certain types of reducible organic matter make necessary the separation of the iodide before its determination. When iodide and iodate are present simultaneously in a sample, both may be determined by polarographic measurement before and after oxidation. LITERATURE CITED
(1) Custer,
J. J., Natelson, S., ANAL. CHEW21, 1005 (1949).
(2) Endres, G., Kaufman, L., Z. physiol. Chem. 243, 144 (1936). (3) Kolthoff, I. M.,Jordan, J., ANAL. CHEM.25, 1833 (1953). (4) Kolthoff, I . &I., Tanaka, K.,Ibid.,
26. 632 --- f 1954). (5) LaTl&en, H. A:, Jennings, J. P., Park, T. D., IND.ENG. CHEW, ANAL.ED. 18, 355 (1946). (6) Lamb, A. B., Bray, W.C., Geldard, W. J.. J . Am. Chem. SOC.42. 1640 > - - - -
I
(i92oj.
( 7 ) Lingane, J. J., IND. ENG.CHEM., ANAL.ED. 16, 329 (1944).
Behavior of Rotating
Gold
Rylich, A , , Collection Czechosin. Chem. Communs. 7, 288 (1935). Shahrokh, B. K.,Chrsbro, R. AI., AKAL.CHEM.21, 1003 (1949). Taylor, J. K.,Ihid., 19, 3 i O (1947). Wooster, W. S., Farrington, I'. S., Sxift, E. H., Ihid., 21, 1457 (1949). Zintl, E., Betz, IC.,2. anal. Chem. 74, 330 (1928).
RECEIVED for review September 5, 1956.. Accepted Sovember 23, 1956.
Microelectrodes
FREDERICK BAUMANN' and IRVING SHAIN Department of Chemistry, University of Wisconsin, Madison 6, Wis.
b Current-voltage curves obtained with rotating gold microelectrodes depend on the surface condition of the electrode. Oxidation of the gold takes place at anodic potentials and may hinder certain electrode reactions. Impurities in the solution have a similar effect. When the electrodes are pretreated consistently, go!d in the absence of complexing anions IS more resistant to oxidation than platinum. This resistance to oxidation permits the direct determination of chrornium(V1) in acid solution.
polarization on the current-voltage curves, but did not determine the effect of these parameters on the residual current in the media under study. Kolthoff and Tanaka (8) have interpreted residual current-voltage curves for platinum electrodes in various electrolytes. They found that currentvoltage curves obtained with platinum microelectrodes are complicated by the formation of platinum oxide films. Similar oxidation of gold electrodes occurs a t anodic potentials. EXPERIMENTAL
R
OTATIKG
GOLD
MICROELECTRODES
have not been used widely in polarography, although they are as generally applicable as platinum electrodes. Several important applications of rotating gold electrodes have utilized the hydrogen overvoltage of gold to investigate reactions a t potentials where platinum electrodes give hydrogen reduction current (5, 6). In the absence of complexing anions gold electrodes are more resistant than platinum electrodes to attack by strong oxidizing agents. This permits the use of gold electrodes where platinum oxidation waves interfere ( l a , IS), and also allows the study of certain reactions-e.g., the reduction of chromium(V1) in acid solution-which do not proceed a t platinum electrodes. Lord and Rogers (10) studied various electrode reactions a t both stationary and rotating gold microelectrodes. They examined the effect of electrode pretreatment, electrode rotation, type of electrode reaction, and direction of Present address, California Research Corp., Richmond, Calif.
