Effect of Surfactants on Polarographic Current-’Time Curves Investigation of Oxygen Maximum and Application to Trace Determination of Cysteine Sidney L. Phillips IBM Corp., Box 390, C-30, Poughkeepsie, N . Y . 12602
MANYWORKERS have investigated the inhibiting effect of surfactants on instantaneous current-time curves obtained a t a dropping mercury electrode (1-5). In most of these cases, the experimental data were obtained in potential regions corresponding to tdiffusional transfer of the surfactant to the electrode surface from the bulk of the solution. Under these conditions, the time a t which the faradaic current falls to zero can be related theoretically to the surface coverage through the equation derived by Koryta ( I ) .
t e = 1.82
x
10’ rez(DA112CA’)-’
(1)
In Equation 1, re is the time a t which the electrode becomes coated with the: equilibrium surface concentration of surfactant (re); ] I A is the bulk diffusion coefficient of the surfactant; and CAo is the bulk surfactant concentration. When the surface concentration corresponds to saturation coverage, reis usu;illy designated by the symbol rm. As noted, Equation 1 was derived specifically for the case of diffusion-limited mass transfer a t a dropping mercury Jectrode, and does not hold for instantaneous current-time data obtai,ied in the region of polarographic maxima where mass transfer app’arently takes place under hydrodynamic amditions ( 4 , 6) In this respect, Barradas and Kimmerle ( 4 ) recently derived a n equation which permits a n approximate estimate of surface coverage from data obtained under streaming conditions corresponding to polarographic maxima of the !1rst kind (6). In [he present work, we apply similar hydroiiyrarnic considenitions to the derivation of a simplified cduat:on, which relates the surface coverage, diffusion coef,‘icient, bulk surfactant concentration and time. COKCE NTRATION-TIME REIATION
Theory. We assume that the diffusional flux of the surFsctant at the elexrode surface can be expressed by the (-lation ( 4 . 6 )
,+
.
= D A l i 2/C,o
- CA = T
7”)
I
(2)
I-iere, 9 is the flux; D,, an6 C‘.: have been defined in Equation , I , c . , , == is the concentration of the surfactant a t the Axtrade surface; and, 2 is defined by Barradas and Kim:iierk. Further, \Ae a s s m e the adsorption rate is so great ,.hat C.I, = 7o = Ci i.hroughout the course of the experiment. r_
~~~
~~
I 1 J , Koryta, C7/lrcriotz Czech. Chem. Cnmmuii., 18. 206 (1953). i 2 ) R. W. Schmlrj m(i C . .N. Reiiiey, J. Am. Chem. Soc., 80, 2087 i 1958). , >,, ,. Kata and 1. Sxmler. Collecrion Czech. Chem. Commrm., 27, 1:3.1‘1(1962). 1.41 R. G. Barradas and F. M. Kimtnerle. J. Efeciroatiai. Chem.. IP, j ‘13 ( 1 966). - 5 ) ii. J . Gross and El. W. Murray, ANAL.CHEM., 38, 40s (1966:. T
>)
v,
: ;ai!,
G. Levich, “Physicochemical Hydrodynamics,” Prentice New Jersey, 1362, i;. 1u7.
Table I. Comparison of Experimental Data From Reference 4 with Equation 3. Supporting Electrolyte, 0.1M KCI; 2.8 = 10-’M Oxygen; Triton X-305 as Shown (CJ) CA0/ex lo6 CAO x 106 r sa M oles/liter Seconds Moles-seconds/liter 1.86 4.3 8.0 8.6 2.47 3.5 3.05 5.88
2.8 1 .7
8.5 10.0
Values estimated from Figure 3 in Reference 4.
This assumption is valid up t o about half-coverage for a planar electrode (7), and is also the boundary condition assumed in the derivation of Equation 1. If the material flux given by Equation 2 is equated to the rate of coverage of the electrode, d r/dt, and integrated, then a n expression relating the various parameters may be derived when 6‘ is not a function of time (3) Equation 3 is similar in form to the equation derived previously for adsorption at a spherical electrode in stirred soluion (8). As in the stirred solution case, the product CAoteshould be a constant if the assumptions used In deriving Equation 3 adequately describe the physicochemical conditions a t the electrode surface. In this respect, the constancy of the product term may be obtained from the current-time curves shown by Barradas and Kimmerle for the effect of Triton X-305 on the oxygen maximum. The necessary data are shown in Table I (9), and, as seen, the values of C , ~ T ~ obtained a t each concentration level are indeed reasonably constant. I n the present work, Equation 3 was tested for the oxygen maximum in 0.002M potassium chloride solution containing various concentrations of cysteine as a surfactant. The low supporting electrolyte concentration used here corresponds to that often employed in work involving suppression of the oxygen maximum (10, 11). EXPERIMENTAL Apparatus. A Model 61-R Wenking potentiostat (Brinkmann Instruments) was used to provide the constant potential. External readout was accomplished by passing the faradaic (7) W. H. Reinrnuth, J . Phys. Chem., 65, 473 (1961). (8) S. L. Phillips, ANAL.CHEM.. 38, 343 (1966). (9) S. L. Phillips, 1st Materials Research Symposium, National Bureau of Standards Institute for Materials Research, Gaithersburg, Nld.. October 3-7, 1966. (10) M. Brezina and P. Zuman, “Polarography in Medicine. Hiochemistry, and Pharmacy,” Interscience, New York, 1955, p. 707. ( i l ) K. H. Mancy and D.A. Okun. ANAL.CHEM.: 32, 108 (1960). ‘JOL 39, NO. 6, M A Y 1967
n
679
0
2
6
4
t, sec.
