3048
J. Phys. Chem. 1983, 87, 3048-3054
agreement with kinetic measurements. Both MO calculations were at the SCF level with 4-31G, or similar, basis sets without configuration i n t e r a ~ t i o n .Electron ~~ correlation dramatically alters the computed saddle point geometry for repulsive surfaces;53 for example, the F-H distance increased from 1.06 to 1.37 A for the F H2 reaction. Higher order terms may similarly effect the dehydrohalogenation geometry, especially since the energy disposal pattern suggests a repulsive potential energy ~urface.~J~,~~
+
Conclusion We have verified the hydrogen halide elimination stereochemistry is predominantly syn and shown the anti (52)We thank a referee for comments about configuration interaction. (53)H. F. Schaefer in "Atom-Molecule Collision Theory", R. B. Bernstein, Ed.,Plenum, New York, 1979,pp 45-78. (54)B. E. Holmes and D. W. Setser, J. Phys. Chem., 82,2461(1978).
contribution is small, perhaps zero. The primary deuterium isotope effect of 2.1 found for threo- and erythro2-bromo-3-deuteriobutane at 590 K agrees with previous work; the @-deuteriumeffect was 1.0. These k ~ / values k ~ concur with the models of Setser and co-workers but are a t variance with ionic models and transition state structures calculated with MO theory. As trajectory or ab initio MO computations including configuration interaction become available the discrepancies may be resolved. Acknowledgment. We are grateful for financial support provided by a Cottrell College Science Grant from the Research Corporation. We thank Mr. Edward E. King for assistance with the data collection, Professor D. W. Kurtz for helpful discussions, and Professor G. D. Renkes for careful reading of the manuscript. Registry No. threo-2-Bromo-3-deuteriobutane, 49623-58-7; erythro-2-bromo-3-deuteriobutane, 27849-07-6; cis-2-butene,
590-18-1;trans-2-butene,624-64-6;deuterium, 7782-39-0.
Orientation of Aromatlc Compounds Adsorbed on Platinum Electrodes. The Effect of Temperature Manuel P. Sorlaga, James H. Whlte, and Arthur T. Hubbard' Department of Chemistty, University of California, a n t a Barbara, Callfornia 93 106 (Received: December 15, 1982)
The adsorption of aromatic compounds on smooth polycrystalline platinum electrodes in aqueous solutions has been studied as a function of temperature. Measurements were made by electrochemicalmethods using thin-layer cells. Eight compounds were studied: hydroquinone ( l ) ,2,5-dimethylhydroquinone(2), 2,2',5,5'tetrahydroxybiphenyl (3), 1,4-naphthohydroquinone (4), anthraquinone-1,5-disulfonicacid (5), 2,5-dihydroxythiophenol (6), 2,3-dihydroxypyridine (7), and 3,6-dihydroxypyridazine (8). The adsorption isotherms of 1-4, which display stepwise transitions to higher packing density with increasing concentration, are influenced by temperature as follows: (i) the transitions are most sharply defined at low temperatures, near 5 "C; (ii) as the temperature is increased, the packing densities at low concentrations increase, while those at high concentrations decrease; (iii) near room temperature (25 < 5" < 65 "C), an additional plateau appears; and (iv) the transitions are barely noticeable at 65 "C, the highest temperature studied. The packing density of 8 is lower at 65 "C than at ambient temperatures, but those of 5-7 are virtually temperature independent. The results are discussed in terms of librational motion within an oriented monolayer.
Introduction Packing density measurements based on thin-layer electrochemical methods have recently demonstrated that aromatic compounds chemisorbed from aqueous solutions onto smooth polycrystalline platinum electrodes adopt various nonrandom orientations.'-' Which orientation is adopted in a particular instance has been shown to depend on adsorbate molecular structure,' adsorbate concentrat i ~ n , ~the , ~ ,presence ' of surface-active anions,2and the pH of the solution; chirality of inert substituents affects the packing density but not the ~ r i e n t a t i o n .Other ~ factors such as electrode potential and the nature of the solvent and substrate are expected to influence orientation. The adsorbed species are desorbed very slowly or not at all when placed in contact with the pure solvent; as such, (1)Soriaga, M.P.; Hubbard, A. T. J . Am. Chem. SOC.1982,104,2735. (2)Soriaga, M.P.; Hubbard, A. T. J. Am. Chem. SOC.1982,104,2742. (3)Soriaga, M.P.; Hubbard, A. T. J. Am. Chem. SOC.1982,104,3937. (4)Soriaga, M.P.; Wilson, P. H.; Hubbard, A. T.; Benton, C. S. J. Electroanal. Chem. 1982,142,317. (5)Soriaga, M.P.; Stickney, J. L.; Hubbard, A. T. J. Mol. Catal., in Dress. c (6) Soriaga, M.
