The Blue Bottle Experiment and Pattern Formation in this System

The Blue Bottle Experiment and Pattern Formation in this System L` Adamčíková and P. Ševčík Department of Physical Chemistry, Comenius University, 842 5 Bratislava, Slovak Republic Z. Naturforsch. 52a, 650-654 (997); received April 2, 997 The methylene blue - saccharide - NaOH system, the so-called "Blue Bottle" experiment was investigated. When this system is poured into an open petri dish, spatial structures start to generate after an induction period. The induction period increases in the order of xylose < glucose < galactose < arabinose < mannose. Introduction The "Blue Bottle" experiment was first popularized by Campbell [] and later by Cook et al. [2]. The experiment consists of a glass flask which is about half filled with colorless solution that turns blue when it is shaken. When left to stand, the liquid turns colorless again. This cyclic process can be repeated many times. The aqueous solution contains sodium hydroxide, glucose and methylene blue. Such an alkaline solution turns colorless upon standing, indicating that the dye has been reduced by glucose. Shaking the flask causes the blue color to return, as the reduced form of methylene blus is oxidized by atmospheric oxygen. Atmospheric oxygen is transported into the bulk solution. CH + OH - <-> C~ + H 2 0, () 0 2 (g) -> O 2 (soln), (2) 0 2 (soln) + 4MBH 4 M B + +2H 2 0, (3) MB + +C" X" + MBH, (4) where CH is saccharide, MB + and MBH are the oxidized and the reduced forms of methylene blue and X - represents the oxidation products from the saccharides. The autooxidation of benzaldehyde illustrates an interphase transport oscillator [3] in which the transport of a gas to a liquid phase competes with the autocatalytic consumption of that gas. When that consumption becomes so rapid that the transport cannot keep up, the dissolved oxygen is depleted and a Reprint requests to L\ Adamcikova. negative feedback reaction turns off the autocatalysis and permits the dissolved oxygen to be replenished by transport. A similar type of behavior was discovered by Burger and Field [4] during the oxidation of alkaline aqueous sulfide by air catalyzed by methylene blue. The catalyst and 0 2 oscillate only in a continuous flow stirred tank reactor (CSTR). Oxygen is present in relatively low concentration; it is essentially consumed in one phase of an oscillation and must be replenished by the flow for the cycle to continue [5]. We report here experimental investigation of an 0 2 - driven system, the methylene blue catalyzed oxidation of saccharides by 0 2 in basic media. We replaced the shaking of the flask by stirring of the solution and demonstrate the influence of the stirring rate and of the effect of air as a direct result of the presence of oxygen. Nonlinear systems with many degrees of freedom can undergo nonequilibrium phase transitions characterized by a large variety of spatial or spatiotemporal patterns. Pattern formation can have different origins. Turing [6] suggested that patterns can be formed in chemical reaction - diffusion systems. These structures arise from a cooperation between molecular transport through diffusion and chemical kinetics. In addition, many other experimental systems have been described which are able to exhibit patterns, the origin of which has been discussed for many years [7]. The density changes can lead to convective motion of the solution; this itself can give rise to some interesting spatial phenomena, and chemical reactions which have suitable color changes can be used to reveal such induced convection [8], but it is separate from the diffusion-driven Turing mechanism. Here we present spatial structures appearing in the saccharide - methylene blue - NaOH system. 0932-0784 / 97 / 0800-650 $ 06.00 - Verlag der Zeitschrift für Naturforschung, D-72027 Tübingen

