literature Cited
Grant,, 11. J. 1\Ianes, 1\1.,IXD.E N G CHEX. . FuXo.\af. 5 , 490 (1966). Grant, R. J., Manes, M., Smit’h, S. B., d.I.Ch.E.J. 8, 3,403 (1962). Pigford, R. L., Baker, B., 111, Blum, D. E., IND.ESG. CHIX. F C N D I n 1 . 8, 144 (1969). Rolke, I
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D E S P I T E the increasing technological imliortance of weak base ion exchange resins, relatively little iiiforniatioii coilcerning their basicities has been published. O n the other hand, many studies have been carried out on the acid-base properties of both linear and crosslinked polycarboxylic acids. One reason for the apparent lack of interest in weak base systems is that the compositions of the resiiis them-elves are not as clearly defined as those of polyacrylate or polymethacrylate resins. Weak base resins possessing a polystyrene-divinylbeiizene (DVB) matrix undergo additional crosslinking via methylene bridging as well as by amine bridging during amination. Weak base resiiis prepared from acrylate or methacrylate copolymers a h ays contain small amounts of carboxylate groups, which are produced by hydrolysis of amide linkages. These undesirable side reactions do not niake the use of weak base resins attractive for comparison of theoretically calculated p K values with experiniental ones. I n studies on weak base resins good experimental data are difficult to obtain because of COn absorption in the alkaline equilibrium solutions employed. T h e n a sample of a resin hydrochloride is shaken with an appropriate amount of a 1 to 1 electrolyte and NaOH, the p H values sometimes drift downward for months without attaining equilibrium. The lack of attainment of equilibrium between the resin and soluI
I
tion phases and the introduction of a small amount of carbon dioxide from the atmosphere during the pH measurement cause serious errors, particularly in the p H 7 to 9 range of interest, in studies on weakly basic resins. Additional problems are encountered because the p H measurements are made on a completely unbuffered aqueous phase. Despite the difficulties involved in such an experimental procedure, several investigators have obtained useful, albeit only semiquantitative, data on a variety of weak base resin structures. Kunin and X y e r s (1947) measured the pH’s of HC1, H2SOa, HSO,, and H3P04 solutions “equilibrated” with Xniberlite IR-4B, a phenol-formaldehyde-triethylenetetramine condensation product, for 48 hours. They found that a 1.6 change in p H was produced by a n ionic strength change from 0.01 to 1.0. Topp and Pepper (1949) made similar measurements, also with a 48-hour contact time, with mphenylenediamine-formaldehyde and poly(viny1 chloride)ammonia resins. They showed that the p K of the aromatic resin (-2.5) was considerably less than that of the aliphatic amine (pK -6) in 0.1V KC1 and that the pK increased upon the addition of neutral salt. Kagasawa et al. (1958) measured the pH’s of Amberlite IR-45 (a polystyrene-polyamine resin) samples which had been equilibrated with NaC1-HC1 mixtures. They fitted the data with a modification of the Henderson-Hasselbach equaInd. Eng. Chem. Fundam., Vol. 9, No. 2, 1970
221
