Effect of pH on Adsorption by Carbons'

August, 1927. INDUSTRIAL AND ENGINEERING CHEMISTRY. 943 of the United States Department of Agriculture6 have resulted in the discovery of several ...
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August, 1927

of the United States Department of Agriculture6have resulted in the discovery of several promising screw-worm fly repellents. One of these, pine-tar oil-obtained by the destructive distillation of the wood of the long-leaf pine, Pinus palusfris L.-is recommended as the most suitable for application upon wounds on animals on account of its cheapness, availability, non-toxicity, and adhesiveness, and is now being used on an extensive scale by ranchmen in Texas and other southwestern states. Experimental During 1926 additional tests were made with the most promising repellents found up to that time. This work was carried on a t Uvalde and Dallas, Texas, where screw-worm flies are abundant. In order to test the strong repellents under as severe conditions as possible, they n-ere diluted, the powders with kaolin and the liquids with medicinal mineral oil, in the ratio of 9 parts diluent to 1 part repellent. The procedure of testing was the same as that previously described; that is, 4-ounce (113-gram) cubes of fresh beef liver, over the surface of which the repellent mixtures were smeared, were exposed from 2 to 5 days in pint Mason jars in an open shed where flies were abundant. The quantity of repellent mixture applied to each bait was 5 cc. for the liquids, or 5 grams for the solids. Untreated meat was exposed a t the same time, and the efficacy of the repellent is estimated by comparing the number of flies visiting the treated jar with the adjacent untreated or check jar. A percentage ratio of 100 indicates no repellent or attractant action; one 6

J . Econ. EntomoL., 16, 222 (1923); 18, 776 (1925); 19, 536 (1926);

U.S. D c p t . Agv., Bull. 1472 (March, 1927).


of 0 represents perfect repellent action, while a percentage ratio of over 100 indicates that the material increased the attractiveness of meat to blowflies. A few of the more significant results are shown in the accompanying table. It is of interest to note that strongly odorous materials, especially the essential oils, which are quite effective in repelling screw-worm flies from meat when applied undiluted, lose nearly all their efficacy or may become actually attractive-. g., bergamot oil-when diluted 1 to 9 with an inert vehicle; whereas copper carbonate, which is entirely inodorous is nearly as effective in 10 per cent strength as when applied undiluted. Conclusion These results, together with other observations, have led to the belief that the most effective blowfly repellents are not necessarily highly odorous materials, such as essential oils, or even highly irritating materials, such as chloropicrin and other “tear gases,” but are materials which can absorb, adsorb, or inhibit the formation of the volatile compounds evolved by decomposing meat which attract the flies to the meat. I n addition to various copper compounds, such strong antiseptics as mercuric chloride, potassium permanganate, sodium salicylate, etc., when applied to meat render it almost entirely non-attractive to blowflies. Tests are now being conducted to determine the practicability as blowfly repellents of copper carbonate and certain other powders when applied upon the wounds of animals under outdoor conditions.

Effect of pH on Adsorption by Carbons’ By S. M . Hauge2 and J. J. Willaman Dlvrsrow



Because of the great discrepancies in the results of various investigators, it has been apparent for some time that some factors in the evaluation of decolorizing carbons are not under control. The present writers have undertaken to show the effect of pH on adsorption by carbons. Data are presented which indicate that the more acid a solution is the greater is the adsorption of caramel and of benzoazurin, both of which are negatively charged colloids, while alkalinity favors adsorption of electropositive substances, such as methylene blue. The adsorption of amphoteric substances, such as proteins, is at a maxim u m in the general region of the isoelectric point of the protein, and is at decided minima in extremes of acidity

and alkalinity. Adsorption of dextrose, a non-electrolyte, is not affected by pH. In the application of carbons to specific requirements as encountered in industries, three factors should be considered-the electrical properties of the substances to be adsorbed, the electrical properties of the available carbons, and the permissible pH of the solutions used. Information concerning these properties should act as a guide in the choice of the carbon best adapted to the specific need, as well as the optimum conditions for adsorption efficiency. Thus, carbons may be prepared for specific requirements.

