ION ADSORPTION BY PECTIN
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A NOTE CONCERNING ION ADSORPTION BY PECTIN GLENN H. JOSEPH California Fruit Growers Exchange Research Department, Corona Laboratory, Corona, California Received July $4, 19Sg
During the past ten years the literature on pectin has been influenced to a large extent by the publications of Tarr and of Miss Spencer concerning the relations between acids and pectin in gel formation. In 1929 and 1930 Miss Spencer published a series of papers dealing with the influence of sugar, salts, and acids on pectin gels. The third paper (2) of that series was devoted to a critical discussion of the 1923 data of Tarr (3). Miss Spencer used the data of Tarr to substantiate her contention that pectin carries a negative charge in solution due to adsorbed anions and to show that pectin does not possess a “buffer action” in acid solutions, as was claimed earlier by Tarr. It has been felt in these laboratories that Miss Spencer erred mathematically in her interpretation of Tarr’s data and that consequently her postulates were without suitable foundation. Since subsequent literature has failed to note the error we desire to call it to the attention of those working with pectin. Tarr presented data showing the pH values of systems obtained by adding various volumes of 0.10 N acids to 100-ml. portions of distilled water and to the same volume of distilled water containing 1.0 g. of pectin. Curves of his data showed that the pH values of the systems containing pectin were higher than corresponding ones where pectin was absent, leading him to conclude that pectin was exhibiting “buffer action.” Similar data have been obtained by us many times during the past decade. Miss Spencer said, concerning Tarr’s data relating to the pH of sulfuric acid systems, ‘ I . . . although the addition of 1 cc. of sulfuric acid to water resulted in an increase in acidity represented by 2.44 (5.50 -3,06),points as against an increase of only 0.15 (4.21 -4.06) points when pectin was present, the nezt added increments, 19 cc. in all, increased the pH value of water only 1.09 (3.06 - 1.97) points as against 1.92 (4.06-2.14) points when pectin was present. That is to say, after the added acid had disposed of some non-pectin influence in the pectin sol, presumably salt impurities, the presence of pectin, instead of “depressing” the dissociation of acid, increased the hydrogen-ion concentration.” The obvious error in the above reasoning centers around the subtraction of pH values and comparison of these differences as numerical entities. The logarithmic nature of pH values prevents such manipulations. The pH values should have been converted to the corresponding hydrogen-ion
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concentrations before Miss Spencer started her calculations. I n the first instance related above, the “2.44 points” represents an increase in hydrogen-ion concentration of 8.7 X while in the second system, where due no pectin was present, the hydrogen ion increase is only 0.15 x doubt, in a small part, to the salt impurities, as pointed out by Miss Spencer. However, for the next 19 ml. of acid addition the hydrogen-ion concentration increased by 98.3 X lo-* for water alone and by 71.6 X when pectin was present, showing just the opposite from Miss Spencer’s conclusion. Pectin did prevent the system from having the hydrogen-ion concentration it would have had in the absence of pectin, hence pectin did “buffer” the system as Tarr contended, although not necessarily by the mechanism he advanced. I n a similar manner it can be shown that in every acid system which Tarr discussed the pectin decreased the hydrogen-ion concentration. Thus it seems that Miss Spencer’s theory of anion adsorption has rested on a false foundation, because, rather than adsorbing anions and leaving hydrogen ions free to decrease pH values as she concluded, we see that actually it was the hydrogen ions that were adsorbed by the pectin, thus increasing the pH. Tarr concluded that anions did not exert a noticeable effect on pectin gel formation. We have data to show that the pH values of pectin-sugar-acid-water systems, in correct proportions for gel formation, go up from 0.06 to 0.15 of a pH unit as a result of actual gel formation (in the region of pH 2.9-3.5 as determined with a glass electrode). This indicates that a definite adsorption of hydrogen ions takes place during the process of pectin gel formation. This has been found true for both apple and citrus pectins. The present conception of pectin structure is one of chain-like aggregates of anhydrogalacturonic acid residues bearing from three to ten times as many methylated carboxyls as free carboxyls. Preliminary work indicates that there are probably from 600 to 1200 of these residues in the molecular aggregate. The physicochemical nature of the pectin molecule is such that one need not assume anion adsorption to account for the fact that in cataphoresis pectin carries a negative charge. The picture of gel formation need not be much different than the one so well presented by Kruyt (1) in 1930. When pectin is in the sol state it is stabilized by water layers, probably held to it by the electrical attraction of the pectin’s negative charge and the unbalanced positive charge of the water dipoles. As sugar is added to the sol, the dehydrating influence of the sugar decreases the stability of the pectin by disturbing the water balance. When the sugar concentration gets up to 50 to 65 per cent, the dehydration of the pectin micells is sufficiently complete so that when an acid is added the hydrogen ions finish destabilization and a gel forms as a result of an unsuccessful attempt to precipitate.
OPTICAL SENSITIZING BY DYES
411
There are a number of frequently overlooked pH changes which occur in aqueous sucrose-pectin-acid systems, especially in the region where gels can form. Data to be submitted in a later paper will show several of the individual factors affecting these pH changes and point out the importance of considering these factors when postulating a mechanism for gel formation. For instance, i t will be shown that when sucrose is added to an acidified pectin sol the pH may decrease as much as 0.10 unit or in some owes may rise 0.20 pH higher than the original value, depending upon the pH region, the sucrose content, and upon whether or not gelation occurs. In the region of proper balance for gel formation, there is, in addition, a time factor which must be considered when making pH measurements. REFERENCES (1) KRUYT,H. R . : Colloids, a Teztbook, 2nd edition, p. 200. John Wiley & Sons, Inc., New York (1930). (2) SPENCER, GENE:J. Phys. Chem. 34, 410-17 (1930). (3) TARR,L. W.: Univ. of Delaware Agri. Expt. Sta. Bull. No. 134 (February, 1923).
PHOTOVOLTAIC CELLS WITH SILVERSILVER BROMIDE ELECTRODES. I11
OPTICALSENSITIZING BY DYES^ S. E. SHEPPARD, W. VANSELOW, AND G . P. HAPP Koduk Research Laboratories, Eastman Koduk Company, Rochester, New York Received July 94, 1039
In the previous papers of this series (6, 7) there have been described the photovoltaic phenomena observed with a cell of two silver-silver bromide electrodes, one exposed to light, the other kept dark; the electrodes were connected electrolytically, usually by a dilute potassium bromide solution, and externally either through a vacuum tube voltmeter or a three-stage amplification oscillograph. In the present investigation these instruments have been replaced by an Einthoven string-galvanometer. A detailed description of the apparatus and operation is being presented elsewhere.2 With this there have been studied the photovoltaic effects obtained when the silver bromide of the illuminated electrode is dyed with a sensitizing l
Communication No. 742 from the Kodak Research Laboratories.
* In preparation.
