THE ADSORPTION OF ELECTROLYTES BY ASH-FREE CHARCOAL VII. A Brief Survey of the Present Status of the Subject, and Evidence that Negative Adsorption of Inorganic Bases decreases with Length of Time of Contact of Charcoal with Solution* BY ELROY J. MILLER
At the Fifth Colloid Symposium1 a summary was presented of the studies that had been carried out on adsorption from solutions of acids, bases and salts by adsorbent charcoal. It was pointed out that the earlier results obtained by Freundlich, Michaelis, Oden, and others in their adsorption studies with blood charcoal were anomalous and contradictory. I n the light of the results obtained with activated sugar charcoal and other ash-free charcoals, these discordant results could be satisfactorily explained on the basis of impurities in the charcoal such as ash, incompletely decomposed organic matter, and unsuspected adsorbed acids. The results with the ash-free sugar charcoal, and with the purified charcoals of animal and vegetable origin, were surprisingly consistent and reproducible, and many new facts were established which since that time have been verified in a number of laboratories. Adsorption from solutions of electrolytes seemed to be on a firm basis and the groundwork was indeed laid for many future investigations, as is evidenced by the scores of papers that have appeared on the subject since that time. It seems desirable again to present a brief survey of the present status of the subject. It will be recalled that ash-free adsorbent charcoal that has been heated a t temperatures around 1000' has the property of adsorbing acids, but it does not adsorb the inorganic bases.2 From many salt solutions the charcoal adsorbs acid hydrolytically with the liberation of alkali. Ash-free charcoal adsorbs large amounts of acids such as benzoic and salicylic. Dicarboxylic acids such as succinic, malic and tartaric are moderately adsorbed. Adsorption of aliphatic acids increases with increasing length of carbon chain. Inorganic acids, such as HC1, HzS04 and HNO,, are relatively slightly adsorbed. Inorganic bases are not adsorbed a t all. The introduction of polar groups such as hydroxyl and amino groups into organic acids decreases adsorption. I n the aliphatic acid series the shorter the carbon chain the greater is the effect of the introduction of the polar groups. Thus the introduction of the amino group into acetic acid cuts down the adsorption completely. Furthermore, the position of the polar groups in the molecule influences the adsorbability of theacid. 0-, m- and p-hydroxy benzoic acids and 0-, m- and p-amino benzoic acids are adsorbed in different amounts depending upon the position of their polar groups; the ortho acid is adsorbed most, the para next and the meta least. Isomeric forms of the fatty acids, butyric, valeric and caproic are adsorbed less
* Contribution from the Chemical Laboratory of the Michigan Agricultural Experiment Station. Published with the permission of the Director of the Experiment Station a6 Journal Article No. IZZ (n.8.)
2968
E L R O P ,I. MILLER
t'han the normal forms. I t should be particularly noted that while ",OH is not adsorbed a t all, tetramethylammonium hydroxide is slightly adsorbed and tetraethylammonium hydroxide is quite appreciably adsorbed. I t is interesting and significant that these organic bases which are practically as strong bases as are NaOH and KOH are positively adsorbed, whereas the latter arc not adsorbed at all or are actually negatively adsorbed. Complete Proof of Hydrolytic Adsorption Methods were developed for Lhe quantitative estimation of adsorbed acids on ash-free ~ h a r c o a l . ~Known amounts of acids were adsorbed on charcoal and the amounts of acid recovered were quantitatively equal to the amounts adsorbed for various types such as aromatic, dicarboxylic, aliphatic and inorganic acids. With the development of these methods for the quantitative estimation of adsorbed acids on charcoal it became possible to furnish complete proof of hydrolytic adsorption. The amount of alkali set free by the charcoal was determined and the charcoals were then examined for the presence of adsorbed acids. The results with salts of a number of organic acids showed that the amount of adsorbed acid recovered from the charcoal was exactly equivalent t o the alkali set free and left in solution. Similar results were obtained with salts of inorganic acids, but in addition, the chloride and sulphate were determined and also found to