A chromatogram of bis(acety1acetonato)beryllium(II) [Be(AA)2]and tris(acetylacetonato) aluminum( 111) [Al(AA),] is shown in Figure l . The aluminum complex appears only after the column temperature is greater than 120' C. whereas the beryllium complex can be eluted at temperatures as low as 75O
L 1, 0
5
10
TIME (MiN)
Figure 2. Gas chromatographic separation of aluminum and chromium acetylacetonato complexes in carbon tetrachloride Sample dze, 0.34 MI, Column, 4 feet X % Inch of 1 % by welght Aplezon L on gloss beadr, 200 mlcronr In dlameter Column temperature, 170' C. Flow rate of argon, 4 3 cc. per minute Detector voltage, 2000 v. Senrltivlty, X10
c.
A chromatogram showing the separation of aluminum and chromium complexes from a carbon tetrachloride solution is shown in Figure 2. The CClr does not appear as a positive peak since it is eluted with the air and lost in the negative air peak. This work ,is being actively extended toward the treatment of samples and the establishment of optimum conditions for the determination of various volatile metal complexes.
LITERATURE CITED
(1) Freiser, H., ANAL. CHEM,31, 1440 (1959). (2) Hishta, C., Messerly, J. P. Reschke, R.F., Fredericks, D. H., Coofte, W. D., Ibid., 32,880 (1960). (3) Juvet, R. S., Wachi, F. X, Zbid.,
32,290 (1960).
W. J. BIERMANN HYMAN GESSER Department of Chemistry University of Manitoba Winni eg, Manitoba Canaci
RECEIVEDfor review July 11 1960. Accepted August 4, 1960. bra,teful acknowled ment is made to the National Research Eouncil of Canada and to the Imperial Oil Coo of Canada for financial sup ort which made possible the purchase ofe!t gas chromatograph.
Alternating Current Voltammetry at Solid Electrodes SIR: Alternating current polarography with a dropping mercury electrode is R well-established technique (1, 2) Few applications of the method have been reported using solid electrodes. A solid electrode is indicated in the study of electrode processes more anodic than the dissolution of mercury. Reproducible ax. response can be obtained by a set method of electrode pretreatment or preparation. A.c. voltammetry consists of the superposition of a small alternating voltage of constant magnitude on a conventional voltammetric scan. A. plot of the alternating current us. the voltage applied to the polarized electrode gives peak-shaped waves. The presence and shape of these peaks provide a qualitative measure of the reversibility of the electrode process, since they are noted only in cases where some reversible process may take place at the frequency of the alternating voltage. Small alternating voltages are necessary for maximum resolution of peaks, or for theoretical treatment of the alternating currents (8, 4 ) . The compounds studied were used to test instrument response and to develop the conditions necessary to obtain usable results. No attempt was made to develop routine analytical methods, or to resolve complex mixtures. Concentrations were 10-6X in electroactive species or higher. I
APPARATUS
The equipment was designed to operate in conjunction with a Leeds & Xorthrup Model E Electro-Chemograph without altering it for use as a normal voltage-scan instrument. This was done by interrupting the polariza1526
ANALYTICAL CHEMISTRY
tion circuit of the Electro-Chemograph, and inserting the auxiliary circuitry between the polarizer and the recorder. A 4PDT switch maintains the normal d, c. circuit, or connects the ax. circuit. The 60 C.P.S. sine wave was .derived from the power lines by mitable transformers. Resistors form a voltage divider that permits selection of alternating voltages from 2 to 20 mv. in 2-mv. increments. A recent modification has replaced the system of transformers with a Hewlett-Packard Model 202A function generator. The ax. circuit is parallel to the d. e. circuit, and makes use of a third electrode to provide a low impedance path
of less than 20 ohms, exclusive of cell resistance, A capacitor prevents direct current in the a x . branch, and a choke limits alternating current, in the d.c. branch. This is shown in Figure 1. A Leeds & Northrup shielded transformer (Std 21178-39) detects the alternating current as a voltage drop across a resistor in series with the electrode. The high impedance side of the transformer is connected to a Dumont Model 407 transistorized preamplifier which has a common mode rejection of 106 for a gain of 10. Further amplification is provided by the vertical deflection amplifier of a Dumont Model 403 oscilloscope. A Heathkit Model 7A VTVM is
TO A C . LINE
L ECG
Figure 1.
Diagram of the alternating current polarograph
ECG-Leeds & Northrup Model E Electro-Chemograph VTVM-Heathkit Model 70 vacuum tube voltmeter CRO-Dumont Model A03 cathode ray oscilloscope PRE-Dumont Model 4 0 7 transistorizer preamplifier C-6000.microfarad 15 WV capacitor 1-1 0-henry choke, 100 ohms d x . resistance RI-1 000-ohm precision resistor RQ-1 to 1 0-ohm precision decade resistor R3-1 0-ohm precision resistor TI General Radio adjustable transformer T2-1 17- to 6.3-volt tilament transformer T3-leeds & Northrup shielded tronsformer (STD 21 1 7 8 - 3 9 )
-
study of these effects on a x voltammetric waves is in progress (10).