... ...... HE well-known property possessed by carbons of alkali-metal ions, according to investigations of Rona and removing coloring materials and other substances myself, and glycocol, according to Abderhalden and Fodor.” from solutions is commonly called adsorption. This Much of the research of recent years has been directed phenomenon is not a characteristic of all carbons to the same toward the production of highly active carbons, increasing degree, nor is it limited to any specific type of substances. the activity of carbons, and the reactivation of carbons. Both organic and inorganic substances, colloids and crystal- As a result several highly active decolorizing carbons have loids, substances which reduce surface tension and others appeared on the market in response to the demand of in~* which do not, are known to be adsorbed. M i c h a e l i ~ ~ ~dustries requiring very efficient carbons. During some writes: “There are only two exceptions known hitherto preliminary work in 1922 on the production of active carbons, which are not a t all adsorbed by charcoal: the sulfates of very irregular and inconsistent results were obtained in 1 Received March 21, 1927. Published with approval of the Director attempting to evaluate these carbons. Some factor was as Paper No. 689, Journal Series of the Minnesota Agricultural Experiment apparently not under control, and it was soon evident that Station. Abridged from a doctor’s thesis submitted by the senior author t o at least one important factor was the p H of the medium in the University of Minnesota. which the carbon was acting. Therefore, the series of ex2 Now a t the Purdue University Agricultural Experiment Station, * Numbers in text refer t o the bibliography at end of article. periments reported herein was begun.




Researches on the evaluation and industrial applications of carbons have not kept pace with the rapid development in the production of active carbons. Attempts to evaluate carbons by determining their adsorptive capacities have given results which are often erratic and sometimes inconsistent. This may be partly explained by the fact that most methods hitherto employed to determine the activity of carbons have been empirical and lacking in control of all important variables. Even with the same type of solution and with similar carbons, the results obtained by different investigators are not always comparable. Therefore, each concern wishing to use carbons as adsorbents has deemed it necessary to evaluate them under the condition in which they are to be used and against substances to be removed, in order to determine their value for their particular need. Carbons have their greatest application in the sugar industry, and they have been used both in direct whitesugar manufacturing and in sugar refining. Their use has been extended to other fields, however, as in the purification of organic and inorganic chemicals, such as lactic, citric, and phosphoric acids, alcohols, acetone, maltose and glucose sirups; decolorization of waxes, gelatin, glue, oils, and fats; purification of petroleum oils and water; recovery of gold from solutions; separation of alkaloids; and preparation of pharmaceuticals.

’5701. 19, KO.8

( 2 ) Mechanical adsorption, where the difference of potential is due to surface tension. Positive adsorption takes place whenever a decrease in surface tension results from an increase in concentration in the interface. (3) Chemical adsorption results when the difference of potential is chemical in nature. (4) . Thermic adsorption results from thermic differences of potential in surfaces. ( 5 ) Photic adsorption occurs when the difference of potential is photic in character.