be equivalent in amount to the acid adsorbed and base set free. Complete proof of hydrolytic adsorption from salt solutions was thus established. The charcoal contained no alkaline impurities, and the acid recovered from the charcoal was the same as that from which the original salt had been derived. Hydrolytic and Molecular Adsorption from Salt Solutions From an invest'igation of adsorption from a series of potassium salts4it was found that the hydrolytic adsorption from solutions of inorganic salts was exclusively hydrolytic, while from salts of organic acids, adsorption was both hydrolytic and molecular. The concentration of potassium after adsorption had decreased in solutions of salts such as pot)aesium benzoate and salicylate, indicating that besides hydrolytic adsorption of acid from the salt, there was adsorption of the salt as such. The concentrations of potassium in solutions of potassium sulphate, nitrate, and chloride were actually slightly higher after adsorption, indicating that only acid (and some water) was adsorbed from t,hese solutions. It was evident from other facts that the increase in concentration was undoubtedly due to adsorption of water along wit,h the acid hydrolytically adsorbed. I t was observed that' while the adsorption of the inorganic salts was exclusively hydrolytic the absolute amount of potassium hydroxide set free was approximately only one-fourth that set, free from sodium benzoate. Negative Adsorption of Potassium Hydroxide By the use of larger quantities of charcoal ( I O grams or more) it was definitely established4that negative adsorption of potassium hydroxide occurs.
ADSORPTION OF ELECTROLYTES BY ASH-FREE CHARCOAL
2969
The concentration of potassium after adsorption was higher than in the original solution, but the absolute amount of potassium was the same before and after adsorption. These results on negative adsorption were all obtained under uniform conditions. The charcoal was always freshly ignited just before use and allowed to remain in contact with the solution for a definite length of time. It was found, however, that when these conditions were varied the result,ant effects were not always in agreement. Decrease of Negative Adsorption with Time It was surprising to find (Table I) that the length of time the charcoal and solution remained in contact determined very largely the magnitude of the negative adsorption. And still more surprising was the fact that the magnitude of the negative adsorption, contrary to positive adsorption, decreased
TABLE I Decrease of Negative Adsorption of NaOH with Time 14 8 . Ash-Free Blood Charcoal (Old Type, 1925) added to IOO cc. 0 . 0 2 N NaOH Duration of Contract
5 minutes 20
’,
’, 35 3 hours 48 ”
Cc. 0.02 NaOH Adsorbed
.............. .............................
-1.4
.................... .............................
+I
.a
Ash-Free Blood Charcoal (Old Type, 1929) added to 100 cc. 0.02 N NaOH 30 minutes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -3.00 30 ” ............. 2 5 hours ............................. -0.60 70 ” g g.
9 g. Ash-Free Blood Charcoal (Old Type, 1930) 20
20
added to IOO cc. 0.02 N NaOH minutes . . . . . . . . . . . . . . . -2.30(2) ............................. ,, - 2 . 3 0 (4)
7s ’, 43 hours 53
,’
.....
.........
............................. .....
-2.90 (I) -0.10 ( 5 )
. . . . . . . . . - 0 . 3 0 (3)
with time, and fairly rapidly. Thus in one experiment, a t the end of thirty minutes 3 cc. of water had been adsorbed while at the end of twenty-six hours the value had decreased to 0.6 cc. I n another experiment, a t the end of fifteen minutes 2.9 cc. of water had been adsorbed and a t the end of twenty-six hours the value had decreased to 0.1 cc. The results were somewhat erratic, however, and it was evident that not all the controlling factors were known. From
ELROY J. MILLER
2970
a considerable number of experiments (Fig, I), however, it was quite evident that negative adsorption does decrease with time. When the adsorption effects were measured within thirty minutes after addition of the charcoal to solution, a considerably higher value for the water adsorption was obtained than when charcoal and solution remained in contact for a period of twentyfour to seventy hours. I n a few instances positive adsorption appeared to be indicated, but it was later found that these were undoubtedly due to the fact that the temperature of ignition was too low, and substances of an acid nature had been formed on the charcoal.
