Analysis of the Potentiometric Titration Curves of Weakly Basic Anionites

Transcription

1 881 Analysis of the Potentiometric Titration Curves of Weakly Basic Anionites A.V. Mamchenko and L.G. Chernova* A.V. Dumansky Institute of Colloid Chemistry and Water Chemistry, National Academy of Sciences of Ukraine, 42, Vernadsky Blvd., Kiev-142, Ukraine. (Received 3 November 2000; accepted 11 December 2000) ABSTRACT: An analysis of the experimental potentiometric titration curves for various classes of weakly basic anionites has been made from the basis of the exchange equilibrium theory. When the NaCl concentration in the external solution differed by more than two orders of magnitude, two curves were obtained for each ionite. Reference to the activities of the chloride ions allowed these two curves to be combined satisfactorily. The acrylic anionite, Amberlite IRA 67, fulfilled the model of ideal exchange in the gel phase. Systematic deviations from such ideal behaviour were observed for the polystyrene divinylbenzene anionite, Lewatit MP 62, while the experimental data for Lewatit AP 49 exhibited an essential deviation from a linear dependence when plotted in terms of the extended Henderson Hasselbalch equation. Comparative analyses of the titration curves for weakly basic Amberlite IRA 67, strongly basic AB 17-8 and Lewatit AP 49 anion exchangers led to the suggestion that Lewatit AP 49 contained weak and strong basic groups whose behaviour may be described by the extended Henderson Hasselbalch equation using various effective equilibrium constants but the same value of n (= 1). An equation for calculating the quantity of strong and weak basic groups in an acrylic anionite has been obtained. An explanation of ideality in terms of weakly dissociating (basic) and strongly dissociating (salt) resinates of weakly basic acrylic anion-exchange resins has been proposed. INTRODUCTION The use of weakly exchanging resins is becoming increasingly common in ion-exchange chromatography and for the removal of inorganic and organic matter from waters. This may be attributed to their distinct advantage in being generally capable of regeneration in situ with basic, acidic and salt solutions and/or regeneratable non-aqueous solvents (Kim et al. 1976). However, comparatively few studies have been undertaken to date on ion-exchange equilibria involving such types of sorbents. The exchange capacity and equilibrium constant are important parameters from the viewpoint of the practical exploitation of such anionites. From a chemical viewpoint, anionites may be regarded as aggregations of exchange groups linked to an inert matrix and also as polyelectrolytes which are ionized but remain insoluble in the equilibrium solution. Hence, the total exchange First presented at the 5th Ukrainian Polish Symposium on Theoretical and Experimental Studies of Interfacial Phenomena and their Technological Application, Odessa, Ukraine, 4 9 September, *Author to whom all correspondence should be addressed. irena@health.gov.ua.

