The selective sorption of acetic acid by a strong base anion exchange resin

J. inorg,nucl.Chem., 1973,Vol.35, pp. 2049-2054. PergamonPress. Printedin GreatBritain

THE

SELECTIVE SORPTION STRONG BASE ANION

OF ACETIC EXCHANGE

ACID BY A RESIN

F. DE CORTE* and J. HOSTE Institute for Nuclear Sciences, Ghent University, Proeftuinstraat 86, B-9000 GENT, Belgium

(Received 31 May 1972) Abma'act-The selective sorption of acetic acid from aqueous solutions by the strong base anion exchange resin Dowex IX8, 100-200 mesh, Ac--form has been investigated. It is demonstrated that acetic acid shows a strong affinity towards the ionogenic groups of the resin. The solvation number of acetic acid for these groups was calculated to be four. INTRODUCTION

IoN EXCHANGEresins of the polystyrene-divinylbenzene type in the dry state are known to sorb strongly a great variety of non-electrolytes besides water[l]. Different types of interactions with the resin constituents can be responsible for this behaviour: interaction with the resin skeleton through London forces [2, 3], interaction with the ions inside the resin through solvation [4], or hydrogen bonding of some types of solutes, e.g. aqueous alcohols, with the hydrate water [5]. Reichenberg and Wall[3] investigated the sorption of both water and acetic acid by sulphonated polystyrene resins in the hydrogen form. They pointed out that the acetic acid concentration in the internal solution of the resin beads was lower than that in the external solution. This effect was ascribed to the hydration of hydrogen and sulphonate ions, thus causing a salting-out effect on the acetic acid. Up to now, no attention has been paid to the acetic acid sorption behaviour of anion exchangers An earlier publication [6] reported a striking swelling of the methylenetrimethylammonium anion exchanger Dowex-lX8, 100-200 mesh, in acetic acid solutions. However, it should be noted that, for practical reasons, in these earlier experiments the anion exchanger, being washed only with glacial acetic acid, was in the chloride form. EXPERIMENTAL

Preparation of the resin in the present work, in order to avoid any ion exchange between the ions of acetic acid and the resin, the exchanger, after purification by a NaOH-HCI conditioning procedure [7], was converted to *Research Associate of the N.F.W.O. 1. R. M. Wheaton and W. C. Bauman, Ind. Engng Chem. 45, 228 (1953);,4nn. N.Y. ,4cad. Sci. 57, 159(1953). 2. J. S. Mackie and P. Meares, Discuss. Faraday Soc. 21, 111 (1956). 3. D. Reichenberg and W. F. Wall, J. chem. Soc. 3364 (1956). 4. H. P. Gregor, J. ,4m. chem. Soc. 70, 1293 (1948); 73, 642 (1951). 5. O. D. Bonner and J. C. Moorefield, J.phys. Chem. 58, 555 (1954). 6. P. Van Den Winkel, F. De Corte and J. Hoste, ,4 nalytica Chim. ,4 cta 56, 241 ( 1971). 7. W. Rieman III and H. F. Walton, Ion Exchange in ,4nalytical Chemistry. Pergamon Press, Oxford (1970). 2049

2050

F. DE CORTE and J. HOSTE

the acetate form by washing a resin bed with seventy-five bed volumes of 1 M sodium acetate. The resin was finally conditioned with glacial acetic acid, followed by thorough rinsing with water and drying in vacuum over phosphorus pentoxide. The residual C1- in the resin was determined by nondestructive neutron activation analysis. The irradiations were performed for a 2 hr period in the Thetis reactor at a neutron flux of 5" 1011 n.cm -2. sec -1, and the induced 38Cl-activities were measured by Ge(Li) 7-spectrometry. A residual chloride capacity of 0"0183 meq Cl-/g dry resin was found, whereas the total exchange capacity was evaluated as being 3.46 meq/g dry resin. From these figures it follows that 99.5% of the exchangeable groups are acetate ions.

Determination of the specific bed volume The swelling of the acetate form of Dowex-lX8, 100-200 mesh, expressed as the specific bed volume p (i.e. the bed volume per g dry resin), was investigated as a function of the acetic acid concentration. Here again the swelling behaviour follows the same pattern as with the chloride form resin [6], P increasing markedly with the acetic acid molarity. From these results it is obvious that the acetic acid molecules interact strongly with one of the resin constituents. Comparison with the bed-volume dependence of the sulphonate cation exchanger Dowex-50WX8, 100-200 mesh, H ÷ form (Fig. 1)[8], eliminates the possibility of interaction with the resin skeleton, the two types of resin having the same matrix composition.