Hours FIG. I
Decrease of Negative Adsorption with Time of Contact of Charcoal with Solution
Effect of Temperature of Ignition of Charcoal on Adsorption from Solutions of Sodium Hydroxide An investigation of the temperature factor (Table 11) definitely indicated that the temperature to which the charcoal is heated prior to the adsorption does influence the results. For example, the charcoal sample No. 4 when heated in the neighborhood of 1200' at the end of thirty minutes gave a negative adsorption of 1.95 cc. and a t the end of two hours 0.96 cc., but when heated at 500' a marked positive adsorption of sodium hydroxide was observed. After washing, drying and re-igniting at I zooo negative adsorption was again obtained. When again heated a t 500' negative adsorption was still observed. The fact that the second heating at j o o o gave a negative adsorp-
ADSORPTION O F ELECTROLYTES BY ASH-FREE CHARCOAL
2971
TABLE I1 Effect of Temperature of Ignition of Charcoal on Adsorption of NaOH Blood Charcoal (New Type) added to IOO cc. 0.02 N NaOH
9 g. Ash-Free
Charcoal Sample
No.
I
1J 11
No. 2 Jl
No. 3 11
No. 4 Jl Jl 11
JJ
Temperature of Ignition 900' 900' 900' 900' 900' 900' 900' 1075'-1200' 1075'-I 200' 5000
1200' 5000
1200' 1200'
1200'
Duration of Contact 30 minutes
30 30
lJ
"
hours 30 minutes 7 2 hours 30 minutes 2 hours 3b minutes 24
20
20
j 1
J
J
Cc. 0.02 N NaOH Adsorbed $1.76 $0.20
-0.40 $7.96 $ 0 . 56 +1.86 $0.80 -0 96 -1.96 +8 00 -3. I O
I5 47 hours
-2.60
minutes 48 hours
-2.50
J
20
f
-1.90 -0.80
tion while the first heating gave a positive adsorption could not easily be explained at first, but Kruyt and de Kadt5 published a note somewhat later which afforded a simple explanation for the apparent discrepancy. They found that charcoal which had been heated a t goo' and which adsorbed acids but not inorganic bases was reheated a t 400' in oxygen, it acquired the property of adsorbing bases. This property could be destroyed by again heating at the higher temperature. Kruyt and de Kadt postulated the formation of substances of an acid nature such as intermediate products of mellitic acid on the surface of the charcoal by the heating in oxygen a t lower temperatures. At the higher temperature the acid substances were destroyed. These acids could easily account for the apparent adsorption of bases as had been repeatedly shown' for adsorbed acids on ash-free charcoal. For example, an ash-free charcoal heated a t 1075' adsorbed a large amount of benzoic acid but no sodium hydroxide. When complex water-insoluble methyl red acid was adsorbed on the charcoal it was able to take up a considerable amount of alkali (molecular adsorption of salt of strongly adsorbed organic acid), and when the charcoal was reheated a t 1075' to destroy the methyl red acid, the charcoal no longer took up any alkali, but still adsorbed benzoic acid. Kruyt's charcoal and charcoal carrying adsorbed organic acids then have properties in common. The apparent discrepancies in the behavior of the charcoal after various heat treatments were eliminated when it was found necessary to heat the charcoal above 900' for a considerable time in order to completely decompose the acid formed on the charcoal. Once the charcoal has been freed from the carbon acids, however, heating a t 500' in the absence of air does not result in
ELROY J. MILLER
2972
the formation of the acid, and negative adsorption of alkalies which decreases with time is still observed. For rapid formation of the complex acid it is necessary to heat the charcoal at 3 o 0 - 4 0 0 ~in the presence of air and water vapor, although the action takes place slowly even a t room temperature. Hecently, Kolthoff6 has attempted to obtain more information on the nature of the acid formed on the surface of Kruyt's charcoal and found evidence of the formation of a complex colloidal acid. If taking up of alkali by Kruyt's charcoal is the same as adsorption of acid, then the 400' charcoal should adsorb KOH hydrolytically from KCl with the liberation of HCl, but Kolthoff could not detect any liberation of HC1 and indeed the chloride content of the solution after adsorption had decreased slightly. The solution had become slightly acid, however, and Kolthoff found some evidence of the presence of a complex colloidal acid.