Apparatus. A Sargent Model XXI recording polarograph was used for measurement of the current-voltage
MERCURY POOL P L A T IN UM WIRE SPRING
-BRASS
ROD
TEFLON
G O L D WIRE
-ti E L E CT R 0 D E Figure 1. Rotating gold microelectrode
curves. The potential scanning rate was 3.7 mv. per second. Potentials were measured with a Rubicon portable precision potentiometer. All potentials are referred to the saturated calomel electrode (S.C.E.). Considerable difficulty was encountered in constructing a suitable rotating gold microelectrode. A satisfactory mercury-tight gold to glass seal could not be made consistently; the electrode design used by Kolthoff and Jordan (6, 6) sometimes produced abnormally high residual currents due to mercury leakage. A satisfactory rotating gold microelectrode was constructed by pressurefitting gold wire (J. Bishop and Co. Platinum Works, 99.9770 pure, 0.81mm. diameter) into a tube machined from Teflon (Figure 1). The exposed portion of the electrode was 5 mm. long. The gold wire was fitted into the Teflon by forcing it into a hole drilled 0.01 inch undersize. Once the electrode was fitted into the Teflon tube, it could not be removed without permanently ruining the gold-Teflon seal. Electrodes could be readily switched by inserting another cap. Although this type of electrode was not as rigid as those made from glass tubing, the slight wobble introduced while rotating did not affect the current appreciably. The electrode was rotated a t exactly 1080 r.p.m. by means of a synchronous motor. Gold-plated platinum electrodes also were satisfactory. They were not used in this work, because it was necessary to be certain that the observed effects were not caused by platinum (due to porosity or imperfections in the gold plate). Electrolysis was carried out in a 260ml. tall-form beaker equipped with a side stem for the introduction of nitrogen. The electrolysis cell was connected to a saturated calomel electrode by means of a double-junction salt bridge containing (in separate compartments) VOL. 29,
NO. 2,
FEBRUARY 1957
303:
the indifferent electrolyte and saturated sodium or potassium chloride. This effectively prevented the diffusion of chloride ions into the electrolysis cell when the behavior of gold electrodes was studied in chloride-free media. The reference electrode and electrolysis cell were thermostated a t 25’ 0.1” C. Materials. All chemicals were reagent rade, used without further purification. inde high purity nitrogen was used to remove oxygen from the solutions. The solutions were prepared F-ith doubly distilled water. The second distillation was carried out in an all-borosilicate glass system from alkaline permanganate. Procedure. The same gold electrode was used throughout. It was stored in air and washed with distilled water before use. Precathodization was carried out for 15 seconds a t -1.0 volt in the electrolyte under study. The same procedure was used for preanodization, except that the potential was 2.5 volts.
*
k
RESIDUAL CURRENT
The behavior of the gold electrode in 1M perchloric acid is shown in Figure 2. Curve A was obtained after preanodizing the electrode and scanning 1.8 volts toward negative pofrom tentials. The behavior of gold in curve A is analogous to platinum electrodes which have undergone the same pretreatment. After preanodizing a t this potential, the electrode is covered with an oxide film upon which oxygen is evolved. As the potential is decreased, osygen evolution ceases and the current returns to zero until the oxide film is reduced. This reduction gives rise to the characteristic peaked cathodic current. The starting potential of the cathodic peak (1.08 volts), measured a t the point of intersection of the residual current line and the beginning portion of the rising cathodic peak, is less positive than the reversible gold-gold hydroxide couple in 1N acid (1.2 volts) as calculated from the standard potential given hy Latimer (9). Curve B in Figure 2 was obtained after first precathodizing the electrode a t -1.0 volt and scanning from -0.6 volt to more positive potentials. After hydrogen evolution ceases, the current is zero until the small anodic prewave beginning a t approximately f0.9 volt is reached. Oxidation of the gold begins a t this potential and continues as the electrode process until oxygen evolution is superimposed. Unlike film formation on platinum, the gold film is porous and oxide formation can continue beyond a unimolecular layer (1). The nearly zero current between the evolution of oxygen and the peak cathodic current in cur1-e A indicates that oxide formation ceases or is very slow after oxide formation has occurred for some time. Other investigators have reported the
+
304
0
ANALYTICAL CHEMISTRY
anodic formation of oxide films on gold. Jirsa and Buryanek (4) reported that auric hydroxide forms upon the anodic oxidation of gold in sulfuric acid. They arrived a t the formula by dehydration studies. Similar results were reported by Armstrong, Himsworth, and Butler ( 1 ) from studies in 0.1M sulfuric acid. Hickling (3) studied oxide formation in basic solution and found the behavior to be fundamentally the same. Silvestroni ( l a ) reported the formation of A u ( O H ) ~in basic solution. Current-voltage curves obtained with platinum electrodes after precathodizing show large anodic currents following the hydrogen evolution waves (8). This has been attributed to the oxidation of hydrogen which has been absorbed by the platinum electrode. The fact that this anodic current is absent with gold electrodes is consistent with the differences between metallic gold and platinum, as gold does not absorb hydrogen. Curve B in Figure 2 is identical to the corresponding curves observed for a “clean platinum electrode” (8). Thus, a “clean gold electrode” may be obtained by preanodization followed by precathodization. However, it is better to short the electrode to the reference electrode to remove the oxide film, as the possibility of contaminating the electrode by deposition of impurities is reduced. Similar results were obtained a t other concentrations of perchloric acid. Shifts in the cathodic peaks were in agreement with theory. For unknown reasons, the cathodic current peak is split into two peaks a t high concentrations of perchloric acid. The useful potential range of gold electrodes is much narrower in 1M hydrochloric acid. Because of the tendency for complex formation between the gold and chloride ion, an oxide film is not formed on the electrode surface. Instead, the gold is oxidized to give the complex auric chloride ion, which causes the gold to be oxidized a t potentials more positive than 0.6 volt. On further anodic polarization the current increases to high values. The residual current-voltage curves shown were not dependent on the speed of rotation of the electrode. This is consistent with the earlier conclusions concerning electrode film formation and removal, as these processes are not diffusion- or convection-controlled. The curves are dependent on the rate of potential sweep. Both the anodic prewave and the cathodic peak change shape when the scanning rate is increased or decreased. If manual ininstruments are used and time is allowed for the current to reach a constant value, neither the anodic premave nor the cathodic peak is observed.