2 882 A.V. Mamchenko and L.G. Chernova/Adsorption Science & Technology Vol. 18 No capacity of such resins may be defined in terms of the number of available active sites which is a constant quantity. In contrast, the operating capacity is defined as the proportion of the total capacity used during an exchange process and depends on a number of process variables including ph, the concentration of the external solution, the types of ions to be adsorbed, etc. Titration curves have been widely used for characterizing ion-exchange materials. Both the exchange capacity and the equilibrium constant can be deduced from such titration curves in terms of the reactions: R N(CH 3 ) 2 + H 2 O + Cl R N(CH 3 ) 2 H + Cl + OH (1) for the formation of a free base with tertiary amino groups and R N(CH 3 ) + 3 OH + Cl R N(CH 3 ) + 3 Cl + OH (2) for strongly basic quaternary amino groups. It has been observed experimentally (Kern 1939; Katchalsky and Spitnik 1947; Katchalsky 1952; Strobel and Gable 1954; Gregor et al. 1955; Fisher and Kunin 1956; Soldatov 1994; Mamchenko and Valuiskaya 1998) that the titration behaviour of many polyelectrolytes conforms to the extended Henderson Hasselbalch equation: ph + pcl = log K* n log[(a/(1 a)] (3) This equation includes the equilibrium constant K* (= ~ K/K w ) and an empirical constant n which is a measure of the deviation of the system from ideal behaviour. When n = 1, equation (3) reduces to the mass action law for ideal resinate solutions. The constant n is related to the structure of the anionite, its capacity and various other properties (Strobel and Gable 1954; Ushakova et al. 1978). The quantity a in equation (3), i.e. the degree of neutralization of the ionite, is equal to e/e where E is the total exchange capacity and e the specific sorption of the resin. Finally, pcl = log a Cl, is the activity of the chloride ions in the external solution. In most cases, solutions in the gel phase of resins are not ideal (Katchalsky and Miller 1947; Katchalsky and Miller 1954; Katchalsky et al. 1954; Strobel and Gable 1954; Mamchenko and Valuiskaya 1998). For varying degrees of anionite neutralization, the order of the equilibrium constant changes by one or two units (Ushakova et al. 1978; Memchenko and Valuiskaya 1998). While an increase in the degree of neutralization of resins leads to increased charge formation, the additional charges associated with the polymer macromolecule limit the transition of fixed ions from a weakly dissociated free base form into a well-dissociated form. EXPERIMENTAL An analysis of experimental potentiometric curves for various classes of weakly basic anionites has been undertaken in the present investigation, with the behaviour of three adsorbents being examined, i.e. Amberlite IRA 67, Lewatit MP 62 and Lewatit AP 49. Amberlite IRA 67 is based on an acrylic gel matrix and according to its manufacturer (Rohm & Haas) contains tertiary amino groups. However, further information regarding the strongly basic groups in this resin was not available. Lewatit MP 62 is based on a macroporous polystyrene divinylbenzene matrix and also contains tertiary amino groups. The method of synthesis employed (Bayer AG) led to only a minimal content of strongly basic groups. Lewatit AP 49 is based on a porous polyacrylic matrix according to its manufacturer (Bayer AG) and contains a small amount of strongly basic groups.

3 Analysis of the Potentiometric Titration Curves of Weakly Basic Anionites 883 Potentiometric curves were obtained for each anionite. For such studies, the resins were converted to the hydroxide form in the absence of CO 2 and washed with deionized water until the effluent gave no indication towards phenolphthalein. The water content of the resins was removed by drying over solid sodium hydrate. Carefully determined amounts of a given anionite were then transferred to a series of appropriate flasks to each of which 100 ml of a combined solution of HCl and NaCl was added. Each flask was then tightly stoppered. The relative amount of HCl employed in the solution in each flask could be varied while that of NaCl was maintained constant in each case. The flasks were maintained at 25ºC throughout and were subject to mechanical tumbling for 15 d. After this time the solution in each flask was separated by decantation and the resultant ph and acid concentrations determined. The data obtained for the solution in each flask constituted one point on the resulting titration curve. Capacities were expressed in equivalents of chloride ion per kg dry hydroxide form of anionite and were calculated from the formula: C 0 V 1 C p (V 1 + V 2 ) e = (4) m which follows from material balance and equations (1) and (2), and where C 0 is the molarity of HCl in the initial solution, C p is the molarity of the acid (or alkali, indicated by a minus sign) in the equilibrium solution, V 1 and V 2 are the respective volumes of the initial acid/salt and salt solutions (dm 3 ) and m is the weight of anionite employed (kg) recalculated on the basis of the completely dried material. RESULTS AND DISCUSSION The total exchange capacities of the anionites, which are required for the calculation of a = e/e, were obtained from the plateaux values of the potentiometric titration curves which occur over a small range of ph values for the external solution (Figure 1). The corresponding data are listed in Table 1. The value of the apparent equilibrium constant K*, corresponding to the change of the anionite from the hydroxide to the chloride form in accordance with the reactions expressed in equations (1) and (2), was calculated from equation (3). Since both the specific sorption and the equilibrium constant also depend on the concentration and ph of the external solution, two potentiometric curves were obtained for each ionite corresponding to NaCl concentrations differing by more than two orders of magnitude, i.e M or 2.0 M. For solutions where the NaCl concentration was 0.01 M, this was comparable in magnitude to the alkalinity (or acidity) of the external solution so that the activity of the chloride ion and the pcl values were calculated in accordance with the equation: pcl = log a Cl = log[g(c 0NaCl + C p )] (5) where C 0NaCl is the molarity of NaCl in the initial solution. The value of the mean activity coefficient, g, was determined in accordance with the relationship (Skorcheletti 1963): log g = m/(1 + m) (6) where m is the ionic strength of the solution which, in this case, was numerically equal to the total molar concentration of electrolyte in the equilibrium solution. Values of pcl in the 2.0 M NaCl solution were calculated disregarding the alkalinity (or acidity) of the equilibrium solution since the concentration of background salt exceeded that of the alkalinity (or acidity) by more than two orders of magnitude. In this case, the values of g were taken from Voznesenskaya (1968).

