ash-graphite matrix and the oscilloscopic traces was postulated. The unusual placement of the deflector plates used for time resolution allowed a method of data reduction which minimized variations caused by sample inhomogeneity. In addition to presenting time resolution as a possible means of studying the excitation of the rf spark source, two uses were postulated which could improve the analytical uses of the technique.
ACKNOWLEDGMENT
The authors thank S. F. Peterson and R. K. Skogerboe for their assistance and comments during this study. RECEIVED for review May 13, 1968. Accepted September 3, 1968. Financial support was provided by National Science Foundation Grant GP-7461X.
Studies on the Anion Exchange Behavior of oxylic Acids and Phenols James S. Fritz and Akira Tatedal Znstitute for Atomic Research and Department of Chemistry, Iowa State University, Ames, Iowa 50010 The uptake of carboxylic acids and phenols by an anion exchange resin is studied from aqueous solutions containing acetone or methanol. Distribution coefficients are higher in basic solutions where the acid anion predominates, but a significant uptake also occurs from acidic solution. A higher proportion of organic solvent in the solution lowers the distribution coefficients. Several practical separations are demonstrated on short columns. Distribution coefficients and capacity measurements are reported for organic acids on a nonionic polystyrene resin and compared with those for the polystyrene anion exchange resin. Also, uptake of organic acids by a quaternary ammonium salt dissolved in cyclohexane is determined and compared with the tw0 resins studied.
nitrophenol by a macroreticular anion exchange resin, Amberlyst A-26, was studied over a wide range of pH in aqueousorganic solvent mixtures. From the data obtained, conditions were selected and successful separations of some of these acids were obtained on a short anion exchange column. Also the sorption behavior of some of these acids on A-26 resin was compared with the sorption on a porous polystyrene-divinylbenzene copolymer, XAD-2, and with extraction by a liquid anion exchanger, Aliquat 336. From these experiments, some conclusions are drawn regarding the ion exchange behavior of carboxylic acids and phenols.
ALTHOUGHanions of carboxylic acids (1-4) and phenols (5, 6) may be taken up and separated by an anion exchange column, a complicating factor is the uptake of some types of nonionic organic compounds by ion exchange columns. Salting-out chromatography and solubilization chromatography are separation techniques based on selective sorption of organic molecules by ion exchange columns. Thus, cation exchange resins have been used for the separation of the lower aliphatic carboxylic acids (7-11), aromatic acids (12), and phenols (13, 14). In the present work, the sorption of some aliphatic carboxylic acids, benzoic acid, o-nitrobenzoic acid, phenol, and o-
Resins. Amberlyst A-26, macroreticular anion exchange resin was obtained from Rohm and Haas Co. The resin was 200 mesh for use in both the ground and sieved to 150 batch and column experiments. The resin was washed several times with hydrochloric acid-ammonium sulfate solution-sodium hydroxide solution cycles and, finally converted into chloride form and air dried. Amberlite XAD-2, a macroreticular resin obtained from Rohm and Haas Co., was treated the same as described above. Aliquat 336 liquid anion exchanger was obtained from General Mills. The concentration of this exchanger was determined by nonaqueous titration with perchloric acid in glacial acetic acid
1
On leave from Kyushu University, Fukuoka, Japan.
(1) C. W. Davies and B. D. R. Owen, J. Chem. Soc., 1956, 1681. (2) K. K. Carroll, Nature, 176, 398 (1955). (3) K. Shimomura and H. F. Walton, ANAL. CHEM.,37, 1012
(1965). (4) S. Egashira, Japan Analyst, 15, 1356 (1966). ( 5 ) M. Magda, R. Chwaszcza, and J. Chmielowoki, Gaz., Woda. Tech. Sunit., 36, 313 (1962). (6) L. T. Clark, J. Chromatogr., 15, 65 (1964). (7) D. Reichenberg, Chem. and Ind. (London),1956,958. (8) T. Seki, J. Biochem. (Tokyo),45, 855 (1958). (9) J. Sherma and W. Rieman, Anal. Clzim. Acta, 20, 357 (1959). (IO) G. A. Harlow and D. H. Morman, ANAL.CHEM., 36, 2438 (1964). (11) . , D. J. Patel and S. L. Bafna, Ind. Enn. - Chem., Prod. Res. Deuelop., 4, l(1965). (12) T. Saki, K. Inamori, and K. Sano, J. Biockem. (Tokyo), 46, 1653 (1959). (13) J. Sherma and W. Rieman, Anal. C/zim.Acta, 18,214 (1958). (14) T. Seki, J. Chromatogr., 4, 6 (1960).
