreement with a previous result on Amberlyst A-26 ( 1 ) in which the effect of pH on the sorption of benzoic and o-nitrobenzoic acids was studied. In addition, it is reasonable to expect that the increase of log D value by the pH change is more pronounced in the case of the derivatives which have relatively high pK, values than in the case of more acidic derivatives (Table I). For example, the increased log D value is 0.18 for o-chlorobenzoic acid (pK, 2.94) and 1.32 for p-chlorobenzoic acid (pK, 3.98). T o obtain more information about the sorption of the acids, the log D values in the KC1-MeOH medium were measured. In the KC1-MeOH medium, where the anion exchange sorption must be reduced because of the presence of the excess chloride ion, all log D values of the acids in 0.1 M KC1-10% MeOH medium were decreased compared to the log D values for H20-10% MeOH medium. I t is also observed that the decrease of log D value by the addition of potassium chloride, contrary to the effect of pH, was more
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
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~~~
Table I. Weight Distribution Coefficients (log D )of Benzoic Acid Derivatives at Various Media Benzoic acid derivatives
p Ka
Benzoic acid o-Nitro ?+Nitro p- Nitr o o- C hlor o ~17C - hloro p- Chlor o o-Hydroxy -Hydr oxy p - Hydroxy o- Methyl ?)?-Methyl p-Methyl o-Amino nz- Amino p-Amino a
KC1-MeOH,
HzO-MeOH
and
4.21 2.17 3.49 3.44 2.94 3.82 3.99 3 .oo 4.08 4.58 3.91 4.27 4.37 4 -95 4.74 4.89
( ) = log D value
-.4
10%MeOH
30% MeOH
2.50 3.13 3 .oo 3.11 2.63 2.84 2.90
2.21 2.84 2.73 2.81 2.25 2.41 2.52 3 -17 2.22 2.01 2.10 2.06 2.09 1.83 2 -81 1.64
...
2.60 2.40 2.27 2.47 2.44 2.16 3.12 1.95
0.1M KC1-
50% MeOH
1.71 2.55 2.24 2.29 1.86 1.88 1.90 2.91 1.81 1.49 1.71 1.50 1.58 1.32 2.50 1.23
...
10% MeOH
1.10 2.16 1.75 1.80 1.17 1.28 1.28 2.10 1.18 0.96 1.10 0.94 1.23 0.78 2.16 0.90
e.*
(2.91)' (2.97) (2.97) (2.04) (2.58) (3.22) (2.96) (2.16) (4.20) (2.51) (2.02) (2.64) (2.97) (2.63) (2.70)
1.86 2.04 1.93 1.68 2.09 2.33
*..
2.11 2.26 1.71 1.89 1.84 1.97 2.45 1.go
a t pH 10.
-*
- e-
P
0 A+
*
4
!--
__
__
Figure 1. Plots of log D vs. ApK, of pderivatives in various H 2 0 -
Figure 2. Plots of log
MeOH media
MeOH
(A) H20-10% MeOH, 70% MeOH
70% ~MeOH
(B)H20-30% MeOH, (C)H20-50% MeOH, (0) H20-
pronounced for the more acidic derivative than that for the more basic one. For example, the effect of KC1 on log D value for nitro- and hydroxybenzoic acid is more pronounced than that for other derivatives. As expected, all log D values measured at the natural pH of the solution decreased with increasing methanol concentration. This is likely due to both the decreases in the dissociation of the acids and to the reduction of interaction between the resin and the acid molecule by the solvation of methanol. Correlation between log D and pK,. The acid strength (or pK,) of the benzoic acid derivatives studied here is expected to be closely related to their chemical structures and to their weight distribution coefficients. This is confirmed by the results given in Table I which show that the smaller the pK, value, the more the acid is adsorbed (higher log D value). In the case of para derivatives, the increasing order of log D values is in accord with, the decreasing order of pK, values of the acids in all media: log D: NO2 > C1> H > CH3 > OH > NH2 PK,: NO2 < C1< H < CH3 < OH < NH2 It is interesting to note that the plots of log D value vs.
.
,
. _-
D vs. ApKa of mderivatives in various
H20-
media
(a) H20-10% MeOH, (b) H20-30% MeOH, (c)H20-50% MeOH, 7 0 % MeOH
(4H20-
ApK, in various water-methanol media give good straight lines (Figure 1).Here, ApK, values are the differences in pK, values (Table I) between benzoic acid and its derivatives. This log D-ApK, relationship helps to explain the fact that the chemical structures of the derivatives contribute significantly to the sorption of the acids on the resin. In the case of meta derivatives, except for m-aminobenzoic acid, the relationship between log D and pK, was as follows: log D: NH2 > NO2 > C1> OH > H > CH3 PK,: NO2 < C1< OH < H < CH3 < NH2 The comparatively good straight lines could also be obtained from the plots of log D values vs. ApK, in various media (Figure 2). This result, except for m-aminobenzoic acid, could be explained by the same manner mentioned above for para derivatives. On the other hand, no agreeable relationship between log D and pK, could be found in the ortho derivatives. The orders of the log D and pK, values are as follows: log D: OH > NO2 > C1> H > CH3 > NH2 PK,: NO2 < C1< OH < CHz < H < NH2 The exceptional large log D value of o-hydroxybenzoic acid is probably due to the hydrogen bonding of the hydroxyl
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
2271
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