65 L\ Adamcikovä and P. Sevcik The Blue Bottle Experiment and Pattern Formation in this System Experimental Results and Discussion Reagents a) The Methylene Blue - Saccharide NaOH Batch System All chemicals were of analytical grade a n d were used w i t h o u t f u r t h e r purification. T h e solutions were p r e p a r e d with d o u b l y ion-exchanged water. A stock solution of M B + was m a d e according to its nominal molecular weight, a n d the final concentration was determined by m e a s u r i n g the a b s o r b a n c e of diluted solution at 668 n m (e = 6.4 x 0 4 M ~ c m - ). T h e stock solution of M B + in w a t e r was kept in the dark; we never observed a n y m e a s u r a b l e decomposition of M B + in water. All m e a s u r e m e n t s were performed at 20 + 0. C. Experiments T h e experiments were carried out in a t h e r m o s t a t e d cylindrical glass reaction vessel (diameter 3.5 cm, height 7.2 cm). T h e v o l u m e of the reaction mixture was 20 ml a n d the free surface area 7.98 cm 2. The reaction vessel was closed with a r u b b e r stopper t h r o u g h which a commercial indication platinum m a c r o e l e c t r o d e (0.5 x 0.8 cm) a n d a reference mercury (I) sulfate electrode were inserted into the solution. In the s t o p p e r was also a hole for a capillary tube (diameter 0.4 cm in the s t o p p e r but 0. cm in the end) by m e a n s of which the gas was bubbled into the reaction vessel. An inert a t m o s p h e r e was reached by bubbling purified n i t r o g e n t h r o u g h the reaction mixture for at least 5 min, followed by a N 2 gas stream which was i n t r o d u c e d a b o v e the reaction mixture. Nitrogen was freed f r o m traces of oxygen by m e a n s of a solution of C r 2 + ions. T h e P o t e n t i o m e t r i e m e a s u r e m e n t s were carried out using a Radelkis O H - 0 5 p o l a r o g r a p h. T h e solution was stirred magnetically with a Teflon-coated stirrer (length 2.5 cm, d i a m e t e r 0.85 cm). T h e r e a c t a n t s were a d d e d into the reaction vessel in the order: a q u e o u s solution of N a O H, saccharide a n d finally solution of M B +. S p e c t r o p h o t o m e t r i c experim e n t s were carried o u t in a 0.5 cm s p e c t r o p h o t o m e t e r t e r m o s t a t e d cell (Zeiss J e n a Specord U V VIS). The time d e p e n d e n c e of the M B + - c o n c e n t r a t i o n was m o n itored by m e a s u r i n g its optical absorbance, at 668 nm. T h e experiments consisting in mixing the reagents a n d p o u r i n g the m i x t u r e i n t o a Petri dish (usually 88 m m in diameter) were p e r f o r m e d at 20 ± 0.5 C. - T h e experimental system consists of a n a q u e o u s solution of s o d i u m hydroxide, saccharide a n d methylene blue. Methylene blue m o n o m e r exists in three oxidation states [5] separated by a single electron. T h e three forms are referred to as M B +, MB" a n d M B H. T h e oxidized f o r m M B + is colored, the reduced form M B H is colorless. In the beginning, the solution is blue. M B + slowly reacts with e.g. D-arabinose generating the reduced f o r m M B H, a n d the potential of the p l a t i n u m redox electrode decreases. Stirring of the solution causes the solution to t u r n blue a n d the potential of the Pt redox electrode a b r u p t l y increases. T h e rate of the bluing occurs in a few seconds. In the absence of stirring, the M B + f o r m reverts to its colorless form. Mixing initiates a f u r t h e r bluing-debluing cycle. Typical responses of a P t redox electrode are s h o w n in Figure. T h e stirring rate was magnetically varied between 50 a n d 000 r p m a n d h a d a n effect on the reaction. A stirring rate a b o v e 300 r p m was sufficiently high to cause blue c o l o r a t i o n which is accompanied by a c h a n g e in the redox potential. E,MV t, m i n Fig.. Potentiometrie recording of the "Blue Bottle" experiment at 20 C. The arrows indicate the time when the stirring rate of 400 rpm is switched on and after 5 s is stopped again. Similarly, arrows indicate the beginning of bubbling of 0 2 (500 ml min - ), which lasts 5 s. Initial concentrations: 0" 4 M MB +, 0.7 M D-arabinose, 0.5 M NaOH.

