Capillary Diffusion Measurements Using Fluorescence Analysis D Values of Some Electrochemically Important Systems Jeff Bacon and R. N. Adams Department of Chemistry, University of Kansas, Lawrence, Kansas 66044 ELECTROCHEMICAL STUDY of organic reactions and electrode processes often requires the knowledge of the diffusion coefficients (D values) of the species being investigated. This is particularly important in determining n values (number of electrons transferred) and in kinetic measurements using simulation methods ( I ) . The D values used in organic electrochemistry can be evaluated from a mass transport controlled current equation where all other parameters in the equation are known. However, as indicated above, very frequently the number of electrons transferred is the primary unknown being sought. Alternatively, D values are obtained by comparison of a limiting current with that of a structurally similar (model) compound whose D value is known. Model compounds whose electrode reactions are sufficiently well understood for these comparisons are by no means abundant. Thus, there is a real need for a simple method for obtaining these D values, under solution conditions as nearly identical as possible to the electrochemical experiment, yet totally independent of the electrochemistry itself. Herein are reported the results of such a simple capillary diffusion technique employing fluorescence analysis. Some earlier work from this laboratory (2, 3) using a capillary method with tritium labeled organic compounds proved to be a reasonable and accurate method for obtaining D values, but had the disadvantages of requiring preparative isotopic labeling for all compounds and a fairly involved analytical procedure. Fluorescence methods do not have these disadvantages. A large percentage of aromatic organic compounds have natural fluorescence or can be made to fluoresce by simple devices such as pH adjustment or complexation. Thus, the time and expense of sample preparation and handling inherent in tracer methods can be avoided with fluorescence techniques with little loss in sensitivity or accuracy. A relative accuracy and precision in the D values of 5-10 is often adequate for the purposes of organic electrode reaction studies. The present procedure was designed to be as simple as possible, consistent with the above limitations. For instance, the usually much more stringent temperature control employed in capillary diffusion work seemed neither necessary nor justified for the present requirements. Since the fluorescence analysis is not precision limiting, more precise results could undoubtedly be obtained with a higher level of temperature control if desired. EXPERIMENTAL
All compounds were obtained from commercial sources or were prepared following literature procedures. Electrochem(1) S. W. Feldberg, “Electroanalytical Chemistry,” Vol. 111, A. J. Bard, Ed., Marcel Dekker, New York, N. Y., 1969. (2) T. A. Miller, B. Lamb, K. Prater, J. K. Lee, and R. N. Adams, ANAL.CHEM., 36,418 (1964). (3) T. A. Miller, B. Prater, J. K. Lee, and R. N. Adarns, J. Amer. Chem. Soc., 87, 121 (1965).
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9
Figure 1. Typical capillary diffusion cell Entire cell immersed in temperature bath at 25.0 f 0.1 “C
ical D values listed are all from this laboratory and derived curves or rotated disk polarograms. from potentiostatic it *Iz Electrode areas were evaluated using D = 0.693 x loW5 cm*/sec for ferrocyanide (4) in 2 M KCI. The electrochemical equipment was standard and has been described elsewhere. Fluorescence measurements were made on an American Instruments spectrophotofluorometer, model 4-8202, with NO. 10-267 photomultiplier microphotometer attached. Capillaries were precision bore tubes (Willmad Corp.) 5.0-5.5 cm in length with 1 mm i.d. and sealed at one end. Temperature was constant at 25 rt 0.1 “C. Diffusion solutions were 1-2mM in diffusant species. The procedure was very similar to those reported earlier (2, 5, 6). The capillaries were filled with diffusion solutions by syringe and placed in a Teflon holder. A large drop of solution was placed on the top of each capillary and the holder was lowered gently into a bath (approximately 1 liter of the diffusion solution minus diffusant). It is important that the capillaries be equal to or slightly lower in temperature than the bath during immersion since a contraction of the solution into the capillary will ruin the experiment. Stirring was then accomplished using a common 300 rpm lab motor turning a glass disk, 2 cm in diameter. The configuration is shown in Figure 1. After an allotted time (2 to 3 days with acetonitrile, 6 to 7 days for aqueous solutions) the tubes were removed from the bath and placed in small beakers containing a measured amount of solvent and rinsed several times with a syringe. The beakers’ contents were then analyzed fluorimetrically. Standard solutions for determining C at t = 0 were obtained by carrying three capillaries through the above procedure except that they were removed immediately after immersion. (4) M. von Stackelberg, M. Pilgram, and V. Toome, 2.Electrochem., 57,342 (1953). (5) J. H. Wang,J. Amer. Chem. Soc., 73,510,4181 (1951). (6) Zbid.,76, 1528 (1954).
1.
2. 3. 4.