It thus appears that a series of different energies may play

a role in adsorption. Since the data secured during the investigation herein reported show that many of the phenomena of adsorption by carbon can be explained on electrical grounds, only that type of adsorption will be discussed here. A possible explanation for lack of parallelism between surface tension as measured against air and adsorption by charcoal is the formation of electrical charges on the surface^.^ In consequence Michaelis divides the total tension at the surface into two classes: “the purely mechanical tension, which causes the usual adhesion and cohesion in the absence of free electrical charges, and an electrical negative tension or expansion effect, due to the covering of the surface with charges of like signs.” In studying the electrical charges on carbons, Perrin38,P.54 found that charcoal was electronegative in alkaline solutions and electropositive in acid solutions. Cylinder carbon was Mode of Action of Carbons found to be electronegative under all conditions by Bethe 4 confirmed ~*~ the earlier work of Adsorption is defined by O s t ~ a l das~ ~“that change in and Toropoff 38,p.54. U m e t s ~5 ~ Concentration which colloids or other dispersed systems Perrin, using blood charcoal, and also found that carbon suffer a t the surfaces where they come in contact with other prepared from sugar was electronegative, similar to the bodies.” He also points out that “this change in concen- cylinder carbon determined by Bethe. Later Ogawa40 tration is the only constantly observed phenomenon that is showed that the charge on sugar char was not only electrocommon to all the myriad manifestations generally grouped negative but could be also electropositive by activation under the term adsorption. After such concentration of the carbon. His endosmotic determinations showed differences’have come to pass, a long series of secondary that the amount of water transported by normal sugar char per minute decreased from O.O348(-)cc. in 0.02 N sodium changes may take place.” hydroxide to 0.0033( -)cc. in 0.02 N hydsochloric acid soh,* the ~ activated 0.0039 j I 1 I 1 1 ~ tions, whiIe ~ ~ sugar ~char transported ~ (-)cc. of water in 0.02 N sodium hydroxide, 0.00025 (&)cc. in 0.002 N sodium hydroxide, and O.O103(+)cc. in 0.02 N hydrochloric acid solutions. This shows conclusively that activation produces effects wlich influence the electrical charge carried by the carbon in solutions. This evidence is in contradiction to M i c h a e l i ~ ’statement: ~ ~ ~ ~ * ~ “Sugar ~ charcoal, on the other hand, does not adsorb the anions at all, not even very easily adsorbed anions of acid orgacic dyes. In agreement with this we find that sugar charcoal is never charged positively by acids but always negatively.” Adsorption tests with carbon show that the basic dyes are best adsorbed by carbons in acid solutions and acid n m ~ /N ,?,Noms dyes are best adsorbed in alkaline solutions. On the basis Figure 1-Rate of Adsorption of Caramel by Norit a t Various pH Values of such observations, Tanners4 presents a discussion on the To bring about this change in concentration, energy is application of the adsorption theory of dyeing to carbons. expended. O s t ~ a l d ~ ~states 1 4 2 that “a whole series of different He likens the impurities in a solution to be adsorbed to the dyes and the charcoal to the fiber. He states: kinds of energy plays a role” in adsorption and makes a generalization of Willard Gibbs’ theorem that is common That charcoal will act like a piece of cloth in removing dye to all forces in adsorption which reads as follows: “Ad- from solution is not new to dye chemists, but the application of knowledge t o the decolorization of sugar sirups and other sorption will take place whenever there exists in the surface this solutions seems to have been entirely overlooked. Industrially, a difference in energy potential which can be decreased charcoal is applied in a most empirical manner. Much assistthrough a change in concentration of the dispersed materials ance could be had in setting the correct conditions for the most efficient usage of charcoal if some of the generalities of dyeing bordering upon this surface.” O s t ~ a l dfurther ~~ classifies the types of adsorption re- be kept in mind. sulting from these differences in potentia1 existing at the The relation of the charge on the adsorbent to the charge surface as: on the adsorbate is possibly the most important factor in dyeing as well as in adsorption by chars. Bancroft2 states: (1) Electrical adsorption, in which the difference of potential is electrical in nature. When the dispersed phase carries an electrical charge opposite t h a t of the solid, substances will be adsorbed by decreasing this difference in potential by neutralization of charges.

A fiber tends to adsorb everything in solution in amounts varying with the nature, concentration, and temperature of the solution, and the nature of the fiber. A basic dye is one which con-

August, 1927



f Suqar char ii




Calaphorcsis 01 caramel

Equilibrium pH WIULS

Iniliol p n values

Figure 2-Adsorption

of Caramel by Various Chars a t Various Hydrogen-Ion Concentrations

tains the color in the acid radical*** I n order to get the maximum adsorption of a n acid dye, we should have present an ion of opposite charge, which is readily adsorbed. I n case of basic dyes, the dye should be more readily taken up in a neutral or alkaline solution. Acids (hydrogen ions) decrease the dyeing of basic dyes and increase that of acid dyes. This action is proportional to the concentration of hydrogen ions. Bases (hydroxyl ions) have just the opposite effect.

electrical properties; therefore electronegative, electropositive, amphoteric, and non-electrolytic substances have been used, with special reference to their adsorption by a number of charcoals throughout an extensive range of hydrogen-ion concentration.

