ADSORPTION OF NITRILESON HYDROGEN ELECTRODES
Nov., 1954
951
[H +I S, mole/l. S/[H+I X 108 acid range 0.1-0.73 N HC104. Kasarr~owsky~ on the other hand considered tellurous acid as a 0,8091 10.20 x 10-3 12.6 weak base which he represented as Te(OH), and ,3892 3.91 x 10-3 10.02 gave for K b the value It must, however, be .0776 0.77 x 10-3 9.92 taken into consideration that a weak polyacid base .0417 0.42 x 10-3 10.08 .0090 0.164x 10-3 18.2 like a weak polybasic acid must dissociate in steps, the first of which, as in the case under consideration The value of K b ' amounting to 3.1 X lo-" as can be represented as computed from the data shown in Table IIIb is Te(OH)4 - H20+Te02HOH +Te02H+ OHundoubtedly higher than that given by Latimer.8 The findings of Schumann were substantiated by However, by determining K b ' from the results obthe following data from which it is apparent that tained by Schumann3 and applying our So value, S/ [H+] is approximately constant between 0.05- 3.2 X mole/l., the values obtained varied from 0.5 N HCl, thus 2.4-2.8 X 10-l1 which compares well with our Kb' value. (9) .I. Kasamowsky, 2. phusik. Chem., 109, 287 (1924).
+
THE COMPETITIVE ADSORPTION FROM AQUEOUS SOLUTIONS OF HYDROGEN AND NITRILES ON PLATINIZED PLATINUM BY THOMAS C. FRANKLIN* AND RAYD. SOTHERN Contribution f r o m the Department of Chemistry of the University of Richmond, Richmond, Virginia Received December IY, 1065
.
The adsorption of a series of nitriles on the hydrogen electrode has been investigated coulometricall The amount of h drogen adsorbed and the potential of the electrode were determined for various concentrations of nitrig. The adsorption orthe nitrile was found to follow the Freundlich e uation. The potential of the cell is proportional to the logarithm of the amount of adsorbed hydrogen. A comparison of %e poisoning effect of various nitriles indicated that the poisoning ability is primarily related to the size of the molecule.
Since i t is known that the presence of poisons such as arsenic, mercury and sulfur compoun~s causes the hydrogen electrode to become erratic, an investigation has been initiated into the adsorption of foreign materials on the hydrogen electrode and their effectupon the potential of the electrode. Poisons as strong as those just mentioned completely replace the hydrogen adsorbed on the electrode; therefore a series of weaker poisons, the nitriles, was selected for this study. The system used in these experiments consisted of platinum black as the adsorbent, 2 N sulfuric acid as the solvent, and hydrogen and the nitrile as competitive adsorbates. The competitive may be expressed as shown in the equations H2 (soln.)
RCN (soln.)
+ HzO (ads.) = 2H (ads.) + HzO (soln.) + HzO (ads,) = RCN (ads.) + HzO (soln.)
The quantity of water in solution as well as the amount Of HZmay be assumed to be constant. The amount of nitrile in solution is considered to be the amount added. Of the three remaining quantities: H, RCN and HzO adsorbed, only the first lends itself to direct measurement, this by a coulometric method. A similar investigation has been made on the adsorption of acetic acid by Oikawa and Mukaib0.l
Experimental Equipment and Reagents.-The apparatus consisted of a small platinized platinum wire, immersed in a solution of 2 N sulfuric acid. This solution was connected by a satu-
* Chemistry Department, Baylor University, Waco, Texas. (1) M. Oikawa and T. Mukaibo, J . Electrochem. Soc. Japan, 90, 568 (1952).
rated ammonium sulfate-agar bridge to a saturated calomel half-cell, which served as a non-polarizable electrode. The cell was connected through switches to a Sargent Model XXI Polarograph and to a Fisher Type S Potentiometer. The electrodes were made by cutting 20 gage platinum wire into 2 cm. lengths which were then sealed into lass tubing. These electrodes were then platinized for &ree minutes in a 3% solution of chloroplatinic acid containing a trace of lead acetate, using an applied potential of 3 volts. The electrodes were then washed briefly in distilled water and in concentrated sulfuric acid and were aged several hours in 2 N sulfuric acid. Just prior to use these electrodes were anodically polarized a t oxygen evolution for 20 to 30 minutes. To determine the effect of pressure, a more elaborate cell was required. This cell consisted of two 200-ml. round bottom flasks as electrode compartments connected by means of a salt bridge compartment. The hydrogen halfcell was connected through a ballast tank to a water aspirator. A mercury manometer was used to measure the pressure. The temperature of the cell was controlled by using a Sargent Model S-84805 constant temperature bath. The nitriles used were the best grade available from the Eastman Kodak Company.z Procedure.-Hydrogen was bubbled over the electrode until a steady potential was attained, as indicated by the potentiometer. The flow of hydrogen was stopped and sufficient time was allowed for equilibrium to be established between the hydrogen dissolved in solution and that adsorbed on the electrode. The adsorbed hydrogen was then removed by electrolytic oxidation with the pen recorder of the polarograph recording a current-time for this oxidation. The number of coulombs passed was determined from the area under this curve. A measured quantity of poison was then added and the process repeated. The decrease in area under the curve was taken as a measure of the amount of poison adsorbed. This procedure was repeated for each poison studied a t several different concentrations of poison. (2) In order to check for the possible effect of impurities Mr. Philip Oglesby made a series of runs, first on the commercially available nitrile, then on the redistilled nitrile. There was no difference in the results.
