with the JOHN D. VOORHIES and STANLEY M, DAVIS Organic Chemicals Division, American Cyanamid Co., Bound Brook, N.
A modification of direct constant current coulometry involving the quantitative adsorption of an electroactive species onto acetylene black from solution and the subsequent quantitative electrolysis of the adsorbed sample is described. Analytical coulometric results for the cathodic reductions of anthraquinone, 4-nitropyridine- 1 oxide, FeS3, and C U + ~ are presented. Carbon electrode coulometry appears to have greatest potential utility in the range of semimicroanalysis of electroactive organic compounds of low water solubility. Samples of 0.1 to 2.0 mg. can be electrolyzed quantitatively on an electrode of compressed acetylene black of about 0.1 gram. The best results were obtained by electrolysis of the adsorbed electrode in Concentrated salt solutions. In dilute aqueous electrolytes, desorption of the electroactive species and possibly the preferential adsorption of hydronium ions are problematic.
-
has found wide application in analytical methods involving direct bulk solution electrolysis, electrochemical generation of titrants, and the electroreduction or oxidation of surface films of oxidized metais or metals, respectively (1, 3, 6, 7 ) . The present paper describes a new coulometric technique involving the quantitative adsorption of organic compounds or inorganic ions from a sampie solution onto an electrode of acetylene black, and the subsequent quantitative electrolysis at constant current or controiled potential. As in the case of the coulometry of surface films, this technique has the advantage of confining the total sample t o be determined to the surface of the electrode, thus eliminating the problem of efficient reactant-electrode contact. Also, since the electrolysis is run in a quiet solution, cross contamination of anode and cathode with their respective electrolysis products is not problematic. A ‘‘redox buffer” or generating medium is employed in most constant current coulometric methods involving oxidation-reduction, In such cases, the species to be determined may exchange electrons directly with an inert electrode or with a chemical intermediate generated a t that electrode. The present technique is unique for organic
J.
compounds in that it is direct constant current coulometry and may have special application to compounds which cannot be dissolved effectively in electrolytic solutions. Certain reducible organic compounds, such as quinones and nitroaromatics, when mixed with carbon black and compressed into an electrode which is saturated with an electrolyte solution, depolarize the electrode at an electromotive force (e.m.f,) close to the apparent reduction potential of the organic compound [as measured by dropping mercury electrode (D.M.E.) polarography]. Constant current coulometric studies in this laboratory and by Glicksman and Morehouse (4) have indicated that electrodes prepared by mechanical mixing of the organic compound with acetylene black do not reduce the sample of organic compound
OULOMETRY
s
completely. However, if small amounts of certain reducible compounds are adsorbed physically onto the acetylene black by passing a sample solution through a cathode compartment containing acetylene black (see Figure l), the sample of adsorbed compound can be reduced quantitatively. Although the present paper describes a technique of adsorption from solution, adsorption from the gas phase and pure liquid phase may also be feasible ways of preparing the carbon electrode. Acetylene black was chosen for an electrode material because of its high purity, low redox blank, high conductivity, and good wettability. Other carbon blacks show better adsorption affinity for organic molecules, but contain reducible and/or oxidizable impurities (8). Also, acetylene black has a high hydrogen and oxygen overvoltage, thus allowing a wide polarization range for both reductions and oxidations in aqueous electrolytes. A constant current technique m s found to be most effective for COLIlometry with the acetylene black electrode. When controlled potential coulometry was applied to the reduction of anthraquinone on acetylene black, a very high background current was encountered making quantitative analysis difficult. The utility of carbon electrode coulometry a t present appears to be in the area of semimicroanalysis of electroactive organic compounds. An example of this application is the quantitative four-electron reduction of 0.4 mg. of 4-nitropyridine-4-oxide adsorbed on acetylene black with a precision to = =kl,9% for five replicate reductions. EXPERIMENTAL
F Figure 1. Carbon blackelectrode compartment A. Acetylene black E. Brass bearing C. Glass electrolysis compartment (length = 18 em., diameter = 1.2 cm.9 E. Electrolyte F. Glass frit G. Graphite sollector electrode S. Rubber stopper W. Weight(1 kg.)
