BRUNO SCROSATP and H. A. LAITINEN Noyes Chemical Laboratory, University of Illinois, Urbana, 111. The electrochemistry of VOf2 in eutectic at 450" C. has been studied by chronopotentiometry, controlled potential eiectrolysis and chronopotentiometry with current reversal. With the aid of these electroanalytical techniques, it has been found that VO+* is reduced to V Q + which undergoes a rapid and irreversible chemical reaction to give VzOs and V(III).
XI-KCI
4
s I-2.5 u
E + -2.0 a
z
vj I-
-1.5
J
0
LiCl-KCl EUTECTIC has been one of the most commonly used media for the study of the electrochemistry of metal chlorides in molten salts, relatively few chemical or electrochemical investigations have been carried out on the behavior of osygenated species in this melt. Laitinen and Rhodes (13) studied the electrochemistry of VZO, and found that vanadium pentoside was reduced to a mixture of a lithium-vanadium bronze oxide and lithium vanadates. An electrochemical study of UO+2 was carried out by Hill, Perano, and Osteryoung (6). Their voltammetric results indicated a two-electron reduction of uranyl ion to solid UOz. A review of the polarography of osyanions has been recently published by Laitinen ( 7 ) . hfolina (16) and Gruen and McBeth (5) reported the absorption spectra of VO+*. Rfolina also discussed qualitatively some aspects of the chemical behavior of the vanadyl ion in LiC1-KC1 a t 4500 6. In this study chronopotentiometry, controlled potential electrolysis, and chronopotentiometry with current reversal were used to investigate the electrochemistry of VOtZ in LiC1-KC1 eutectic at 450" C. LTHOUGH
EXPERIMENTAL
A Hevy Duty Type AI-3012-5 Split Tube furnace (Hevy Duty Electric Co., Milwaukee, Wis.) was used. The fused salt cell was similar to that previously used (9) except that a borosilicate glass cell head with five standard taper joints was used in place of the rubber stopper. The methodology and the procedure Present address: Istituto Di Chimica Fisicn Ed Elettrochimica, Universita lli Roma, Roma, Italy. '1894 * ANALYTICAL CHEMISTRY
3 -1.0
: E I-
-0s
o a 0.1
0
0.2
Figure 1 .
0.3 Curve A.
0.4
0.6 0.7 TI M E, SEC.
0.S
09
0.8
1.0
Chronopotentiogram of the solvent
I = 6.0 X 10-* amp.
Curve B.
Typical chronopotentiogram of VOSOl in LiCI-KCI at I = 7.6 X
450' C.
lova amp. cm.-2
C = 6.18 X 10-8M
of making the measurements have been described elsewhere ( 2 , 8). For the chronopotentiometric studies a P t foil indicator electrode of total area 0.5 cm2 was suspended just below the surface of the melt. A larger P t foil, in a separate compartment, was used as the counter electrode. The constant current source was similar to that designed by Deron (4). Chronopotentiograms were displayed on a Tektronix 535-S5 oscilloscope and photographed with a Tektronix type C-12 Polaroid-back camera. For the coulometric studies a large Pt foil was used an as indicator electrode. The measurements were carried out using a conventional operational amplifier potentiostat (2) ; the current-time curves were recorded with a Sargent AIR Recorder. The reference electrode used was a platinum foil in contact with Pt(lI), and has been described previously (11). The LiC1-KC1 eutectic mixture (melting point 352' C.) was prepared and purified by Anderson Physics Laboratories, Inc., Champaign, Ill., according to the method of Laitinen, Tischer, and Roe (24).
