which was superimposable on admixture with that identified with the DD and LL diad end groups. Because the end groups of these polymers are derived from random cleavage, they should reflect the structure of an undegraded polymer chain which then must consist of either all L or all D units. Previous x-ray crystallographic studies of crystalline racemic PPO could not distinguish between a polymer chain of alternating D and L units and one of either all L or all D units ( 7 ) . Extension of the technique described here to the analyses and study of other hydroxyl containing polymers would seem obvious. In addition, certain information on the backbone stereochemistry of PPO's as a function of catalyst can be obtained with this method utilizing racemic materials while the same information has in the past
required preparation of optically active materials. Work in progress indicates that this technique is much more satisfactory than the time-consuming competitive kinetic methods (2-4) for estimating the relative amounts of primary and secondary hydroxyl groups in PPO materials partially terminated with ethylene oxide. One important advantage of this technique is the small amount of sample required. Csually about 30 to 50 mg. of a PPO (mol. wt. 4000) on trifluoroacetylation gives adequate signal on a Varian A-56/60 spectrometer. LITERATURE CITED
(1) Cross, A. D., Landis, P. W., J. Am. Chem. SOC.84, 1736, 3784 (1962). (2) Crummett, W. B., ANAL. CHEM.34, 1147 (1962). (3) Davis, D. R., Lutz, R. P., Roberts, J. D., J. Am. Chem. SOC.83,246 (1961).
(4)Hanna, J. G., Siggia, S., J. Polymer Sci. 56, 297 (1962). (5) Hendricksen, J. G., ANAL.CHEM.30, 126 (1964). (6) Manatt,' S. L., J. Am. Chem. SOC.88, 1323 (1966). (7) Stanley, E., Litt, M., J. Polymer Sci. 43, 453 (1960). (8) Takahashi, M., Davis, D. R., Roberts, J. D., J.Am. Chem.SOC.84,2935 (1962). (9) Tsuruta, T., Inoue, S., Yoshida, N., Yokota, Y., Makromol. Chem. 81, 191 (1965). STANLEY L. MANATT D. DAVIDLAWSON JOHN D. INGHAM NEVILLE S. RAPP JIMP. HARDY Space Sciences and Propulsion Divisions Jet Propulsion Laboratory California Institute of Technology Pasadena, Calif. 91103 RESEARCH sponsored by the National Aeronautics and Space Administration under Contract No. NAS7-100.
Voltammetric Determination of Aluminum by Oxidation of Its Solochrome Violet RS Complex at the Rotated Pyrolytic Graphite Electrode SIR: Solochrome Violet RS (5-sulfo2-hydroxy-benzene-az0-2-naphtho1, also known as Pontachrome Violet SW, and Eriochrome Violet B, C. I. 15670) and similar di-o-hydroxyazo dyes have been used for the polarographic determination of aluminum ( 2 , 6 , 8 , 9) and several other metals. These methods rely on the ability of the metal to form a Solochrome Violet RS (SVRS) complex which is reduced a t the dropping mercury electrode a t a potential more negative than that of the free dye reduction. A solution containing a mixture of free SVRS and its metal complex exhibits two discrete reduction waves, and the concentration of metal may be determined by measuring either the height of the metal complex wave, or the decrease in height of the free dye wave. Single sweep polarography has unique advantages for the determination of metals with di-o-hydroxyazo dyes, and aluminum has been determined a t the 0.5-p.p.b. level using a Southern Instruments cathode-ray polarograph ( 3 ) . Conventional polarographs, however, are limited to a sensitivity of about 100 p.p.b. of aluminum (9). This is partly because of their lower inherent sensitivity, but mainly because a t low concentrations these instruments cannot adequately resolve the free dye and dye complex waves which, in the case of aluminum, are separated by only 0.17 volt. We have found that the sensitivity of the method may be increased considerably by using the oxidation, rather
CONTACT TO POLAROGRAPH 25 crn. GLASS TUBE 0.6 crn. O.D.
0-4crn. I.D. MERCURY
than the reduction, waves of SVRS. At a rotated pyrolytic graphite electrode both the dye and its aluminum complex produce well-formed oxidation waves, which are separated in an acetate buffer by 0.35 volt. This allows aluminum to be determined a t concentrations as low as 10 p.p.b., even with quite simple polarographic apparatus. Other advantages are that the test solution need not be deaerated, and several metals that interfere in the reduction method have no effect when the oxidation waves are used for measurement. The current-voltage curves formed a t the rotated pyrolytic graphite electrode are reproducible, and the electrode requires no pretreatment other than to remove the dye oxidation products from the electrode surface a t the end of each determination. EXPERIMENTAL
O,5crn. PY ROLYTI c GRAPHITE PLUG
0.3 crn. DIAM. Figure 1.