Apparatus. A block di’agram of the apparatus for constant current coulometry is given by Voorhies and Furman (8). The current supply has been modified to deliver constant currents in the 0- to 2-ma. range. Some of the initial work was done without a recorder by plotting millivolt readings from the pH meter a t time intervals recorded on a stop watch. Carbon Electrode Preparation, Figure 1 shows the carbon electrode compartment. This compartment is inserted into a filter flask for the purpose of applying a controlled vacuum t o the glass frit. A weighed VOL. 32, NO, 13, DECEMBER 1960
e
1855
-~ -
\ C
Table 1.
Summary of Analytical Results End Point, Coulombs to E.P. Current, Volt VI. Theory Sample, Mg. Ma. B.C.E. Observed (4Z) Blank A. Anthraquinone adsorbed onto 0.12 gram of acetylene black from CH&N solution (0.150 gram of anthraquinone/250 d.), Electrolyte = 33% ZnCle, 20% "&I, 47% Hz0 0.80 0.80 1.04 1.20 1.20
-0.2
o
Figure 2.
+az -0.4 -02 VOLTS va S.C.E.
o
0.673 1.093 1.091 1.915 1.528
+a2
Typical potential vs. time
curves Left. 0.40 mg. (0.18 mL/aqueous rolutlon) of 4-nitropyridine-1 -oxide adsorbed onto 0.1 0 grom of acetylene black 47% Electrolyte. 33% ZnClz, 20% "&I,
Ha0
Current. 1.901 ma. Right. 1.20 mg. (2.00 ml. CHaCNsolution)of anthraquinone adsorbed onto 0.1 2 gram of acetylene black 47% H z 0 Electrolyte. 33% ZnClz, 20% "&I, Current. 1.915 ma.
RESULTS
Typical potential us. time curves for the cathodic reduction of anthraquinone and 4-nitropyridine-1-oxide adsorbed onto acetylene black are shown in Figure 2. The time to a potential occurring a t the potential break, C, from the initial current-on point, A , is multiplied by the constant current to give an analytical value in coulombs. This value includes a blank made up of the charging current and the electroreduction of inipurities in the sample solution and on the acetylene black. This blank (see Table I) i s somewhat variable, but an average value of 0.1 coulomb for the 0.10- to Q.12-gram
56
e
A ~ A ~ Y ~ ~CHEMISTRY CAL
I
1.483 1.483 1.928 2.225 2.225 2.225 2.225 2.225 2.225 2.225
$0.169 $0.078 $0.062 -0.257 +O. 171 +O. 136 +O ,084 +o. 118 +O ,193 t-0.238
B. FNitropyridine-1-oxide adsorbed onto 0.10 gmm of acet -lene black from HzO solution (0.40 gram/lOO ml.). Electrolyte = 33% ZnCl,, 20& NHaC1, 47% H20. 0.40 1.901 -0.15 1.315 1.207 +O ,108 1 .go1 1,332 1.207 +o. 125 1.901 1,901 1.901
1.268 1.289 1.298
1.207 1.207 1.207
+0.061
+0.082
+0.091
adsorbed onto 0.10 gram of acetylene black from aqueous ferric nitrate solution (0.34M Fe+*). Electrolyte = 1M KNOa. Fef3 + Fe+2 1.90 (0.10 ml.) 1.874 $0.20 2.11 3.28 -1.17
C.
amount of acetylene black is added to the compartment and tamped firmly into the bottom. A known amount of test solution is ther; added and drawn through the acetylene black at a roughly controlled flow rate such that the compound to be determined is adsorbed quantitatively. The graphite collector electrode, which carries the current to the compressed acetylene black, is then introduced carefully so that all of the acetylene black is trapped under it. Then n small amount of the electrolyte is added above the collector and several drops are drawn through the compartment. The electrode is placed in the electrolysis cell and the weight is positioned. Leads are connected and the electrolysis current is applied. Chemicals. Acetylene Black. Shawinigan Products Corp., 5070 compression. Acetonitrile. Matheson Coleman & Bell, Practical (b.p. 80" to 82" C.), Anthraquinone. Purified sample, ni.p. 284" to 286.6" C. 4-Nitropyridine-1-oxide.Prepared in this laboratory, m.p. l6Ooto 161.5' C.