L\ Adamcikovä and P. Sevcik The Blue Bottle Experiment and Pattern Formation in this System 652 652 The behavior of a reaction mixture which is thoroughly deaerated with nitrogen is i n d e p e n d e n t of the stirring rate. Neither bluing n o r the changes of potential are recorded. This fact indicates that the dependence on the stirring rate is due to oxygen. T h e rate of t r a n s p o r t of oxygen into the reaction mixture d e p e n d s on the stirring rate. At the stirring rate 300 rpm, at which the solution t u r n s blue, we noticed a dimple in the surface which began to increase its area. At still greater stirring rates, the dimple became a vortex which pulled air into the solution a n d obviously would be expected to increase the rate of t r a n s p o r t between the phases. We used a cylindrical reactor in which the rates of r o t a t i o n of the liquid are c o m p a r a ble at all depths. N o y e s a n d co-workers [9] p r o p o s e that the t r a n s p o r t of molecules between gas a n d surface takes place in a single translational step a n d t h a t the t r a n s p o r t between surface a n d bulk solution is accomplished by physical mixing without significant c o n t r i b u t i o n f r o m molecular diffusion. 02(gas) o 0 2 (surface layer) o 02(bulk) Interrupted b u b b l i n g of 0 2 into the reaction mixture (Fig. ) has a similar effect as interrupted stirring. The only difference is in the length of time in which the solution stays blue. The time the solution stays blue (t) d e p e n d s on the time of mixing f m i x (which is p r o p o r t i o n a l to [0 2 ]), a n d under o u r experimental conditions this dependence is linear in the c o o r d i n a t e s \/t against l / f m i x. T h e reduction of M B + by saccharides in the presence of dissolved 0 2 is complex. Figure 2 shows the a b s o r b a n c e at 668 nm due to M B + when M B + is mixed with glucose (Fig. 2 a) or with a r a b i n o s e (Fig. 2 b) as a f u n c t i o n of time. T h e r e is an induction period (IP) d u r i n g which [ M B + ] decreases only very slightly. After the I P the reduction of M B + occurs quite rapidly. We evaluated the induction period and the m a x i m u m rate of the decrease of M B + vmax = Ac/Ar, as the m a x i m u m slope of the rapid reaction course for glucose and a r a b i n o s e (Table ). T h e induction period decreases in the order of arabinose > glucose, a n d vmax decreases in the o r d e r of glucose > arabinose. b) Pattern Formation Saccharide - NaOH in the Methylene System Fig. 2. Kinetics, at 20 C of the reaction of MB + with D-glucose (a) and D-arabinose (b) in the presence of 0 2. Initial concentrations: 2.7 x 0" 5 M MB, 0.08 M saccharide, 0.5 M NaOH. A is the absorbance at 668 nm. p o u r e d into a Petri dish (8.8 cm diameter). After a few minutes {IP) at 20 C, colorless structures in the otherwise blue solution became visible in the u n d i s t u r b e d and uncovered solution (Figure 3). T h e i n d u c t i o n period is defined as the time between the p o u r i n g of the solution into a petri dish until the beginning of the p a t t e r n formation. In the course of time a slowly changing but globally stable mosaic p a t t e r n developed. Table 2 lists several saccharides t h a t have been observed to undergo this type of p a t t e r n f o r m a t i o n. T h e IP increases in the order xylose < glucose < galactose < arabinose < manose, a n d in the s a m e order the rates of the reactions of saccharides with M B + decrease in the presence of 0 2. T h e activation energies of these reactions have been determined [2] a n d the order correlates also with o u r results (Fig. 2, Table ). T h e pattern f o r m a t i o n was affected by the thickness / of the reaction mixture layer. T h e listed thicknesses are a p p r o x i m a t e because n o correction was Table. The induction period and the maximum rate of decrease of MB + for two concentrations of D-glucose and Darabinose in a solution containing 0.5 M NaOH and 2.7 x 0" 5 M MB +. 20 C. Blue - A t h o r o u g h l y mixed solution c o n t a i n i n g 0.5 M N a O H, 0.08 M saccharide a n d 3.4 x 0 " 5 M M B + is c/mol IP/s i m a x / m o l " s~ D-glucose D-arabinose 0.08 0.7 50 82 7.4 x 0 " 6 7.0 x 0 " 0.08 250 4.0 x 0 " 7 0.7 98 9.3 x 0 ~ 7