tion. Selegny and Merle (1963, 1964) obtained titration curves for poly(viny1 alcohol), poly(dimethylaminostyrene), and ail amine-containing cellulose resin. Weiss et al. (1966) and Bolto et al. (1968) measured p H titration curves for a large number of weakly basic resins. They found that most homofunctional secondary or tertiary amine resins exhibited relatively flat titration curves, whereas heterofunctional resins (those prepared from polyamines such as triethylenetetramine or tetraethylenepentamine) showed steeply sloping titration curves. Keiss et al. also investigated variations of p K with ionic strength, crosslinker content, temperature, and the nature of amino group substituents. They applied these data and similar information concerning n eakly acidic resins to the development of an ion exchange deionization process involving thermal regeneration. I n the present study, the basicities of six weak base resins of different chemical structure have been measured a t various ionic strengths and temperatures by equilibration of the resins with aqueous amine-amine hydrochloride buffer solutions followed by determination of the degrees of neutralization, CY, of the resins. Since both a and the p H of the buffer solution may be measured with excellent precision, p H titration data reproducible to ~ t 0 . 0 3unit may be obtained for monofunctional resins. The measurements of titration curves a t several temperatures thus permit the determination of enthalpies of neutralization with an accuracy not possible with the classic titration procedure. Experimental
Resins. T h e resins used were : poly(N,N-dimethylvinylbenzylamine)-3yc D V B (DMVBA) ; poly [N,N-bis(a-aminoethyl) vinylbenzylamine]-3% D V B (AEVBA) ; poly(2methyl-5-vinylpyridine)-5, 7 , and 10% D V B (MVP) ; poly(2-N,N-dimethylaminopropylacrylamide)-2, 4, 6, and 8yoDVB ( D M X P X ) ; a condensation polymer of epichlorohydrin and triethylenetetramine ( E P I - T R I E K ) ; and a condensation polymer of phenol, formaldehyde, and triethylenetetramine (Amberlite IR-4B). All the resins were treated alternately with 1 M solutions of HC1 and N a O H in order to remove impurities and low molecular weight polymeric fragments. T h e properties of t h e various resin samples are given in Table I. Equilibrations. Small samples (3 t o 4 meq.) of t h e resins were held in jacketed 30-ml. filter tubes through which water of appropriate temperature was circulated during
CHZ H,c-N-cH,
I
DMVBA
AEVBA
I
CH,
MVP 222
r'"
HN-(CH,),-N
DMAPA
Ind. Eng. Chern. Fundam., Vol. 9, No. 2, 1970
CH,
I
I hi,
Table 1.
Properties of Weak Base Anion Exchange Resins
Resin
% Solidsn
DllVB&3% DVB AEVBA-3% DVB 31VP-5% DVB lfVP-77c DVB llVP-lO% DVB DhIAP.4-2yc DVB DlIAPA-4% DVB D&fAP.k-6% DVB DhfAPA-8% DVB EPI-TRIEN Amberlite IR-4B
48 5 58 7 45 2 50 4 508 29 8
a
41 47 56 37 55
7 8 3 8 4
Anion Exchangea Capacity, Meq./G.
4 5 7 7 6 6 5 5 4 11 10
30 30 63 80 82 07 96 27 92 42 23
COOH" Capacity, Meq./G.
0 0 0 0
18 56 91 96
Resin weighed in free base form.
t h e equilibrations. T h e total number of milliequivalents of resin used in each experiment was determined b y passage of 500 ml. of 1N HC1 through the resin, followed b y a rinse with 200 ml. of methanol to remove occluded acid, elution of t h e chloride by passage of 1 liter of 0.5JI Ka7SOs, and subsequent coulometric determination of the chloride content with t h e Aminco-Cotlove chloride titrator. Next, buffer solutions of appropriate p H and ionic strength were passed downflow through the resin samples until equilibrium between the two phases was obtained. Although in most cases an equilibration time of only several hours is required, u p to 20 liters of solution were passed through each resin sample for a 2- to 4-day period. At the end of this time, the resins were washed with methanol and the anion (chloride or sulfate) content of each resin was determined following elution with 0.5-11 NaS03. The sulfate analyses were carried out by back-titration with E D T A after the addition of standard BaC12 (Schwarzenbach, 1955).