EXPERIMENTAL
N
(15).
Distribution CoefPleients. The distribution coefficients of organic acids on resins were measured by a batch method. Twenty milliliters of organic solvent-water mixture which contained about 0.4 milliequivalents of organic acid, inorganic salt and acid, or base to adjust the pH of the solution was added to 1.5 grams of resin in a 125-ml glass-stoppered flask, shaken mechanically for 1-2 hours and allowed to stand for several hours. After the liquid phase was filtered free from resin, an aliquot of the liquid was taken and analyzed for the organic acid present. The distribution coefficient was calculated from the differencein the concentration of organic acid before and after achievement of equilibrium. Analysis of lower aliphatic acids employed titration with sodium hydroxide solution. In the experiment for alkaline pH solution, the carboxylic acid in resin phase was determined as follows: The resin which filtered free from the liquid was washed into a small column with 85% acetone-0.01M HC1 solution, and the same solution was passed through the (15) S. Siggia, “Quantitative Organic Analysis via Functional Groups,” John Wiley and Sons, New York, N. Y . , 1954. VOL. 40, NO. 14, DECEMBER 1968
e
2115
q0 0 C
I
0
I 0.2
,,
, 0.4 0.6 NaCI, M
I 0.8
i
I
1.0
kO E
0
0.4
0.2
0.6
0.8
1.0
0.8
1.0
Y o CI, M
Figure 1. Distribution coefficients of aliphatic carboxylic acids on Amberlyst A-26 (a) 0
Methanol
(b) 25 % iMethano1 (c) 50 % Methanol 0 Acetic acid
Propionic acid A Butyric acid Valeric acid
column to elute the carboxylic acid. The effluent was titrated potentiometrically with 0.05M to 0.001M sodium hydroxide solution; the second potentiometric break was used to determine the concentration of carboxylic acid. The concentration of other acids was determined spectrophotometrically by reading the absorbance of the organic acids at the peak wavelength of the absorption curve in acidic solution with a Spectronic 600 spectrophotometer. Because acetone interfered, solutions containing phenol, benzoic acid, or o-nitrobenzoic acid, were diluted to be below 5 acetone. Sodium hydroxide was added and acetone was removed completely by heating at 65 75 "C on a water bath for 1.5 hours. Column Separation Procedure. For column experiments, the air dried resin was soaked in the eluent, added to the column, and washed with the eluting solution. The volume of sample added to the column was 0.5 1.0 ml. A flow rate of approximately 0.5 ml per minute was employed to adsorb the sample onto column and also for the elution. Extraction Procedure. Ten milliliters of acetone-water mixture, which contained a known quantity of organic acid and was adjusted to the desired pH, was added to a 60-ml separatory funnel. Ten milliliters of 2 0 x solution of Aliquat 336 in cyclohexane was added also, and the mixture was shaken for 2 to 3 minutes and the phases allowed to separate. An aliquot of the acetone-water phase was taken and analyzed. To prevent volume changes, each phase was preequilibrated with the other before use.
0.2
0.4
0.6
NaCI, M
100
L
.-
1"
601
2
40
9-NITROPHENOL \
0-NITROBENZOIC ACID
Y
-
-
RESULTS AND DISCUSSION Distribution Coefficients of Organic Acids on Amberlyst A-26. The distribution coefficients of acetic, propionic, butyric, and valeric acids on Amberlyst A-26 were measured as a function of the content of methanol and the concentration of sodium chloride. The results are shown in Figure 1. The sorption of these lower carboxylic acids increases with increased concentration of sodium chloride and decreases with a higher proportion of methanol in the solution. The acids are probably in their molecular form, and the increased sorption caused by sodium chloride might be a salting-out effect. Carroll ( 2 ) showed that an organic acid can be eluted 21 16
0
ANALYTICAL CHEMISTRY
CAPRYLIC ACI
B U T Y R I C ACID
-
d I
I
I
I
I
I
I
I
I
I
I
I
I 2 3 4 5 6 7 8 9 1 0 1 1 1 2 P"
Figure 2. Effect of pH on distribution coefficients of organic acids on A-26 from 25 acetone solution
faster from an anion exchange resin with a methanol-water mixture than with water alone. Distribution coefficients of organic acids on Amberlyst A-26 from acetone- or methanol-water mixtures were studied as a function of pH of the solution. The solution contained 0.1M ammonium chloride and the pH was adjusted with hydrochloric acid or ammonia. The pH was apparent pH value of methanol or acetone solution measured with pH meter. In methanol solution experiments in the acidic pH range were not completed because the organic acids are partially converted into their esters. Results are shown in Figures 2-5.