P.; Stickney, J. L.; Hubbard, A. T. J. Electroanal. Chem. 1983. 144.207. (7)Chia,V. K: F.;Soriaga, M. P.; Hubbard, A. T.; Anderson, S. E. J. Phys. Chem. 1983,87,232. (8) Stickney,J. L.; Soriaga, M. P.; Hubbard, A. T.; Anderson, S. E. J . Electroanal. Chem. 1981,125, 73. 0022-3654/83/2087-3048$01.50/0
they may be regarded as surface organometallic compounds. Orientational transitions observed when the adsorbate concentration was increased3v4or surface-active anions were introduced2suggest that alternative modes of binding exist analogously to well-characterized organometallic complexes."" Molecular orientation has important implications in the chemistry of adsorbed compounds. For instance, the path of electrochemical oxidation of aromatics chemisorbed on Pt electrodes depends on ~rientation.~-' The present article describes the influence of temperature on the packing density and orientation of aromatic compounds adsorbed on smooth polycrystalline Pt electrodes. Eight compounds were studied (Table I). It may be mentioned that a few studies have been reported on the effect of temperature on the reversible adsorption of organic compounds at mercury-electrolyte interfaces.'2-'s At (9)Calderon, J. L.;Cotton, F. A.; DeBoer, B. G.; Takats, J. J . Am. Chem. SOC.1971,93,3592. (10)Rogers, R. D.; Bynum, R. V.; Atwood, J. L. J . Am. Chem. SOC. 1978,100,5238. (11)Ugo, R. Catal. Reu. 1975,11, 225. (12) Damaskin, B. B.; Petrii, 0. A,; Betrakov, V. V. 'Adsorption of Organic Compounds on Electrodes"; Plenum Press: New York, 1971. (13)Damaskin, B. B.; Survilla, A. A.; Kybalka, L. E. Elektrokhimiya 1967,3,146. (14)Danilov, F. I.; Panasenko, S. A.; Sechin, L. G. Elektrokhimiya 1981,17, 1191.
0 1983 American Chemical Society
Aromatic Compounds Adsorbed on Platinum Electrodes
The Journal of Physical Chemistry, Vol. 87, No. 16, 1983 3049
material, after which the desorbed amount was quantitated least one study has been publishedl9 on the temperature by coulometry; (ii) after rinsing with pure electrolyte and dependence of benzene adsorption at a platinized Pt a waiting period for desorption, the surface was rinsed electrode, but the work was limited to concentrations at which only flat-oriented species would be p r e ~ e n t . ~ ? ~ ~ again to remove species desorbed from the surface; the amount desorbed (I'des) was found from the amount adMeasurement of Adsorbed Amounts sorbed (Ard when the now-partially-coated electrode was reexposed to the surfactant solution of the same concenA method for accurate measurement of the packing tration at which the initial chemisorption was carried out: density of aromatic compound adsorbed on Pt electrodes using thin-layer electrochemical techniques has been de(5) Arads = (8- Qb) - (81- Qib)/nFA scribed in detail elsewhere: and only a brief summary will where Q1 represents the charge for electrolysis of dissolved be presented here. In the thin-layer cells2'@ employed for surfactant after only a single filling and Q the charge after this study, the solution is contained between a cylindrical multiple rinses (cf. eq 3). Pt electrode and surrounding precision glass tubing. Spent liquid is removed by application of pressurized inert gas; Experimental Section filling occurs due to capillary action when the pressure is The preparation of electrodes, electrolytes, and surfacreleased. When the thin-layer cavity is filled only once, tants has been described previ~usly.'-~The adsorption the Pt electrode is exposed to a single aliquot of solution temperature was controlled by immersing the cells in a containing a precise molar amount of adsorbate. If all of thermostated water bath. The temperature was varied at the solute is taken up during the first filling, the amount intervals from 0 to 65 "C, as determined by a thermometer of material adsorbed is given by eq 1,where rl (mol/cm2) placed in the reference compartment of the H cell. The rl = VCO/A (1) temperature fluctuated less than *0.5 "C below 50 "C and f l "C above 50 "C. is the surface concentration, C" the bulk solute concenFor five of the subject compounds (1-5) the packing tration, V the thin-layer cell volume, and A the electrode density was determined, for a given temperature, at consurface area. Complete uptake by the surface is verified centrations from 50 pM to 3 mM. The upper concentraby