4 884 A.V. Mamchenko and L.G. Chernova/Adsorption Science & Technology Vol. 18 No Figure 1. Experimental data for the potentiometric titration of weakly basic acrylic anionites at different ionic strengths in the external solution: Lewitat AP 49, curves 1 and 2; Amberlite IRA 67, curves 3 and 4. Ionic strengths: 0.01 M NaCl, curves 1 and 3; 2.0 M NaCl, curves 2 and 4. TABLE 1. Total Exchange Capacities of Anionites Anionite Total exchange capacity, E Standard deviation, S Confidence level a, D (mol/kg) (mol/kg) (mol/kg) Amberlite IRA Lewatit MP Lewatit AP AB a With f = 0.05.

5 Analysis of the Potentiometric Titration Curves of Weakly Basic Anionites 885 It can be readily seen from estimates of the uncertainty error involved in the determination of the specific sorption of the chloride ions (e) that the relative inaccuracy in the calculations amounted to 2 5% and increased as the magnitude of the parameter a increased. This resulted in a relative error of % in the calculation of the a values. Hence, the relative error in the calculation of the quantity a/(1 a) using equation (3) exceeded 40% when a > 0.9. For this reason, further calculations of the magnitude of ~ K and similar parameters were undertaken using experimental data where a did not exceed 0.9. Figures 2 and 3 depict the potentiometric titration curves for weakly basic anionites plotted on the basis of the Henderson Hesselbalch equation. In such plots, allowance has been made for the chloride ion activities in the external solution and this has enabled the curves obtained with different background salt concentrations for a given anionite to be combined quite satisfactorily. The systematic deviation from ideal behaviour observed for the polystyrene divinylbenzene anionite, Lewatit MP 62, is similar to that observed for carboxyl cationites containing a rigorously identical Figure 2. Experimental data for the potentiometric titration of weakly basic anionites at different ionic strengths in the external solution as expressed in terms of the Henderson Hasselbalch equation (X = log[a/(1 a]): Lewitat MP 62, curves 1 and 2; Amberlite IRA 67, curves 3 and 4. Ionic strengths: 0.01 M NaCl, curves 1 and 3; 2.0 M NaCl, curves 2 and 4.

6 886 A.V. Mamchenko and L.G. Chernova/Adsorption Science & Technology Vol. 18 No Figure 3. Experimental data for the potentiometric titration of weakly basic acrylic anionites at different ionic strengths in the external solution as expressed in terms of the Henderson Hasselbalch equation (X = log[a/(1 a]): Lewitat AP 49, curves 1 and 2; weakly basic groups in Lewatit 49, curves 3 and 4; Amberlite IRA 67, curves 5 and 6. Ionic strengths: 0.01 M NaCl, curves 1, 3 and 5; 2.0 M NaCl, curves 2, 4 and 6. exchange group composition (Mamchenko and Valuiskaya 1998). It will be seen from the figures that the experimental data for anionites with tertiary amino exchange groups were described quite well by the generalized Henderson Hasselbalch equation. The slopes of the related linear plots corresponded to values of n equal to ca. 2.5, i.e. the apparent equilibrium constant was affected to a significant extent by the degree of neutralization of the anionite (see data recorded in Table 2). The deviation from ideality which occurred when n = 1 could be described within the parameters of the exchange equilibrium theory (Mamchenko and Chernova 1999). For the weakly basic anionite involving an acrylic matrix (Amberlite IRA 67), the values of n corresponded to the theoretical value of unity to within experimental error and were independent of the concentration of background electrolyte in the equilibrium external solution (Table 2). In this respect, Amberlite IRA 67 provides a model example of ideal exchange in the gel phase. However, the introduction of quaternary amino-based fixed ions into the exchanger (as occurs in Lewatit AP 49) leads to typical deviations from a linear relationship for the experimental dependence ph + pcl versus log[a/(1 a)] when the latter is plotted in terms of the generalized Henderson Hasselbalch equation (Figure 3). Such deviations occurred when only a small fraction of the anionite was present in a salt form, i.e. over the range of small values for log[a/(1 a)],

