M. W. Massey and Z. A. Schelly
2450
Dissociation Field Effect and Temperature-Jump Kinetics of Ethanolic and Aqueous Phenolphthalein1 M. W. Massey, Jr., and 2. A. Schelly’ D8parfment of Chemistry, University of Georgia, Athens, Georgia 30602 (Received March 11, 1974; RevLwd Manuscript Received August 20, 7974)
Dissociation field effect and temperature-jump results are presented on ethanolic and aqueous phenolphthalein. It is shown that the steady-state approximation cannot be applied to the hydrolysis equilibria in pure ethanol.
Introduction titrated with 0.1 N HCl standard Titrisol in water solution. The electrolyte used in the ethanolic temperature-jump The mechariism of proton transfer reactions in aqueous experiments was tetramethylammonium chloride obtained solution can be described by three possible paths: protolfrom J. T. Baker. ysis, hydrolysis, or direct proton exchange between donor Apparatus. Our square wave dissociation field effect and acceptor. The unique structural properties of water (DFE) pulse generator used in the electric field-temperaallow direct participation in the reaction through seconture-jump apparatus closely resembles that described by dary and tertiary hydration, where structural diffusion is Olsen, et al. usually the rate-limiting step in the drift mobility of the Single 32-50-kV square wave pulses (Figure 1)were genproton.2 The same is true only to a more limited extent in erated by discharging a low-inductance 0.005-pF capacitor the lower aQ,iphaticalcohols, e.g., ethanol, where structural from Plastic Capacitors Inc. through EG and G GP-15B diffusion and hydrogen bonding is less pronounced. Nevertriggered spark gaps. The pulse width is variable between 0 theless, the requirements for diffusion-controlled proton and about 30 psec. The rise time of the pulse is ca. 100 exchange rates may be still fulfilled, unless other factors nsec, but the fall time is less than 20 nsec (Figure 2). Typibecome prevalent, cally 40-kV pulses 2-3.5-psec long were used in the DFE In the present work we report a study of proton transfer experiments where the chemical relaxations following the relaxation kinetics involving the acid-base indicator phentrailing edge of the perturbation pulse were observed, ratholphthalein (d i-p -dioxydiphenylphthalide) in aqueous and er than the in-field ones7 ethanolic solutions. In a previous T-jump study of this sysSpectrophotometric detections was used for monitoring tem in H20, structures for the species in equilibrium were the suggested3 different from the ones generally a ~ c e p t e d . ~ , ~ transient concentration changes in the relaxing reaction mixtures a t the visible absorption maximum of phenThe present study was undertaken to establish the effects olphthalein (A 552 nm in water and X 563 in ethanol). The caused by different types of perturbation, and the use of a optical system consisted of a 250-W Osram halogen lamp solvent less polar than water. An investigation using differpowered by an Electronics Measurements RE40-1OML regent solvents can reveal the intricate details of the reaction ulated power supply, a quarter meter Jarrell Ash monomechanism characteristic to pseudo acids, where intramochromator, and a mirror box to focus the light beam passlecular electronic rearrangement accompanies the proton ing through the sample cell to the end of a glass fiber optic. transfer. In the case of phenolphthalein, the extent of this The fiber optic terminated at the window of an electromagrearrangement is drastic and can be directly followed under netically shielded Fairchild KM2433 photomultiplier tube specifically controlled conditions. Information about the operated with a 15-kilohm dynode chain, and powered by a “polarization” of the molecular states has been obtained by Power Design 2K20 power supply. The photomultiplier the alternate use of‘ electric field and temperature-jump output was fed directly into a Tektronix 7503 oscilloscope perturbations. with a 7A13 differential comparator and a 7B52 dual time base. Oscilloscope traces were photographed with a TektroExperimental Sectnon nix C-50 camera, and the pictures evaluated graphically in Chemicai‘s. Phenolphthalein (certified ACS) was obthe usus1 manner. Using 1.5-kilohm terminating resistor tained from Vischei., and purified through precipitation the detection system had a rise time of 30 nsec. from ethanol with water, and then dried under vacuum. The Plexiglas DFE reaction cell used had an outside diThe solutions used in the kinetic experiments were preameter of 63.5 mm, an optical path length of’ 29 mm, and a pared from 3.14 X or 5.70 X M ethanolic stock volume of 9 ml. Highly polished flat surfaces 180° apart solutions. served as windows for the analyzing light beam. The TAll water used in the experiments was distilled and doujump cell with 1-cm path length was manufactured of Tefbly deionized with research cartridges Model I from the 11lon and had conical quartz windows. A minimum of 5 ml of linois Water Treatment Co. US Industrial Chemical Co.’s solution is needed for an experiment, but only 1 ml is heatabsolute ethanol. was further dried with molecular sieve ed. Both cells were equipped with highly polished nonmagLinde Type 3 h . netic stainless steel electrodes located 5 and 10 mm apart J. T. Baker’s analytical grade NaOH was washed with in the DFE and T-jump experiments, respectively. The liqand dissolved in absolute ethanol, then a sample of it was uids in both cells were thermostated to f0.1’ by circulating The Journal ot Physical Chemistry, Voi. 78, No. 24, 1974
RotonTransfsr Relaxation Kine(icsfaFlmnolphthaleln
High-voltage square pulse with 3.14 X M phenolphmlein and 4.76 X lo-' M NaOH ethanolii solution in the cell at 25O: 5 X lo3 Vldivision. 1 psecldivision.
