amounts of water. The absorbance curves obtained from solutions containing approximately 17yowater and higher show that water causes an increase in the intensity and a decrease in the sharpness of the absorbance a t 2.02 microns. As the concentration of water increases, the amine band a t 2.02 microns becomes a shoulder of the water band until it is completely obscured by the broad water band. Several different types of desiccants and adsorbants were used in attempts
to remove the water without affecting the amines present. None Of the materials used produced satisfactory results. The differential analysis technique was extended to water* For samples containing reasonably large amounts of water, a reference solution must hold the same amount of water as the sample solution before it is placed in the reference beam. I n this manner, compensation can be made for the water present in the sample.
LITERATURE CITED
( 1 ) Kaye, W., Spectrochim. Acta 6, 257 (1954). (2) Kaye, W., Ibid., 7, 181 (1955). (3) McDonald, 1. R. C., Nature 1749 703 (1954). (4) Robinson, D. Z., ANAL.CHEM.24,619 (1952). (5) Washburn, W. H., Scheske, F. A., Ibid*,299 346 (1957)*
RODNEY J. QUENTIN Research Division Wyandotte Chemic& carp. Wyandotte, Mich.
Reverse-Current and Current-Cessation Chronopotentiometry with a Two-Component System SIR: This note examines the chronopotentiometric behavior of solutions containing two separate reactants A and C which can be consecutively reduced and then reoxidized a t the electrode. The cathodic electrode reactions are A
+ nle-
+.
B
(1)
+
C n2e-+. D (2) The two-step reduction of a single starting species according to A B
+ rile+ n2e-
-+
B
(3)
4
D
(4)
is mathematically equivalent to the first case so that the equations presented will apply to both cases. Rouse (2) and Testa and Reinmuth (3) have given general theoretical treatments of current-reversal and currentcessation chronopotentiometry for multicomponent systems as well as some experimental results. However, there is no record of previous experimental study of systems in which both the oxidized and reduced forms of all species are soluble in the solution. We have studied the behavior of solutions of Fe(II1) and Cu(I1) in 1F HC1 and Ce(1V) and Fe(II1) in 3F HzSO4. I n both cases four chronopotentiometric waves are obtained, two preceding and two following current reversal. As pointed out by Testa and Reinmuth (S), the occurrence of the reaction nzA
e.g., Fe(II1)
tion times following current reversal, should also not be influenced by the occurrence of reacton 5 if all M u s i o n constants are assumed equal. This becomes apparent if a new concentration variable, C*, is used:
+ nlD = nlC + nnB Fe(I1)
+ Cu(I1)
+ nZC$
(7)
CA and CD are the time-varying concentrations of A and B and CCO is the initial concentration of C. C* obeys the same equations and boundary conditions that describe the chronopotentiometric behavior of a simple one-component system. Namely
ac* - -D-a"* bt
C*-+C*OasX-+
m;
C* = C*O at t = 0
where D is the common diffusion coefficient of all species, and io is the current density. Thus C* will obey the well known chronopotentiometric equations derived for a one-component system (I)-i.e., a t the electrode surface
C* =
c*o [l -
(5)
(-& t'
+ 1)1/' + = t + >0 (71
72)
(11) (6)
in the solution near the electrode does not affect the observed values of 71 or r2except for differences in diffusion coefficients of the various species. This is true regardless of the rate of this reaction. Analogous arguments can be used to shon- that r3 and r4, the transi-
The four transition times correspond to the following values of C*: 71:
c* = ?&@
72:c*
Rouse (8) expressed the quality in Equation 12 in the form
-
0
For the case where all concentrations and n-values are equaI, Equation 12 gives 7 ) = 0.0717 (71 72) which, r2)/(n 4taken with the ratio (71 r4) = 3, corresponds to 7 3 = 0.215 (r3 r4). Because, for this special case, r3 corresponds to the point a t which the concentration of the reduced species a t the electrode surface is just one half of its maximum value, the value of r3 could also be calculated from Delahay's equation 8-35 (1) with the same result. (The relation t' = 0.222 7' given by Delahay contains an arithmetic error; the correct relation is
+ +
+
t'
c* = nlCAo + mcco
:
= 0.215 T'.)
To test Equation 12 experimental data were obtained for Fe(II1)-Cu(I1) solutions in 1F HC1 and for Ce(1V)Fe(II1) solutions in 3F H804. A series of experiments with a solution approximately 0.01M in Fe(II1) and Cu(I1) gives the following results: 7,
= 2.88 sec.;
=
5.56 sec.;
78:C* = n2CP
74
+
+
3x2
2(*2)1/2],
+ Cu(1) =
- nzCD
C* = nlCn
The values to be expected for the successive transition times can be calculated from Equations 10 and 11. The results for r1and rn are well known (1). The fact that (78 r4) is one third of (rl T ~ in ) the absence of complications is also familiar (1). The value of 73which results from Equations 10 and 11 is
T~
TI
= 0.92 sec.
calculated from Equation 10 = 0.96 sec.
VOL. 34, NO. 9, AUGUST 1962
1171
Similar experiments with Ce(1V)-Fe (111) solutions resulted in the following typical set of transition times: 71
78
= 0.84 sec.; rz = 4.08 sec.; = 0.44 Bee., 7 8 calculated = 0.48 sec.