The effect of acid and alkali on the decolorization of sugar and molasses solutions has been known in a qualitative way In fact, for a long time by many inve~tigators.'0,23,~j,54,~~,~* it has been a common factory practice to decolorize sugar solutions in a slightly acid medium rather than in one that is neutral or alkaline. However, the literature shows that very little quantitative work has been done on the relation between hydrogen-ion concentration and decolorization. Brewster and Raines12 studied the effect of pH on the decolorization of cane juice. They found that decolorization increased with hydrogen-ion concentration. In order to reduce inversion of sugar to a minimum, they advocated acidifying with phosphoric acid to p H 4, allowing action for a time, and then neutralizing with lime to p H 6.4. Later they's advocated the application of hydrogen-ion control in sugarhouse liquors, and finally took out a patent14 on the control of hydrogen-ion concentration in the use of decolorizing carbons. Recently Blowski and Bon7 reported similar effects on raw sugar liquor using a range of p H 4.5 to 8.5. Williams and G e b e l i r ~advocate ~~ p H between 6.8 and 8.5 for good clarification. Further data on the accurate determination of hydrogen-ion concentration in sugarhouse control have been recently published.lS*

THECARBOKS-The carbons chosen for this investigation were of two types-animal charcoals high in mineral and relatively low in carbon, and vegetable carbons low in mineral and high in carbon. By such a choice it is possible to compare representative carbons which have been and are used extensively in commercial practices. The following carbons were used:

The Present Problem

In order to test the importance of p H in the relation between carbon and the substances to be adsorbed] it was believed necessary to investigate adsorbates of various

Materials and Methods

1-Apple char, prepared from apple pomace (made available through courtesy of Best-Clymer Company of St. Louis). 2-Blood charcoal, Merck. 3-Bone black, blerck. 4-Carbrox, prepared from rice h ~ l l s manufactured , ~ ~ ~ ~ ~ by Carbrox Co., Inc., New Orleans. j-Darco, a vegetable carbon purchased from the Darco Sales Corporation, New York. 6-Norit, a vegetable carbon prepared from a secret material, which Tanner66believes to be birch wood. 7-Sugar char, prepared from purified sucrose by caramelizing until a char is produced and then heating the char in a nickel crucible with limited air a t 1000" C. for 2 hours in a muffle furnace. 8--Superiiltchar, a chemically prepared decolorizing carbon, probably of vegetable origin, sold by the Industrial Chemical Co., Tyrone, Pa. PHYSICAL AND CHEMICAL ANALYSES O F CARBONS-SinCe considerable emphasis has been placed on the general characteristics of carbons, determinations were made of ash, nitrogen, and density (Table I). Ash. The vegetable carbons contain ash with percentages ranging from 0.22 in the case of sugar char to 23.09 in apple