All the chemicals were Analytical Reagent Grade and were dried under high vacuum before addition to the melt. RESULTS A N D DISCUSSION
Chronopotentiometry has been shown to be a particularly useful technique for the study of reaction mechanisms in molten salts (10). The advantages of chronopotcntiometrg over more classical methods of analysis in fused media have been discussed ( g ) , and the theory was recently considered in detail (3) and extended for various reaction mechanisms (17, 20-28). The basic equation for chranopotentionietry is the Sand equation (19)
142 =
?Z
F
rl'z
2
D1Iz C
(1)
where I is the current density in amp. cni.+, T the transition time in sec., C the bulk concentration of the reacting species in mole cm.-3, D the diffusion coefficient in cm.2 sec.-l, and n, F , and n have their usual meanings. According to Equation 1, for a semiinfinite diffusion controlled process, the
I
IT1/12
quantity __ should be independent of
I
I
I
I
1
I
C
the current density and the concentration. Figure 1, curve B, shows a typical chronopotentiogram of VOSOa in LiClKC1 a t 450" C. Three reduction steps were observed with Et,4 values of about -0.65 volt, -1.45 volts, and -1.80 volts us. a Pt(I1) (lX)/Pt reference electrode. Chronopotentiometric data for the first reduction step at different current densities and concentrations are reported in Table I. The transition times were calculated according to the method proposed by Reinmuth (18). For various current densities and concentrations
I
IT'"
the quantity __ remained constant C within +370, showing the applicability of Equation 1 to the reduction of VOf2. From the average value of the transition time constant, the product nD1/lzwas calculated to be (5.2 + 0.1) X 10-3 cm. sec.-l'z. Assuming n to be 1, the diffusion coefficient of VOf2 a t 450" C. is estimated to be (2.68 i 0.03) x sec.-', a value which is comparable to those obtained by Laitinen and Gaur see.-"), (10) for Cd+2(2.08 X cmn2sec.-l) and Pb+lz C o f 2 (2.42 X (2.18 X sec.-l). With ?z taken equal to 2 or more, the calculated value of D was too lo^^ compared to the values obtained for comparable ions in LiCIKC1 a t 450' C. Some doubt is cast upon the accuracy of this value, however, by the apparent trend toward slightly higher values of transition time constant at higher concentrations and longer transition times (Table I). As vi11 be further discussed below, this trend may be attributed to an effective n somewhat greater than unity under these conditions. The values of T presented in Table I were obtained within about 20 minutes from the dissolution of VOSOl in the melt. After this period, the values of the transition times, at a given current density, started to decrease with time. VOSOe, when dissolved in LiCl-KCI eutectic, gave bright green solutions. This color changed slowly to a yellow green, suggesting a decomposition of the vanadyl ion. Evidently sulfate stabilizes vanadyl ion, perhaps by ion-pair formation, because solutions of V0Cl2 showed evidence of much more rapid decompositions. Even freshly prepared solutions of T'0Cl2 gave chronopotentiograms indicative of complications due t o decomposition. When a stream of oxygen-free argon mas bubbled for about 2 hours through a concentrated solution of VOS04 in the melt, a black insoluble product precipitated and there was no evidence of any volatile compound in the effluent gas. An x-ray powder pattern of the black
product was obtained, and the d-spacings and intensities of the lines agreed with those for VOZ obtained by Andersson ( I ) . One possible decomposition reaction is
Vo+2 * v 0 2 c
+ 2 c1-
2 vo+2
-4- v(lV)
(3)
The cathodic-anodic current reversal chronopotentiogram of VO+2 is shown in
-+
VO,(S) 4- V(II1) Table 1.
(2)
To clarify the results obtained by chronopotentiometry for the reduction of VOf2, a controlled potential electrolysis of the vanadyl ion in LiCI-KCI eutectic was carried out. Electrolvsis a t -0.65 volt of solutions of VOS& produced a black precipitate which was
+ Clz
Chronopotentiornetric Data for the Reduction of VOf2 in LiCI-KCI Eutectic at 450" C.
cx
103
moles/liter 6.18
5.63 6.47
7.85
I x 103 amp./cm.Z 7.6 7.6 7.6 3.0 4.2 6.0 6.0 7.0 8.0 9.0 10 4.0 4.8 5.0 6.0
r sec.