L\ Adamcikovä and P. Sevcik The Blue Bottle Experiment and Pattern Formation in this System 653 Table 2. The induction periods IP/s for different saccharides and for the various solution depths (0- Initial concentrations: 3.4 x 0-5 M MB +, 0.8 M saccharides, 0.5 M NaOH. 20 C. Saccharides //mm.97 D-xylose D-glucose L-glucose D-galactose D-arabinose L-arabinose D-mannose Fig. 3. Pattern in a Petri dish filled to a height of 2.5 mm with a solution containing 0.5 M NaOH, 0.08 M D-glucose, 6.8 x 0" 5 M MB +, 450 s after the start of the experiment. Fig. 4. Pattern in a Petri dish filled to a height 3.29 mm with a solution containing 0.5 M NaOH, 0. M D-glucose, 0. ml of % toluidine blue per 20 ml of solution, 240 s after the start of the experiment. m a d e for the solution a d h e r i n g to the wall of the petri dish by capillary forces. T h e average IP for various d e p t h s of the solutions are given in Table 2. N o patterns were observed for / =.6 m m or less. If the petri dish was protected f r o m air with a glass cover, no spatial structures a p p e a r e d in the reaction mixture. This indicates t h a t oxygen which affects the chemical reaction a n d / o r e v a p o r a t i o n play a f u n d a m e n t a l role in the p a t t e r n f o r m a t i o n. 60 240 250 290 300 500 640 2.3 2.5 230 450 2.96 50 205 225 260 280 390 540 3.29 4. 8.2 75 240 235 390 360 283 P a t t e r n f o r m a t i o n m a y arise primarily f r o m a chemically driven convective instability, as is exc h a n g e of m a t t e r across the air-reaction m i x t u r e interface. Because of the molecular n a t u r e of chemical systems, fluctuations in the c o n c e n t r a t i o n s are always present. O t h e r physical factors might also be i m p o r t a n t to p a t t e r n f o r m a t i o n in a chemically reacting system. Evidence t h a t a convective instability plays a role in the p a t t e r n f o r m a t i o n is given in Table 2. T h e lack of mosaic structures in reaction mixtures with d e p t h s of.6 m m a n d less was unequivocal. T h e convective instability is primarily a surface p h e n o m e n o n. This conclusion is s u p p o r t e d by the small dependence of the size of the "small-cell structures" o n the solution of the depth. T h e m o s a i c p a t t e r n was observed also in the cases when dishes of varying shapes were used. Also toluidine blue catalyzes the o x i d a t i o n of an alkaline a q u e o u s solution of glucose w h e n mixed with oxygen a n d spatial mosaic structures are generated (Figure 4). In s u m m a r y, it is d e m o n s t r a t e d t h a t the "Blue Bottle" is quite rich in its d y n a m i c a l behavior. In order to construct a m e c h a n i s m it is i m p o r t a n t to investigate as m a n y c o m p o n e n t reactions as possible a n d to develop m e c h a n i s m s for each of them. T h e equilibrium c o n s t a n t of reaction () is.2 x 0 " 3 at 0 C a n d I = 0.03 M [0]. T h e reaction (2) is discussed in [9, ], the rate c o n s t a n t of the reaction (3) (direct oxidation of M B H by 0 2 ) is d e t e r m i n e d to be.62 x 0 2 M _ s _ at p H 9.0 [2]. T h e reduction of M B + by saccharides in the presence a n d absence of 0 2 as the i m p o r t a n t p a r t of the m e c h a n i s m will be the subject of f u t u r e research. Acknowledgement This research was s u p p o r t e d by G r a n t N o /463/ 94 by the Scientific G r a n t Agency of M E S R.

654 L\ Adamcikovä and P. Sevcik The Blue Bottle Experiment and Pattern Formation in this System 654 [] J. A. Campbell, J. Chem. Educ. 40, 578 (963). [2] A. G. Cook, R. M. Tolliver, and J. E. Williams, J. Chem. Educ. 7, 60 (994). [3] J. H. Jensen, J. Amer. Chem. Soc. 05, 2639 (983). [4] M. Burger and R. J. Field, Nature London 307, 720 (984). [5] P. Resch, R. J. Field, F. W. Schneider, and M. Burger, J. Phys. Chem. 93, 88 (989). [6] A. M. Turing, Philos. Trans. Roy Soc. London B. 237, 37 (952). [7] R. Kaprai and K. Showalter, Chemical Waves and Patterns, Kluwer Academic Publishers, Dordrecht 995. [8] U. Kummer and G. Baier, Naturwissenschaften 83, 522 (996). [9] R. M. Noyes, M. B. Rubin, and P. G. Bowers, J. Phys. Chem. 96, 000 (992). [0] B. Perlmutter, B. Hayman, and A. Persky, J. Amer. Chem. Soc. 82, 276 (960). [] J. Wang, F. Hynne, P. G. Sörensen, and K. Nielsen, J. Phys. Chem. 00, 7593 (996). [2] P. Sevcik and H. B. Dunford, J. Phys. Chem. 95, 24 (99).