Vol. 19, No. 8

From a common s t o c k solution of caramel, solutions of equivalent concentration of coloring matter but with differently adjusted p H values were prepared. A series of tests was made from each solution. One hundred cubic centimeters of the solution were placed with 1 gram of carbon in each of a series of Erlenmeyer flasks a n d t h e treatment period varied from 1 to 110 minutes. At the termination of the different p e r i o d s the solutions were rapidly filtered by means of a Buchner funnel with aid of suction. The filtrate was a n a l y z e d with a Duboscq colorimeter. The apparent percentage of decolorization was calculated by the following formula: Per cent decolorization = 100readine of standard X 100 reading of unknown The results are given in Figure 1. It is apparent that adsorption is very rapid and that equilibriumis established 2 1 4 5 6 7 R 9 lo I1 within a few minutes. These pH Values results agree with those of Figure 3-Adsorption of Benzoazurin b y Various Chars at Various Hydrogen-Ion Concentrations other investigators. Firthss char, while in t.he animal charcoals the percentage of ash found that iodine was adsorbed by charcoal Gery rapidly the ranges from 19.36 to 83.47. first few minutes and then rather slowly. He classified the Nitrogen. The Kjeldahl method was used in the deter- first as true adsorption and the latter as absorption. Yoesl mination of nitrogen. The results show t8hat the animal found that the rate of adsorption of arsenious acid on alucharcoals contain many times as much nitrogen as the minum oxide was quite rapid, about 50 per cent of the total adsorption taking place within the first 5 minutes. Blowski vegetable carbons. Sugar char was free of nitrogen. Apparent Density. The importance of the density of and Bon7 found that approximate equilibrium was attained carbons has been pointed out by several inve~tigators.2~.46 after 15 minutes’ contact between carbon and sugar liquor, The apparent densities of carbons were determined for and that no further color adsorption took place after one the various carbons in both air and toluene. hour. Thirty minutes appeared to be ample time for equilibrium Table I-Physical a n d Chemical Analyses of the Carbons to be established between the carbon and the solution, APPARENT DENSITY and this period was generally adopted. CARBON ASH NITROGEN In air I n toluene Apple char Blood charcoal Bone black Carbrox Darco Norit Sugar char Superfiltchar

Per cent 23.09 19.36 83.47 10.34 22.08 2.25 0.22 3.47

Per cent 0.56 8.20 1.13 0.70 0.46 0.18 0.00


0.372 0.864 1.017 0.333 0.510 0.306 0.670 0.284

1.896 1.833 2.632 1.796 1.732 1.865 1.534 1.522

The apparent density in air of these samples of charcoal was determined by filling a 100-cc. graduated cylinder with carbon and tamping the cylinder until constant volume was observed. The volume of carbon was then weighed and the density calculated. The apparent density in toluene was determined by measuring the volume of toluene displaced by a weighed portion of carbon in a pycnometer and from this the density was calculated. RATE OF ADSORPTION-In making adsorption tests it is important to know the time required for equilibrium to be established between the adsorbent and the solution. A number of tests were made to determine the rate of adsorption of caramel by Norit at various pH’s.

Adsorption of Electro-Negative Substances

ADSORPTION OF CARAMEL-Twenty-four grams of sucrose were placed in a 400-cc. beaker and heated for a half hour at 220’ C. in a sulfuric acid bath. The caramelized sugar was dissolved in warm water and made up to a volume of 3.5 liters. To insure a uniform stock solution, a sufficient amount was prepared a t once to supply the necessary solutions for all tests. A series of solutions of different hydrogen-ion concentrations was prepared by taking 140 cc. of the stock solution, adding hydrochloric acid or sodium hydroxide to adjust the hydrogenion concentration, and bringing the final volume up to 160 cc. Exactly 100 cc. were transferred from each solution to an Erlenmeyer flask containing 1 gram of carbon, shaken occasionally for a half hour, and filtered. A portion of the remaining 60 cc. was used for the electrometric determination of the initial hydrogen-ion concentration, and the remainder was used as a standard in the Duboscq colorimeter in analyzing the filtrate for the degree of color removed. By using the corresponding standard with each filtrate the error due