the absence of voltammetric peaks due to dissolved tion limit in most cases was imposed by the solubility of (unadsorbed) material. If k repetitive fillings are required the aromatic compound in aqueous 1M perchlorate, while to build up the organic monolayer, the adsorbed amount the lower limit was the inconvenience of introducing the accumulated at that stage is adsorbate through many fillings of the thin-layer cavity rl + r2+ ... rk = ~ V C O / A (2) (volume, 4.08 pL; area, 1.18 cm2). For the remainder ( 6 4 , packing density varied only slightly with concentration The next (k + 1) filling leads to an excess of dissolved adsorbate and the last increment of amount adsorbed, rk+', over the accessible range and was therefore measured at only a single concentration at various temperatures. is found by thin-layer coulometry from the dissolved exAdsorption was carried out at controlled potential: 0.200 cess: V (vs. Ag/AgCl in 1 M C1- reference electrode) in 1 M r k + l = [(Qk+2- Qb,k+d - Qb,k+i)l/nFA (3) HC104or -0.220 V at pH 7 (phosphate-buffered 1 M NaClO4'). A waiting period of 180 s was allowed for adwhere Q represents the charge for electrolysis of the dissorption. However, no significant changes were noted solued material, Qb is the electrolytic charge due to backwhen adsorption was carried out at open circuit or when ground reactions under identical conditions except that the adsorption time was varied from 60 to 300 sa4 In beunadsorbed surfactant is rinsed from the thin-layer cavity tween experimental trials, the electrode was cleaned by with pure supporting electrolyte, and n is the number of electrochemical oxidation in 1M HC104 at 1.2 V and reFaradays F consumed per mole of reactant. Since the final duction at -0.2 v.' Linear-potential sweep voltammetry (k + 2) filling leads to no further adsorption, it may be used and potential-step coulometry were done by using a conto determine the adsorbate concentration, C". Using ventional multipurpose electrochemical circuit based on Faraday's law operational amplifiers, as described previously; 21 the (4) co= (%+2 - Qb,k+P)/nFV electrolytic charge Q was recorded with a four-place digital The above method is applicable whether or not the disvoltmeter. solved and adsorbed forms of a given surfactant display Results similar electrochemical rea~tivities.~ Figure 1shows thin-layer cyclic current-potential curves The amount of aromatic desorbed from a bound layer for the reversible quinone/diphenol couple of unadsorbed has been determined by two methods: (i) the aromatic1,4-naphthohydroquinone,4. The dashed curve was obcoated electrode was allowed to stand in pure supporting tained after a single filling of the thin-layer cavity, while electrolyte for a predetermined length of time; a thin-layer the solid curve represents that after multiple fillings. The cyclic voltammogram was then obtained to detect desorbed amount of reversibly electroactive material was smaller for species which display peaks characteristic of dissolved a single filling of the thin-layer cavity than for multiple fillings because the adsorbed species reacted at more (15)Eronko, 0.N.;Lizogub, A. V.; Afanasev, B. N. Elektrokhimiya 1981,17, 1732. positive potentials than unadsorbed naphthohydro(16)Mordovchenko, I. P.; Loshkarev, M. A. Elektrokhimiya 1965,1, quinone;I4 but, after several fillings of the thin-layer cavity 94. (in which case the surface had become saturated with (17)Conway, B. E.;Gordon, G. M. J. Phys. Chem. 1969, 73, 3609. (18)Retter, U.;Lohse, H. J. Electroanal. Chem. 1982, 134, 243. aromatic), the amount of reactivity expected from the bulk (19)Heiland, W.;Gileadi, E.; Bockris, J. O'M.J. Phys. Chem. 1966, concentration was observed. The area difference between 70, 1207. the dashed and solid curves (obtained by potential-step (20)Kazarinov, V.E.; Frumkin, A. N.; Ponomarenko, E. A.; Andreev, N. V. Elektrokhimiya 1975 11, 860. coulometry) is a measure of the amount of 4 taken up by (21)Hubbard, A. T.In "Critical Reviews of Analytical Chemistry"; the Pt surface, eq 3. Figure 1 is typical of results obtained Meites, L., Ed.; Chemical Rubber Publishing Co.: Cleveland, OH. 1973: for all eight compounds. p 201. The results are summarized in Table I. Packing density (22)Lai, C. N.;Hubbard, A. T. Znorg. Chem. 1972,11, 2081.