100 0-NITROBENZOIC ACID
O-NITROBEN:
\
60
\ LL
w v
0
20
z
d 2 0.5
I
I
I
I
I
I
I
I
I
I
2
3
4
5
6
7
8
9
I
I
I
Figure 3. Effect of pH on distribution coefficients of organic acids on A-26 from 50 acetone solution
Distribution coefficients of organic acids in alkaline solution are generally larger than those in acidic solution. The pH at which the uptake of an organic acid increases seems to correspond to the pK value of that acid. For example, in Figure 2 the sorption of o-nitrobenzoic acid (pK = 2.2) increases at pH 1-3, butyric acid (pK = 4.8), caprylic acid (pK = 4.85), and benzoic acid (pK = 4.2) increase at pH 3-6, o-nitrophenol (pK = 7.2) increases at pH 6-8, and phenol is constant over the pH range studied but begins to increase from pH 9.8. The distribution coefficients of organic acids from methanol solution are higher than from acetone solution but show a similar behavior pattern. Therefore, the sorption behavior of organic acids on A-26 may be considered as anion exchange in alkaline solution and as molecular sorption of the free acid in acidic solution. Several possibilities for the separation of mixtures of carboxylic acids and phenols are apparent from Figures 2-5. One technique for separation of binary mixtures is to choose a pH where one compound is largely present as the anion while the other is present as the molecular acid. Elution of a ~
~~
I
I
I
I
I
I
I
I
I
2
3
4
5
6
7
8
9
12
1011
PH
~~
I
~
I
I
I
1011 12
PH
Figure 4. Effect of pH on distribution coefficients of organic acids on A-26 from 25 methanol solution
I h
-
PI00 E
=
I
I
I
1
1
.
E
40-
B
20-
I
I
I
I
I
I
I
0 - N ITROPHENOL
-+- 6 0 --
0 LL
1
0-NITROBENZOIC
0 0
5
l0-
5m E
5-
HENOL
BENZOIC ACID
I 2
4
3
5
6
7
8
9 1011
12
P!
Figure 5. Effect of pH on distribution coefficients of organic acids on A-26 from 50 methanol solution ~~
~
~~~
~~
-
Compounds separated Butyric acid Caprylic acid Phenol o-Nitrophenol o-Nitrophenol Benzoic acid Benzoic acid o-Nitrobenzoic acid Phenol Benzoic acid o-Nitrobenzoic acid o-Nitrophenol Phenol o-Nitrophenol Benzoic acid o-Nitrobenzoic acid
Table I. Separation of Organic Acid Mixtures Resin: Amberlyst A-26, 150 200 mesh, C1-form Flow rate: 0.5 ml/min Eluent Recovery of compound Column size Vol, ml Composition Taken, mg Found, mg Recovery, % 1.01 X 14.1 cm 20 25 % Acetone, pH 5.1 1,708 1.696 99.3 35 25 % Acetone, pH 5.1 2.081 2.097 100.7 0.80 X 7.4 cm 20 50% Methanol, pH 8.6 0.983 0.994 101.2 120 50% Methanol, pH 8 . 6 1.433 1.475 102.9 0.80 X 17.0cm 20 50% Acetone, pH 6.3 0.295 0,295 100.0 50 50 % Acetone, pH 6.3 3.791 3.855 101.7 0.80 X 17.0cm 25 50 % Acetone, pH 4.1 3.627 3.407 93.9 50% Acetone, pH 4 . 1 0.715 0.702 98.2 65 1.01 X 16.9 cm 60 50 Methanol, pH 8.6 0.274 0,273 99.6 120 50% Methanol, pH 8.6 0.873 0,860 98.5 0.179 0.172 96.1 170 50 % Methanol, pH 8 . 6 0.080 0.084 105.4 205 50 Z Acetone, pH 2 . 7 1.01 X 17.9cm 25 50% Acetone, pH 7 5 4,300 4.300 100.0 50 50 % Acetone, pH 6.3 0.997 0 993 99.6 75 50 % Acetone, pH 4.15 3,126 3.100 99.2 105 50% Acetone, pH 4.15 0.647 0,638 98.7
VOL. 40, NO. 14, DECEMBER 1968
e
51 17
0.5 50%
I
50%
I
i
50% ACETONE:
BENZOIC ACID
w
y 0.3
g 0.2 4
0
IO
20
30
40
0.I
MILLILITERS OF ELUENT
Figure 6. Separa~ionof butyric acid and caprylic aci Eluent: 25 acetone, pH 5.1 Column size: 1.01 X 14.1 em
0 0
20
40
60
80.