No measurable dissociation of HC1 from the resins equilibrated with the buffers occurred during the methanol rinse. However, some dissociation of HC1 from the hydrochloride forms of the AEVBX and EPI-TRIETU' resins was observed during the methanol wash in the determination of the total capacities of these samples. The average deviations in the determinations of the capacities of these resins were 0.9 and 1.3%, respectively, whereas the average deviation in the capacity of D l I A P h mas only 0.5y0.The contributions of such errors are small, however-for instance, a 2% capacity error would introduce the following errors in the pK determinations: DMXP.1, 0.02; DMVBX, 0.02; XEVB.4, 0.05; EPI-TRIEN, 0.08; IR-4B, 0.07; MVP, 0.02. Thus, analytical errors in the determinations of the various resin capacities are expected to introduce errors less than 0.01 pK unit in the cases of DMAPA and DSIVBX, 0.03 pK unit for XEVBX, and 0.03 to 0.05 pK unit for IR-4B and EPI-TRIEX. Most of the buffers were 0 . l N in total electrolyte and 0.02M in ammonia, triethanolamine, or pyridine. A sufficient quanwas added to produce the detity of standard HC1 or sired pH. Finally, to obtain the desired ionic strength, an appropriate amount of KC1 or K2S04was added. The concentrations of ammonia and triethanolamine in the 0.3 and 0.03 ionic strength solutions were 0.06 and 0.006, respectively. The pH's of the buffers were measured with a glass-calomel electrode system and a Beckman Model G pH meter. The meter was calibrated with National Bureau of Standards buffer solutions (0.05J1 potassium hydrogen phthalate, 0.025M KH2P04-0.025M Na2HP04, and 0.01X sodium tetraborate) using p H values tabulated by Bates (1964).
10
,
I
I
3
I 10
-10
Figure 1. Henderson-Hasselbach plots for weak b a s e anion exchange resins in 0.1M chloride solutions at 25°C.
Calculations
The dissociation of the protonated form of a weakly basic resin may be described by the equilibrium +
Ka
RNH
K
a -
+ H+
DRZBP.1
(RN:)(H+)
where the parentheses indicate activities. This expression may be written as
where a is the fraction of the resin in the dissociated form, if activity coefficients are neglected. T h e hydrogen ion concentration term refers to the resin phase which is, of course, inaccessible to measurement. It may be determined indirectly from the measurement of the p H of the aqueous phase by the use of the Donnan relationship, r =
PK 8 32 6 70 6 16 6 08 5 30 3 92
Resin
e RK:
(RXH)
[H+lr[Cl-I
Table II. Values of pK and n for Weakly Basic Anion Exchange Resins at 25" in 0.1M Chloride Media
[H'Is [Cl-ls
(2)
in which activity coefficients and the pressure-volume term have been neglected. Subscripts r and s refer to the resin and solution phases, respectively. Substitution of Equation 2 into 1 gives
or
DMVBA AEVBA EPI-TRIEN IR-4B h1VP a Slope of line tangent to plot at
CY
=
n
1 1 3 4 3 0
35 04 0 5 7. 89
0.5.
+
being equal to pK, log ([Cl-],/[CI-],). As a rough estimate, neglect,ing volume changes in the resin and activity coefficient terms, a tenfold reduction in the concentration of a 1 to 1 electrolyte, say from 0.1 to O . O l X , will reduce the value of pK by one unit, since the chloride concentration in the solution phase is reduced by a factor of 10 while that in the resin phase remains essentially constant. Data in the range CY = 0.25 to 0.75 will often conforni to the linear equation pH
=
pK
+ n log a / ( l - a )
(3
The slope n is a result of the electrostatic interactions of neighboring functional groups. I n the discussions that follow, p K is defined as equal to the p H of the aqueous solution in contact with the resin a t a = 0.5. Results
(3) This is similar to the commonly used Henderson-Hasselbach equation p H = pK
+ log a / ( l - a )
(4) except that it is obvious that the p K of Equation 4 is not a constant but rather is markedly dependent upon ionic strength,
Plots of pH vs. log [ a / ( l - a ) ] obtained for DAIAPA, DLIVBA, AEVBA, E P I - T R I E S , Amberlite IR-4B, and b l V P resins in 0.1-11 chloride media a t 25.0" are shown in Figure 1. The values of pK and n are summarized in Table 11. Several striking features are apparent. D1IhP.i has a pK much higher than that of any of the other resins studied. It is more than 1.5 units greater than that of D;\IVBX, although both materials are tertiary amines. Part of this effect is due to the fact that benzylamines are less basic than analInd. Eng. Chern. Fundorn., Vol.