2.70 Dowex

50Wx 8 - - R H

2-60

~:

2.50

'~ .a

2.40

2.30 Q.

2-20

t 0

1 2

,

I 4

,

I 6

,

I 6

,

I I0

,

I 12

,

I

i

14

I 16

I

I 18

Molarity acetic acid

Fig. 1. Specific bed volume of Dowex 1 × 8, 100-200 mesh, Ac--form and Dowex 50 W × 8, 100-200 mesh, H+-form [8], as a function of HAc molarity (25°C).

Selective sorption of acetic acid In order to obtain more detailed information about the selective sorption of acetic acid from aqueous solutions, the acid concentration inside the resin beads was determined as a function of the external concentration. After equilibrating a known quantity of dry resin with an acetic acid solution, the mixture was separated by centrifugation at 400 G for 30 min, as described by Pepper et al. [9]. The resin was weighed and then washed with water to remove the acid from the internal solution, Finally, the acetic acid washed from the resin was determined by titration with 0" 1 N NaOH. A correction was applied for the incomplete separation of resin beads and surface-adhering solvent, which was assumed to have the same composition as the external solution. This was accomplished by centrifuging a similar volume of glass beads (diameter: 0.09-0.1 mm). The correction was found to be 0.033 ±0.002 ml per ml of resin bed, with no observable change as a function of acetic acid concentration. 8. S. K. Jha, F. De Corte and J. Hoste, To be published. 9. K.W. Pepper, D. Reichenberg and D. K. Hale,J. chem. Soc. 3129 (1952).

Selective sorption of acetic acid

2051

The solvent uptake at 25°C, expressed as a weight increase per gram of dry resin, is given in Table 1. Each value is the average of four experiments. In order to check the above centrifugation method by an independent technique, the water uptake per g of dry resin was determined thermogravimetrically. About 0.7 g wet resin was uniformly spread on the bottom of an uncovered porcelain crucible and placed on one arm of a thermobalance. The temperature of the oven was kept constant at 65°C. The drying curves were registered photographically to constant weight of the resin (ca. 0.3 g). As shown in Fig. 2, the curves of weight vs time can be subdivided into three different regions [ 10]: (i) The warming-up period of the resin sample. (ii) The constant-rate period, corresponding to dehydration of the surface of the resin beads. (iii) The falling-rate period, corresponding to removal of internal water. At point P, where the constant rate ends and the drying rate begins to fall, dehydration of the surTable 1. Solvent uptake of Dowex1X8, 100-200 mesh, Ac--form, as a function of the external acetic acid concentration (25°C)

Outside Mr~?,c

Solvent uptake g/g dry resin

0 1.005 4.114 7.375 10.25 13.29 15.41 16.12 16.63 17.14

0-643 0.670 0.720 0.769 0.786 0.801 0.817 0.819 0.818 0.821

I wr 4o0 ~ - ~ - - ~ m

E 300 ~p Drysornpleweight0.2997g 200 ~ e " 65*C o IO0

i 0

I

I

I Time t

i 2

I

I 3

hr

Fig. 2. Thermogravimetric analysis of Dowex 1 × 8, 100-200 mesh, Ac--form. 10. J. H. Perry, Chemical Engineers' Handbook, 4th Edn. McGraw-Hill, New York (1963).

2052

F. DE CORTE and J. HOSTE

face can be considered to be complete, and at this point the ordinate corresponds to the internal water content of the resin when fully swollen in water. For the water uptake per g of dry resin an average value of 0.64 g/g was found, with a standard deviation of 0-01 g/g (six experiments.) This result is in good agreement with the value of Table 1. From the data of Table l, and from the results of the titrations with 0.1 N NaOH, the internal acetic concentration could be computed by the method of the successive approximations, the density of the inner solution, originally accepted as equalling that of the external solution, being the quicklyconverging value. The results of the calculations are summarized in Table 2, which refers to experiments in quadruplicate. From Table 2 it follows that Ko, i.e. the ratio of the internal to external acetic acid concentrations, is quite high at low external acid molarities, tending to unity at high concentrations (Fig. 3.) Table 2. Inside HAc-molarity of Dowex, lX8, 100-200 mesh, Ac-form, as a function of external HAc molarity (25°C) Outside M,xc

