Light Induced Liesegang Type Patterns in Batch and Flow Reactors

Journal of Scientific & Industrial Research Vo 1.60, January 200 I, pp 10-22

Ishwar Das * and Alpana Bajpai

Department Of Chemistry, DDU Gorakhpur University, Gorakhpur 273 009, India

Received:29 May 2000; accepted: 3 November 2000

Recent work on light induced Liescgang type patterns of living and non-living systems has been reviewed Experimental set ups tor hght 1I1duced expenments !low reactors and electric field experiments have been described Mechanistic aspects or . 111 precIpItatIon processes 111 the presence and absence of light have also been examined

hand, comparable graphs of nearly all known Liesegang Introduction patterns are straight lines. Structure of the mineral limo Inter-diffusion of one electrolyte into another re nite that grows in the form of concentric rings during the weathering of sedimentary rocks yield graphs that acting electrolyte may ·lead to a rhythmic deposition of precipitate which is known commonly as Liesegang almost given straight line. phenomenon'. When a solution of large concentration Frequently occurring types of self-organised order are periodic in time (oscillation ) or space (repetitive of K Cr 0 is allowed to diffuse into a less concentrated 2 2 1 solution of lead nitrate containing a lyophilic gel (agar- structures) and in time and space as demonstrated by agar) medium, periodic deposition of precipitate appears spatio temporal pattern formation in the well known B after several hours. Similar type of periodic precipita Z reaction . Wave patternmay evolve in an oscillatina or tion has been investigated and studied by many work in excitable medium. The propagation of such wave� of ers2 '2. The pattern formation or Chemical instability or chemical activity is one of the most stricking examples inhomogeneity in nucleation of colloidal particles may of pattern formation due to coupling of reaction and dif arise due to several factors. Inter-diffusion due to con fu sion. The most common geometries of two dimensional centration gradient may be one of them. wave fronts are a circle , a set of concentric circles called target patterns or rotating spirals. Figure shows Liesegang ring type patterns ob I Tu ring type spatial structures arise when the ef served during crystallization of ascorbic acid from its fects of diffusion cause a stable homogeneous steady alcoholic solution" ,periodic precipitation of copper chro state of a chemical system to become unstable to infini mate in agar agar gel ,25 agate bands, growth of the bac tesimal inhomogeneous perturbations Turing struc teria Pr vulgaris '40n agar plate and the fungi Necteria :16,37. tures are stationary,i.e., the spatial patterns give a char cinnaharina." Rings of this kind are observed in nature acteristic wavelength that is independent of the size of in minerals such as malachite , limonite, variscite , chal the system. Nearly all theoretical treatments of Turing cedony and wings of certain colourful butter fl ies. structure in a chemical system employ an activator- in Liesegang suggested that the rings and bands of agate hibitor kinetics. The activator accelerates the reaction arose when the Liesegang phenomena operated in natu while the inhibitory species decreases the effect the ral silica gels. Geologists agree that agate patterns can of activator making the reaction slow. Castets ef whi be attributed to the successive deposits of layers of silica 1l1.3X le working with an open unstilTed gel reactor observed gel and impurities. When the number of each band in the first experimental evidence for Turing structure in the pattern of these materials is plotted against the loga chemical systems, the chlorite iodide- malonic acid rithm of the distance of the band from the origin, the (CIMA) reaction. In the subsequent section we describe resulting graph is usually a curved line. On the other flow reactors employed for the study of Liesegang

* Author for correspontlence patterns. DAS & BAJPAI: LIGHT INDUCED LlESEGANG TYPE PATTERNS II

(a) (b)

Figure 2-Experimental set up of: (a) continuolls-nuw reactor1X. The petridish contains per cent agar-agar gel and I a reactant of lower concentration., (b) Gel-ring reactor2", (e) I per cent aqueous agar-agar was used to fo rm gel-ring, Figure I-Periodic patterns of non-living and living systems: outlet nozzles; [B1. 8 ] Corning burets, (RI,R2) (a) concentric rings of ascorbic acid Crystallized (01 ,02) 2 reservoirs containing aqueous solutions of reactants from methanolic solution, (b) copper chromate.(c) agate thin slab, (dl.Pmtells vulgaris,(e) Necteria were always kept constant. The kinetics of propagation cinnabari17({ of the precipitate front of PbCrO in DPL reactor has • been studied which satisfied the relation d = cf'" and the Liesegang Patterns in Flow Reactors simple spacing law. The DPL reactor was fu rther modi Although, various of experiments have been per fied by Das and coworkers)' " to make it open with re formed to study temporal oscillations and spatial pat spect to the flow of both the reactants. This reactor is ternsin flow reactors for homogenous reactions but little identical with that developed by Noszticzius and co

