Studies on the Anion Exchange Chromatography of Salicylic Acid and Its Derivatives Kil Sang Lee and Dai Woon Lee Department of Chemistry and Natural Science Research Institute, Yonsei University, Seoul, Korea
The chromatographic mechanism of a series of salicylic acid and its derivatives is studied in the aqueous ferric chloride and the ferric chloride-organic solvent media. The elution behavior of the acids is markedly influenced by some factors, such as the stability of the ferric salicylate complex, the ionization of the acid, and the interaction between the organic solvent and the acid or the resin. The effect of the substituents on the distribution data is also investigated and an empirical equation, which can be used In calculating the ionization constants of substituted salicylic acids, is obtained from the distribution data.
A considerable amount of study about the sorption and the separation of aliphatic and aromatic carboxylic acids by anion exchange resins has been carried out in various experimental conditions. As is known well, it is very difficult to interpret the chromatographic data because there are many complicated factors which influence the elution behavior of carboxylic acids. A brief review about the elutions of carboxylic acids by the anion exchange resin and its mechanism were presented in our previous papers ( I , 2). On the other hand, the separation and adsorption of the hydroxybenzoic acid isomers were studied by salting-out chromatography using Amberlite CG-50 (Na+ form) and sodium chloride solution ( 3 ) ;ligand-exchange chromatography using Amberlite CG-120 (Fe3+,Ti4+,and Hg2+form) and dilute aqueous ammonia or water ( 4 , 5 ) . The mechanisms of separation and adsorption, they concluded, were correlated with the basicity of the ligand and the complexation between metallic ion and the isomers in the resin phases. The aim of this work was to study the elution mechanism and the factors which influence the elution behaviors in comparison with the elution data of a series of salicylic acid and its eight derivatives which can form stable complexes with ferric chloride contained in the eluent. In addition, the correlation between the distribution coefficient and the effect of the substituent was investigated. EXPERIMENTAL Materials. Dowex 1-X8 (chloride form, 200-400 mesh) was used for the measurement of the distribution data. This resin was washed and purified by the procedure performed in our previous papers (I, 2). The stock solutions of the acids (0.02M) were prepared by dissolving the guaranteed reagent chemicals in deionized water and water-methanol. In the FeC13-organic solvent eluents the concentration of organic solvent indicates the percentage by volume. The organic solvents (methanol, ethanol, 1-propanol, and acetone) used were all of analytical reagent grade. .411 eluents must be freshly prepared to prevent the hydrolysis of (1) K . S. Lee. D. W. Lee, and E . K . Lee, Anal. Chem., 42, 554 (1970). (2) K . S. Lee, D. W. Lee, and Y. S. Chung, Anal. Chem., 45, 396 (1973). (3) W. Funasaka, K . Fujimura, and S. Kushida, J. Chromatogr., 64, 95 (1972). (4) W. Funasaka, T. Hanai, K . Fujimura, and T. Ando, J. Chromatogr., 78, 424 (1973). (5) K. Fujimura, T. Koyama, T. Tanigawa, and W. Funasaka, J. Chromatogr., 85, 101 (1973).
Fe(II1). However, this possible hydrolysis could be avoided by making the eluents slightly acidic. Ion Exchange Column. For the elution method, the resin was placed in a borosilicate glass column (50 X 0.6 cm in diam.) furnished with Teflon (Du Pont) fittings and porous Teflon bottom. The column was washed thoroughly with aqueous organic solvent using a Serva Elutionspumpe, and then the resin bed was adjusted to 45.5 cm. Volume Distribution Coefficient. The details of the procedure for the measurement and the calculation of the volume distribution coefficients (D,) were the same as described earlier (I, 6). The amount of the acid stock solution introduced to the resin bed was 0.4 ml. The flow rate during the elution was adjusted to 1.0 f 0.1 ml per minute. The colored eluates were collected and analyzed by measuring the absorbance a t h,,,(nm) of the Fe(II1)-salicylate complex (Table I). Weight Distribution Coefficient. The weight distribution coefficients ( D ) of the acids were measured by a batch method. Thirty milliliters of water-methanol solution containing 0.02 milliequivalent of the acid were added to 1.0 gram of resin in a 50-ml glass-stoppered flask. The flask was mechanically shaken for just ten minutes. An aliquot taken from the supernatant liquid was then analyzed by spectrophotometry. All D values were calculated from the following relation:
a m o u n t of a c i d a b s o r b e d on r e s i n / g r a m of d r y r e s i n D=(1) a m o u n t of a c i d i n s o l u t i o n / m l of s o l u t i o n where the dry weight of the resin was corrected from the moisture content of the resin. All D , and D values were measured a t room temperature (20').