THOMAS C. FRANKLIN AND RAYD. SOTHERN
952
The areas under the curves were measured by a planimeter, the quantity of electricity measured being between 6 and 2000 microcoulombs. In order to obtain reproducible results the current-time curves were taken with the voltage increasing over a short span, 0.6 v., rather than at a constant applied potential. This voltage span, chosen from previously run currentvoltage curves, was chosen to take in the oxidation peak of hydrogen without taking in subsequent oxidation processes.
Data and Results The data can be put into two groups: (1) The effect of the nitrile on the amount of hydrogen adjacent to the electrode. (2) The resultant effect on the potential of the hydrogen electrode. Figure 1 shows the effect of propionitrile on the
&
0.8
1
0.7
1
hydrogen electrode. Figure 2 indicates a linear relationship between the logarithm of the amount of hydrogen adsorbed (as measured by the area under the current time curve) and the e.m.f. of the cell. As will be noticed there is a good deal of scatter to the data. All of the e.m.f. data exhibited the same sort of scatter. This scatter is probably due to failure of the system to reach final equilibrium since potential measurements were made with hydrogen bubbling over the e l e ~ t r o d e . ~ The Nernst equation for the hydrogen electrode shows a linear relationship between the e.m.f. and the logarithm of the pressure of the hydrogen. Therefore it follows that there should be a linear relationship between the amount of adsorbed hydrogen and the pressure. This relationship was found to be true and is shown in Fig, 3.
0.6 .
20.5
:
7 0.3
.
3~ 0 . 4. 0
3
0.2 . I
0.1 }
L
0.8 1.0 1.2 1.4 1.6 1 log mg. propionitrile. Fig. 1.-The adsorption isotherm for propionitrile.
+
amount of hydrogen adjacent to the electrode. Plotted along one axis is the logarithm of the mil& grams of propionitrile in 140 milliliters of solvent. Along the other axis is plotted the logarithm (Aa A)/&. A0 is the area under the current time curve when H2alone is adsorbed. A is the area under the current-time curve when both H2 and the nitrile are adsorbed. Ao - A therefore corresponds to the amount of hydrogen displaced by the nitrile. AOis a measure of the adsorbing area of the electrode. Therefore (A0 - A ) / & can be pictured as representing the amount of nitrile adsorbed per unit area of adsorbent. This graph then corresponds to a log-log plot of the Freundlich equation and the resultant straight line confirms the applicability of the Freundlich equation. The other phase of the problem was the measurement of the resultant effect on the potential of the 2.0
0.4
c
i.
0.280 0.284 0.288 0.292 Electromotive force (volts). Fig. 2.-Variation of potential with amount of adsorbed hydrogen (acetonitrile).
Vol. 58
40
120
t '
4100 . .r(
80-
e
4
s
6040 .
NATIVE DEXTRAN
Nov., 1954
size is probably the primary factor in the nitrile series in blocking the electrode surface. I n summary it can be said that: (1) The effect of the nitriles on the potential of the hydrogen electrode seems to be due t o a displacement of the hydrogen adsorbed on the surface. The potential of the hydrogen electrode is governed by the concentration of adsorbed hydrogen not by the pressure
953
of hydrogen above the solution. (2) The displacement ability of the nitrile seems to be primarily a function of its size. We wish to take this opportunity to express our thanks to the Research Corporation for the financial assistance which they are furnishing for this project and to Mr. Samuel L. Cooke, Jr., for his aid in preparing this article for publication.