EFFECT OF SURFACE CONDITION ON CURRENT VOLTAGE CURVES
Oxide Films. When the surface of the gold electrode is oxidized, electrode reactions which ordinarily take place on a clean gold surface are hindered. An esaniple of this behavior is shown in Figure 3 for the oxidation of iron(I1). This ouidation does not occur until the ouide film is removed a t $0.9 volt. Then the current quickly reaches the limiting value for the oxidation of iron(I1). The complete oxidation wave is obtained upon decreasing the potential further. A similar effect is observed for the reduction of chromium(V1) in 9.2M perchloric acid (Figure 41. The reduction wave of chromiuni(Y1) occurs in the same potential region as the oxidation of gold. After preanodizing the electrode and scanning ton-ard more negative potentials (curve B ) , no current is observed for the reduction of chromium(Y1) until the oxide film is removed a t $1.0 volt. Once the oxide film is removed, the current almost imniediately reaches the limiting value for the reduction wave of chromium(Y1). When the current-voltage curve is scanned in the opposite direction (curve A ) , the limiting current begins to decrease a t f0.85 volt corresponding to the start of the oxidation of the gold electrode. The decrease in the limiting current is greater than can be attributed to the anodic current from the osidation of the gold electrode. This distortion of the current-voltage curves is believed to be due to the fact that the gold oxide film acts as a barrier to the transfer of electrons. Because of the presence of this oxide film, the useful potential range of gold electrodes is -0.2 to +0.8 volt in osygen-free 1M perchloric acid. Contamination. Because impurities exert a profound effect on currentvoltage curves when gold electrodes are used, gold electrodes have to be pretreated. The effect of the accumulation of impurities on the reduction wave of chromium(V1) is shown in Figure 5 . This effect was also evident for the reduction of iron(II1) in 1X perchloric acid. Theoretically shaped current-voltage curves could not be obtained in spite of pretreatment unless the reagent n-as of the highest purity. The most satisfactory method of pretreatment Fyas a 15-second anodization at f2.5 volts, followed by a 5second treatment a t 0 volt to remove the oxide coating. The preanodizatioI1 removes the impurities by ouidation. Escellent reproducibility was obtained by following the above pretreatment. For example, a series of eight current voltage curves was obtained on different days using freshly prepared solutions containing 6.60 x 10-6111 chromium(V1) in 1.11 perchloric acid. The average
I
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I 1.2
I
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1.0
0.8
0.6
i
T
20 110.
I2 W
a a 3 0
I 1.5
I 1.2
I 09
I
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i
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06
03
0
-0.3
I
VOLTS VS S.C.E
we 2. Current-voltage curves obtained with d microelectrode in oxygen-free perchloric acid; I
A. Preanodized 8. Precathodized Direction of voltage sweep indicated by arraws
I
I
I
I
1
I
I4
V O L T S VS. S.G.E.
I
Figure 4. Behavior of gold microelectrodes in reduction of 2.44 X lO-6M chromium(V1) in 9.2M perchloric acid A.
Scanned from negative to positive
8. Scanned from positive to negative
ANALYTICAL APPLICATIONS
1.4
1.2
1.0
08
06
0.4
0.2
V O L T S VS S.CE
Figure 3. Effect of gold oxide films on currentvoltage curves for oxidation of 2 X 10-44 iron (11) in 1M perchloric acid Scanned from positive to negative voltages
half-wave potential observed was 0.674 volt with an average deviation of 0.0034 volt. The limiting current also was very reproducible. The average observed in these same determinations was 7.39 pa. with an average deviation of 0.10 pa. The direction of voltage sweep was from the limiting current region to the foot of the wave. Equally reproducible results were obtained by scanning in the opposite direction. The conditions of the pretreatment could be varied
over a considerable range without affecting the results significantly. Even though very pure solutions were used, these results may not be completely free from the effects of impurities. Bockris (2) states that the concentration of impurities must be reduced to 10-10 M to avoid completely the influence of impurities. However, the important conclusion reached in this study is that the effect of impurities is made constant by following the above method of pretreatment consistently.