Pyrolytic graphite electrode
Designed for mounting in a Sargent synchronous rotator
Apparatus. The pyrolytic graphite electrode ( 7 ) is shown in Figure 1. It was mounted in a Sargent synchronous rotator that had a rotation speed of 600 r.p.m. The electrode should be cleaned of oxidation products a t the end of each measurement, or low currents will result. The cleaning can be performed simply by allowing the electrode to rotate for a few seconds in warm 1M NaOH, followed by warm 5ill HC104. The test solution was contained in a water-jacketed cell, fitted with a Teflon cap drilled to take the electrode shaft and a reference anode. All results were obtained a t 30.0" =t0.2' C., and potentials quoted refer to the S.C.E. VOL. 38,
NO. 8,
JULY 1966
1065
The pen-recording polarograph was a 2-electrode O.R.N.L. Model Q-1338, equipped with an RC derivative circuit. Reagents. The grade of Solochrome Violet RS obtained from dye manufacturers-e.g., Geigy Pty. Ltd.-may be purified sufficiently for analytical use by recrystallizing once from 50% ethanol. The material used in this work was synthesized by coupling diazotied o-aminophenol-p-sulfonic acid with 2-naphthol in sodium hydroxide solution. The dye was precipitated by adjusting the solution to p H 4,it was filtered, then recrystallized twice from 50% ethanol and dried in an air oven at 100' C. Elemental and controlled potential coulometric analysis showed the purity to be 98-99% as ClJ311?rT~OsS. Na. Procedure. Fume the sample almost to dryness with HC104 to ensure that all elements are oxidized to a high valency state. Transfer an aliquot of the sample solution, containing 3-15 pg. of aluminum, to a 25ml. volumetric flask. Add 2.00 ml. of a 6.25 X 10-4;M aqueous solution of SVRS, and 5.0 ml. of a lM, pH 4.7 sodium acetate-acetic acid buffer mixture. If the amount of aluminum in the sample aliquot is outside the range given, use a proportionally smaller or larger concentration of SVRS.' Cool, transfer t o the voltammetric cell, and record the current-voltage curve between +0.3 and + 1 . 1 volts. Both the height of the aluminum complex wave (Eliz = + O M volt), and the decrease in height of the blank free dye wave (Eli2 = +0.53 volt) are proportional to the aluminum concentration. The measured wave heights should be compared with those of a standard containing the same total concentration of SVRS as the sample.
Figure 2. Current-voltage curves of Solochrome Violet RS and its aluminum complex at the rotated pyrolytic graphite electrode (pH 47.0) 5 X 1 O-sM SVRS, 0.2M acetate buffer 1.5 X 10-%4 AI3+, 0.2M acetate buffer (6) 5 X 10-sM SVRS
+
(a)
of the dye oxidation wave recorded under the same conditions (Table I), which suggests that SVRS undergoes a 2-electron oxidation to an azoxy derivative (I@,
Ell2 = +0.720
- 0.040 pH (pH 2.5-10.5).
The corresponding equation for the aluminum-SVRS complex was,
-03s
RESULTS AND DISCUSSION
Solochrome Violet RS is reduced a t the dropping mercury electrode in a 4-electron step involving formation of an unstable hydrazo compound which disproportionates a t the electrode surface (4, 5). The reduction wave produced a t the rotated pyrolytic graphite electrode has nearly twice the magnitude
-0,s A plot of Eli2 against pH for oxidation of the free dye was a straight line which fitted the equation,
Table I. Voltammetric Behavior of Some Azo Compounds at the Rotated Pyrolytic Graphite Electrode
Compound"
Limiting current (pa.) Oxid&- Reduch/~ V. 0s. , S.C.E. tion tion Oxidation Reduction c c
Azobenzene*
Azobenzene-4-sulfonic acid 4-Hydroxyazobenzene-4'sulfonic acid
1.42d 1.31
e c
+-0.805
n
Value for reduction at the D.M.E.e
-0.36d -0.345
2 2
-0.335
2
1.58
1.75
1.97
2.60
+0.695
-0.361
2
