Adsorption of Iodine and Bromine by Carbon Black J. W. WATSON AND D. PARKINSON Research Centre, Dunlop Rubber Co., Ltd., Birmingham, England
D
Experimental. The carbon blacks used [all commercial samples (6) except Graphon ( I C ) ] were first rendered free of tarry matter by extraction with pure benzene, then dried a t 120 C. for 12 hours, and finally stored over calcium chloride. Possible incomplete wettin by the water pump method ( 1 3 ) was discounted by removing a8 physically adsorbed gases from the black by applying a high vacuum and then adsorbing water onto the black to wet it completely. There was no detectable difference between the two techniques when applied to a furnace and channel black. Filtering the black from solution was only effective with certain blacks ( I S ) and could not be used as a general method of separation. A centrifuge technique wa9 developed which gave good results. Glass-stoppered tubes were used to prevent loss of iodine. The temperature was controlled within 1" C. by carrying out the adsorption in Dewar flasks. The black was wetted with 25 ml. of water, then 50 ml. of 0.015M potassium iodide solution, saturated with iodine a t 25' C, was added and the mixture shaken a t 25' C. The potassium iodide concentration was standardized a t 0.01M during adsorption because a t higher concentrations the equilibrium constant tends to fall as saturation is approached ( 1 2 ) . After adsorption and centrifuging, 50 ml. of the iodine solution was pipetted off for estimation; 50 ml. of 0.02N sodium thiosulfate solution was added to the black and the remaining iodine solution and shaken a t 25" C. for 15 minutes. Finally, the black was separated and the residual sodium thiosulfate solution estimated. Sodium thiosulfate solution was eminently suitable as a desorbing reagent because it was not adsorbed by the black, and its strength did not influence the amount of iodine desorbed.
ETERMINATION of the specific surface of carbon black has been made possible in recent years by the low temperature nitrogen method ( 7 )and t o a limited extent by the electron microscope. These methods have the disadvantage of being both complex and tedious in operation, and therefore the development of a simpler one was desirable. Consequently, the investigation into the adsorption of iodine by carbon blacks, described here, was undertaken in the hope that such a method might be evolved. ADSORPTION O F IODINE
Although the adsorption of iodine from aqueous solution had been used for comparing blacks for many years (6, 16, I7), no systematic work known to the authors had been attempted until 1946 when Kendall ( I S ) showed that the problem was more complex than previous workers had assumed. In a detailed investigation Benson and Sanlaville ( 1 ) concluded that the adsorption of iodine by blacks is a very slow process and that multimolecular layer adsorption takes place. These claims contradicted those of all previous workers. However, their failure to recognize the adsorption's dependence on the free iodine concentration rendered the shape of their isotherms ( 1 ) entirely artificial. Consequently their results, based on a B.E.T. (4)fitted to these isotherms, can be, a t the most, only approximate. SUMMARY OF KENDALL'S WORK
An aqueous solution of iodine in potassium iodide was known t o contain potassium iodide, potassium tri-iodide and iodine molecules because of the equilibrium KI
.
+
12
e KIs
Hence the identity of the adsorbed species needed consideration. Blank experiments showed that potassium iodide was not adsorbed. Adsorption isotherms were plotted for a number of blacks, and the isotherms were only independent of the amount of potassium iodide in solution when the adsorption was expressed as a function of the free iodine concentration. These facts proved that only neutral iodine molecules had been adsorbed. to the isoThe ~~~~~~i~ adsorption equation was therms for several blacks, and a satisfactory fit was obtained. This equation was used to estimate m1, the mass of iodine for a monolayer, for several blacks. The ratio mljarea (both nitrogen and electron microscope values) varied widely. The black was wetted by evacuation under water with a water pump. An aqueous solution of iodine was then added, and the mixture was shaken for 30 minutes a t room temperature. Finally, the black was separated from solution by filtration. SORPTION FROM AQUEOUS SOLUTION
The nature of the interaction between iodine and carbon black was obscure mainly because previous workers had made no attempt to desorb the iodine. Thus the extent Of any chemisorption was unknown. An endeavor was made to clarify the situation by developing techniques for the determination of both adsorption and desorption isotherms from solution.