9, No. 2, 1970 223
70 - 8
- G
- 4
0
- 2
2
.4
6
a
10
log 5 1-a
Figure 2.
Dependence of basicity of DMAPA resins on DVB content Measurements in 0.1M chloride solutions a t 25'C.
ogous alkylamines. The pK, of benzylamine (Bruehlman and Verhoek, 1948) in 0.5JI KN03 a t 25" is 9.62, whereas the pK,'s of methylamine (Bjerrum and Lamb, 1950) and ethylamine (Carlson et al., 1945) are 10.72 ( p = 0.5 a t 25") and 10.61 (in 0.5X KKO, a t 30"), respectively. In addition, the concentration of functional groups in DMAPA is less than that in DAIVBA. As shown later, the pK's of DilIAPX resins increase significantly as the degree of crosslinking decreases. The chemical nature of these resins is important also. For every 100 tertiary amine groups, there are nine carboxylate groups in the DRIAPA sample used, aiid 14 quaternary ammonium groups in DAIVBA. The former groups are introduced via hydrolytic cleavage of the amide groups; the latter, by crosslinking through amino groups during synthesis. These properties affect the resins in opposite directions. The presence of the quaternary ammonium groups in D;\lVB.k enhances the ease of renioval of a proton from neighboring -CH2-S +H(CH3)? groups
because of a mutual repulsion effect, thus lowering the basicity relative t,o that which the resin would possess if the quaternary groups were absent. Conversely, the -COO- groups in partially dissociated DRIAPX hinder the renioval of protons from neighboring -CH2-N+H(CH3)2 groups because of mutual at'tract,ion,thus raising the basicity. The pK of the MVP resin is low (3.90), as was expected. The pK, of 2-methylpyridine itself is 6.20 in water a t 25" (Bruehlman and Verhoek, 1948). The n values of all the resins containing monofunctional amines were close to unity, indicat'ing little interaction between neighboring functional groups. I n contrast, AEVBA, EPI-TRIEN, and Amberlite IR-4BI which contain diethylenet'riamine or triethylenetetramine, show high values of n, consistent with the fact that a wide range of functional group basicities is present. Variation of pK with Crosslinking. The effect of varying the crosslinker content upon the basicit,ies of D M A P A resins may be seen in Figure 2 aiid Table 111. As the moisture content of the resin increases, the basicity also increases. Part of this change is produced by the reduced interactions between neighboring chains as the resin swells. More significant, however, is the decreased chloride ion concentration as t,he moisture content of the resin increases. This results in a higher value of [Cl-],/[Cl-], in Equation 3, producing a higher value of p H a t any value of a . The dependency of pK upon the DVB content in D l I X P h resins is linear (Figure 3). This result' may be somewhat fortuitous, since the amount' of carboxylic capacity varies from resin to resin. The presence of occasional negatively charged groups along the polymer chains should exert a significaiit effect
Table 111. Values of pK and n for DMAPA Resins Crosslinked with Varying Amounts of DVB %J
0
2
4
6
8
10
Yo D V B Figure 3. resins 224
Plot of pK vs. DVB content for DMAPA-DVB
Ind. Eng. Chem. Fundam., Vol. 9, No. 2, 1970
DVB
2.0 4.0 6.0 8.0
PK
n
8.48 8.32 8.02 7.82
1.14 1.35 1.31 1.17
5 2
Table IV. Values of pK and n for 2-Methyl-5-vinylpyridine Resins Containing 5, 7, and 10% DVB at 25.0"C. and p 0.10
/
=
% DVB
PK
n
5.0 7.0 10.0