The agreement is well within the experimental error involved in measuring the transition times. Current Cessation. If the current is turned off rather than reversed following the second cathodic transition time, rZ, a single anodic transition time, 73, is observed (2). I n this case T~ is the time required for the consumption of all of species D at the electrode by means of reaction 5. It can be readily shown that r3 is given by 73
= rz2/4r1(where ra is meaaured from
TZ)
(14)
(All diffusion coefficients are again assumed to be equal.) Equation 14 was derived on the assumption that the purely chemical reaction, which proceeds after the current is turned off, is rapid and quantitative at the electrode surface or in the body of the solution. The reaction will be quantitative at equlibrium so long as the waves are well separated and the couples are reason-
ably reversible. Furthermore, the rate of the reaction at the electrode will have to be rapid if the electrode is to be at equilibrium with respect to both couples following current cessation. Thus, even though the chemical reaction may be slow in the body of the solution, the electrode surface can act as a catalyst for the attainment of equilibrium. Under these circumstances current-cessation chronopotentiometry will not provide information about the kinetics of reaction 5. If the electrode is not in equilibrium with respect to both couples, the concentrations of both A and D could be simultaneously appreciable a t the electrode surface. In such a case the potential inflection, if one occurs, would correspond to the concentration polarization a t the electrode surface of the couple with the larger exchange current. Thus, the observed transition time would be influenced by the rate of the chemical reaction between A and D, and kinetic information might be obtainable (3). Typical results of a series of current-cessation chronopotentiograms for Ce(1V)-Fe(II1) solutions are given below. The current was turned off after the second cathodic transition
time, 72, and the single well defined anodic transition time, 73, was measured. 71
7a
=
1.2sec.; T Z = 2.44sec.; rg = 1.00 sec.
calculated from Equation 14
=
1.24sec.
The agreement between theory and experiment was always poorer for current-cessation chronopotentiometry than for current-reversal chronopotentiometry with Ce(1V)-Fe(1II) solutions. Lack of the assumed equality of diffusion coefficients rather than chemical kinetic complications probably is responsible for this discrepancy. LITERATURE CITED
( 1 ) Delahayl
P., “New Instrumental Methods in Electrochemistry,” Chap. 8, Interscience, New York, 1954. (2)Rouse, T.O.,Ph.D. thesis, University of Minnesota, 1961. (3) Testa, A. C., Reinmuth, W. H., ANAL. CHEM.33, 1324 (1961). b-ILLIALI E. PALKE CHARLES D. RUSSELL FREDC. AMON
Gates and Crellin Laboratories of Chemistry California Institute of Technology Pasadena, Calif. Contribution No. 2828.
FIuo rimetric Determ ina tio n of 3-0-To1y 1oxy -1, 2- pro pa ned ioI (Mephenesin) Using the 580 mp Fluorescent Peak detergent (Aquet, Emil Greiner Co., New York) and rinsed with glass distilled water prior to their use.
SIR: A fluorescent peak displayed by 3-o-tolyloxy-1,2-propanediol (mephenesin) at 315 mp has been cited previously as a means of quantitative assay of this compound (9). In measuring mephenesin in biological fluids, however, contaminants interfered in this fluorescent reaction. This communication is a description of a quantitative analytical method in which a previously undescribed fluorescent peak at 580 mp, produced by activation of the drug a t 275 mp, was utilized.
RESULTS AND DISCUSSION
APPARATUS AND MATERIALS
An Aminco-Bowman spectrophotofluorometer (American Instrument Co., Inc., Silver Spring, Md.) with a 1P28 multiplier phototube (Radio Corp. of America) and silica cells of 10 mm. square base and 45 mm. height (internal dimensions) was employed. Mephenesin, commercially obtained, was purified by crystallization from isopropyl ether and recrystallization from hexane. Glass distilled water or phosphate buffer (O.lM, p H 7) was used as the solvent for fluorescence. The silica cells were washed with a mild
1 172
ANALYTICAL CHEMISTRY
200
300500
600
700
MI LLI M ICRO NS Figure 1. Fluorescent and activation spectra of 3-o-tolyloxy- 7,2-propanediol
..-. Activation spectrum - Fluorescent spectra
at 2, 1, 0.5, and 0 pg./ml, in decreasing peak height, respectively The conditions for fluorescence were: activating wavelength 275 mp, slit arrangement number 4 with ‘/S-inch slits to reduce scatter, sensitivity 25, multiplier 0.03, and recorded at 50 mv.
The activation and 580 nip fluorescent spectra of 3-o-tolylo~~-i,2-propanediol are given in Figure 1. The measured fluorescence was found to be linearly proportional to the amount of compound present in the range of 0-2 p g , and sensitive to less than 0.5 pg, of compound per ml. In contrast, a reported spectrophotometric method for 3-o-tolyloxy-1,2-propanediolis not reproducible below about 20 pg. per ml. (1). LITERATURE CITED
(1) Mass, A. R.,Carey, P. L., Heming, A. E., ANAL. CHEW.31, 1331 (1959). (2)Udenfriend, S., Duggan, D. E., Vmta, B. M., Brodie, B. B., J . Pharmacol. Ezptl. Therap. 120,26(1957). W. H.BRADSHAW J. F. DOUGLAS
Wallace Laboratories Division of Carter Products, Inc Cranbury, N. J.
