THE ADSORPTION ISOTHERM? SAWCASPE,200 WEST738.D STREET, NEWYORKCITY
Charcoal i n commerce i s n o t e chemically pure form of carbon. Its impurzties effect many side reactious and cause phenomena not attributable to pure chorcoal. The errors inherent i n the adsorption isotherm experiment performed i n colleges are pointed out, and it is s h m that these errors t a d to disprme the hypothesic (which the experiment is de.~ifnedto prooe) that charcoal absorbs acids.
. . . . . . The word charcoal, used alone, implies an allotropic form of carbon. In commercial practice, it is used to designate one of the most impure products. Even though commercial charcoal is neither a single nor unvarying set of substances, producers of it furnish merely a trade name or a prefixed "animal," "bone," "blood," "purified," but never a complete analysis of their products. Has a chemist the right to accept "bona fide" the extraordinary qualities of a charcoal? Noyes (1)has already called the attention of chemists to the necessity of purifiying charcoal before using it for the purification of organic compounds. Can one imagine the chagrin an investigator experiences when, in place of purifying a biochemical substance by charcoal tiltration, he finds his sensitive product destroyed by some impurity, such as iron, contained therein. Truly he seldom thinks of charcoal as anything but a purifier and general benefactor of his product, and therefore hesitates to blame it for the damage. Iron, however, is not the only impurity of charcoals; animal and bone chars contain calcium ? and magnesium phosphates, and here we meet a case where the impurity constitutes the greater portion (2)of these active adsorbing mixtures. It is even to be deduced that some properties attributable to charcoal might be explained on the basis of these impurities. In filterhg a dilute sulfuric acid solution of an amino acid through a bed of bone black, we observed the formation of aystals which proved, upon analysis, to be calcium sulfate. The calcium sulfate must have been formed from the reaction between the sulfuric acid and the calcium phosphate of our bone black. From this observation, it dawned upon the writer that the adsorption of acids, such as acetic and oxalic, by charcoal was open to direct experimental rebuttal. "The Absorption Isotherm" constitutes an experiment to be found in Findlay's (3) and other standard textbooks on physical chemistry. In these college experiments the students shake a weighed grade of animal or bone charcoal with dilute solutions of acetic or oxalic acid of known strength for a definite period of time. At the end of this period, the charcoal is iiltered off, and the acidity is titrated using phenolphthalein as an indicator. A decrease in total acidity is found and is explained by assuming that some acid must have been adsorbed by the charcoal. The 907
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amount adsorbed is dignifed by the mathematical expression X/m = ac"" where ' X / m represents the amount adsorbed per unit weight of charcoal, c represents the equilibrium concentration, and a and n are qmstants. Mathematics, however, does not indicate the correctness of the fundamental hypothesis: that in this case we have an adsorption. Are we dealing here with an adsorption phenomena? In order to fathom this tantalizing problem, experiments were designed. The basis for these experiments was predicated upon the fact that magnesium and calcium phosphates react with dilute acid as follows:
+ 6HCzH36 = 3Mg(GH~02)2+ 2H1P04
Mga(P04)r
We see in the above reaction that phosphoric acid replaces acetic acid and that it is a stoichiometrical change. If we should titrate the total acidity of the acetic acid used in dissolving Mg3(PO& and then titrate the H3P01 stoichiometrically produced bythis reaction, we should find in the latter case a titer of 2/3 that of the former (theend-point for phosphoric acid titration. using phenolphthalein as indicator, is noticed when two of the three hydrogens contained in the molecule are replaced). Stoichiometrically speaking there is no reduction in total acidity, but titrametrically there is. The question then arises4oes not this reaction occur in charcoal where, as we have pointed out, several phosphate impurities exist? If this reaction does occur in charcoal, then it would follow that what one considers adsorption is merely a change in titrametric acidity. With these notions to the foregrougd the experiment was pursued. The ashes of several charcoals were determined and all were found to be above 65%. The ashes were tinted pink, faint brown, or faint blue depending on their source. They were completely soluble in either strong hydrochloric acid or concentrated nitric acid. It would then he essential to purify a charcoal and experimentally determine whether the purified product still manifested acid-adsorbing properties. By treating charcoal with hydrochloric acid, filtering, and washing thoroughly with water to free it from acid, and then with alcohol and ether to dry it, a charcoal was obtained which contained 5 to 10% ash. The acid treatment evidently removed most of the readily soluble portions of the ash. By refluxing the charcoal with hydrochloric acid for a long time, a purer grade can be ohtained. Weighed samples of charcoal (65% ash) and purified charcoal (10% ash) were treated respectively with 100 cc. N/20 acetic acid and were shaken for twenty minutes. They were then filtered and the filtrates titrated. There was a reduction of 35 to 40% in the total acidity after treatment with charcoal (65% ash) but, in contrast, there was no reduction after treatment with purified charcoal. It was interesting to note that the solution of the reduced acidity gave a pronounced qualitative test for phosphates while the
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one of the unreduced acidity gave none. I n view of the fact that a question might be raised whether the purified charcoal was changed or perhaps inactivated by the hydrochloric acid treatment, color adsorption tests were run upon both samples. One gram of charcoal (65% ash) was taken in comparison with 0.38 gram of charcoal (10% ash). These quantities were used to insure the same amount of absolute charcoal in each, namely, 0.35 gram. The purified charcoal adsorbed the color from 20 cc. of a saturated picric acid solution diluted thirty-two times while charcoal (65% ash) failed t o give complete color adsorption of 10 cc. of the same picric acid solution. The greater surface of the charcoal exposed in the purified charcoal was perhaps the reason for the greater color adsorption efficiency. This experiment tends to prove that the color adsorption properties (on the basis of the absolute carbon content of the charcoals) are enhanced in the purified charcoal, and that color adsorption is a particular charcoal phenomena. It becomes apparent that acid adsorption by charcoal, as usually determined, is no adsorption. In order finally to clinch the point, the natural alkalinity of the charcoal and the phosphoric acid formed by the treatment with N/20 acetic acid were determined, and the two determinations practically accounted for what has previously been considered the adsorption of acids by charcoal. On the basis of the evidence furnished, it appears that the so-called adsorption of acids by charcoal is the antithesis of adsorption and, in reality, merely the effect of impurities upon the acid. The results of this experiment lead one to recommend the replacepent of "The Adsorption Isotherm" experiment found in textbooks by an actual color-adsorption experiment which illustrates a distinct property of charcoal.
, . . . . . The author appreciates the courtesy of Dr. Horatio S. Krans of the American University Union and Dr. Francis Perrin of the University of Paris for the privilege afforded him in permitting the pursuit of this work. a t the University of Paris. Literature Cited ( 1 ) NOYES,W. A,, "Textbook of Chemistry," Henry Holt & Co., New York City, 1914, 278 pp. ( 2 ) L ~ L EA., D., "Animal Charcoal," J. CHEM.EDUC.,7,2185 (Sept., 1930). ( 3 ) FINDLAY,"Practical Physical Chemistry," 4th edition, Longmans, Green & Co., London, 1925, 286 pp. ,