,ci"
- c=o )o
Oxyd A
Oxyd B
B und C sir innrres Sal&
Oxyd 0
Oxyde
einheitliob
FIG.z Schilom's Oxides of Carbon
Schilow's Oxide Theory About the time that Kruyt and de Kadt made their discovery, Schilow and c o - ~ o r k e r also s ~ observed that charcoals heated a t lower temperatures took up bases. From this and other facts they postulated a theory that adsorption of acids, bases and salts by charcoal takes place through interaction with acidic and basic oxides of carbon on its surface. Schilow8 pictures these oxides (Fig. 2) as A which with water gives a slightly basic carbon hydroxide, oxide B which forms a strongly basic hydroxide and oxide C which is acidic in nature. An inner salt of oxide B and C is also pictured. Oxide A is stable at all temperatures and in presence of oxygen up to z rqm. pressure. Oxide B is formed from A at z mm. oxygen pressure and is stable up to very high pressures. Oxide Cis formed from oxide B by heating in oxygen at 300-700'. Schilow's explanation of the mechanism of hydrolytic adsorption is as follows :
:
/OH :C \OH
+zKCl-+:
/cl : C +zKOH \Cl
The chloride ion of the KC1 replaces the OH ions and KOH remains in solution. Hydrochloric acid would of course react with oxide B to leave water in place of KOH, while potassium hydroxide would not react at all with it.
ADSORPTION OF ELECTROLYTES BY ASH-FREE CHARCOAL
/OH
:C
+zHCl+:
\OH
:C
2973
+zHzO
\Cl
SchilowB found also that the oxygen pressure on the charcoal influenced markedly the adsorption of hydrochloric acid. I n Fig. 3 are his curves for the adsorption of hydrochloric acid in a number of concentrations as affected by the oxygen pressures. The action is stepwise and independent of the adsorption isotherm for oxygen. At pressures up to z mm. the curves are horizontal,
then there is a sharp break followed by another horizontal section. The lower section represents the adsorption by oxide A and the steep part marks the formation of oxide B, and the second flat part the adsorption by oxide B.
Frumkin's Gas Electrode Theory
I
Frumkin and co-workers,*Ohowever, obtained entirely different results for the relation between oxygen adsorbed and hydrochloric acid adsorbed. They found that within certain limits the milliequivalents of hydrochloric acid adsorbed were equal to the milliequivalents of oxygen adsorbed. Their curve for oxygen adsorption (Fig. 4) against pressure was smooth, and the adsorption of HC1 was equivalent to the oxygen taken up (Table 111). They have an entirely different explanation for the adsorption of acids, bases and salts." Their theory is essentially that activated charcoal in the presence of hydrogen or oxygen functions as a gas electrode. In the presence of oxygen, hydroxyl ions appear in solution and the positive charge on the charcoal then attracts anions and adsorbs acids but not alkali. Neutral salts are hydrolytically ad-
ELROY J. MILLER
2974
TABLE 111 Milliequivalents HC1 Adsorbed per g Charcoal
Milliequivalents of Ox gen per g. 8harcoal 0.012
’
0.012
0,033
0.035
0.044
0.043 o.og1
0.052
FIG.4 Frumkin’s Curve for Adsorption of Oxygen at Different Pressures
sorbed with the liberation of alkali. In an atmosphere of hydrogen, hydrogen ions appear in solution, leaving the charcoal negatively charged and in a position to attract cations. Alkali is adsorbed but not acid. Neutral salts are hydrolytically adsorbed with the liberation of acid. Frumkin’s early experiments11with charcoal alone did not entirely support his theory, but whm platinum was incorporated with the charcoal the adsorption of HC1 and NaOH in atmospheres of hydrogen and air was in keeping with the theory (Table IV). HC1 was adsorbed in the presence of air but not in hydrogen, while NaOH was not adsorbed in air but was adsorbed in hydrogen. Through improvement in technique, however, FrumkinI2 was able to show (Table V) that charcoal even in the absence of platinum behaved in keeping with the theory, and that from potassium chloride in an atmosphere of hydrogen KOH was hydrolytically adsorbed, while in air HC1 was taken up. Hydrochloric acid was not adsorbed in hydrogen but was adsorbed in air.