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05
Flgure 1.
Typical E-jump relaxation 01 3.14 X IO-' M ethanoiic phenolphthalein at 25' caused by a 2-psec 40-kV puise. The location of the perturbation pulse is indicated by arrows. NaOH concentration is 4.76 X IO-' M. Direction of increasing analyzing light ( A 563 nm) intensity is toward top Of page: vertical axis 100 mvldivision. horizontal axis 1 psecldivision. Two traces superimposed. Due to the high frequency Of the noise, relaxation times could b e determined from a single trace wiih a precision of bener than 5 % in terms of deviation between two extreme possibilities. The reproducibility of the traces was usually bener than f 7 % in terms of average deviation from the mean.
Flgure 2.
water through the hollow upper electrodes with a Paratherm U4 thermostat. Experiments. The E-jump experiments in ethanol were done a t 25'. typically using 3 psec or shorter perturbation pulses. Concentration changes during and after the pulse were recorded, however, only the curves obtained a t zero field were evaluated. A representative curve is shown in Figure 2. The resistance of the solutions measured in the cell was always greater than lo4 ohms caused by a maximum ionic strength of 1.03 X M. Maximum heating during the pulse was 0.2O. All T-jump experiments in ethanol were done a t 24.5O as the final temperature, a t a 0.3 M tetramethylammonium chloride concentration, as the conducting electrolyte. By
15
10
pH-+PhHJ x IO4 M Flpun 3. Concenlrah dependence of the relaxation tkne T DFE experiments k H a at 4S0.
In the
not firing the second spark gap, all the energy stored in the capacitor a t 35 kV is dissipated in the cell having a resistance of 330 ohms within less than 1 met, causing a temperature rise of 1.6'. The DFE experiments in water were the most difficult ones reported here, because of the relatively low resistance of the solutions and the long relaxation times observed. Since the second spark gap in most cases would not fire after 8-10 psec, it was necessary to evaluate the in-field relaxation curves. In order to speed up the reactions the perturbation was done a t 45O, where the minimum resistance of the solutions in the cell was 4 X lo3 ohms, representing a maximum heating of 1 . 2 O within e minimum heating time of 10 psec, using 35-kV pulses. The simultaneous increase of the temperature and the electric field strength causes opposing shifts of the equilibria, therefore the experimental conditions had to be carefully optimized in such a way that the effects of the temperature were minimal. Thus only the first 4-5 psec of the relaxation curves were evaluated, where the heating is about 0.5'. The plots of In A(signal) us. time yielded straight lines and so did the plots of 7-l us. the appropriate concentration variable (Figure 3). The pH of the reaction mixtures in these experiments was measured immediately before each perturbation with a h d s and Northrup No. 7401 pH meter using No. 117208 and No. 117169 reference and measuring electrodes. respectively. The maximum ionic strength of the solutions never exceeded 2 X lo-' M,where the Debye-Hiickel activity coefficients are so close to unity that they can be neglected in the calculations.
Results and Discussion All our experiments can be interpreted based on Eigen's general hydrolysis mechanism* (with some modification in pure ethanol) PhH'
+ OH-
112
PhH-.. . OH'
b21
1
'23
Ph2'
+
H20(1)
'32
2
3
where the meaning of the symbols is explained in Figure 4. There are two possibilities for the kinetic treatment of this The J ~ ~ r n s l o l P h y g i ~ s l C h ~ m iVOI. s b y78. . NO. 24. 1974
M. W. Massey and Z. A. Schelly
2452 1-10
+ H20 PhH-
blsphenolate anion
5
1-1
+OH-+
three purple-coloured resonance forms
colourless carbinol
base
Ph--
c3-
Figure 4. Detailed mechianism of proton transfer and color change of aqueous and ethanolic phenolphthalein.
I /'
mechanism, depundiiig on whether the intermediate encounter complex PhH[-- OH- is present in an infinitesimal concenti~ation,that is necessary for a steady-state approximation. The steady-state approximation yields for the overall forward and ireverse rate constants k f = k12k231 (1221 f k 2 3 ) and I z , =- k 3 2 k 2 1 / ( h 2 1 k 2 3 ) . For normal proton transfer Iz 2 3 >:> k 2 1 and the frequency of encounter is rate As, however, the activation energy of step 2-3 increases, caused by internal hydrogen bonding or intramolecular electronic rearrangement accompanying the reaction, the chemical transformation step 2-3 becomes rate determining. Thus if k 2 3