4.0 3.92 3.81
1.05 0.89 0.87
upon the dissociation of hydrogen ions as well as the salt uptake and swelling characteristics of the resins. Data illustrating the relationship b e h e e n pK and DVB content for 11VP resins are shown in Figure 4. 'The pK and n values arc sumniarized in Table IV. The pK values may be considered precise (within ~ t 0 . 0 2 unit), although the values of n are only approximate, since they were based on only two reliable experimental points. It is assumed that the plot is linear in the region of interest. At any rate, there is no doubt that the changes in pK produced by changes in the crosslinker level are less than those observed for the D1IXP:i resins. Variation of pK with I o n i c Strength. Henderson-Hasselbach plots (Figure 5 ) have been obtained a t ionic strengths of 0.03, 0.10, and 0.30 using the D M A P A - 4 ~ o DVB resin. A tenfold increase in ionic strength produces a 0.9 to 1.0 increase in the pH of the solution in contact wit,h t,he resin a t any given value of a . Equat'ion 3 predicts a 1.00 increase. T h e small deviation from this value is produced by differing degrees of electrolyte invasion, activity coefficient variat,ions in both the solution and resin phases, and changes in the electrostatic interactions within the resin phase as the external electrolyte concentration changes. Equation 3 is good as a rough approxiniation in order to extend Henderson-Hasselbach plots to higher and lower ionic strengths. The importance of an understanding of the ionic strength effect may be seen in the following example. Suppose that an
- 2
0
2
4
6
8
10
I 2
14
Figure 4. Henderson-Hasselbach plots for 2-methyl-5vinylpyridine-DVB resins in 0.1 M chloride solutions at 25°C.
anion has been adsorbed 011 the protonated form of the DMAP-k-4y0 DVB resin at 25OC. If a 1.OS p H 9.0 buffer is used as an eluent, the resin will behave much like a quaternary ammonium resin, since 60% of its functional groups will be protonated. If the adsorbed species has a high affinity for the resin, elution might be difficult. However, if a O.1N p H 9.0 buffer is used as an eluent, the resin, upon being equilibrated with an appropriate excess of the buffer solution, will be only 20% protonated. A further decrease in ionic strength to 0.01 produces a reduction in charged sites to only 9% of the total. pK's in Sulfate Solutions. Henderson-Hasselbach plots obtained from experiments carried out in O.1ON (0.050M) sulfate buffer solutions a t 25OC. gave the d a t a shown in
6.2
log
&
Figure 5. Henderson-Hasselbach plots for DMAPA-470 DVB anion exchange resin in various ionic strength chloride solutions at 25°C. Ind. Eng. Chem. Fundam., Vol. 9, No. 2, 1970
225
9
8
DMVBA
AEVBA
7
8
6
z7
5
6
I
n
5
d
- 1.0
-0.5
.o
0.4
0.0
log Ai-s
.2
.4
.6
.6
1.0
1.2
l o !g- aA
9.0
9
8.0
8
2
7.0
7
6
6.0
5
5.0 .I
.2
.3
log
.4
s
.5
.6
7
Figure 6. Henderson-Hasselbach plots obtained in 0.1M chloride solutions at various temperatures for DMAPA, DMVBA, AEVBA, and EPI-TRIEN resins
226 Ind.
Eng. Chem. Fundam., Vol. 9, No. 2, 1970
15
Table V. Values of pK and n in 0.10N Chloride and Sulfate Solutions at Ambient Temperature Resin
D&lAP.\-270 DVB D11LkpAk-470DVB DRIAPa-6% DVB DL1.kPAAx-8% DVB
(0.1 N
PK
PK
I
13 14
n
n
SO,*-) (0.1 N CI-) (0.1 N SO,'-) (0.1 N CI-)
8 62 8 42 8 20 7 93
8 8 8 7
48 32 02 82
1 10 1 15 1 14
-
1 1 1 1
12
14 35 31 17
t-
II
r AY
"I..