August, 1927

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duced the effects already mentioned. Finally, an apparatus was designed w h i c h gave very reliable results. A modification of the Burton apparatus (Figure 4) was used in these tests. The a r m s of the U-tube were about 12 cm. in length and about 1.5 cm. in diameter. At the bottom of the Utube was connected a fine d e l i v e r y t u b e with a stopcock and a funnel, arranged so t h a t t h e tube was bent round behind the a r m s a n d with the funnel slightly above the level of the arms. At about 3 cm. from the top of the arms, side tubes were connected downward to f o r m a b r i d g e between the electrode cells and the U-tube. This arrangement prevented the accumulation of gas bubbles, which often break the circuit in liquid bridge Experiments in Electrophoresis c o n n e c t i o n s . The The data thus far presented have shown that the hydrogen- electrode cells were 9 ion concentration of the solution has a decided effect on the cm. long and 3 cm. in adsorption' of electronegative substances. Since either diameter, with a cathe adsorbent or the adsorbate, or both, could be affected pacity of about 50 cc., by the reaction of the medium, experiments were instituted assuring ample volt o determine the nature of the electrical charges on the two ume for the accumulat i o n of electrolysis compounds. Careful surveys of the literature on the electrical prop- products. These cells erties of dispersed substances, the theories of cataphoresis, were so arranged as the velocity of migration of particles in an electric field, the to be of t h e s a m e charge on particles, etc., have been presented by B ~ r t o n , ~ ~height , ~ g as the arms of the U-tube. T h e y Svedberg,L3B a n ~ r o f tTaylor,56 ,~ and many others. Many methods and many types of apparatus have been were fitted with s t o p used for the measurement of cataphoresis. Svedbergs3 pers containing two classifies the methods into three types: "(1) rate of migration holes, the larger hole of boundary between sol and dispersion medium measured; for the escape of gases (2) change in concentration of dispersed phase in a volume a n d t h e smaller for near the boundary measured; (3) migration of the individual the electrode. particles measured directly in the ultra-microscope." The METHOD-A series Grst appeared to be the most promising for suspensions of of solutions h a v i n g caramel and of benzoazurin. the same specific conAPPARATuS-V~~~OUS types of U-tube apparatus were ductivity, but differtried in these experiments to determine the velocity of mi- e n t p H values, was gration of caramel and benzoazurin in solutions of different prepared of the differpH values. Since the migration is slow and, therefore, the ent materials to be period of time must be long, many difficulties were encoun- tested. Since an a p tered. With the ordinary U-tube having the electrodes in preciable error may be contact with the liquid in the arms of the tube, the chief introduced by using interference was due to (1) convection currents around the solutions of different electrodes which displaced the boundary line of the colloid, specific conductivity .and (2) the accumulation of electrolysis products which pro- f r o m t h o s e of the

to the effect of acidity on color change is largely eliminated. The final or equilibrium p H value was determined on portions of the filtrate. Decolorization is expressed in percentage of color removed. The results of these tests are given in Table 11. A detailed comparison of the data is shown in Figure 2, It is to be noted that with decrease in p H value adsorption increased and vice versa. In the case of sugar char and blood charcoal adsorption took place only in acid solutions. Differences in the general characteristics of the curves of various carbons must be ascribed to differences in certain characteristics of the carbons. In all cases adsorption was accompanied by a decided change in the p H value of the solution. ADSORPTION OF BENZOAZURIN-Astock solution was prepared containing 1.0 gram of the dye per liter. A series of solutions of different hydrogen-ion concentrations was prepared by measuring with a pipet 400 CC. of the stock solution into a 500-cc. volumetric flask, adding definite quantities of hydrochloric acid or sodium hydroxide to adjust the p H value, and making up to 500 cc. with distilled water. This gave a dye concentration of 0.08 per cent. One hundred cubic centimeter portions of these solutions were tested according to the procedure described under Adsorption of Caramel. Owing to the change of color in strongly alkaline solutions, reliable results could not be obtained beyond the limits reported in this investigation. The results of these tests are given in Table 111and shown in Figure 3. These curves emphasize the similarity in adsorption of the dye by the different carbons as affected by the hydrogen-ion concentration. Furthermore, caramel and benzoazurin behave similarly. In the tests with both materials, data were obtained which show that the efficiency of the different carbons was increased enormously by increasing the acidity.

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irig a long time, increasing in amoiiut wibli incre:isiug acidity. However, in iioiie of the solutions above pH 2.5 was there any sign of precipiMion, even after a year and a half. With the bcrisonzurin wlu t,iiui the PI-1was decreawl the rate of migration fell gracliinlly 11 a pH of 2.5 mas renc~hrd :it which point there w i i s an abrupt fall in the niig Kith the part,icular solutions involved, it was not, f i to go lower on the p E scale, wliere possibly iin nctiial isoeleet.ric point of the dye might Imrc beeii foiind. ELEcTl{o-Esni)SlloSlS OF ( ~ . 4 l ~ B O ~ S - - ~ T l l:lttel,lpt ? \