+
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The Journal of Physical Chemistry, Vol. 87, No. 16, 1983
Soriaga et al.
TABLE I: Adsorption Data for Aromatic Compounds Adsorbed o n Platinum from Solution at Various Temperaturesa
r,
r,
T,"C
5
25
35
5
25
5
nmol/ cmz T , " C
C,mM
C,mM Hydroquinone (1)
0.030 0.069 0.114 0.167 0.224 0.244 0.313 0.330 0.400 0.536 1.19 2.45 0.0025 0.0075 0.020 0.028 0.070 0.115 0.131 0.153 0.187 0.219 0.257 0.261 0.300 0.349 0.350 0.420 0.445 0.460 0.511 0.697 0.774 1.70 1.86 2.00
0.296 0.303 0.310 0.350 0.392 0.447 0.501 0.517 0.557 0.583 0.606 0.616 0.277 0.294 0.313 0.317 0.317 0.338 0.353 0.381 0.395 0.432 0.465 0.454 0.484 0.501 0.498 0.519 0.533 0.555 0.588 0.592 0.597 0.606 0.606 0.606
nmol/ cmz
35
0.120 0.193 0.266 0.349 0.456 0.556 0.718 1.28 2.49
0.364 0.411 0.451 0.484 0.494 0.524 0.569 0.588 0.606
40
0.272 0.331 0.392 0.503 0.631 0.818 1.38 2.38 0.039 0.060 0.086 0.125 0.203 0.253 0.286 0.345 0.534 1.06 1.48 2.13 0.0131 0.056 0.0773 0.162 0.250 0.597 1.38 2.40
0.447 0.470 0.482 0.489 0.494 0.508 0.550 0.573 0.341 0.348 0.360 0.392 0.416 0.454 0.456 0.461 0.475 0.482 0.479 0.484 0.343 0.353 0.371 0.390 0.400 0.411 0.411 0.418
45
65
0.014 0.329 0.055 0.331 0.089 0.345 2,5-Dimethylhydroquinone( 2 ) 0.019 0.200 25 0.905 0.054 0.209 1.10 0.151 0.235 1.31 0.223 0.261 1.50 0.339 0.303 1.63 0.566 0.331 1.80 0.770 0.357 45 0.025 0.892 0.374 0.076 0.998 0.418 0.126 1.13 0.442 0.182 1.31 0.472 0.322 1.44 0.494 0.427 1.70 0.496 0.685 2.04 0.496 0.842 0.021 0.214 1.05 1.48 0.0786 0.223 0.118 0.230 1.81 0.144 1.90 0.247 0.202 0.263 65 0.019 0.348 0.324 0.117 0.420 0.343 0.168 0.381 0.474 0.380 0.500 0.390 0.654 0.409 0.557 1.65 0.666 0.425 0.705 0.437
0.442 0.447 0.468 0.494 0.494 0.501 0.226 0.247 0.256 0.294 0.362 0.404 0.432 0.447 0.447 0.456
r,
T. "C C.mM
25
5
25
0.461
0.461 0.242 0.289 0.308 0.348 0.376 0.378
2,2' ,5,5'-Tetrahydroxy biphenyl ( 3 ) 0.011 0.188 25 2.43 0.573 0.038 0.202 2.80 0.573
nmol/ cm2 T . "C C.mM
0.084 '0.221 0.013 0.137 0.249 45 0.037 0.193 0.291 0.050 0.236 0.324 0.059 0.296 0.334 0.085 0.334 0.347 0.108 0.424 0.338 0.132 0.518 0.343 0.158 0.647 0.353 0.210 0.371 0.710 0.312 0.800 0.395 0.387 0.916 0.439 0.446 0.494 1.00 0.530 1.16 0.555 0.587 1.40 0.564 0.710 1.58 0.564 0.815 0.005 0.190 0.983 0.020 0.197 1.15 0.067 0.216 1.38 0.101 0.247 1.57 0.132 0.277 2.05 0.191 0.322 3.01 0.242 0.343 0.020 0.314 0.353 65 0.060 0.450 0.353 0.087 0.554 0.362 0.154 0.605 0.376 0.265 0.747 0.404 0.337 0.832 0.425 0.470 0.914 0.479 0.830 1.14 0.505 1.11 1.45 0.543 1.63 1.78 0.564 2.83 2.20 0.573 1,4-Naphthohydroquinone( 4 ) 0.025 0.233 35 0.050 0.057 0.244 0.090 0.112 0.259 0.130 0.150 0.329 0.158 0.187 0.414 0.206 0.226 0.472 0.316 0.350 0.529 0.417 0.511 0.550 0.595 0.605 0.555 1.oo 0.850 0.559 45 0.034 1.68 0.564 0.062 1.98 0.569 0.085 0.117 0.026 0.235 0.151 0.045 0.247 0.317 0.100 0.249 0.561 0.118 0.298 1.02 0.125 0.345 1.30 0.153 0.376 1.90 0.178 0.416 0.248 0.421 65 0.029 0.295 0.421 0.098 0.327 0.421 0.192 0.363 0.447 0.551 0.380 0.456 1.05 0.478 0.479 1.81 0.534 0.498 0.727 0.533 0.934 0.559 1.88 0.564