1 . 652b 1.559* -0.510 1,990* -0.53 1 968b 2.396 -0.53 12.361 2.309 2.343 2.418 2.463 Av. = 2.382 (r = i 2 . 3 2 %
-0.50a -0.52a
&+a
1.874 1.874
+O,20
2.13 2.38
+o. 20
3.28 3.28
-1.15 -0.90
D. C u f 2adsorbed onto 0.10 gram of acetylene black from aqueous cupric sulfate solution 1.19 (0.10 ml.)
(0.187M Cufa). 1.880 1.880 1.880 1.880
Electrolyte 1M KNOa. Cu+z-+ Cu" -0.20 -0.20 -0.20 -0.20
3.83 3.75 3.70 3.99 3.87 Av. = 3.83 u = f3.0% a Taken from manual potential vs. time plots (pH meter and Average of four to six measurements.
acetylene black cathode could be used for reasonab!y accurate quantitative analyses. The slight prewave, B, for anthraquinone reduction cannot be explained. It is possibile that it represents a different state of adsorption from that represented by the main depolarization region from B to C. This prewave is not very reproducible but appears at all sample sizes and current levels investigated. Table I shows the analytical results obtained for two representative organic compounds and two inorganic redox systems. Although the examples shown are all cathodic reductions, it is also possible to do anodic oxidations on this electrode. The cathodic reduction of ferric ion adsorbed from ferric nitrate solution onto acetylene black in LW KETO8 (see Table I) showed a well defined potential us. time curve. The e.m.f. values a t the half wave point (where time time to end point) should corre-
-
3.61 3.61 3.61 3.61 3.61
+0.22 $0.14 +0.09 +O .38 +0.26
stop watch).
spond roughly to the standard redox potential of the ferric/ferrous couple. TO study this correspondence, a sample of ferric ion adsorbed onto acetylene black was reduced cathodically and subsequently oxidized anodically by reversing the leads to the current source. The half wave potentials observed are shown below: Eli2
(anodic)
+O. 57
E o (6)
(cathodic)
f0.53
+o. 34
--
(ill1 values are reduction potentialsvolts us. S.C.E.) Thus the ferric/ ferrous couple appears to be dectrochemically irreversible under these experimental conditions. The low coulometric value for the ferric reduction (Table I, C) in 1M KN08 is probably indieative of incomplete adsorption of the ferric ion onto acetylene black. When the ratio of acetylene black to ferric was raised by B factor of two in order to increase the number of adsorption sites, the
coulometric result was still 20 to 30% low. An attempt to precipitate the ferric ion onto the carbon black by adding dilute NaOH to the cathode compartment was also unsuccessful. Cupric ion, on the other hand, was adsorbed quantitatively onto acetylene black from aqueous solution and the cathodic reduction of the adsorbed sample was quantitative (Table I , D)in 1JP KNOB. The adsorption of atmospheric oxygen onto the acetylene black as a possible source of error in cathodic COUlometry was studied for the case of anthraquinone. Air was drawn through an anthraquinone-acetylene black cathode compartment for about 4 minutes a t a negative pressure similar to that used to draw the sample solution through. The constant current electrolysis of the resulting electro2e showed no measurable air blank. DISCUSSION
The success of carbon electrode COUlometry depends on establishing the conditions for quantitative adsorption, choice of electrolyte, ratio of sample to carbon black, and electrolysis current. The quantitative adsorption requirement limits the applicability of this techrique. Xany electroactive organic compounds have a low affinity for acetylene black and even if they can be adsorbed from the test solution, they may be desorbed in the electrolyte. For this reason, it is necessary to use small volumes of concentrated test solutions in the adsorption step and concentrated salt solutions for the electrolyte. In the case of anthraquinone a concentrated zinc chloride-ammonium chloride-water solution (33 * 20 :47 by weight) with a zinc anode gave good analytical results. When 1N HCI was used as an electrolyte, the carbon black electrode polarized to the hydrogen overvoltage upon application of the current followed by slow back-polarization to the potential of anthraquinone reduction. It is probable that this phenomenon is caused by the adsorption of large amounts of hydrogen ions onto the eiectroreduction sites in the cathode nhen dilute aqueous solutions of low pH (0 to 5 ) are used. In concentrated salt solutions such as the zinc-ammonium chloride electrolyte (pH = 3.9), hydrogen ions are excluded from the electrical double layer a t the electrode surface by virtue of the large excess of zinc and ammonium ions in solution. The high electrolyte concentration may also serve to keep the electroactive adsorbed species salted onto the electrode surface. The ratio of sample to carbon black is important in both the adsorption and electrolysis steps. Because of the