effort has been made fo r heterogeneous systems. Tam workers)' h for B-Z reaction in many respect. In the open and coworkers)" introduced a Continuously Fed Unstirred gel - ring reactor developed by Das and coworkers�" a Reactor (CFUR) which acted as a tool fo r systematic gelring (Figure 2b), separates the two electrolyte solu studies of spatial pattern formation in the same way as tions and the systems which have been investigated with this set up are CuCrO and HgI '·)II. The two solution: the CSTR has served in the study of homogeneous reac . / tions. The CFUR could be maintained indefinitely at a were fed continuously at the same flow rate. These re fixed distance away from the equilibrium by the con actors have many advantages over the closed or batch tinuous feed of reagents and made possible to study the reactor in which the concentration of diffusing electro stability of chemical patterns. Two dimensional experi lyte decreases due to its consumption during less diffu ments on periodic precipitation of. PbCrO , CuCrO and sion and chemical reaction processes. The reaction ulti • • 4 d. HgI have been carried with a simple and inexpensive mately stops after or l 3 Das-Pushkarna-Lall (DPL) flow reactor (Figure 2a) in Experiments were also performed under various al. troduced for the first time by Das 1<, . The reactor is experimental conditions namely, variation of the initial el open with respect to the flow of one reactant as other electrolyte concentration and temperature and pH of the electrolyte was taken in the gel. In this reactor the con medium. Kinetics of propagation of precipitate front has centration and level of the electrolyte in the empty space been studied. The relation d = c("'was obeyed at various 12 J SCI IND RES VOL.60 JANUARY 200 1

value of solubility product.It suggested that outside the range of pH 5.0-5.5 the precipitate began to dissolve and the ionic concentration product was not sufficient to exceed the solubility product. It suggested that outside the range of pH 5.0-5.5 the precipitate began to dissolve and the ionic concen tration was not sufficient to exceed the solubility prod uct. An experimental set up as shown in Figure 3a , was employed to study the variation of pH during pre cipitation. The progress of the reaction was followed by recording pH of the solution as a function of extent of (CI) propagation of the precipitate front from the junction. As soon as the advancing front approaches a region little

10·0 above the tip of the electrode the pH shoots up and in course of time attains a steady state. No appreciable change in pH was observed in curves I and 6 of Figure 3b which predict the equilibrium states. These show the existence of bistability or multiple stability in a narrow pH range.

Copper Chromate System � '2·0L� ��_1.0 '2.·0L- _3·0'-:-_",-:-�:'-:-- 40 5·0 -:-:-::6·0 - o Copper chromate precipitates in agar agar gel by THE JUNCTlON d(cm ) DISTANCE OF PRECIPITATE FRONT FROM the diffusion of aqueous K2CrO. solution into agar agar Figure 3(a).- Experimental set up P= pH. eleclrode, A= gel containing Cu(NO,\ using the method described ear aqueous potassium chromate solution (O.02M), lier29. As a result of diffusion and chemical reaction, B= lead nitrate solution (0.001M) containing 1.5 precipitation started in the form of bands. Band loca per cent agar-agar gel M= pH meter, (b) Plots of pH of the medium vs distance of precipitate front tions were measured from the fi nal precipitation patterns from the junction (d). [K2Cr041 =0.02M, and experiments were carried out under different condi [Pb(N03hl= O.OO IM. (0) pH 2.5, (M 3.95, (D) tions such as initial electrolyte concentrations, orienta 5.3 ,(0) 6.75 , (Ll)7. 0 , (0) 8.0 tion of tubes, flow conditions (DPL, gel-ring continu ous flow reactors), temperature , and pH. electrolyte concentrations, temperature and pH of the Light effect on precipitation pattern of copper chro medium. It is also found that by increasing the concen mate in gel media was studied by Swami and KanPliand tration of the entering reagent or the temperature, bands later on by Das and coworkers 25-31 . For this purpose are observed at a larger distance from the junction and experiments were carried out in triplicate with 21 tubes. (n)ln + C the simple spacing law x" = k is obeyed. In each set out of seven tubes, four were wrapped with It has been observed that [H+] plays an important different coloured transparent papers (blue, green, yel role in the macroscopic structure fo rmation of illumi low , and red) and the fifth tube was placed inside a tube nated lead chromate system. The patterns are formed in of larger diameter containing I / CCI as shown in Fig- 2 • the pH range 3.0-6.0. Width of the diffused portion was ure 4 . The sixth tube was wrapped with black paper to found to increase with increase in pH, passes through a prevent light to enter the tube. CuCrO. precipitated peri maximum and decreases with further increase in pH. The odically either in the complete darkness or when exposed maximum of the curve lies in the pH range 5.0-5.5. The to red , yellow , green and blue light. The periodicity dissolution of the precipitate may be due to the change was disturbed when the tubes were exposed to violet of ionic product with ionic strength. Lead chromate dis (405nm) and white light. Considering these observations solves in acetic acid as well as in alkaline medium which the experiments were carried out in the complete dark reduce the product of the [Pb>+] and [CrOn below the ness. DAS & BAJPAI: LIGHT INDUCED LIESEGANG TYPE PATTERNS 13