100
120
MILLILITERS OF ELUENT
Figure 7. Separation of phenol, o-nitrophenol, benzoic acid, and o-nitrobenzoic acid Table 11. Comparison of Distribution Coefficients on A ~ ~ e r l y A-26 5 t and Amberlite XAD-2 from 25 Acetone Qne gram of resin was added to 28) ml of solution Compound Benzoic acid
D INgl Acidic solna Alkaline soha
Resin A-26 9.12 22.8 XAD-2 15.2 -0 Phenol A-26 8.65 8.66 XAD-2 13.9 13.3 o-Nitrophenol A-26 10.7 143 XAD-2 72.1 6.S5 pN of the solution are 2.7 for benzoic acid, 2.9 for phenol, and 2.5 for 0-nitrophenol. b pM of the solution are 8.3 for benzoic acid, 8.6 for phenol, and o-nitrophenol. 0,
sorbed acid is facilitated by using a more acidic eluent containing 5 0 z acetone. Data for quantitative separations on A-26 columns are given in Table I; typical elution curves are shown in Figures 6 and 3. Sorption Behavior of Organic Acids. Diamond, Chu, and Whitney (16) emphasized the importance of solvation in determining the selectivity of anion exchange resins for carboxylic acid anions. Our studies on the effect of pH on distribution coefficients on A-26 anion exchange resin (Figures 2-5) suggest that organic acids are sorbed by an anion exchange mechanism from basic solution and by molecular sorption of the free acid from more acidic solution. To obtain further evidence regarding the sorption mechanism, several acids were studied using Amberlyst XAD-2 resin. This is a highly porous polystyrene-divinylbenzeneresin that is physically similar to A-26, but it has no quaternary ammonium or other functional group necessary to make it an anion exchange resin like A-26. Distribution coefficients of benzoic acid, phenol, and o-nitrophenol from acidic and basic solutions with XAD-2 resin are given in Table 11and are compared with distribution coefficients using 8-26 resin. The distribution coefficients with XAD-2 are much larger from acidic solution than from basic solution for benzoic acid and o-nitrophenol Because anion exchange is impossible with this resin, the uptake must (16) R. M. Diamond, B. Chu, and D. G . Whitney, J. Inorg. Nucl. Chem., 24, 1405 (1962).
Column size: 1.01 X P7.9 em
be explained by some form of chemical interaction between the acid and the polystyrene matrix. The anionic forms of the acids are more strongly solvated by the aqueous-acetone solution and thus have much lower distribution coefficients than do the acidic forms. Phenol is largely in its acidic form at both pH values studied. To obtain more information about the sorption on XAD-2 resin, the capacities of benzoic acid and o-nitrophenol on XAD-2 and A-26 from both acidic and basic solutions were measured by column experiments and compared. One gram of the resin was packed into a small column and washed with 25% acetone of the desired pH. Benzoic acid or o-nitrophenol (0.005 M-0.1 M ) , dissolved in 25% acetone of the desired pW, was passed through the column containing one gram of resins with a very slow flow rate until the concentrations of organic acid entering and leaving the column were equal. The organic acid on the resin was desorbed with 5 Q z acetone containing 0.2M hydrochloric acid, and the acid content of the column was determined. The volume of free, interstitial liquid in the column was determined and a correction was applied to take into account the acid in this liquid. The corrected capacities of A-26 and XAD-2 resins are shown in Table III. In alkaline solution, where benzoic acid and o-nitrophenol dissociate completely to their ions, the quantity of each acid sorbed onto Amberlyst A-26 is slightly larger but nearly the same as the exchange capacity of the resin measured for the chloride form. In acidic solution, where each acid exists as the undissociated molecule, the quantities of benzoic acid and o-nitrophenol on Amberlyst A-26 are significantly smaller, 0.56 and 0.55 milliequivalent, respectively. For the sorption on XAD-2 higher capacities are obtained from acidic solution, the opposite from the 8-26 capacities. If the basic solution capacity of XAD-2 is subtracted from the basic solution capacity of A-26, the difference is 3.51 milliequivalents/grarn for benzoic acid and 3.55 milliequivalents/gram for o-nitrophenol. These values are in rather good agreement with the capacity for chloride ion which is 3.52 milliequivaPents/gram. Thus it appears that the uptake of these acid anions by A-26 resin is primarily an anion exchange mechanism, but there is a small amount of another type of sorption which contributes to the resin capacity. Davies and Qwen ( I ) noted the excess or nonionic sorption of aliphatic carboxylic acids and phenylacetic acid on resins.
