COVALEST ADSORPTION ON BASE-EXCHANGE RESINS
697
COVALENT ADSORPTION OY BASE-EXCHANGE RESINS. I1 ADSORPTIOS
FROM
BUFFERSOLUTIONS
OF
STRAIGHT-CHAIN ACIDS
JOHN A . BISHOP Department of C h e m i s l r ~A'ewark , College o,f Engineering, .Yewark, S e w Jersey Received August 2 4 , 1949
I n the first paper of this series (l),it was suggested that the ion product should be substituted for the total concentration in plotting the Freundlich isotherm when acid adsorption was being studied. The adsorption equation becomes log X
=
fi log u H + ' a A -
+ const,ant
(1)
where X is the adsorption in millimoles of acid per gram of resin adsorbent. If this equation holds, adsorption from buffer solutions should be greater than from nonbuffered solutions of the same acids, and should increase with increasing salt concentration vhen the pH is held constant. I n addition, the points for a given acid should all fall on the same line regardless of pH or salt concentration. To check this equation and to find out whether all acids 7%-ouldfall on the same line regardless of degree of ionization, it was decided to run adsorption isotherms of tLyo acids which were far enough apart in pK that they would not fall on the same line fortuitously. Acetic acid and chloroacetic acid were chosen. EXPERIJIESTAL
Materials The ion exchanger used in these experiments was Amberlite IR-400 A.G., which is produced by Rohm and Haas Company, and has been sized for use in analytical determinations. The resin iyas washed by extraction as explained previously (l), since it was found that a pronounced color developed which increased with increased acid and/or increased salt concentration. Jenny (4) has remarked on this, but Kunin and Meyers ( 5 ) made no mention of it. The acids used were of C.P. grade and gave no test for chloride. Method of analysis Solutions were made up from a concentrated solution of the acid, and were buffered either by partial neutralization with sodium hydroxide, or by adding a solution of the sodium salt of the acid. The initial solutions \rere titrated for total acidity, and the pH determined using a line-operated pH meter. The apparent concentration of the anion was calculated from these measurements, since log ah-
pH - pK
+ log
uH.~
('4
where pK v-as the thermodynamic pK. The concentration of unionized acid was used in place of the activity. This was not used with the idea of obtaining the individual ion activity, but simply to correct for the slight variations of anion between individual solutions.
6 98
JOHN A . BISHOP
After 100.0 ml. of the solution had been allowed to come to equilibrium with 1.000 g. of the washed and dried resin, samples were taken and analyzed for pH and for total acidity. In making the plots of log X us. log uH+'uA-, the final pH was combined with the value of log u,- from equation 2. In the case of chloroacetic acid a correction was made for the concentration of anion due to the dissociation of the acid. DISCUSSION OF RESULTS
In figures 1 and 2 adsorption is plotted against pH for varying buffer ratios. From these graphs it can be seen that addition of buffering salt does increase the adsorption for any pH. Kunin and Meyers ( 5 ) have shown that the same thing is true for the adsorption of hydrochloric acid from potassium chloride solution,
I Od 3.0
4 0
5.0
60
PH
LO
40
6.0
P+r
FIG.1
FIG.2 FIG.1. The effect of buffer salt concentration on adsorption of acetic acid on Amberlite IR-100. Curve I, no buffer; curve 11, acetic acid sodium acetate (0.02 M ) ; curve 111, acetic acid sodium acetate (0.04 M ) ; curve I V , acetic acid sodium acetate (0.227 '11); curve V, acetic acid sodium acetate (0.453 M ) . FIQ.2. The effect of buffer salt concentration on adsorption of chloroacetic acid 011 Amberlite IR-400. Curve I, no buffer; curve 11, chloroacetic acid sodium chloroacetate (0.10 M ) ; curve 111, chloroacetic acid sodium chloroacetate (0.50 31).