r,
nmol/ cm' 0.204 0.221 0.223 0.233 0.247 0.270 0.289 0.313 0.342 0.360 0.360 0.360 0.376 0.388 0.414 0.432 0.482 0.494 0.498 0.498 0.510 0.519 0.247 0.259 0.291 0.317 0.327 0.334 0.338 0.367 0.381 0.400 0.414 0.261 0.273 0.364 0.400 0.411 0.425 0.451 0.494 0.512 0.270 0.280 0.287 0.338 0.400 0.423 0.442 0.447 0.451 0.465 0.294 0.308 0.324 0.343 0.353 0.357
Anthraquinone-l,5-disulfonic Acid ( 5 )
25 45
0.923 1.90 0.162 0.833 2.24
0.127 0.127 0.141 0.141 0.146
65
1.05 2.38
0.143 0.150
The Journal of Physical Chemisiry, Vol. 87, No. 16, 1983 3051
Aromatic Compounds Adsorbed on Platinum Electrodes
TABLE I (Continued) T , "C 5 25 5 25
C, mM
I', nmol/ cm'
r,
r,
T, "C
C, mM
2,5-Dihydroxythiophenol (6) 45 0.75 0.75 0.571 0.569 65 0.75 0.75 2,3-Dihydroxypyridine (7), pH 7 b 1.0 0.702 45 1.0 1.0 0.702 65 1.0
nmol/ cm 0.569 0.564
T,"C 5 25
C,mM 0.96 0.96
cm2
0.4
Flgure 1. Thin-layer current-potential curves for 1,4naphthohydroquinone at a polycrystalline platlnum electrode: (- - -1 first filling; (-) presaturated surface. C o = 0.08 mM In 1 M HCIO,. Thln-layer volume, V = 4.08 pL; A = 1.18 cm2;rate of potential sweep, r = 2.00 mV/s; T = 25 "C.
vs. concentration curves for hydroquinone (l), 2,5-dimethylhydroquinone (2), 2,2',5,5'-tetrahydroxybiphenyl(3), and 4 at various temperatures are shown in Figure 1. The data obtained at room temperature have been discussed previously: the concentration-induced transitions from low to high coverages are attributable to changes in the orientation of the adsorbed species from flat (lower plateau) to edgewise (upper plateau) structures. Several important features may be noted in Figure 2: (i) the transitions are most sharply defined at the lowest temperature studied, 5 "C; (ii) as the temperature is increased, the packing densities at low concentrations increase, while at high concentrations the packing densities decrease; (iii) at intermediate temperatures (25 < T < 65 "C), an additional plateau appears; and (iv) transitions in packing density are barely noticeable at 65 "C, the highest temperature studied. Graphs of packing density vs. temperature for various concentrations of 1 and 4 are given in Figure 3. From the figure it is apparent that at a certain concentration, Ciso, the packing density is virtually independent of temperature up to about 45 "C. At concentrations below Ciso, packing density increases with temperature, whereas above Ci,the opposite temperature dependence is observed. The effect of temperature on packing density is most pronounced for conditions corresponding to the edgewise ($) orientation. The temperature dependences of the packing densities 394
C,mM
nmol/ cm 0.639 0.588
0.698 0.689
POTENTIAL, VOLT VS. AgCl
0.2
T,"C
3,6-Dihydroxypyridazine (8) 0.667 45 0.96 65 0.96 0.660
The average relative standard deviation in r was ?3% from 0 to 45 "C and *6% at 65 "C. means of a 1 0 mM phosphate buffer in 1 M NaClO,.'