RESULTS AND DISCUSSION Nature of the Ferric Salicylate Complexes. I t is well known that the ferric chloride and substituted salicylic acid react in a 1:l mole ratio to form a stable complex in the acidic solution with p H value less than 3. In order to interpret the elution data, the previously reported ionization constants of the acids and the stability constants of their complexes are illustrated in Table 11. I t can be observed from Table I1 that the substitution in the salicylic acid molecule may alter its ionization constant and the stability constant of the complex it forms-Le., the larger the pK value of acid, the higher the stability of the complex. Sorption of Salicylic Acid and Its Derivatives on the Resin Bed. In order to investigate the sorption mechanism of the acids on the resin bed before their elutions begin, the weight distribution coefficients of the acids were measured in the condition which is similar to that of the sample loading in the elution (Table 111). If the shaking is performed for an extended period of time for equilibration, it is hard to compare the effects of the substituents on the sorption since all the acids will have similar D values. Therefore, the shaking time was kept to just ten minutes in order to compare the differences in D values of the acids. Consequently, the D values measured here are nonequilibrium distribution ratios, the arbitrary data for the comparision of D values of the acids used only this particular case. (6) 0. Samueison, "Ion Exchange Separations in Analytical Chemistry," Almqvist and Wiksell, Stockholm, Wiley, New York, N.Y., 1963, p 126.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
1903
Therefore, the interaction between the resin ion and the dipole of the acid molecule, and van der Waal's interaction between the resin matrix and the acid molecule should be taken into consideration in interpreting the above results. The latter mechanism is similar to that of the sorption of phenols and salicylaldehyde in which the ionization cannot be considered from their pK1 values (2,16). The change of the concentration of methanol, from 30 to 50%, decreased the D values of all acids (Table 111). I t is thought that these decreases are also attributed to the decreases of the ionization of the acids and the solvation effect of methanol on the interactions between the resin and the acid molecules.
Table I. M a x i m u m Wavelengths of Complexes of Salicylic Acid a n d Its Derivatives Acid
Wavelength, n m
pH range
Salicylic acid 4-Methyl 4-Hydroxy 4-Nitro 3-Methyl 3-Nitro 5-ChlOro 5-Nitro 5-Sulfo
515
530 517 485 550 493 535 498 500
1.8-2.8 1.8-2.5 1.5-2.4 1.8-2.5 1.8-2.5 1.6-2.4 1.8-2.5 1.8-2.5 1.8-2.5
Table 11. Investigated Ionization and Stability C o n s t a n t s Salicylic acid and its derivative Constant
pK1 (-COOH)
3-Nitro
1.87
4-Nitro
5-Nitro
5-Chloro
Salicylic
3-Methyl
4-Methyl
4-Hydroxy
2.31
2.32
2.71
3 .00
3.02
3.11
3.23 3.12 14.01
2.67"
17.31
11.74b 14.41
10.33 pK?(-OH)
10.34
12.95
13.59
12.66 14.339
15.66 16.842
16.59
14.59 P(KIK?)
12.30 14.193
log K,(FeL+ complex)
2 .92c
30% MeOH
D
Sorption, %
-100
1300 1300 11.3 103 67 . 5
-100 -100 -100 97.7 97.7 79 . O 77 . 0 69.2
50% MeOH
D
Sorption, %
769 370 333 278 333 180 65.4 137 47.1
96.2 92.5 91.7 90.2 91.7 85.7 68.5 81.9 60.9
Resin: 1.0 g (dry);solution: 30 ml; shaking time: 10 minutes.
As shown in the results (Table 111), it is thought that first, the sorption of the acids on the resin is due to the ion exchange between the acid anion and the resin ion because the acids have relatively large ionization constants. This sorption, based on ion exchange, could be verified by determining the concentration of the chloride ion removed from the resin bed. Meanwhile, the following results cannot be explained by the sorption mechanism solely based on the ion exchange reaction: The acids show unexpected large D values from the view point of their pK1 values. Any direct relationship between the D values and the pK1 values of the acids is not generally found except for three nitro derivatives. The D values of the nonpolar groups such as methyl were larger than those of the polar groups such as nitro. 1904
2.99?
3 , 14c
14.42
(14) (12) (151
a
Medium
(I
17) (8 (9, 12) (9,10) (11) (10, 12)
5-sulfosalicylic acid. 'log ( K I ' = [ H + l[FeL+I/[Fe3'1[HL-l).
T a b l e 111. Weight Distribution Coefficients (D)of Salicylic Acid and Its Derivatives in Methanol Medium9
3-M et hyl 5-Chloro 4-Methyl 5-Sulfo 5-Nitro 4-Hydroxy 4-Nitro Salicylic 3-Nitro
2.5&
Ref.