0.130 0.129 0.133 0.663 0.338 0.153 0.229 0.167 0.131 0.102 0.081 0.832 0,525 0.595 0.378
1r1'2/C X 10-2 amp. cm. sec.1'z/mole 4.43 4.42 4.48 4.33 4.33 4.23 4.44 4.42 4.48 4.43 4.39 4.65 4.61 4.71 4.69 Av.-4 Std. dev. 0 . 1 3
VOL. 38, NO. 13, DECEMBER 1966
1895
I
I
I
I
I
I
I
I
TIME,SEC.
Figure 4. Curve A. trolysis of VO+2
Chronopotentiogram after complete elec-
3.65 X 1 0-aM in LiCI-KCI at 450’ C. I = 6.0 X 1 O-a amp. cm.-2
Curve B.
Typical chronopotentiogram of VCls in LiCI-KCI at
450’ C. Figure 3.
Potential-time analysis of the reduction of VO+2 in LiCI-KCI a t 450’ C.
Figure 2. If the current was reversed a t t equal to r , when the surface concentration of VOf2 had reached zero, the reverse transition time was about 0.01 second (negligible compared to the forward electrolysis time). If the current was reversed a t successively shorter electrolysis times, the fraction of VO+ which did not undergo the chemical reaction increased. For example, et t equal to 0.4 T , the ratio T ~ ~ was ~ / found to have a value of 0.28, which is close to the value expected for a reversible process with both oxidized and reduced species soluble. No wave for the oxidation of V(I1I) to V(IV) was found in the anodic region, in agreement with the results of Laitinen and Pankey (12). This behavior, while indicative of a chemical decomposition of the primary electrolysis product, is not consistent with a first-order decomposition mechanism. The primary reaction, however, is reversible, as shown by the plot of E us. log (+iZ - W 2 )in Figure 3 (16). The points in this plot fall in the time range following r/4, when the potential change primarily reflects changes in the surface concentration of diffusing oxidant, proportional to (# - W ) ,and is insensitive to the decomposition of the reduction product. The experimental slope of 0.148 volt is in satisfactory agreement with the theoretical value of 2.303 RT/F of 0.1435 volt a t 450’ C. Returning now to the apparent n value for the first reduction wave of VO+2, it could vary between extreme values of 1.00 and 1.33, depending upon the rate a t which V(II1) is produced a t the electrode surface, assuming that it is further reduced to V(11). It should be 1896
ANALYTICAL CHEMISTRY
I = 4.0 X amp. crn.-a c = 3.06 x 1 0 - 3 ~
noted that, owing to the possibly complex decomposition path of VO+, the rate of formation of V(II1) is not necessarily proportional to the rate of disappearance of VO+ a t the electrode surface. Qualitatively, however, it would appear that the first transition time of VO+2 would most closely approach the value for n = 1 a t short transition times and low concentrations, when the effects Tof ~secondary ~ ~ decomposition would be least important. The true value of the diffusion coefficient may therefore be somewhat lower than that calculated from the average value of the transition time constant. If mechanism (4)is correct, after the complete electrolysis of VO + z , the solution should contain V(II1) ions. ,4 chronopotentiogram of 3.06 X 10-aM VC13 in the eutectic mixture (Figure 4, curve B) shows two reduction waves corresponding to the processes V(II1) e +.V(I1) and V(I1) e - -,V(0) (12). Curve A, Figure 4 shows a chronopotentiogram taken after complete electrolysis of a 3.65 X 10-2AVsolution of VOSOl which, according to Equation 4 should yield a 1.22 X 10-2Jf solution of VC&. The two waves are comparable to those seen for VC&, and their transition times increased proportionately with concentration when VCls was added to the solution. Thus, the formation of VC13 upon electrolysis of VOSOa was demonstrated, but its amount was considerably less than that expected from Equation 4. The preparative electrolysis was carried out using a stream of argon for stirring, during a period of about 30 minutes. It appears likely that some reduction of IrCla occurred
+
+
during the exhaustive electrolysis, and that the solution of VOS04 underwent some decomposition during the electrolysis to yield an electron number close to unity. Some loss of VCla by volatilization might also have occurred under these experimental conditions, although in the absence of a gas stream, vanadium (111) solutions had previously been observed to be stable (12). The second step of the chronopotentiogram of VOS04 (see Figure 1) was not very well defined and only an approximate evaluation of the transition time was possible. Interpretation of this wave was also complicated by its irreproducibility. Possibly the wave was due to the reduction of that fraction of VOf or other intermediate species present a t the electrode surface which had not yet undergone the complete chemical reaction. The third step was probably due to the reduction of the sulfate ion. The value of the quarter wave potential and the shape of the curve were similar to those found by Woodhall (23). He observed that sulfate was reduced a t potentials ranging from -1.1 to -1.75 volts us. a Pt(1I) ( l M ) / P t reference electrode and the chronopotentiograms gave long drawn out waves. Similar behavior was observed in the present case. LITERATURE CITED
( I ) Andersson, G., Acta Chem. Scand. 8, 1599 (1954). (2) Bankert, R. D., Ph.D. Thesis, University of Illinois, 1966. (3) Delahay, P., “New Instrumental
Methods in Electrochemistry,” Interscience, New York, 1954.