ADSORPTION OF ELECTROLYTES BY ASH-FREE CHARCOAL
2975
TABLE IV Milliequivalents HC1 adsorbed in Presence of Air Hydrogen
Per cent Pt. 0.00
0.227
0.004
0.298 0,333 0.358 0.376 0,393 0.406
0.032
0.16 0.80 4.00 10.00
Milliequivalents NaOH adsorbed in Presence of Air Hydrogen
0.190 0.116 0.048
0.000
0.000
0.000
0.000
0.000
0.426
0.000
0.000
0.521
0.000
0.000
0,557
0 IO00
0.000
0.000
0.000
0.576 0.582
TABLE V Charcoal saturated with Charcoal saturated with Hz adsorbs KOH Air adsorbs HCl
I I1 I11
KCl Solution 0 . 0 2 16
0.025 0.025
0.021
0 ,0205
Charcoal saturated with H I adsorbs HCl
Charcoal in Presence of Air adsorbs HC1
HC1 Solution IV V VI
0.14 0.739
0 .o 0.0
0.0
Charcoal saturated with Charcoal in presence of H Zadsorbs KOH Air adsorbs KOH
VI I
VI11
KOH Solution 0.66 0.0655
0.0 0.0
TABLEVI Amount of HC1 adsorbed Millimoles In vacuo After contact of Charcoal with air 0 0.140 0
0.140
Amount of NaOH adsorbed In vacuo
After contact of Charcoal with air
0
0
0
0
Potassium hydroxide was adsorbed in hydrogen but not in air. Frumkinla was finally able t o demonstrate (Table VI) that in a vacuum, in the absence of both hydrogen and oxygen, neither acid nor alkali was adsorbed. Schilow,14 however, always maintained that even in vacuo the charcoal adsorbed hydrochloric acid through the agency of his oxide A which was stable even at low pressures and high temperatures, and that KC1 was not hydrolytically adsorbed by this oxide. Frumkin16 questioned Schilow's results and pointed out
2976
ELROY J. MILLER
that it is necessary to continue the heating of the charcoal for twenty-four hours in a high vacuum in order to get rid of all the oxygen. But a t the same time with improved technique Frumkin found that the hydrogen atmosphere no longer prevented the adsorption of hydrochloric acid when the concentration of the acid solution was above 0.1Normal. In concentrations above tenth-normal the adsorption increased rapidly in spite of the presence of hydrogen. This Frumkin attributed to molecular adsorption. Recently Bretschneider,16 working in Ruff’s laboratory, has reported that the adsorption of succinic acid in air and in vacuo is the same (Table VI). This substantiates Frumkin’s findings” that the adsorption of benzoic acid is not
o
ar.
0.2 03 a4
05
0.6
07 N o r N a I ENdkONZeNtrat~ON
FIQ.5 Bretsohneider’s Curves for Adsorption of HC1
influenced by the presence of hydrogen or oxygen, even by platinized charcoal. Contrary to Frumkin, Bretschneider found that hydrochloric acid is quite appreciably adsorbed by charcoal in vacuo but in the presence of air the adsorption is increased, practically to the same value for the original charcoal in air. From Bretschneider’s curves (Fig. 5 ) it is apparent that the original charcoal and the evacuated charcoal t o which air had afterward been admitted had identical adsorption capacities. The practically constant difference between the values for evacuated charcoal and charcoal in air Bretschneider attributed to the presence of Schilow’s oxide B which can bind hydrochloric but not succinic acid. He favors Schilow’s theory and states that Frumkin’s theory does not afford any better explanation.