(13)
17.435
a , 5 pKn,p K 7 , valuesfor
Salicylic acid and its derivative
17.61 18.131
5-Sulfo
The Value a n d the Complexation between Fe(II1) a n d Salicylic Acid a n d Its Derivatives. The volume distribution coefficient (D,) of the acids was measured with various concentrations of ferric chloride eluent (Table IV). It is obvious that the differences between D, values of the acids are correlated with the differences in their pK1, pK2, and log K1. The complexation in the column also plays an important role in the elution of the acids. It can be made clear by comparing the D, values of the acid in two different eluents: salicylic acid has a D, value of 0.82 in the elution with 0.05M FeC13-30% MeOH and 15.3 in the case of 0.05M CuC12-45% MeOH (2). This large difference in D, value is likely due to the difference in the stability of the two complexes. From the results, in general, the D , value of the acid decreases with the increase of stability constant of the complex. Correlation between 0,a n d Eluent Concentration. The correlation of log D, value with the concentration of FeC13 is shown in Figure 1. The curves of acids are classified into three types. In the first type of 3-nitro, 4-nitro, 5-nitro, 5-chloro, and salicylic acid, their curves show that log D, values decrease within the range of 0.03 0.07M or 0.03 0.10M of ferric chloride, and increase above 0.07M or 0.10M. These acids have comparatively small pK1, pK2, and log K1 values,
-
-
(7) L. G. Bray, J. F. J. Dippy, S. R. C. Hughes, and J. W. Laxton, J. Cbem. Soc. London 2405 (1957). (8) G. E. Dunn and Fei-iin Kung, Can. J. Cbem., 44, 1261 (1966). (9) P. K. Migaland A. V. Ivanov, Zb. Obsbcb. Khim., 37, 380 (1967). (10) 2. L. Ernst and J. Menashi, Trans. Faraday Soc., 59, 1803 (1963). (1 1) 2. L. Ernst and J. Menashi, Trans. Faraday Soc., 59, 230 (1963). (12) A.Agren. Acta Cbem. Scand.. 8, 266 (1954). (13) 2. L. Ernst and J. Menashi, Trans. Faraday Soc., 59, 2838 (1963). (14) 2. L. Ernst and J. Menashi, Trans. Faraday SOC., 59, 1794 (1963). (15) M. V. Park, J. Chem. Soc. A, 816 (1966). (16) M. G. Chasanov, R. Kunin, and F. McGarvey, Ind. Eng. Cbem., 48, 305 (1956).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
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Table IV. Volume Distribution Coefficients (0") of Salicylic Acid and I t s Derivatives i n Various Concentration of Ferric Chloridea Salicylic acid and its derivatives Eluent
Salicylic
0 . 0 3 M FeCl,-H20 0 . 0 5 M FeCl,-H20 0 . 0 7 M FeCl,-H20 0 . 1 0 M FeCl,-H20 0 . 1 5 M FeCIB-H,O 0 . 2 0 M FeCl3-H20 0 . 3 0 M FeC13-H20 ': Flow rate:
1.0
3
4-Methyl
1.17 1 .09 1. 0 1 1.05 1.24 1.56 2.26
1.72 1.87 1.95 2.17 2.70 ...
4-Hydroxy
&Nitro
3-Methyl
3-Nitro
5-Chloro
5.54 6.24 6.80 7.41 8.58
6.24 4.84 3.58 3.90 4.21 4.68 5.34
4.80 5.07 5.46 6.24 7.08
8.19 5.85 3.98 3.51 3.04 3.28 3.74
8.00 5.93 4.45 5.07 6.15 7.41 9.80
..
...
...
... ...
6-Sulfa
5-Nitro
16.8 9.05 5.93 4.29 3.90 3.20 2.73
8.50 6.63 5.50 6.30 7.02 8.19 9.91
0.1 In1,'min; resin bed: 0.6 X 45.5 cm; volume of loaded acid: 0.4 ml of 0.02M methanol solution.
1
I
I
I .2
3
>
a
(3
0 0.8
a
PKI
2 0.4
0.0 0.0
0.2
0.I
0.3
1 1 0.0
1
1
I
0.4
a8
K
vs. concentration of aqueous ferric chloride
(1) salicylic, (2) 4-methyl. (3) 3-nitro, (4) 3-methy1, (5)4-nitro, (6) 4-hydroxy. ( 7 ) 5-chloro, (8) 5-nitro, ( 9 ) 5-sulfo
compared with other acids, and have electron-attracting groups. Plots of log D , us. the FeC13 concentration give good straight lines. This leads to the empirical equations: l o g D , = log Dvo
(low c o n c e n t r a t i o n )
sMe
--
(2) l o g D , = l o g Dvi' + ?%Me
(high c o n c e n t r a t i o n )
(3) where Me represents the molarity of the eluent and log DvO is the value of log D ,when Me = 0, and x and y are characteristic constants of the acids. The values of log D,O are ohtained by extrapolation. In the range of low concentration, the increasing order of the x value agrees with the order of decrease in both pK1 values of the acids and stability of their complexes except for 5chlorosalicylic acid: salicylic