limited number of adsorption sites on acetylene black, only small samples can be adsorbed effectively. Although no quantitative studies have been performed to determine the maximum sample to electrode weight ratios, the approximate maximum ratio for the adsorption of anthraquinone from acetonitrile solution is 2 mg. of anthraquinone t o 100 mg. of acetylene black. It is possible that an improvement in the analytical results of this technique may be obtained by variations of the carbon black electrode and some revision of the adsorption step. Approaches to improvement of the carbon black electrode include preparation of a high surface area carbon black whose surface is not reducible or oxidizable in aqueous electrolytes. Such a purified high surface area black would be capable of adsorbing larger samples of an organic compound than the corresponding acetylene black electrode. This large ratio of adsorbed compound to carbon black might result in better precision and accuracy of the coulometric analysis. The minimum effective sample to electrode ratio is determined by the nature of the electrolysis step. If this ratio is too small, the electrode is not depolarized effectively during constant current electrolysis and the polarization break at the end point is not well defined. Current density is also an important factor in the electrolysis step. The definition of the polarization break a t the end point improves with increasing current density. However, in the electroreduction of adsorbed anthraquinone, the coulometric value is lorn a t high current density (see Table I), indicating an incomplete four-electron reduction. The cathodic reduction on acetylene black of both anthraquinone and 4nitropyridine-1-oxide proceeds by a four-electron change and the probable products are represented as follows: 0
hydroanthraquinone. Polarographic studies in this laboratory have shown that anthraquinone reduces by a twoelectron change, probably to 9,lOdihydroxyanthracene a t pH 2 5, but the keto form of the product1,4-dihydroanthraquinonenamely, also reduces by a two-electron change a t about the same potential. In coulometric experiments, it is possible that the initial reduction product of anthraquinone, while still adsorbed onto acetylene black, ketonizes in contact with the acidic zinc electrolyte to 1,4dihydroanthraquinone, which reduces by another 2-electron change to 1,4dihydro - 9,lO - dihydroxyanthracene. Moreover, the low coulometric result shown in Table I, A (1.915 ma.), may be a manifestation of s l o ~ketoat nization of 9,lO-dihydroxyanthracene low electrolysis time. ACKNOWLEDGMENT
The authors acknowledge the contribution of James Qoddera who carried out a large part of the experimental work. They also thank James Parsons and Charles Maresh for their suggestions and encouragexent. LITERATURE CITED
(I) Campbell, W. E., Thomas, U. B.,
Trans. Electrochem. SOC.76, 303 (1939). (2) Drushel, H. V., Hallurn, J. V., J . Phys. Chem. 62, 1502 (1958). (3) Furman, N. H., J . Electrochem. Soc. 101, 19@ (1954). (4) ?licksman, R., Morehouse, 6. IC.> Ibad., 105,299 (1958). (5) Latimer, W. If., “The Oxidatio!i
States of the Elements md Their Potentials in Aqueous Solutions,” Prentice-Hall, New York, 1938. (6) Lingane, J. J., “Electroanakftical Chemistry,’’ 2nd e!,. Chapters XIX, XX. XXI. Interscience. New York,
1958. ( 7 ) Robson, H., Xuwana, T., ANAL. CHEW32,567 (1960). (8) Voorhies, J. D., Furman, N. H., Ib%d.,30,1656 (1958).
OH
0
I’
It is unusual that the electroreduction of anthraquinone adsorbed on acetylene black proceeds by a fOUr-eleCtron Change In an apparent single step to tetra-
F :2$z?. ~ ~ , ; r ? ~ ~ p r g Analytical Chemistry, 138th Meeting, ACS, Xew York, N. Y., September 1960. VOL. 32,
NO. 13, DECEMBER 1960
* 1857