Table 111. Capacity of Benzoic Acid and o-Nitrophenol on Amberlyst A-26 and Amberlite XAD-2 from 25 Acetone Amberlyst A-26 Amberlite XAD-2 pH of solution 2.25 10.40 2.25 11.50 Benzoic acid capacity. mea. 0.561 3.634 0.951 0.126 pH of &uti& 2.60 11 .o 2.60 12.0 o-Nitrophenol capacity, meq. 0.548 3.642 1.995 0,093 Table IV. Comparison of Distribution Coefficients of Organic Acids on Amberlyst A-26 and Aliquat 336-Cyclohexane from 25 Acetone The concentration of Aliquat 336 in cyclohexane was 0.365M. The condition of Amberlyst A-26 is same as Table II. Acidic solutiono Alkaline solutionb Doc Dd Dd [ml/meq] Doc Dd Dd[ml/meq] Benzoic acid Aliquat 336 0.56 10.0 27.5 0.02 18.3 50.0 A-26 2.50 6.13 Phenol Aliquat 336 0.79 14.4 39.4 0.80 14.6 40.1 A-26 2.36 2.36 o-Nitrophenol Aliquat 336 12.7 2.82 7.72 1.08 183.7 56.4 A-26 2.93 39.1 0 pH of the solution are 2.3 for benzoic acid and phenol, and 2.5 for o-nitrophenol. b pH of the solution are 8.3 for benzoic acid and phenol, and 8.5 for o-nitrophenol. Do is the distribution coeEcient with cyclohexane alone represented ml/ml as dimension. D is the distribution coefficient with Aliquat 336-cyclohexanecalculated by subtracting the quantities of organic acid extracted by cyclohexane alone from the total quantities in organic phase extracted. 0
Anderson and Hansen (17) also investigated the excess sorption of phenols on resin, and showed that the uptake of phenols is by the functional group of the resin. It was felt that studies with a high molecular weight, liquid quaternary ammonium chloride might be interesting. Such a compound has the same functional group as an anion exchange resin like A-26, but it lacks the polystyrene matrix and any special effects due to the physical characteristics of a solid polymer would be missing. Accordingly, the extraction of benzoic acid, phenol, and o-nitrophenol from 25% acetone solution was studied with a solution of Aliquat 336 in cyclohexane. Because the organic acids are somewhat extracted by cyclohexane alone, the distribution coefficients for the extraction with Aliquat 336-cyclohexane are calculated by subtracting the concentration of organic acid extracted by cyclohexane alone from the concentration extracted by Aliquat 336 in cyclohexane. To compare the distribution coefficients on Amberlyst A-26, the distribution coefficients are calculated in terms of uptake per milliequivalent of ion exchange capacity instead of uptake per gram of ion exchange resin. The results in Table IV show that although the acid anions are more strongly extracted by Aliquat 336 than are the free acids, extraction of the free acids also occurs to a considerable extent, Addition of the more polar Aliquat 336 to cyclo-
Our experiments show that uptake of organic acids may occur by anion exchange of the acid anion, and chemical interaction (possibly solvation) of the acid or acid anion by the resin matrix (polystyrene), liquid solvent (cyclohexane), and/ or the quaternary ammonium functional group. Opposing these forces are: (1) Solvation of the acid or acid anion by the solution outside the resin. For organic acids a higher proportion of acetone or methanol in the outside, aqueous solution increases this solvation and lowers the distribution coefficient. (2) Other anions (such as chloride) in the outside solution will compete with the organic anions for the cation sites of the resin or liquid ion exchanger and thus reduce their distribution coefficients. However, inorganic ions may increase the distribution coefficients for free acids by a salting out effect.
(17) R. E. Anderson and R. D. Hansen, Ind. Kng. Chern., 47, 71 (1955).
RECEIVED for review August 8, 1968. Accepted September 19, 1968. Work performed at the Ames Laboratory of the U.S. Atomic Energy Commission.
hexane increases the extraction of the free acid in every case studied, but the increase in much less for o-nitrophenol. The constants in the last column of Table IV indicate that Aliquat 336 in cyclohexane appears to be a more efficient extractant for acid anions than A-26 resin, at least when the two are compared on the basis of an equivalent amount of quaternary ammonium functional group. CONCLUSIONS
VOL. 40, NO. 14, DECEMBER 1968
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