n
0
10
9
Table V for several D M A P A resins. Values of p K and n obtained in 0.10N chloride solutions are shown for comparison. T h e pK's measured in sulfate solutions are 0.15 f. 0.02 greater t h a n those measured in chloride media. K O a t t e m p t will be made t o explain these observations, even on a qualitative basis, since so many factors contribute t o changes in both directions. Effect of Temperature upon pK. T h e results of measurements of pK's of D M A P A , DRIVBA, AEVBA, a n d EPIT R I E X a t various temperatures are shown in Figure 6. F r o m these d a t a values of t h e enthalpy change, 4H0, for t h e dissociation reaction were calculated by t h e relationship
a t various cy values, using the three combinations of pairs of experimental data. The average values of AHC are plotted as a function of cy in Figure 7 . The enthalpy change is relatively insensitive to variations in the degree of neutralization of the monofunctional resins, DllIXPX and DLIVBh. The AHc values for D l I V B h are 2 to 3 kcal. per mole higher than those calculated for DA1AP.I. All values are in the range 10 to 14 kcal. per mole, which is in the range reported in the comprehensive review of Christensen et al. (1969) for ammonia and a variety of aliphatic amines in aqueous solutioni t . , ammonia = 12.4 to 12.5, ethglamine = 13.6 to 13.7, and ethylenediamine = 11.0 and 11.9 for dissociation of the first and second protons, respectively. I n contrast to the above-mentioned cases, the resins which contain polyfunctional amines show a marked increase in 4Ho-i,e., become more endothermic-as cy decreases. Thus, the dissociation of hydrogen ions from the least basic functional groups shows the least dependence on temperature. Similar trends of decreasing values of 4H0 with decreasing pK are seen by comparing values for pyridine (pK, = 5.45 at 25O, AHo 4.7 to 5.0 kcal. per mole) and aniline (pK, = 4.6 a t 25", AH0 = 7.2 kcal. per mole) with those for more basic ammonia and aliphatic amines. Discussion
Use of W e a k Base Resins in Deionization Systems. Weak base resins are of interest in a variety of applications involving acid adsorption because of their ease of regeneration with alkali solutions. A novel brackish water deionization scheme, t h e DESAL process, utilizes a bed of Amberlite IRA-68 (a DLIAPA resin) in the bicarbonate form t o convert NaCl t o XaHCOa, after which t h e sodium bicarbonate is adsorbed b y a bed of a weak acid ion exchange resin. T h e successful use of this procedure is dependent upon efficient binding of COz at moderate pressures b y the weak base resin component in order to form the bicarbonate salt. The extent of binding of COz by the DLIXP*k, XEVBA, DLfVBA, and E P I - T R I E N resins has been calculated as a function of the equilibrium CO;!concentration in aqueous solu-
8 7 6 5 .2
3
.4
6
.5
.7
,8
.9
a
Figure 7. Plots of AH" vs. degree of neutralization for weak base ion exchange resins in 0.1M chloride solutions 100 I
t
? I ?j0
0
-I
70 2
12 v)
a 60
z
0 0 I-
z W
50
V
a w a
40
01
1
I
1
I
I
02
03
04
05
06
MOLARITY
CO,
Figure 8. Extent of COz binding b y weak base ion exchange resins vs. COz concentration in aqueous solution
tion. I value of K = [H'] [HCOa-]/ [HzCOS] = 10-6.a5a t infinite dilution in water a t 25OC. was used in the calculations. T o determine the apparent pK's of the various resins at ionic strengths of 6 X to 1.6 x lop4,which is the range of interest for 0.01 to 0.06JI COZ solutions, it is necessary to construct Henderson-Hasselbach plots in this range. I t has been assumed that plots of p H us. log [oc/(l - a ) ] at' various ionic strengths are parallel to one another, that the plot' obtained a t an ionic strength of 0.01 is displaced downward by 0.90 unit relative to the p = 0.10 plot, and that each succeeding 10-fold decrease in ionic strength is accompanied b y a 1.00 p K unit decrease. Plots of the extent of CO, binding us. COz concentration (Figure 8) show that the efficiency of carbonation decreases in the order DLIAPh >> DMVBA > AEVBX > EPI-TRIEN. Consideration of the volume capacities of these resins shows that the practical volume capacities for COZ decrease in the order DMAPA (1.75 meq. per ml.) >> EPI-TRIEN (2.9 meq. per ml.) > AEVB.1 (2.05 meq. per ml.) > DMVBX (1.45 meq. per ml.). The values in parentheses are the volume capacities of the free base forms of the various resins. These Ind. Eng. Chem. Fundam., Vol. 9, No. 2, 1970
227
80
80
I
70
70
60
60
w 50
50
DMVBA
W
a Y
a
W -1
40
40
5 30
30
20
20
10
IO
k 2 W V
a
0
0
20
0
30
40
50
70
60
80
0
IO
20
40
30
80
M”
-.