(4) Deron, S., Ph.D. Thesis, University of Illinois, 1964. (5) Gruen, D. RI., McBeth, R. L., J . Phys. Chem. 6 6 , 57 (1962). (6) Hill, D. L., Perano, J., Osteryoung, R. A., J . Electrochem. Soc. 107, 698 (1960). ( 7 ) Laitinen, H. A., Talanta 12, 1237 (1965). (8) Laitinen, H, A., Ferguson, W. S., ASAL.CHEW29, 4 (1957). (9) Laitinen, H. A., Ferguson, W. S., Ostervoune. R. 8.. J . Electrochem. Soc. 104, 516 (i957). ’
(10) Laitinen, H. A., Gaur, H. C., Anal. Chim. Acta 18, 1 (1958). (11) Laitinen, H. A.. Liu. C. H., J . A m . Chem. Soc.’80. 1015 (1958). (12) Laitinen, H. A, ‘ Pankey, J. W., J . Am. Chem. Soe. 81, 1053 (1959). (13) Laitinen, H. A, Rhodes, D. R., J . Electrochem. SOC.109, 413 (1962). (14) Laitinen, H. A., Tisher, R. P., Roe, D. K.. J . Electrochem. SOC.107. 546 (1960): (Id) Molina, R., Bull. SOC.Chim. France, 301 (1961). (16) Reinmuth, W. H., ANAL. CHEY. 32, 1514 (1960). ~
(17) Reinmuth, W. H., Ibid., 33, 322 (1961). (18) Ibid., p. 485. (19) Sand, H. J. S., Phil. Mag. 1, 45 (1901j. (20) Testa, A. C., Reinmuth, W. H., ANAL.CHEY.32, 1512 (1960). (21) Ibid., 33, 1320 (1961). (22) Ibid., p. 1324. (23) Woodhall, B., unpublished experiments, University of Illinois. RECEIVEDfor review April 18, 1966. Accepted Bugust 8,1966. Work supported by the Army Research Office, Durham.
Rapid, Sensitive Kinetic Method for Detection and Determination of Phenolic Compounds GEORGE G. GUILBAULT,1 DAVID N. KRAMER, and ETHEL HACKLEY Research labs, U . S. Army Edgewood Arsenal, Edgewood Arsenal, Md.
b A rapid, sensitive method i s described for the detection and determination of phenolic compounds in the range of 1 to 100 pg. per ml. of solution. The method is based on the measurement of the initial rate of reaction of N-(benzenesulfony1)quinonimine with the phenols to yield the indophenol. Phenol, o-chlorophenol, 2,3-dirnethylphenolr m-aminophenol, a-naphthol, 5-amino-a-naphthol, and @-naphtholwere analyzed with a deviation of about 1.2y0. The color formed i s produced in 3 minutes, and i s stable with time. The reagent, N(benzenesulfonyl)quinonimine, i s stable, and can be used for several days.