100
EXPERIMENTAL RESULTS
00
60
40
a-
7 20
1
0'1
E Figure 2.
vs.
om2
03
0 4
0,5
S. C , E.
Alternating current voltammetric waves
Oxidation of 3.36 X lO-*M o-dianisldlne In 1 M HzS04 (8.mv. a.c.) E , I . Oxidation of p-hydroxydlphenylamlne in Britton and Robinson buffer of pH 2, In molar sodium sulfate (8-mv. a.cJ 2. Reverse scan of same solutlon
A.
used to detect and rectify the amplified signal, which is then fed as a current output to the Electro-Chemograph recorder. All recorder range selections, damping and zeroing functions, and voltagespan selections of the Electro-Chemograph are available while the unit is functioning as a n a x . instrument. Observation of the signal wave form or Lissajous figures on the oscilloscope is possible a t all times. Substitution of precision resistors in place of the cell showed linear recorder deflection us. a.c. All circuit leads were shielded and grounded. The electrode system consists of a polarizable electrode, platinum foil, and a saturated calomel electrode with a salt bridge. The polarizable electrode and S.C.E. are in the d.c. branch while the ax. branch consists of the polarizable electrode and the platinum foil in the form of a cylinder. A platinum wire, a carbon paste electrode (9), and a graphite rod have all been used successfully as polarizable electrodes for a x . voltammetry. Typical background media are 111 H2S04, 1114 ?;a2SOd, and Britton and Robinson buffers in 1J1 SasS04. The high concentration of electrolyte is necessary to keep the solution resistance lon-. The acetonitrile used contained less than 0.2 gram of water per liter. The platinum electrode was pretreated by dipping in chromic acid with subsequent cathodization. Hydrogen was then removed by anodization a t 0.1 volt cs. S.C.E.The a.c. response of platinum electrodes is altered by the prcsencc of oxides on the electrode. A
+
The ax. waves for the oxidation of iodide at a platinum electrode have been reported (6, 61, and these were verified with this apparatus. I n addition, these waves were obtained with the carbon paste electrode, and with a graphite rod. Good a x . waves were obtained at platinum electrodes for the oxidation of ferrocyanide in 1M Na2S04 so long as the electrode was free from oxides of platinum. A.c. voltammetric waves have been recorded for the osidation of o-dianisidine (Figure 2 4 ) ) p-hydroxydiphenylamine, o-tolidine, X,S-dimethyl-pphenylenediamine, several naphthylaminesulfonic acids, dimethylaniline, and p-phenylenediamine. The a x . technique detected products which were formed during the voltammetric scan in the cases of p-hydrosydiphenylamine and dimethylaniline. The osidation of p-hydroxydiphenylamine gave one peak on the initial scan, and two peaks for subsequent scans (Figure 2 , B ) . The oxidation of dimethylaniline gave one peak for the initial scan, and three peaks for subsequent scans. The new peaks were identified as the peak for tetramethylbenzidine, and a n unknown coupling product formed by the osidized tetramethylbenzidine and excess dimethylaniline. The a x . peaks for
I E Figure 3.
vs.
S. S.E
Alternating current voltammetric waves
1,l '-Diethylferrocene, 2.7 X 1 O-5M Ruthenocene, 1.63 X 1 0 - 4 M 3. Osmocene, 7.44 X 1 O-5M
7. 2.
All in acetonitrile with 0.2M lithium perchlorate (8-mv. 0.c.)
VOL. 32, NO. 1 1 , OCTOBER 1960
1527
the dimethylaniline oxidations were obtained with all three types of polarizable electrodes. Chronopotentiometric studies have indicated that the rate of the electrode process is a function of the metal for metal cyclopentadienyl compounds (7, 6 ) . A.c. voltammetry of these compounds in acetonitrile gave well-defined waves for ferrocene, 1,l'-diethylferrocene, and phenylferrocenylcarbinol. Osmocene and ruthenocene gave no peaks (Figure 3). Hence, a qualitative indication of reversibility was easily obtained. CONCLUSIONS
A.c. voltammetry at solid electrodes extends the ax. method t o anodic processes and shows promise for the study of the oxidation of many organic compounds. It facilitates the study of electrode mechanisms by permitting the detection of small amounts of reaction products formed during the voltammetric scan. The technique is sen-
sitive to variations in electrode surface conditions, and can be used to evaluate electrode pretreatment methods. The technique may be used in nonaqueous media. A.c. voltammetry is not greatly influenced by stirring (6). I n conjunction with solid electrode systems, this feature may prove advantageous for continuous process-stream monitoring. ACKNOWLEDGMENT
Acknowledgment is made of the assistance given by Theodore Kuwana in the investigation of the metal cyclopentadienyls. Acknowledgment is also made of the recommendations given by J. U. Eynon of Research and Development Department of the Leeds & Northrup Co. for the connection of the a x . apparatus to the Electro-Chemograph. LITERATURE CITED
(1) Breyer, B., Gutman, F., Bauer, H. H., &err. Chemiker-Ztg. 57, 67 (1956). (2) Breyer, B., Gutman, F., Hacobian, S.,
Australian J . Sci. Research 3A, 558 (1950). (3) Breyer, B., Hacobian, S., Australian J. Chem. 7,225 (1954). (4) Delahay, P., Adams, T. J., J. Am. Chem. SOC.74,5740 (1952). (5) Juliard, A. L., Delaware Valley Con-
ference, Philadelphia, Pa., American Chemical Society, 1957. (6) Juliard, A. L., Nature 183,1040 (1959). (7) Kuwana, T., Bublitz, D. E., Hoh, G., Chem. & Ind. 1959,635. (8) Kuwana, T., Bublitz, D. E., Hoh, G., J . Am. Chem. SOC.to be published. (9) Olson, C., Adams, R. N., Anal. Chim. rlcta. 22, 582 (1960). (10) Walker, D. E., Adams, R. N.,
University of Kansas, Lawrence, Kan., unpublished data. DONALD E. WALKER RALPH?IT. A D A J ~ S Department of Chemistry University of Kansas Lawrence, Kan. Houdry Process Corp. Linwood, Pa.