+
+
+
+
+
+
also using a base-exchange resin. Previously it was found that salts having an anion ion in common with the acid also increase adsorption of acids on both activated charcoal and activated alumina (2, 6). Figures 3 and 4 show the result of using equation 1 as the basis for plotting adsorption from buffers. In figure 5 a graph has been made using the results of Iiunin and Meyers ( 5 ) . Points were taken from their graph a t rounded pH values, using dividers, since they gave no data. The chloride concentration \vas taken as that of the salt, except for the pure acid. In that case, as with acetic and chloroacetic acids, the pH was doubled to get -log U ~ + ' U * - . These graphs show clearly that equation 1 can be used for solutions of acids containing a salt with an anion in common with the acid. Figure 6 shows the data of Graham and Horning for the adsorption of oxalic acid on hydrous alumina from various concentrations of potassium oxalate. They started with a neutral solution of
COV.4LENT ADSORPTION O N BASE-EXCK4NGE RESINS
699
potassium oxalate (pH = 7) and observed the change in pH (2). From this change the author of the present paper has calculated the amount of acid adsorbed; the resulting plot is shown in figure 6. This would indicate that there is a possibility that equation 1 may be applied to general acid adsorption.
0.a.
as.
X U
0
a 04.
a2
,
7.0 - L O U a ,, ' aA2.0 40 6o - L O G a", naFIG. 3 FIG.4 FIG.3. ddsorption of acetic acid from buffer solutions on Amberlite IR-400. 0 , no buffer; b , acetic acid sodium acetate (0.02 M ) ; A , acetic acid sodium acetate (0.04 M ) ; X , acetic acid sodium acetate (0.227 M ) ; acetic acid sodium acetate (0.453 M ) . FIG. 4 . Adsorption of chloroacetic acid from buffer solutions on Amberlite IR-400.0 , chloroacetic acid, no salt; 9 ,chloroacetic acid 0.10 M sodium chloroacetate; 6 , chloroacetic acid 0.50 M sodium chloroacetate.
5.0
60
+
+
+
9,
+
+
+
a7
P5L
XM. v
0
0 J
e 0;s.
Irn-
PI.
,
/
FIG 5 FIG.6 FIG 5. Adsorption of hydrochloric acid from potassium chloride solutions on Amberlite IR-4OO. 0, no salt; X , hydrochloric acid potassium chloride (0.01 M ) ; 0 , hydrochloric acid potassium chloride (1.0 M). FIG 6 . Adsorption of oxalic acid from potassium oxalate on alumina
+
+
In the previous paper (1) the author suggested that the acids were held to the exchangers by covalent adsorption of the hydrogen to the nitrogen of the adsorber, with the anion being held by electrostatic attraction. Kunin and Meyers ( 5 ) objected to this, interpreting it as a "mechanism" of adsorption. They proposed that the adsorption of the anion was simply a matter of displacement of
fOO
JOHK A . BISHOP
the hydroxide ion of the resin by the anion in question, with subsequent reaction of the hydroxide ion with the hydronium ion of the acid solution. They quoted Jenny (4), who has called attention to the large amount of water adsorbed by these resins when used with water solutions. This has been pointed out by others (1, 7). Since mechanism can not be determined from equilibrium measurements, it seems to be a matter of opinion as to whether displacement or neutralization takes place first. The brief discussion by Heyman and O’Donnell (3) seems to cover this point adequately.
FIG.7. Adsorption of acetic acid from buffer solutions on Amberlite IR-400.0 ,no buffer; sodium acetate (0.02 M ) ;A , acetic acid sodium acetate (0.04 M ) ; X , acetic acid sodium acetate (0.227 .%I);?, acetic acid sodium acetate (0.453 M ) .