7 Analysis of the Potentiometric Titration Curves of Weakly Basic Anionites 887 TABLE 2. Calculated Values of Henderson Hasselbalch Equation Parameters Parameter Anionite Amberlite IRA 67 Lewatit MP 62 Lewatit AP 49 a AB M 2.0 M 0.01 M 2.0 M 0.01 M 2.0 M 0.01 M NaCl NaCl NaCl NaCl NaCl NaCl NaCl n S(n) D(n) log K* S(log K*) D(log K*) a Weakly basic groups only. which may be connected with an appreciable sorption of chloride ions in the alkaline range (Figure 1). When the fraction of the salt form increased, i.e. over the range of high values for log[a/(1 a)], the potentiometric curves for Lewatit AP 49 and Amberlite IRA 67 were virtually coincident provided that the salt concentration in the external solution was equal in both cases (see Figures 1 and 3). Although the relationship ph + pcl versus log[a/(1 a)] for Lewatit AP 49 shows a pronounced curvature (Figures 2 and 3), this cannot be taken as an indication of imperfect behaviour for the resinate solutions. Since the generalized Henderson Hasselbalch equation is linear and approximates to that part of the function ph + pcl = f{log[a/(1 a)]} which is close to linearity, the imperfection of the resinate solution may be expressed as the deviation of n from unity at higher values. Furthermore, the total exchange capacity for Amberlite IRA 67 in acid regions exceeds the corresponding total exchange capacity for Lewatit AP 49 and vice versa at lower degrees of anionite neutralization if the same function is expressed in terms of the specific adsorption of chloride ions versus the ph of the external solution (Figure 1). These observations indicate that the anionite Lewatit AP 49 contains not only weak basic groups but also strong ones. It should be noted that the anionites Amberlite IRA 67 and Lewatit AP 49 exhibited almost identical potentiometric curves over the region where large values of a existed for both concentrations of background salt in the external solution (Figure 2). This suggests that the resinate solution involving the weakly basic groups of Lewatit AP 49 is also perfect. Analysis of the titration curves obtained for the strongly basic anionite AB 17-8 under the same experimental conditions shows that the data may be described satisfactorily by employing the generalized Henderson Hasselbalch equation with n = 1 (Figure 4, Table 2). Similar data for other strongly basic anion-exchangers have been obtained by Stroble and Gable (1954). On this basis, it is reasonable to assume that the ion-exchange reaction in the gel phase of Lewatit AP 49 which contains both strong and weak basic groups can be expressed in terms of equation (3) for each kind of group when n = 1. Such an assumption allows the potentiometric titration curves for Lewatit AP 49 to be described by two equations similar to equation (3) containing different equilibrium constants but with the parameter n identical and equal to unity. Let us designate the total exchange capacity of the weakly basic groups in the anionite as E w, the strongly basic groups as E s, the respective specific sorption of chloride ions of any magnitude ph