Results and Discussions. The time required for equilibrium a t 25" C. was investigated by plotting isotherms for one black (Spheron C, a conductive channel black) after adsorption times of 15 minutes, 1 hour, 5 hours, and 25 hours, the desorption time being constant a t 15 minutes. I n the first 15 minutes both the reversible and total adsorption increased rapidly, becoming constant in the interval between 1 and 5 hours. A further small, but definite increase in total adsorption occurred after 25 hours (Figure I), suggesting that a slow reaction was taking place in addition to the main rapid reaction. This was consistent with the findings of Benson and Sanlaville ( I ) . No such increase in reversible adsorption took place which disproves the suggestion ( 1 ) that the Slow reaction was due to the formation of a second layer. The result a190 Proves the reversible adsorption to be complete in 1 hour. Analysis of the iodine solution, before and after adsorption, showed only a small decrease in the total Potassium iodide concentration ( K I 4- Kid, proving that the adsorbed species were nearly all free iodine molecules; consequently sorption is plotted as a function of the free iodine concentration. The Langmuir Plot5 (Figure 2) of the isotherms (reversible) (Figure 3) of a number of blacks showed that only furnace blacks gave the straight line demanded by the linear form of this equation. The equation can be applied to adsorption from solution by assuming that ( a ) the solute can be treated as a perfect gas and ( b ) no adsorption of the solvent takes place. The equation 8 = bp/l
f bp
where 8 is the fraction of surface covered, constant, becomes
m 1053
= mlKlc/l
+ Klc
(1)
the pressure, and b a (2)
1054
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 5
200.
5
160-
E
-TOTAL 0- n
120-
ADSORPTION AFTER Ihc U
II
0-REVERSIBLE
II
11
A
11
11
11
.-HYSTERESIS
II
II
-
v-
Ihr. 25hn
3
A-VULCAN
Ihr. 25hrr
II
I1
11
25hrr.
0-PHILBLACK 0 X
-GRAPHON
401
=I ===
0'
Figure 1.
0.1
02r 0 3 04 CONCENTRATION m.moli/litre
-
0
0 5
Iodine sorption isotherms us. adsorption time-Spheron C
e having been expressed as the ratio of
m, the mass adsorbed, to ml, the mass required to form a monolayer; p is replaced with c, the concentration, and the constant b is adjusted to K1. Brunauer has shown ( 3 ) that the B.E.T. equation reduces to a Langmuir-type equation when the number of adsorbed layers is equal to unity. The constant, b, in Equation 1 is then c / p ~ . By an analogous treatment the corresponding equation for adsorption from solution is (3)
PO,the saturation pressure of the adsorbate now becomes CO, the solubility of the adsorbate in the solvent, because this is the concentration a t which an infinite number of layers can be formed. K is given approximately by exp. A H / R T
(4)
where AH is the net heat of adsorption (3). Equation 3 shows that adsorption from solution depends on the solubility of the adsorbate in addition to the concentration. This point must be
Figure 2.
-
E
s&
160-
E
O-TOTAL
ADSORPTION: SPHERON C e-RNERSI0LE rn
-
E 1605 5E' I
z
@-TOTAL
ADSORPTION
25°C A-REVERSIBLE
II
11
0°C
0-
11
H
2 5°C
11
II
0°C
II
11
2Pc
lI
v-HYSTERESIS
ao TOTAL *DXIRPTION.VULCAN 3 REVERSBLE I O-TOTAL
V-RMRSl0LE B -TOTAL e-REMR510LE
(I
STERLING 95
I ID
AT 0°C
120-
%
40
0.0
Langmuir plots for va/rious blacks
0 s1
0.6
04
taken into consideration when isotherms are compared under conditions of varying solubility-Le., different solvent or temperature. For instance, comparison of the sorption isotherms, plotted against concentration, for Spheron C at 25" and 0" C. (Figure 4) indicates that the temperature reduction had markedly increased both adsorption and desorption, especially at low concentration. But, if the isotherms are compared a t the same relative concentration the large increase will be seen t o be only apparent (Figure 5 ) . The net heats of adsorption were calculated from Equation 4 for the HAF blacks, Philblack 0 and Vulcan 3, and for the graphitized channel black, Graphon. They were all about 2000 cal. per mole, showing the reversible adsorption for these blacks to be physical in nature. The deviation from the Langmuir equation shown by channel blacks (Figure 6) suggested that their surfaces might be heterogeneous. This conjecture was investigated by attempting to fit the equations of Freundlich (Figure 7) and Sips (16). A satisfactory fit could not be obtained with either equation. Applica-
200
200
0.2
RELATIVE CONCENTRATION
40
e-
1
n
ADSORPTION
TIME
EXTRACTION
II
Ihr.
15minr
SPHERON 6
II
0'
0. I
0 2
0.3
CONCENTRATION
Figure 4.