IMOLECULAR WEIGHT, MOLECULAR WEIGHT DISTRIBUTION AND MOLECULAR SIZE OF A NATIVE DEXTRAN BY LESTERH. AROND AND H. PETER FRANK Institute of Polymer Research, Polytechnic Institute of Brooklyn, Brooklyn, N. Y. Received December 18, 1063
Native dextran, as produced by a subculture of Leuconostoc mesenteroides NRRL B-512, was separated into a number of fractions. The fractions were characterized as to their molecular weight and size by means of light scattering and intrinsic viscosity in aqueous solutions, and their degree of branching, in terms of l-6/non 1-6 linkages, by periodate oxidations. The niolecular weights ranged from 12 to 600 million. The intrinsic viscosities were fairly low due to the high degree of branching. Flory's viscosity theory did not satisfactorily explain the experimental data. This discrepancy is robably due to the highly branched nature of the dextran molecules and the molecular inhomogeneity of the fractions. &in, a recent theoretical development by Benoit, the molecular inhomogeneity of the fractions and the deviation of their radii of gyration from the radius of a hypothetical linear molecule of the same molecular weight were estimated from the angular dependence of scattered light.
The polyglucose dextran can be produced by Leuconostoc mesenteroides using sucrose as starting material. We felt that it would be of interest to investigate the molecular weight and molecular weight distribution of this polymer more intensively than has been done in the past.' The structures, however, of several types of dextran have been the subject of previous study.2 Therefore, it also seemed worthwhile t o attempt a correlation of the structure (degree of branching) of native dextrans and its fractions with their molecular weight and size.
Experimental Fractionation.-A sample of native dextran produced by the so-called whole culture method with Leuconostoc mesenteroides (NRRL B-512) was obtained from the Dextran Corporation. The dextran is produced by massive inoculation of a medium containing approximately 10% sucrose plus cornsteep liquor and mineral salts as nutrients. The population of Leuconostoc reaches a maximum of about 1 billion per milliliter. The culture is kept as sterile as possible in order to keep out other organisms a8 for instance molds. An approximately 1% solution (pH 7.6), which proved to be quite hazy, was used in the fractionation. The solution was centrifuged (approximately 20,000 X gravity) for 50, 90 and 145 minutes. A pellet-like sediment of 2.47, (based on total solids) was obtained, independent of the time of centrifugation. This sediment, apparently an insoluble carbohydrate polymer, was not further investigated (the very low Kjeldahl, N I0.12%' ruled out the possibility of a proteinic composition). The centrifuged solution was used for fractionation. It proved to be extremely difficult to fractionate dong conventional lines. Evidently the molecular weights are so large that there are only. very small differences in the solubility of the various species. However, it was possible to prepare five main fractions by carefully adding methanol and varying the temperature. These fractions were further divided into 10
-~
(1) F. R. Senti and N. N. Hellman, Report of Working Conference on Dextran, July, 1951. (2) I. Levi, W. L. Hawkins and H. Hibbert, J . A m . Chem. SOC.,64, 1969 (1942).
(3) Performed in the laboratory of the Dextran Corporation.
subfractions. I n Table I the precipitation conditions for the 5 main fractions are given.
TABLE I PRECIPITATION CONDITIONS OF THE FIVEMAIN FRACTIONS Fraction
A B C D E
Methanol (vol. %)
42.5 42.5 42.5 42.6 43.3
Temp., O C .
Yield, %
36.6 35.5 34.6 32.3 5.0
20.0 15.6 13.4 24.3 20.8
In addition to fractions A-E, fraction 11 was separated by concentrating the supernatant of fraction E and adding a very large excess of methanol. Fraction 12 was obtained by evaporation of the final supernatant. The latter two fractions seem to be of an irregular character: 11 is very small and shows an extremely large intrinsic viscosity (Table II)4; 12 seems to contain essentially low molecular weight contaminants. The original native dextran contained 0.028% N, 0.14% of reducing sugars in terms of glucose, and lost approximately 10% on dialysis through Visking cellulose casing. Some of the low molecular weight comtaminants were probably occluded in fractions 1-10. The actual yields of the fractions are given in Table I1 (second column). In the third column the weight fractions are adjusted for losses and for the neglect of the centrifuged sediment and of fractions 11 and 12. (B) Viscosity.-Viscosities of aqueous solutions we;e measured in an Ubbelohde dilution viscosimeter at 32.7 In all cases 4 or 5 dilutions were made and qBp/c was extrapolated to c + 0. The intrinsic viscosities are given in Table IT. (C) Light Scattering.-Optical clarification of the polymer solutions proved to be difficult. The customary methods of filtrat,ion and centrifugation both failed. (Filtration using Selas 04 and ultrafine lassinter resulted in clogging of the filters; centrifugation Sed to partial molecular sedimentation of the very large molecules in the comparatively high centrifugal field.) The following procedure was found to be satisfactory: clear water was prepared by distillation and consecutive double filtration (Selas 04 filter). Comparatively concentrated dextran solutions were used as master solutions and were filtered (medium glassinter) in small increments directly into the clear water. The dilu-
.
(4) The reason for thia irregularity ia unknown.