T
and still the most sensitive, method for the identification of phenolic compounds is based on the intense blue solutions of the indophenol salts formed from these materials ( 2 , 6). Gibbs (4)developed qualitative and quantitative tests for as little as 5 p g . per ml. of phenols using 2,B-dichloro- and 2,6-dibromo-Nchloroquinonimine. Both of these substrates are unstable, and unless they are stored in the dark, they become highly colored after several hours standing. Another good spectrophotonietric method for the determination of phenol involves the reaction of phenol with 4-arninoantipyrine in the presence of a n oxidant, KbFe(CIT)O, to yield a red antipyrine dye ( 3 ) . Burnistrov and Titov (1) reported the synthesis of indophenols using the benzenesulfonyl derivatives of quinonimine. Preliminary experiments inHE
OLDEST,
Present address, Chemistry Department, Louisiana State University in New Orleans, New Orleans, La.
dicated that these compounds could be used as the basis of sensitive qualitative and quantitative methods for a number of phenols. The readily available reagents were stable for days, and by measuring the initial rates of reaction, an ailalysis could be performed in minutes, rather than in a half hour or more as with the Gibbs reagent using the total color formation. The indophenols formed are stable, and complete color is produced in 3 minutes. Sensitivities comparable to other methods were obtained. EXPERIMENTAL
Reagents. A series of benzenesulfonyl derivatives of quinonimine (I) were prepared, where R = p-H, p-CHa, p-OCH3, p-N02, m-SOz, p-C1 and R” = H, C1, and Br, by a procedure similar to that described previously. Improved yields of the products were obtained by oxidation of the sulfonamides to the sulfonimides using potassium dichromate-glacial acetic acid reagent. All phenolic compounds tested were of the highest purity available commercially and were purified by either sublimation or recrystallization. Apparatus. All recordings of the change in absorbance with time were made using a Beckman Model DB spectrophotometer. A constant temperature room was used to maintain a constant temperature during all kinetic runs. Procedures. PROCEDURE 4 . To 1 ml. of a 0.0005X methyl Cellosolve eolution of the LV(benzenesulfonyl) quinonine is added 1 ml. of 1.5N ”,OH (pH 11.7) in a 1.0-cm. cell, a n d the absorbance is adjusted t o read zero at the appropriate wavelength (see below). At zero time, 0.5 ml. of a solution of t h e phenol t o be analyzed (1 t o 100 pg.) is added, and the rate of change in the absorbance of the
solution with time is autoniatically recorded. From calibraticn plots of A A . per minute LIS. time, the amount of phenol present in the solution can be calculated. This procedure is to be used for the determination of phenol, o-chlorophenol, 2,3-dimethylphenolJ maminophenol, and P-naphthol. PROCEDURE B. The procedure is followed as above, except that the absorbance of the solution is read after 3 minutes. From calibration plots of absorbance vs. concentration, the amount of 5-amino-a-naphthol and a-naphthol may be calculated. This niethod must be used for 5-amino-anaphthol and a-naphthol, but it can also be used for any of the compounds determined by procedure ii. RESULTS
The results of the determination of various phenols using M-(benzenesulfony1)quinonimine are indicated in Tables I and 11. Using procedure A, 1 to 100 pg. per nil. of total solution of phenol, 10 to 250 pg. per nil. of o-chlorophenol, 3 to 300 pg. per nil. of 2,3-dimethylphenol, 1 to 50 pg. per ml. of m-aminophenol, and 5 to 100 pg. per ml. of p-naphthol can be determined with deviations of about +1,3%. From 5 to 100 pg. per ml. of &aminoa-naphthol and 1 to 100 pg. per ml. of a-naphthol may be determined by procedure B with a standard deviation of about +1.3%. Each result reported is based on a n average of 3 to 5 determinations and good repeatability and reproducibility were obtained. DISCUSSION
Substrates for the Determination of Phenol. Burnistrov and Titov (1) reported that phenols form indophenols when reacted with N-(arylVOL. 38, NO. 13, DECEMBER 1966
1897