Compound Anthracene Naphthalene Biphenyl N,N-Dimethylaniline
Table I. Results of Diffusion Measurements Solvent" D X 106 2.91 f 0.10 AN 2.96 f 0.15 AN 2.46.f 0.10 AN 0.68 f 0.03 B&R 0.65 f 0.04 Acetic acid
X 10' 2.W
Dtr
Del
X 10'
2.41c,d
3. lod
...
2.80d 0.67 1.09
...
...
0.47
0 . 45b
...
O.O5M,
pH 4.45 5. o-Dianisidine 6. 9,lO-Diphenylanthracene 7. 9,10-Dihydro-9,10dimethylphenazine 8. N,N-Dimethyl-panisidine 9. o-Anisidine 10. N,N,N',N'-Tetramethylbenzidine 11. Benzo(a)pyrene 12. 6,6'-Bibenzo(a)pyrenyl 13. Perylene 14. Aniline
AN AN
0.43 f 0.03 2.12 f 0.08 2.29 f 0.02
B&R
0.70 f 0.03
1M B&R
0.64 f 0.03 0.50 f 0.02
1M His04
... ... ... ...
...
... AN 2.11 f 0.04 ... AN 0.78 f 0.03 ... AN 2.25 f 0.11 ... AN 2.90 f 0.14 B&R ... 0.73 f 0.03 B&R ... 0.81 f 0.04 15. Phenol Diffusion coefficients in cm2/sec. D ; this study. Dt,; ref. (2, 3). D,I = electrochemical, this study. a AN = acetonitrile; B & R = Britton and Robinson buffers, pH 2.4. b Values determined by chronoamperometry. c Determined by rotating disk. d Viscosity corrected to zero electrolyte.
The D values were calculated from the equations used in previous studies (2, 3, 5-7):
where CA"is the average concentration of diffusant in the capillary after time t , and L is the capillary length. Bearman has shown theoretically that electrochemical D values and tracer (also called self-diffusion) coefficients, while not identical, are ordinarily equal within experimental limits of limiting current equations (8). The present experiments seem to be even more closely related to electrochemical conditions. RESULTS AND DISCUSSION
Results are shown in Table I. For all compounds, a minimum of two runs were made. Each run involved 6 capillaries. The D values and their average deviations are shown in the table. Along with several electrochemical models (compounds 5-8), the compounds listed include those used in the previous tracer studies as a check. All new values involving acetonitrile as a solvent were obtained with no electrolyte present. Other values from the tracer and electrochemical studies have been converted to zero electrolyte content for Table I by using the viscosity ratio 1.13, corresponding to 7 for acetonitrile with 0.1M tetraethylammonium perchlorate (3.90 millipoise) divided by 7 for the pure solvent (3.45 millipoise). The product (07)has been shown to be constant for a given compound (3). The table shows good agreement with previous tracer work (7) R. A. Robinson and R. H. Stokes, "Electrolyte Solutions,"
2nd Ed., Butterworths, London, 1959, pp 261-264. (8) R. J. Bearman, J. Phys. Chem., 66,2072(1962).
2.09C~d 2.23btd 0.74b
...
0.4P 2.23"~d
...
2.26Crd 2.96"d 1.20 1 .oo
and verifies the electrochemical D values of several model compounds, which heretofore lacked independent checks. An interesting discrepancy is seen in compound 4, dimethylaniline. At the two pH's listed, this study shows D to be the same within experimental precision, while the tracer values show a marked increase with pH. An increase might be expected because of solvation changes caused by a loss of a formal positive charge on the amine at pH's above its pK, (5.06). However, the 62 increase shown in Dt,going from pH 2.4 to 4.45 seems inordinately large. This suggests a loss of activity with increasing p H in the tracer study rather than an effect on the diffusion coefficient. However, any activity loss must be due to something other than sinple exchange reactions, since they would be expected to have an opposite pH dependence (9, IO). Nevertheless, it seems reasonable to assume the previously determined tracer values at high pH are incorrect. Another important result is the ratio of diffusion coefficients for monomers and dimers. A comparison of compounds 4 with 10 and 5 with 9 shows an intuitively reasonable decrease in D of about 30 in going from monomer to dimer. However, the D values of benzo(u)pyrene (compound 11) and its dimer (compound 12) are drastically different. Assumptions about relative D values should be made with care.
RECEIVED for review November 24,1969. Accepted February 2, 1970. Support of this work by NSF uiu grant GP-12169 and partially uiu NIH grant NB-08740 is gratefully acknowledged. (9) M. S. Karasch, W. G. Brown, and J. McNau, J. Org. Chem., 2, 36 (1938). (IO) W. G. Brown, M. S . Karasch, and W. R. Sprowls, ibid., 4, 442 (1939).
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