b, acetic acid
+
+
+
+
The ideas of Kunin and Meyers do imply, however, that resin adsorption may be regarded as a type of Donnan equilibrium. This would result in an equation of the following type:
where X = adsorption of acid in millimoles per gram of adsorbent ( u o H - ) ~= activity of hydroxide ions on the resin ( U O H - ) ~ = activity of hydroxide ions in solution uA- = activity of the anion of the acid adsorbed. If both numerator and denominator are multiplied by uH+, the right-hand member of the equation is changed into the ion product divided by K,, resulting in equation 4:
If X is plotted against the ion product, a curve should result which should not be a straight line, and the slope of which should be equal to (aoH-)/K,. This has
701
COVALENT ADSORPTION OK BASE-EXCHANGE RESINS
been done for the solutions which mere plotted above, the results being shown in figures 7,8, and 9. It is again noticeable that all solutions for one acid fall on the same curve. Values of the slope were determined and (aOH-), calculated (table 1).
n
50
150
100
a,. a,*d
so
25
7 5 uHta,.,io,
FIG. 8 FIG.9 FIG.8. Adsorption of chloroacetic acid from buffer solutions on Amberlite IR-400. 0 , chloroacetic acid, n o salt; 9, chloroacetic acid 0.10 M sodium chloroacetate; 6, chloro0.50 Y sodium chloroacetate. acetic acid FIG.9. Adsorption of hydrochloric acid from potassium chloride solutions on Amberlite potassium chloride (0.01 X ) ; 6, hydrochloric IR-400. 0 , no salt; X, hydrochloric acid acid potassiumchloride ( 1 , O M ) .
+
+
+
+
TABLE 1 Values of (aOH-)r CHICOOH
I
CHiClCOOH
i
HCI
I n the previous paper (1) an equation was obtained hy considering the adsorption as one of salt formation:
x
~.
7'- X
_. c,+.c,___
KO+
where T - X represents the total number of millimoles of possible adsorption minus the amount adsorbed (per gram of resin). At first glance K,+, which is really the classical hydrolysis constant of the salt formed, would appear to be equal to K,, from the resemblance of equations 4 and 6 . Actually this would be true only if T - X were composed entirely of ionized hydroxide ions, held to the resin by electrostatic forces. That this is not so can be seen by comparing the
702
JOHN A. BISHOP
values of the total possible adsorption on resins with the values of the hydroxideion “activity” calculated from the above equation and tabulated in table 1. It would seem therefore that there may be adsorption due to direct reaction aa well as adsorption due to reaction with displaced hydroxide ions. If the logarithm of both sides of equation 4 is taken, an equation is obtained which resembles equation 1: log
x = log
aHA’aA-
+ log
(aOH-)?/Kw
(6)
Since the hydroxide-ion activity varies as the value of X changes, this may explain why there is always a constant in the logarithmic equations for adsorption. The intercept changes continuously; consequently the slope will be expected to change. Since the value of the last term in equation 6 will vary only gradually as X changes, it may also be expected that there will be a close approximation to a straight line if the range of concentrations is not too great. SUMMARY
The equation proposed previously in which might adsorbed is plotted against ion product, using a log-log plot, has been tested for buffer solutions. It has been found that results predicted from this equation are obtained experimentally, and that all points for a single acid fall on the same curve, both those for the acid solutions and those for the buffers. Further support for the method of plotting has been found using the data of other observers. A reason why the slope is not unity has been suggested, based on the similarity of the equation to one which may be obtained by considering the adsorption as a type of Donnan equilibrium. REFERENCES
BISHOP: J. Phys. Chem. 60,6 (1946). GRAHAM AND H O R N I N G : J. Am. Chem. SOC.69, 1214 (1947). HEYMAN .4ND O ’ D O N N E L L : J . Colloid sci. 3, 479 (1948). JENNY:J. Colloid Sci. 1, 33 (1946). KUNINAND h f E Y E R S : J. Am. Chem. soc. 69, 2874 (1947). SVEDBERG: Colloid Chemistry, p . 243. The Chemical Catalog Company, Inc., Kew York (1928). (7) WALTOS:J. Phys. Chem. 47, 371 (1943).
(l$ (2) (3) (4) (5) (6)