Discussion I n view of the uncertain and controversial status of much of the evidence for the oxide and gas electrode theories it is difficult, if not impossible, to evaluate them at present. One thing, however, appears t o be certain, and that is the section of the field of adsorption of electrolytes from solution to which they apply is very limited. Frumkin and Schilow have both recognized this fact and emphasize that molecular adsorption is not excluded by their theories.
ADSORPTION OF ELECTROLYTES BY ASH-FREE CHARCOAL
2977
It should be pointed out that, on the whole, the experimental evidence indicates that only those substances which are least adsorbed or not adsorbed a t all in the presence of air are influenced by the nature of the gas adsorbed on the charcoal, and these only in lower concentrations. The adsorption of HC1 from dilute solutions by charcoal apparently can, under certain conditions, be influenced by the presence of hydrogen, but in higher concentrations it is not affected. Only the first two 0"l. three members of the aliphatic acids," which are relatively slightly adsorbed, are appreciably influenced, but the effect falls off rapidly with increase in length of carbon chain. The more strongly adsorbed organic acids such as benzoic and succinic are not affected by the adsorbed gas. The reason for this, Frumkinll states, is that acids such as benzoic are like non-electrolytes in their adsorption by charcoal. This explanation encounters serious difficulties when the adsorption behavior of bases is considered. It is difficult to explain, by either of the theories, why sodium and potassium hydroxides are not adsorbed in the presence of air, while the equally strongly dissociated organic base, tetraethylammonium hydroxide is adsorbed; and again, on the other hand, the weak base, ammonium hydroxide, is not adsorbed. Equally difficult to understand is the fact that the adsorption of KCl, which is exclusively hydrolytic, can be affected by the gaseous atmosphere, while the greater hydrolytic adsorption of potassium benzoate is not affected. Under the circumstances it would be necessary to account for hydrolytic adsorption of potassium chloride by one theory, and hydrolytic adsorption of potassium benzoate by another. The effect of introduction of polar groups on adsorption does not seem to be explainable by either of the theories, nor is the negative adsorption of sodium and potassium hydroxides and its decrease with length of time of contact of charcoal with solution. Roychoudhury and Mukherjee's have put forth the argument against the gas electrode theory that Kruyt's charcoal in oxygen is negatively charged whereas charcoal in oxygen according to Frumkin's theory should be positively charged. If, however, Kruyt's charcoal does contain carbon acids with the carboxyl groups extending into the water phase, the negative charge could arise from the dissociation of the COOH group. This is in keeping with the fact that this charcoal is easily wetted by water whereas the same charcoal heated to a high temperature is very difficult to wet. This charcoal after destruction of the carboxyl groups could, in oxygen, be positively charged according to Frumkin's scheme. It is clearly possible to have charcoal either positively or negatively charged in oxygen, depending upon the conditions of the previous heat treatment, and the argument, therefore, is not a valid one against the gas electrode theory. Furthermore, it seems possible that some of the experimental evidence and procedure employed by R o y c h o ~ d h u r yare ~~ open to question. I n the first place his charcoal was heated at 600' which is not high enough to destroy or drive off the acid products on the surface of the charcoal and they probably are even slowly formed a t that temperature, for the optimum is in the neighborhood of 400'. As is evident from the data presented in this and previous papers it is necessary to heat the charcoal
ELROY J. MILLER
2978
around 1000’or above to free it from its acidic properties and to obtain a charcoal that does not take up alkali. Roychoudhury’s charcoal, therefore, probably carried some acid products. Another procedure which is open to question and which may have vitiated some of Roychoudhury’s results is that of washing the charcoal with conductivity water to effect purification. In the early work with ash-free charcoal it was repeatedly pointed out1~20J1 that it is practically impossible to wash out easily measurable amounts of acid by repeated extraction with conductivity water alone. In Table VI1 are given the results of one experiment which illustrates this fact fairly well. Methods for the quantitative estimation of adsorbed acids on charcoal were d e v e l o ~ e dand , ~ procedures for detecting and measuring acidic and alkaline impurities were devised.*I In view of the results obtained with these methods it is exceedingly doubtful whether conductivity measurement as carried out by Roychoudhury is any indication of the quantity of acid adsorbed on the charcoal. How futile such a measurement is VI1
TSBLE
Removal of Adsorbed HC1 from Charcoal by Extraction with Water Each Extraction with 200 cc. Conductivity HzO Extraction No.