8 888 A.V. Mamchenko and L.G. Chernova/Adsorption Science & Technology Vol. 18 No Figure 4. Experimental data for the potentiometric titration of the strongly basic anionite AB 17-8 with 0.01 M NaCl in the external solution as expressed in terms of the Henderson Hasselbalch equation (X = log[a/(1 a)]). The dotted line corresponds to the potentiometric titration of the strongly basic groups in Lewatit AP 49 with 0.01 NaCl in the external solution as calculated on the basis of equation (9). + pcl as e w and e s, and the equilibrium constants for reactions (1) and (2) as K w and K s. Hence, according to equation (3), the specific sorption of chloride ions by the weakly basic and strongly basic groups are, respectively: and e w = E w [K w /(K w + 10 ph + pcl )] (7) e s = E s [K s /(K s + 10 ph + pcl )] (8) Since the total exchange capacity and the specific sorption are summations of the total and specific sorption of the weakly basic and strongly basic groups, respectively, i.e. and e = e w + e s (9) E = E w + E s (10)

9 Analysis of the Potentiometric Titration Curves of Weakly Basic Anionites 889 it is possible to obtain the degree of conversion into the salt form of the anionite as: a = a w E w /E + a s E s /E (11) where e w = a w /E w and e s = a s /E s. The fraction of strongly basic groups involved in the total capacity of the anionite may be written as: and combining equation (11) with equations (12), (7) and (8) yields: b = E s /E (12) a = b[k s /(K s + 10 ph + pcl )] + (1 b)[k w /(K w + 10 ph + pcl )] (13) It is possible to obtain the parameters b, K s and K w by using equation (13) in conjunction with least-squares methods. It should be noted that, when calculated on the basis of potentiometric titration curves obtained in the presence of 0.01 mol/dm 3 NaCl in the equilibrium solution, all the parameters have significant values whereas unambiguous values of K s could not be obtained from the data corresponding to a 2.0 M NaCl external solution because the values of a obtained were all in the region a The experimental potentiometric titration curve for the strongly basic polystyrene resin AB 17-8 calculated on the basis of the apparent dissociation constant K* (Table 2) has been compared in Figure 4 with the curve for the strongly basic groups in Lewatit AP 49 calculated on the basis of the constant K s. It will be seen that both dependencies are linear and that the corresponding lines are parallel. However, the dependence ph + pcl versus log[a/(1 a)] for the acrylic resin lies above that for the polystyrene resin. This indicates that the strongly basic groups in Lewatit AP 49 have a greater strength than those of AB This may also be confirmed by comparison of their log K s values listed in Tables 2 and 3, where the difference amounts to 0.76 units. Divergencies between the values of b for solutions where the concentration of background NaCl differed by more than two orders of magnitude cannot be explained in terms of calculation errors despite the fact that the b values are quite close. The reason for this difference is the same as that which led to a significant error in the determination of K s in the presence of 2.0 M NaCl. Determination of K w for Amberlite IRA 67 and for the weakly basic exchange groups in Lewatit AP 49 for the two concentrations of NaCl employed in this study shows that significant differences exist in the two cases (see data in Table 3). Such differences also accord with the data in Table 2 which show that, for the ion-exchanger Amberlite IRA 67, the logarithms of the apparent equilibrium constants decreased appreciably on increasing the concentration of NaCl in the external solution. Such differences cannot be explained in terms of experimental errors but are probably due to swelling of the resin induced by the concentration changes of the background electrolyte, a possibility which needs to be investigated further. However, despite the appreciable differences between the magnitudes of the parameters in equation (9) as derived for the two NaCl concentrations in the external solution, the use of the parameter determined in the presence of 0.01 M NaCl is sufficient to allow a satisfactory explanation of all the experimental data for Lewatit AP 49 (Table 3). It has been verified by the present study that the acrylic anionite Amberlite IRA 67 contains only weakly basic groups. Hence, the ideality of the basic group salt transition in systems involving this resin solution cannot be explained from the viewpoint of the chemical irregularity of the fixed ions. There are difficulties in suggesting that weakly basic resins based on polystyrene divinylbenzene (e.g. Lewatit MP 62), which contain chemically homogeneous tertiary amino functional groups, behave non-ideally (Mamchenko and Chernova 1999) whereas similar anionites synthesized on the basis of acrylic matrices exhibit ideal behaviour.