70
70
5 % CITRIC A C I D
50
60
L//”-.
BA
5 % PHOSPHORIC ACID
60
60
50
50
40
40
w 30
30
20
20
10
IO
a Y a W k 2 W V
[L
a
0 0
IO
20
30
40
I/
/
0
50
60
0
B E D VOLUMES SOLUTION TREATED
Figure 9. Efficiency of removal of and MVP resins in column operations
HCI from 5%
Ind. Eng. Chem. Fundam., Vol. 9, No. 2, 1970
IO
15
20
25
SOLUTION TREATED
aqueous solutions of acetic, lactic, citric, and phosphoric acids b y DMVBA
Flow rate = 2 gal./ft.’/min.
228
5
BED VOLUMES
results are in agreement with the experinient'al data reported previously (Kunin and Vassiliou, 1964) for column operations in which it was shown that the volume capacities in the chloride-bicarbonate exchange step decreased in the order hniberlite I R I - 6 8 (DAL\I'-I type) > Xmberlite IR-45 (AEVBA type) > -\niberlite IRA-93 (DXVBAIt,ype). I t is apparent, on the basis of these calculations and the reported p K determinations, that there are no resin structures, other than analogs of DllAPh, which will approach lOOyo bicarbonate conversions :It C 0 2 pressures of less than 5 to 10 atm. The polyamine resins, such as AEVBX and EPIT R I E Y , simply have too great a percentage of relatively weakly basic functional groups, a property which manifests itself in a high value of the slope of a plot of p H us. log [ a / (1 - a ) ] . The higher capacities of these materials tend to make up some of the deficit, but, such resins are still inadequate. Benzylamine struchres, by their very nature, are less basic than aliphatic amines and are unable to approach the performanee of D X 4 P h . Weiss et al. (1966) have taken advantage of the large AH" values of weakly basic resins in their development of the Sirotherm process. Briefly, a mixed bed containing a polycarboxylic acid resin and a polyamine resin is used to desalinate water according to the reactions: RCOOH KaC1 e RCOO-, N a + HC1 RY: HC1 e R N + H , C1Following saturation of the resin bed with salt a t the ionic strength, pH, and temperature used, the above equilibria may be reversed to some degree by a n increase in temperature of the eluting solution, which may be the same untreated water used as the feed. The above authors obtained tit,ration data at 20" and 80°C. for a wide variety of weakly basic and weakly acidic resins. Resins which showed a large difference in pK (or in p H at various values of a ) were considered to be of interest for the Sirotherni system. Unfortunately, because of the deficiencies in the method used for the determination of pK values, particularly a t 80°, their ApK values were often not of the correct magnitude. For instance, in the following table the values of ApH a t CY = 0.5 for several commercially available resins, as determined b y Weiss et al., have been compared with our data obtained with the same resins.
+
Resin
+
ApHa at a = 0.5 (Weirs et of.)