Figure 2. Plots of data obtained from Figure I according to: A. Equation 1, B. Equation 3
Figure 1. Effect of cysteine on instantaneous current-time curves of oxygen Supporting electrolyte, 0.00 2 M potassium chloride; cysteine concentration x 10-GM: a. 0. b. 1.00 c. 2.00 d. 3.00 e. 4.00
f.
5.00 g. 6.00 h. 7.00 i. 8.00
current through a decade box load resistor, and displaying the resulting IR drop on a Tektronix Type 535A oscilloscope. The oscilloscopic trace was then photographed using conventional techniques. The electrochemical cell used was a n 85-ml capacity weighing bottle terminating in a f50!12 groundglass joint. A Teflon (polytetrafluoroethylene, Dupont) lid was machined t o fit snugly over the top of the cell, and the electrodes projected through this lid into the cell. The dropping mercury electrode was conventional; the counter electrode was a sheet of platinum with a total area of about 2 cm2. The saturated calomel electrode contained a flowing junction which served t o separate the saturated potassium chloride solution from the supporting electrolyte. The solution in the flowing junction was 0.1M potassium chloride, while 0.002M poiassium chloride was contained in the arm of the electrode which was immersed in the supporting electrolyte.
Table 11. Comparison of Experimental Data from Figure 1 with Equation 3 Supporting Electrolyte, 0.002M KCI; Equilibrium Oxygen Concentration; Cysteine Concentration as Shown (CAO) CA" x IO' Moleshter 1
oc'
2 oc 3 00 4 CY! 5oc'
6 W 700 8 K l
.e
Seconds
CA"fe x 1 0' Moles-Seconds/iite:
>6 5 8 4 3 2 2 1 1
11 12 13 $3 13 13
2 3 7 2 9 7
Mean Std dev
680
0
ANALYTICAL CHEMISTRY
6 6 2 5
2 3 I3 6 1 3 Ci 0 6s
Chemicals. All chemicais were reagent grade. Stock 1.00 X lO-3M cysteine solutions were prepared daily by dissolving the appropriate amount of cysteine chloride monohydrate (Fisher Scientific) in doubly distilled water. Only small aliquots (typically 0.05-0.10 mi) of the stock solution were used so that the change in chloride content or pH of the supporting electrolyte was negligible. This is important because the oxygen maximum is sensitive t o changes in the SUP porting electrolyte. Double distiiied water, with the seconu distillation made from alkaiine permanganate, was used throughout. Experimental Test of Theory. Typicai instantaneous c ~ i r rent-time curves depicting the experimentally observed effec-t of cysteine on the oxygen maximum at a constant appiied potential of -400 mV us. SCE are shown in Figure 1 . Fc.. each concentration, the time at which the current fell to a constant value was taken as a measure of f,, and the quantity CAot, calculated. The experimental data are shown :r3 Table 11, and, as can be seen, are in good agreement with ti;? predictions of the simple relation given in Equation 3 According t o Equation 1, for diffusion-limited adsorption, z plot of t , us. (C,.)-' will give a straight line which passethrough the origin. On the other hand, Equatior. 3 which derived for adsorption under streaming conditions ( 4 , predicts that a comparable linear plot passing through the origin wilt be obtained if r , is a function of (C,c~)-l. Thus. z s a further test of Equation 3, the data in Table I1 are plotted according to both equations in Figure 2. As seen from Figure 2 straight lines are obtained in both cases, but only thr line corresponding to Equation 3 passes through the origin. From the data shown in Table I and I1 and the plot i; Figure 2, we conclude that Equation 3 adequately describes the physicochemicai situation corresponding to the oxygen maximum under the experimental conditions used her;:. However, it should be remembered that Equation 3 does nor hold when 6' is a function of time. Analytical Applications. The linear relation betweer: C s e and f, can be made the basis of a sensitive method for analyzing traces of surface active substances. AS shown by the data in Table 11, quantities of cysteine as iow as 2.00 X 10-6M can be determined under relatively simple experimentai conditions. This lower level is about a n order of magnituu: smaller than that obtainable f;om diffusion-limited electrode. reactions, and is a reflection of the mhanced mass transfer due lo the electrode streaming. For the particular case of cysteine analysis the proposed metnod is not as sensitive as the
catalmc cobalt melhod ( 1 3 , but it has the advantage of presenting a linear relationship between bulk concentration and the measured parameter. By contrast, the catalytic cobalt method requires plottmg empirical currentconcentraa o n curves (12). It should be noted that oxidation of cysteine to cystine at the electrode surface is ai distinct possibility under the prevailing experimental condiQons. That is, at the applied potential used,hydrogen peroxide will form and a concomitant increase -________ (12) W. C. M y , h a .CHEM.,36,29A (1964).