10 890 A.V. Mamchenko and L.G. Chernova/Adsorption Science & Technology Vol. 18 No TABLE 3. Parameters for Equation (13) as Obtained from Potentiometric Titration Curves for Lewatit AP 49 with Different Electrolyte Concentrations in the External Solution Parameter NaCl concentration (M) in external solution b S(b) D(b) K s S(K s ) D(K s ) K w S(K w ) D(K w ) F f log K s log K w It should be noted that the two parameters pk and n are strongly dependent on the structure of the resin and the environment into which it is placed. The value of n increases in magnitude as the configurational entropy of the polymer chain decreases due to increasing electrostatic repulsion between neighbouring charged groups. The electrostatic repulsion between various charged groups in the resin is greatly enhanced by an increase in the degree of neutralization of the anionite. This effect has been analyzed theoretically both for linear and crosslinked polyelectrolytes (Katchalsky 1952; Katchalsky and Miller 1954; Katchalsky et al. 1954; Lifson and Katchalsky 1954; Katchalsky and Michaely 1955). In the present study, in those anionites synthesized on the basis of polystyrene divinylbenzene, the amino groups are linked to the polymer matrix via small mobile bridging chains containing a benzene ring and methylene groups. In acrylic anionites, the bridge between the three-dimensional polymeric matrix and the amino group consists of a much longer mobile chain containing linear bonds such as CO NH CH 2 CH 2 N(CH 3 ) 2. The high mobility of such a chain allows the ready separation of fixed ionic groups from one another as the resin proceeds from the weakly dissociated free base form to the strongly dissociated salt form. Thus the compensation of electrostatic interaction between neighbouring fixed ions is increased by the increasing distance between such ions and the accommodation of co-ions in the internal matrix structure. For anionites synthesized on the basis of the macroporous polystyrene divinylbenzene matrix, the operation of a similar mechanism for lowering the electrostatic interaction between neighbouring fixed ions (which must be linked with the ideal behaviour of the resin solution) is hindered by the high rigidity of the bridge between the polymeric matrix and the ionic groups. REFERENCES Fisher, S. and Kunin, R. (1956) J. Phys. Chem. 60, Gregor, H.P., Hamilton, M.I., Buher, J. and Bernstein, F. (1955) J. Phys. Chem. 59, 879.

11 Analysis of the Potentiometric Titration Curves of Weakly Basic Anionites 891 Katchalsky, A. (1952) J. Polym. Sci. 7, 393. Katchalsky, A. and Michaely, I. (1955) J. Polym. Sci. 15, 89. Katchalsky, A. and Miller, I.R. (1954) J. Polym. Sci. 13, 57. Katchalsky, A. and Spitnik, P. (1947) J. Polym. Sci. 2, 432. Katchalsky, A., Shavit, N. and Eisenberg, H. (1954) J. Polym. Sci. 13, 69. Kern, W. (1939) Biochem. Z. 301, 338. Kim, B.R., Snoevink, L. and Saunders, E.M. (1976) J. Water Pollut. Control Fed. 48, 120. Lifson, S. and Katchalsky, A. (1954) J. Polym. Sci. 13, 43. Mamchenko, A.V. and Chernova, L.G. (1999) Khim. Tekhnol. Vodi 21, 451. Mamchenko, A.V. and Valuiskaya, E.A. (1998) Khim. Tekhnol. Vodi 20, 451. Skorcheletti, V.V. (1963) Theoretical Electrochemistry, Goshimizdat, Leningrad. Soldatov, V.S. (1994) Dokl. Akad. Nauk 336, 782. Strobel, H.A. and Gable, R.W. (1954) J. Am. Chem. Soc. 63, Ushakova, G.I., Znamenskiy, Y.P. and Artushenko, G.U. (1978) Gyrn. Fiz. Khim. 52, Voznesenskaya, E.I. (1968) Questions of Physical Chemistry of Electrolyte Solutions, Khimiya, Leningrad, p. 172.