Period of Contact
Temp.
hours
Room Boiling
I
I5
2
I
’*
3 4 5 6
30 minutes
Y
5 minutes
8 9
I
hour
2
”
IO
2
”
60
”
IO
I’
Ij
?l JI
hours
11
2
”
I2
I
”
I3
I5
”
I4
IO
15
I
16
I
”
8
”
I
”
IY I8
)1
Room Boiling 11
I>
.
))
11
,, Room Boiling
minutes hour
>?
11 11 1)
Total HCl extracted by water. . . . . . . . . . Extracted by boiling with NaOH . . . . . .
Acid Removed Cc. 0.02 N 0.00
1.5C
0.80 1.00
0.40 0.00
0.10 0.20
1.10
0.55 0.40 0.20 0.00
0.10 0.15 0.20
0.30 0.20
7.20
7.09
-
Acid recovered from charcoal equivalent t o . . . . . . . . . . . . . . . .14.29 cc. 77 1) . . . . . . . . . . . . . . . .I4.19 ” Chloride ” ” Acid originally adsorbed on charcoal. . . . . . . . . . . . . . . . . . . . 14.34 ” )’
0.02 ” ”
N
ADSORPTION OF ELECTROLYTES BY ASH-FREE CHARCOAL
2979
as a means of detecting impurities on the charcoal can be judged from the fact that large quantities of acids can be irreversibly adsorbed on the charcoal, leaving no detectible amount of acid in solution. I n fact the charcoal will even adsorb acid from a slightly alkaline solution of KCl,4and it requires more than one extraction with a considerable excess of alkali a t boiling temperature to extract all the adsorbed acid, From the data in Table VI1 it is evident that equilibrium a t room temperature left no acid in solution. At boiling temperature, however, there was an appreciable quantity in solution. The amount of acid removed with each extraction was dependent, not upon the length of time the charcoal was boiled, but upon the temperature at which it was filtered. I n fact it is considerably more efficient to extract the acid by percolating boiling water through the charcoal, thus preventing the establishing of equilibrium. This, however, is not to be recommended as a reliable method for removing adsorbed acids, except possibly for removing the bulk of adsorbed acid from a small amount of charcoal. I n the experiment summarized in Table VII, 14.34 cc. of 0.02 N HC1 was (at room temperature) irreversibly adsorbed on 2.3 g. of charcoal. This furnishes some idea of the quantity of even a slightly adsorbed acid that can be adsorbed without leaving detectible amounts in solution. That oxides do exist on the surface of the charcoal seems certain from the well known fact that when oxygen is adsorbed on charcoal it is not recoverable as desorbed oxygen but as COz a t lower temperatures and as CO at higher temperatures, and that the heat of adsorption of small amounts of oxygen on charcoal is much higher than that of larger amounts. On the other hand, it is not so easy to determine definitely the form in which these oxides exist on the charcoal. Kruyt’s idea of intermediate products of mellitic acid seems plausible, for his charcoal, as pointed out above, does have properties in common with charcoal carrying strongly adsorbed complex organic acids. The idea of basic oxides of carbon does not seem so easily accepted in any case and it is rather difficult to understand why such a basic oxide as Schilow’s B should react with HCl but not with benzoic or succinic acids16 when the fact is that the latter are much more strongly adsorbed by charcoal than is hydrochloric acid. Additional evidence of the existence of acid oxides of carbon was discovered in the course of the study in this laboratory of methods for the quantitative removal of adsorbed acids from charcoal. An attempt was made to remove the acids by electrodialysis. The following results were obtained: I. Charcoal heated a t 1o75OC. (not electrodialyzed) did not adsorb alkali. 