ANDRI~ L. JULIARD
RECEIVEDfor review Junp 10, 1960. Accepted July 25, 1960. Work supported in part by the Research Corp.
Sulfonated Phenylstearic Acid as a Polarographic Maxima Suppressor SIR: Attempts to determine the critical micelle concentration of sulfonated phenylstearic acid (SPSA) by polarography did not yield precise information; however, the effect of this compound on the polarographic maxima of several simple metal and complex ions was studied. The results of the influence of SPSA on the polarographic waves for the reduction of Si+2, C O + ~iodide-cad, mium complex, Pb+2, cystine, copperglycine and copper-biuret complexes, and in the Ni+2-Co+2mixture are described. EXPERIMENTAL
SPSA was prepared (4) by condensing pure oleic acid and benzene (dry and free from thiophene) in the presence of anhydrous aluminum chloride and sulfonating the product. Its strength was determined by titration against caustic soda solution of known strength. Solulions of the required strength were obtained by diluting with doubly distilled water. The salt solutions were obtained by dissolving analytical reagent grade samples in doubly distilled Fater. Solutions of cystine and glycine (British Drug Houses) were obtained by dissolving knoa-n amounts in 0.02N HC1 and doubly distilled water, respectively. The biuret was prepared (1) in this laboratory. A Langcs Polarometer, Model 111, 1528
ANALYTICAL CHEMISTRY
KO. 46 was used with a multiflex galvanometer Type MGF2 (at sensitivity 1:lO). The dropping mercury and saturated calomel electrodes were immersed in a water thermostat maintained a t 30' C. Purified nitrogen was passed through the cell to maintain an inert atmosphere. Conductivity measurements were carried out a t 30' C. using a conductivity bridge No. L 350140 and a conductivity cell K =
Table I. Data from Polarogram Ion or Com- Maximum Polarographic Suppression Micelle plex Point, M a Concn., M b Studied
Pb +2 Ni +2 c o +2
Iodidecadmium Copperglycine Copper biuret Cystine-, c o -2-h
1 +2
17.38 X 43.65 X 17.38 X
9.8 X 8.2 X 4.9 X
16.98 X lo-'
12.1 X
16.6 X 8 3 . 3 X 10-5 39.81 X 10-5
4.9 X 13.2 X 9 . 1 3 X 10-6
mixture 36.31 X 10-4 12.1 X im/id vs. log c 4 Obtained by plottin and extrapolating to i m j d = 1. Value taken from the point where the first sharp discontinuity appeared. These values are comparable t o the critical micelle concentration value for SPSA as determined by conductivity measurements. The value in m-ater is 45.4 X 10 - b M .
0.1; L. 355216 (Cambridge Instruments CO.). RESULTS AND DISCUSSION
h typical polarogram is shon-n in Figure 1. SPSA was nonreducible a t the dropping electrode and its polarogram gave only a hydrogen wave. Additional data are given in Table I. I n all cases the maximum suppression point values are almost of the same order except for Cd14-2 and the Cof2-Sit2 mixture. This is because maxima in these cases are pronounced and a large amount of the suppressor is needed. Lingane and Kerlinger (3) observed two maxima for the cobalt-nickel mixture using an automatic recording polarograph. They suppressed the first maximum (smaller one due to nickel) with 0.01% gelatin but were unable to suppress the second one (due to cobalt) even with 0.05% gelatin. Using a manual polarograph, we could not realize the maximum due to nickel, but could suppress the second maximum with 55.5 X lo-*.If SPSA. A reversible wave for cobalt in pyridine with A/,= -1.08 volts was obtained after elimination of the maximum. Kolthoff and Lingane (2) found that the iodide-cadmium complex (probably Cd14-2) gave a pronounced maximum 11-hich was difficult to suppress. By