in the pH around the electrode surface will take place (13). Both effects will favor oxidation of cysteine (14) so that the proposed analytical method may not distinguish between these substances.
RECEIVED for review January 21, 1967. Accepted March si, 1%7. (13) I. M. Kolthoff and K. Izutsu, J. Am. Chem. Soc., 86, 1275 (1964). (14) I. M. Kolthdand W. Stncks, ANAL.CHEM., 23, 763 (1951).
Purification of Carbon Dioxide for Mass Spectral Analysis by Gas-Liquid Chromatography R. B. Jordan' and A. L. OdeW Department of Chemistry,Stanford University,Stanford, Calif. Det e ctor
irJ OXYGEN-^^ ssfn-0~1~ ANALYSIS t& major problem is often the purification of c v b o n dioxide prior to its introduction into the mass spectnwneter. This note reports a simple a n d widely applicable gas chromatographic method for the purification of C G while retaining the isotopic punty of the gas.
f-
Helium
ExPmuhlENTAL
Chnwnatographic ~mlutnnswere prepared using standard techniques, by packing acid-washed Chromowrb P (Wilkens instrument Co., Walnut Creek, Calif.) with 10% by weigh: of adsorbed liquid phase in a I@foot long, 0.25-inch diameter copper tube. The columns were coiied and wrapped with heating tape. The apparatus is shown SChemaiicaIly in Figure 1. A standard G ~ w - ~ U at cl i e d conductivity detector was used in .conjunction with a l a d s & Northrup Model H recorder. The helium carrier gas was maintained at a flowrate of 30 cc per minute. Isotope ratio m s u r e m e n t s on carbon dioxide were made OR an Atlas M86 mass spectrometer. The mass spectrometer was also used t o confirm the purity of the chromatographed carbon dioxide. As shown in Figure 1, each sample was attached to the vacuum system a t A and the helium was pumped out of the inlet system. The sampie was then condensed at liquid nitrogen temperature into the inlet trap. The inlet trap was then c l C off from ,the vacuum system and the sample was swept onto the chronlatography column. The carbon dioxide was trapped in the radiator trap at liquid nitrogen ternperature. Following this, the helium stream was bypassed around the radiator trap, the helium pumped out of the trap, and the sample condelnsed into a previously evacuated sample tube a t outlet B. Dimethylformamidc ( D M n and sec-butyl phthalate were tried as liquid phases for the chromatographic separation. The see-butyl phthalate columns were dried for 1 hour at 120-150" C. These materials were tested initially for retention of the isotopic purity of the A sample of enriched in W by equilibration with W-enriched water, was passed over the coiumn and the ratio of masses
a,
a.
'Present address, Department of Chemistry, University oi Alberta Edmonton, AltPrta, Canada 'Present address, Cepaltment of chemstry. University of Auddaod, Auckland, New zealanci
F i 1. Iolet and trapping system for CO, purificatiOab~gaschrornatograph~
+ 45) was compared before and after passing over the
46/(44
COlUmn.
To chesk for memory effects on the column, samples of CO, with enriched and normal (from dry ice) oxygen isotopic content were passed through the column in success~on, and the isotopic content before and after contact with the d m was compared. Memory effects would be indicated by enhancement of the enrichment of the normai sample by the preceding enriched sample and depletion of the enriched sample by the preceeding n o d sample. The results are shown in Table I. RESULTS k h D DISCUSSION
The D M F column was found to be unsuitable because it showed a iarge memory effect and consistently gave aboct 10% depletion in 180content due to exchange on the column. The cause of these ef€ectswas not investigated further. The results with the sec-butyl phthaiate column indicate that it is completely satisfactory in retaining the isotopic purity of the Subsequent routine use indicates that column treatment has less than 0.2zeffect on the measured isotopic ratio of samples. Previously Boyd et ai. (Z},
a.
R. H. Boyd, R. W. Taft. Jr., A. P. Wolf, and D. R, Christiac, 1.Am. Chem. Soc., 82.4729 (1%).
(1)
VOL 39, NO. 6, M A Y 1967
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