0 4
- rn.rno1sJlitre
0 5
Influence of temperature on iodine sorption-Spheron C
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1955
tion of the multilayer equations of B.E.T. ( 4 ) and Huttig (IO) revealed that while the former deviated widely in the direction of too little adsorption the latter deviated only slightly. Comparison of the Langmuir and Huttig plots (Figure 6) shows their deviations to be almost equal though in opposite directions; this suggests that the adsorption was limited to a finite number of layers. The possibility was investigated by attempting to fit the modified B.E.T. for n layers. Figure 7 shows that a very satisfactory fit was obtained when the number of layers was equal to two. I n spite of this remarkable agreement the concordance is considered to be coincidental because ( a ) the theoretical minimum value of m1 (317 mg. per gram corresponding to a molecular area of 30.1 square A.) is much greater than the 160 mg. per gram given by the B.E.T. plot, and ( b ) it is possible to devise a dual surface Langmuir equation that gives just as satisfactory a fit. The notion that the isotherm might be interpreted by a dual surface Langmuir equation arose from the consideration that the two layers, suggested by the modified B.E.T. plot, might be explained by the existence of two different types of adsorption sites on the surface. Walker and Zettlemoyer (19) used a similar idea to interpret deviations from the B.E.T. equation. If these sites were of equal area and the heat of adsorption of iodine by one type was much greater than that of the other, then Sadsorption on the more active half of the surface would appear as the formation of the first layer and adsorption on the remainder as the second layer. The Langmuir equation for adsorption on a dual surface may be written
1055
Equation 5 furnishes the value 320 mg. per gram for the mass for a monolayer, and this value lies within the theoretical limits 317 and 345 mg. per gram corresponding to iodine molecular areas 30.1 and 27.65 square A., respectively, showing Equation 5 to be more satisfactory than the B.E.T. for two layers. Calculation of the net heat of adsorption on the more active part of the surface from K = exp. AHIRT gives about 2000 cal. per mole suggesting that the adsorption was physical. This was confirmed by calculating the total heat of adsorption from the adsorption data a t two temperatures (Figure 4)-5500, and 4150 cal. per mole when the adsorptions were 140 and 180 mg. per gram, respectively. The volatile contents for the channel blacks (Spheron C, 3, 6, and 9 ) are all about 5% by weight, and their specific surfaces (6) fall progressively from 227 square meters per gram for Spheron C to 100 square meters per gram for Spheron 9. Assuming that the volatile content is responsible for the less active part of the surface the proportion of active sites must fall in this order. Thus
I
- 0.008
/ .
0,004
I
I L4L-J
0,006
0.003
0004
where the subscripts refer to the two types of sites. We will choose site 1 as the more active, and therefore m: and K 1 must correspond to m1 and K from the B.E.T. plot. The two parts of the surface are considered to be of equal extent so mi = mi. The remaining parameter, Kz, was determined by substituting in Equation 5 the known value of mi, mi,and K I with the value of m a t an arbitrarily chosen high relative concentration. Comparison of the plot of Equation 5 with the desorption isotherm showed that it gave just as satisfactory a fit as the B.E.T. equation for two layers.
PLOT
0-LANGMUIR
0.00I
0,002
0.2
0
0 6
0 4
CONCENTRATtON
Figure 6.
1.0
08
- m.molr/lltre
Langmuir and Hiittig plots-Spheron
24C
C
/
2001
J
20c
i
T T
16c
E I
z 0 !-
Y VI m W
>
W
80
I2C
A
a
-
Q-
E
9 ADSORPTION
TIME
A
8C
-
BE.%
ei
x -FREUNLICH
II
-
I hr.
PLOT
11
0 LANGMUIR
EXTRACTION TIME 15minr.
II
-REVERSIBLE
40-
n52 n=3
ADSORPTION
4c
LL
C
0 2
0 4
0 6
Q8
I *Q
RELATIVE CONCENTRATION
Figure
7.
Comparison of adsorption Spheron C
equations-
INDUSTRIAL AND ENGINEERING CHEMISTRY
1056
Vol. 42, No. 5
L
-
t-
P
a
SPHERON
A- SPHERON V
0
- SPHERON
-
SPHEAON PHILBLACK 0
@-GRAPHON
,
.
#
2
I
0’
4
3
EXTRACTION TIME- hours Figdre 9.
0
Iodine hysteresis adsorption us. extraction time-Spheron C
I hc
-ADSORPTION TIME
Shrr.
$1
IS mlnr. EXTRACTION AT
~SOC
l
x - ADSORPTION
AT 25°C
15 mins. EXTRACTION AT 25°C n
0.05
I
I
010
0.1 5
I
CONCENTRATION-mmols/litre. Figure 10.
Iodine hysteresis adsorption at low concentration-Spheron C
the progressive decrease in adsorption per unit area (Figure 8) of these blacks is in agreement with the dual surface theory. The logical extension of the theory to furnace blacks shows their Langmuir-type isotherms correspond to the case when m12 becomes very small (volatile content