+
ApHa a t a = 0.5 (Our Data)*
Aniberlite IRX-93 1. o 1.6 Amberlite IR-45 0.5 1.1 hmberlite IRA-68 -0.1 1.4 Duolite A-30T 0.8 0.8 a Difference in pH's calculated at 20" and 80°C. *Values of pH at a = 0.5 at 80°C. obtained by ext,rapolation of data obtained a t 25', 45', and 65'C.
.imberlite IRA-68, whose pK was found by Reiss et al. to be independent of temperature, actually has a AH" nearly equal to that of hmberlite IRA-93. Removal of Strong Acids from Weak Acids. B y t h e use of weak base resins 11hich have p K values approximating, or preferably lower than, those of weak acids, it is possible t o remove strong acids, such as hydrochloric, nitric, a n d sulfuric, from reasonably concentrated solutions of the weak acids. I n general, the separation will be more effective as t h e pK, of t h e weak acid increases a n d t h a t of t h e resin decreases. Columnar separations of HC1 from 5% solutions of acetic (pK, = 4.76), lactic (pKl = 3.86), citric (pK1 = 3.13, pKs = 4.76, pKs = 6.40), and phosphoric (pK1 = 2.15, pKs = 7.20, pK3 = 12.33) acids have been carried out. In each case approximately 30 nil. of resin was pre-equilibrated with a 5y0 solution of the appropriate weak acid, after which the resins were transferred to l.l-cm.-diameter columns. The influent solutions, which were 0.027X in HC1, were passed through the columns a t a 2.0 gal./ft.3/minute flow rate (approximately 16 bed volumes per hour) a t room temperature. Plots of HC1 leakage us. the amount of solution treated are shown in Figure 9. The efficiency of HC1 adsorption in the various solutions decreased in the order lactic > acetic 2 citric > phosphoric and, except in the treatment of the HC1-phosphoric acid solution, the performance of the poly(2-methyl-5-vinyl-pyridine) resin was superior to that of the more basic benzyldimethylamine resin. Literature Cited
Bates, R . G., "Determination of pH," Wiley, New York, 1964. Bjerrum, J., Lamm, C. G., Acta Chim. Scand. 4, 997 (1950). Bolto, B. A., McNeill, R., Macpherson, A. S., Siudak, R., Weiss, D. E., Willis, D., Australian J . Chem. 21,2703 (1968). Bruehlman, R. J., Verhoek, F. H., J . Amer. Chem. SOC.70, 1401 1948).
Carlson,' G. A., McReynolds, J. P., Verhoek, F. H., J . Amer. Chem. SOC.67, 1334 (1945).
Christensen, J. J., Izatt, R . M., Wrathall, D. P., Hansen, L. D., J . Chem. SOC. 1969 A, 1212.
Kunin, R., Myers, R . J., J . Amer. Chem. SOC.69,2874 (1947). Kunin, R., Vassiliou, B., Ind. Eng. Chem. Proc. Design Develop. 3, 404 (1964).
Sagasawa, SI., Ishigai, H., Kagawa, I., X e m . Fac. Eng., Sagoya Univ. 10, 105 (1958). Schwarzenbach, G., "Die Komplexometrische Titration," Ferdinand Enke Verlag, Stuttgart, 1955. Selegny, E., hlerle, Y., Bull. SOC.Chim. France 1963, 1138, 1964. 1306.
Selegny, E., Merle, Y., Compt. Rend. 256, 674 (1963). Topp, N. E., Pepper, K . W., J . Chem. SOC. 1949,3299. Weiss, D. E., Bolto, B. A., McNeill, R., hlacpherson, A. S., Siudak, R., Swinton, E. A., Willis, D., Amtralian J . Chem. 19, 561, 765, 791 (1966).
RECEIVED for review February 13, 1969 ACCEPTEDDecember 9, 1969
Ind. Eng. Chem. Fundam., Vol. 9, No. 2, 1970
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