0.0
r,
nmol/
A pH of 7 was maintained by
for 5-8 are given in Table I. The packing density of anthraquinone-1,5-disulfonicacid ( 5 ) , a compound which always adopts primarily the flat orientation, increases slightly with temperature. The observed increase is probably due to increased packing efficiency in view of an appreciable void area found to be present at 25 0C.2 The packing densities of S-q'-oriented 2,5-dihydroxythiophenol (6) and N-7'-oriented 2,3-dihydroxypyridine (7) were not influenced by temperature. In contrast, N-V'-oriented 3,6-dihydroxypyridazine(8) gave a higher packing density at room temperature than at 65 "C. Data relating to the irreversibility of adsorption and reorientation appear in Table 11. Note the following, in particular: (i) When an electrode precoated with $-oriented hydroquinone at 25 "C was warmed to 65 "C,additional adsorption occurred such that the final packing density was the same as when the adsorption was carried out entirely at 65 "C. However, when the predominantly $-attached layer formed at 65 "C was cooled to 5 "C, no detectable desorption occurred after 15 min. (ii) When a layer of q2-oriented hydroquinone formed at low temperatures was warmed to higher temperatures, a small amount of 1 was desorbed and the final packing density was nearly that observed for adsorption entirely at the higher temperature. The reverse was also true: when the layer formed at 65 "C was cooled to ambient temperature, increased adsorption occurred such that the final packing density was comparable to that resulting from adsorption entirely at 25 "C. However, at a fixed temperature, the degree of desorption from an $-oriented layer was small, amounting to less than 4% after 15 min. (iii) The decrease with inin packing density of +oriented material, rdes, creased temperature is the same whether determined directly by voltammetry after heating or indirectly by reuptake measurements (eq 5). The former method revealed that desorption produced starting material. The latter method ruled out the possibility that heating produced fragments having different packing density behavior than the starting material.
Discussion The temperature dependence of gas adsorption on solids has been extensively investigated, and numerous reviews have a p ~ e a r e d . ~ ~ The - ~ effect ~ of temperature on the properties of fatty acid monolayers at air-water interfaces is well studied; at low temperatures, the pressure (23) Flood, E. A. "The Solid-Gas Interface"; Marcel Dekker: New York, 1967. Trapnell, B. M. W. "Chemisorption";Butter(24) Hayward, D. 0.; worths: Washington, DC, 1964. (25) Adamson, A. W. "Physical Chemistry of Surfaces";Wiley: New York, 1976. (26) Harkins, W. D.; Young, T. F.; Boyd, E. J. Chem. Phys. 1940,8, 954. (27) Nutting, G. C.; Harkins, W. D. J . Am. Chem. SOC.1939,61, 2040. (28) Boyd, G. E. J. Phys. Chem. 1958, 62, 536.
3052
The Journal of Physlcal Chemistty, Vol. 87, No. 16, 1983
Soriaga et ai.
TABLE 11: Adsorption and Desorption Data for Platinum Precoated with Hydroquinone desorption datab rdes, nmol/cm2
initial pretreatmenta
r re
initial temp, "C
nmol/cm2
25 65
0.322 0.371
45 65 5 5 25 25
0.479 0.411 0.616 0.616 0.606 0.606
adsorption dataC Arads,
temp, " C
eq 5c
direct
final packing density, nmol/cm2
temp, " C
nmol/cmz
65
0.050
0.372 0.371
25 25
0.113 0.183
0.592 0.594 0.594 0.588 0.521 0.451
0.08 mM Hydroquinone
5