Chromium(V1) can be directly determined in acid solution with gold electrodes, because gold is not oxidized under these conditions. With rotating gold electrodes, the limiting current for the reduction of chromium(V1) in 1M perchloric acid could be measured a t $0.4 volt without interference from the reduction of oxygen. Current-voltage curves were obtained on a series of 24 chromium(V1) solutions in 1M perchloric acid to test the limiting current-concentration relationship. The chromium(V1) concentration ranged from 1.08 X 10to 2.93 X lop4M . The average ratio of limiting current to concentration (in microamperes per micromole per liter) was 1.11 with an average deviation of 0.013 for the 24 solutions. These results also suggest amperometric methods involving chromium(V1) as the titrant. Gold electrodes have proved very satisfactory in the Kolthoff and May (7) titration of iron(I1) with chromium(V1) ; the current readings are more stable than with platinum, as gold is not oxidized a t the potential used in this titration. COMPARISON OF GOLD AND PLATINUM ELECTRODES
Poorly defined and unreproducible current-voltage curves were obtained VOL. 29, NO. 2, FEBRUARY 1957
305
in this laboratory for the reduction of chromium(V1) in acid solution using platinum electrodes, regardless of the pretreatment used or the direction of voltage change. This is believed to be due to the oxidation of the electrode surface by the chromium(VI), and formation of an oxide layer. The existence of this film has been shown by Kolthoff and Tanaka (8) and Ross and Shain (11). The oxide film probably lowers the rate of the electrode reaction by acting as a barrier in the electron transfer process. Catalytic reactions, in which platinum is first oxidized by chromium(V1) and then reduced a t the electrode surface, probably do not occur to any great extent, because of the low rate of the platinum oxide reduction reaction. The behavior of gold electrodes is very similar to platinum electrodes. In the absence of complexing media, gold electrodes have greater stability toward oxidation.
I-
z
W
a LT
3 0
LITERATURE CITED
Armstrong, G., Himsworth, F. R., Butler, J. A. V., Proc. Roy. SOC. 143A, 89 (1933). Bockris, J. O W . , “Modern Aspects of Electrochemistry,’’ p. 260, Academic Press, New York, 1954. Hickling, A., Trans. Faraday SOC.42, 518 (1946).
I
I
06
04
I 02
VOLTS VS
Figure 5. duction o f acid
I 0
I -02
I -04
S C E
Effect of electrode pretreatment on re-
5 X 1 0-SM chromium(V1) in 1M perchloric
A. Preanodized a t 2.5 volts and shorted a t 0 volt for 5 seconds B,C. Re-use of gold electrode without further pretreatment
ACKNOWLEDGMENT
The authors %-ish to thank E. I. du Pont de Nemours & Co. and the Wisconsin Alumni Research Foundation financial assistance.
I 08
D. E.
After repeated trials without further pretreatment Residual current with same pretreatment as A
Jirsa, F., Buryanek, O., Chem. Listy 16, 189 (1922). Kolthoff, I. M., Jordan, J., AKAL. CHEM.24, 1071 (1952). Kolthoff, I. M., Jordan, J., J. Am. Chem. SOC.74, 4801 (1952). Kolthoff, I. )I., May, D. R., ANAL. CHEM.18,208 (1846). Kolthoff, I. bl., Tanaka, X., Ibid., 26, 632 (1954);( Latimer, W. M., Oxidation States of the Elements and Their Potentials in Aqueous Solutions,” 2nd ed., Prentice-Hall, Yew York, 1952.
Lord, S. S., Jr., Rogers, L. B., A N ~ L . CHEM.26, 284 (1954). Ross, J. W., Shain, I., Ibid., 28, 648 (1956). Silvestroni. P.. Ann. chim. (Rome) 44,464 (1954). Silvestroni. P.. Troili., hf.., Ricerca Sci. 24, 116 (1954). RECEIVED for review May 17, 1956. Accepted September 26, 1956. Based on the Ph.D thesis of Frederick Baumann, University of Wisconsin, 1956.
Differential Thermal Analysis and Thermogravimetry Applied to Potassium PerchIorate-AIuminumBarium Nitrate Mixtures VIRGINIA D. HOGAN, SAUL GORDON, and CLEMENT CAMPBELL Pyrofechnics Chemical Research laboratory, Picafinny Arsenal, Dover, N. J.
b A thermogravimetric study indicated that barium nitrate catalyzes the decomposition of potassium perchlorate to potassium chloride. Differential thermal analyses of the system yielded curves characteristic of the individual compounds and their relative quantities. These complementary techniques have been used to develop a simple 306
ANALYTICAL CHEMISTRY
but rapid method for analyzing the pyrotechnic composition potassium perchlorate-aluminum-barium nitrate. Thermogravimetric curves provided the data for determining the optimum temperature for perchlorate decomposition. A filtering crucible was used to permit the quantitative aqueous removal of potassium chloride and bar-
ium nitrate, leaving a residue of aluminum. Barium nitrate may be determined b y difference or b y titration for barium ion.
D
IFFERENTIAL thermal
analysis has been widely used in the study of minerals, clays, and soils (9, 16).