2. Charcoal after electrodialysis did not adsorb alkali. 3 . Adsorbed HC1 was quantitatively removed by electrodialysis. 4. Charcoal from which adsorbed HC1 had been removed by electrodialysis did adsorb alkali. 5 . Carbon dioxide appeared in both anode and cathode chambers during electrodialysis. While it was expected that the adsorbed hydrochloric acid would be removed by electrodialysis it was surprising to find that the charcoal after re-
2980
ELROY J. MILLER
moval of the acid was still able to take up alkali. This was quite contrary to the behavior of the charcoal after adsorbed hydrochloric acid had been removed by other methods. The appearance of relatively large quantities of COZin both anode and cathode chambers during the process of removal of the acid was unexpected. Further investigation will have to be made to elucidate the factors involved, such as the nature of the electrode and membrane materials, and the voltage applied. It is not easy to explain the fact that the charcoal through adsorption and removal of HCl by electrodialysis should acquire the ability to take up alkali. These and many more perplexing facts must await further work before they can be satisfactorily explained.
Summary and Conclusions A brief survey of recent trends and developments in studies on adsorption of electrolytes by ash-free charcoal has been presented. 2. Data have been presented which indicate that negative adsorption of alkali decreases with length of time the charcoal is in contact with the solution. 3. Data have been presented which show that charcoal must be heated a t approximately 1000' or above to produce negative adsorption from solutions of sodium hydroxide. 4. Quantitative data have been presented illustrating the practical impossibility of removing even a feebly adsorbed acid such as hydrochloric from charcoal by repeated extraction with boiling water. 5 . Adsorbed hydrochloric acid can be quantitatively removed from charcoal by electrodialysis. 6. No one theory alone seems capable of explaining satisfactorily all the known facts of adsorption of electrolytes by adsorbent charcoal. I.
East Lansing, Xzchagan.
Literature cited Miller: Colloid Symposium Monograph, 5, 55 (1928). 2 Bartell and Miller: J. Am. Chem. SOC.,44, 1866 (1922). a Miller: J. Am. Chem. SOC.,46, 1150(1924). Miller: J. Am. Chem. SOC.,47, 1270 (1925). 6 Kruyt and de Kadt: Kolloid-Z., 47, 44 (1929). 8 Kolthoff: Personal Communication. 7 Dubinm: Z. physik. Chem., 140, 81 (1929). 8 Schilow, Schatunowskaja and Tschmutow: Z. physik. Chem., 149, 2 1 1 (1930). 9 Schilom, Schatunowskaja and Tschmutow: Z. physik. Chem., 150, 31 (1930). 10 Frumkin, Burstein and Lewin: Z. physik. Chem., 157, 442 (1931). 11 Bruns and Frumkin: Z. physik. Chem., 141, 141 (1929). 12 Burstein and Frumkin: 8. physik. Chem., 141, 158 (1929). 13 Burstein and Frumkin: Z. physik. Chem., 141, 219 (1929). 14 Schilow and Tschmutow: 2. physik. Chem., 148, 233 (1930). 15 Burstein, Frumkin and Lawrowskaja: Z. physik. Chem., 150, 421 (1930). 16 Bretschneider: 2. physik. Chem., 159, 436 (1932). 17 Lepin: 2. physik. Chem., 155, 109 (1931). 18 Roychoudhury and Mukherjee: Z. physik. Chem., 157, 435 (1931). 1 9 Roychoudhury: J. Indian Chem. Soc., 8,433 (1931). 2 0 Miller: J. Phys. Chem , 31, 1197 (1927). 21 Miller: J. Phys. Chem., 30, 1162(1926). 1