Inside MH~c

0 1.005 4.114 7.375 10-25 13.29 15.41 16.12 16-63 17.14

0 3-10 8.20 11.21 12.95 14.95 16.21 16.70 17.20 17-39

3

2

2

i 0

i 2

i

I 4

I

I 6

i

I 8

i

I

I

I0

Outside molorfty oce'he

I

i

12

i 14

i

I 16

i

I 18

ocid

Fig. 3. Ka ratio of HAc--form, as a function of outside H A c - molarity (25°C). DISCUSSION

Solvation of the ionogenic groups Using the model of Gregor[4], we can assume that the internal solution of the anion exchange resin consists of two different parts: firstly, the hydration and

Selectivesorption of acetic acid

2053

solvation molecules of water and acetic acid, respectively, with no solvent power for other solutes, and secondly the free acetic acid solution, for which there is no reason to accept a composition different from that outside. As the strong interaction between the H A ? molecules and the ionogenic groups of the resin has been proved in this paper, it is plausible to accept a higher mole fraction of the acetic acid in the solvating solution than in the free solution. These considerations explain the increased "overall" molarity of the acid inside the exchanger, when equilibrated with an aqueous acetic acid solution of a given composition. To a first approximation, at high acetic acid molarities, we can consider the ionogenic groups as being exclusively solvated by acetic acid molecules. Using this assumption, it is possible to calculate the internal volumes of solvating and free solution per gram of dry resin by solving a set of two equations with two unknowns:

VsMs + VIMI Vs+ Vr

= M~

V~d~5 + Vfd~ 5 = S

(I) (2)

where Vs -Vj = M8 = Mf = M~ = d8e5 = d~ 5 = S=

internal volume of solvating solution per g dry resin; internal volume of free solution per g dry resin; molarity of the solvating solution (= 17.38 M); molarity of the free (i.e. external) solution; over-all molarity of the internal solution; density at 25°C of the solvating solution (= 1-044 g/ml); density at 25°C of the free solution in g/ml; solvent uptake, weight per g dry resin.

From the computed Vs values the solvation number ~ of acetic acid can be calculated from the equation: ~s =

V~.M~ Q.

(3)

where Q, is the total weight capacity of the exchanger (= 3.46 meq/g dry resin). The results of the calculations for some high external acetic acid concentrations are summarized in Table 3. It is apparent that the internal volume of the free solution tends to zero at high acetic acid molarities, whereas the solvation number ~s reaches a maximum value of about four at 17-38 M H,~c. This means that, in the case of glacial acetic acid, the swelling of the resin is due only to the tendency of the ionogenic groups of the exchanger to be surrounded each by four acetic acid molecules. This conclusion seems reasonable, since it is logical to assume that the forces causing distortion of the resin skeleton can only result in the case of such a high swelling from the very strong affinity of the acetic acid molecules towards the

2054

F. DE CORTE and J. HOSTE Table 3. Determination of the solvation number ~ of acetic acid (Dowex1X8, 100-200 mesh, Ac--form); (25°C)

External MnT.¢ 16.12 16.63 17.14

External tool H Ac mol H20 3.2 5.4 16.2

V~ml

Vsml

"~¢' molecules/functional group

0.420 o. 188 ~0

0.359 0.594 0.786

1.80 2.98 3.95

ionogenic groups, leaving no space for free solution. I f the solvent u p t a k e per g r a m o f dry resin (Table 1) is e x t r a p o l a t e d to a n h y d r o u s acetic acid (17.38 M), a value o f 0.822 g is obtained, c o r r e s p o n d i n g to h, = 3.96. T h u s , it m a y be conc l u d e d f r o m these figures that the solvation n u m b e r o f acetic acid for the ionogenic g r o u p s o f D o w e x - 1 X 8 , 1 0 0 - 2 0 0 mesh, A c - - f o r m , is four.

Acknowledgements-Grateful acknowledgement is made to the Nationaal Fonds voor Wetenschappelijk onderzoek for financial support. Thanks are also due to T. De Wispelaere for technical assistance and to F. Martens (Institute for Crystallography) for the helpful advice concerning the thermogravimetric experiments.