1----4 M 1------1

JUNCTION (b)

(0) Figure 5 (a)-Experimental set up for electrical ti eld experiments in batch reactor. PI. P2 platinum electrodes. RI R2. Reservoirs containing aqueous copper ni trate (0.0 and potassium chromate fcilIau,O C=lif:lo- FIllER I M) (O.IM)solutions respectively. B, DC voltage source , M, digital multi meter, T, tubular reactor of length 16.5 cm. ShS2 magnetic stirrers ,(b) experimental set up for electrical field experiments in a continuous tlow reactors: Figure (i). Experimental set up for producing monochromatic 4- (PI.P2) bright platinum electrodes, BI,B2 burettes light using a liquid fi lter. (RI,R2), reservoirs containing aqueous Cu(NO,)" (ii).Experimental set up to study the light effect on (0.0 I M)and K2Cr04 (0. 1 M) solutions the precipitation of lead chromate. [Pb(N03)21= respectively; C, D, electrode chambers, 01.02, 0.00 M( lower portion ) and [K2Cr041 0.0 15M. outlets. E, DC source; M, digital multimeter ; T, I ° = (upper portion) at 28.0±0. 1 C. Tube (iii) was tubular reactor containing 1.5 per cent agar-agar wrapped with black paper gel; S hS2 magnetic stirrers () vel y concemra()on. ne Olsrance Del ween me elec lOW 1 Electric field effects on one and two dimensional trodes was kept the same in each experiment A DC out precipitation of copper chromate in batch, DPL and gel put source was connected with two bright platinum elec ring reactors31a were studied. The reactors employed for trodes (P , and P)2 . The field intensity was varied with experiments in the absence of electric field were modi the help of a potential divider. fied for carrying out experiments in the presence of elec tric field. One dimensional experiments were performed Continuous Flow Reactor with batch and continuous flow reactors as shown In It consisted of two glass cylindrical electrode cham Figure S. bers C and D (Figure Sb) containing electrolyte solu Description of One Dimensional and Two tions separated by a tubular glass reactor (inner diam. Dimensional Electric Field Experiments 6.8 mm) filled with I.Sper cent agar agar gel. The COIl centrations of electrolytes were maintained cOllstant by One Dimensional Studies a continuous influx of electrolytes from reservoirs R , and R 'at a constant flow rate = 2ml/h) which was main- Experiments related to one dimensional precipita 2 ( tained with the help of two burettes B and B . The level tion in gel media in the presence of an electric field, in , 2 of solution in the chambers was also maintained with batch and continuous flow reactors were carried out by out let nozzles, 0 and 0 .Electrolytes were continuously Das et atH. , 2 stirred using magnetic stirrers S , and S 2. The DC voltage Batch Reactor source was connected with bright platinum electrodes P and P . Salient features of one dimensional experi- An experimental set-up (Figure Sa) consisted of , 2 • ment in batch and continuous tl ow reactors are as fol two cylindrical glass chambers R and R that contained , 2 lows : the two reactants separately. These were separated by a tubular glass reactor T containing 1.5 per cent of gel of Two types of one dimensional experiments were length 16.5 cm. The gel also contained a reactant of rela- carried out in a batch reactor (Figure Sa) in the presence 14 J SCI IND RES VOL.60 JANUARY 200 1

of an electric field . In experiment (i) the solutions were In Batch Reactor separated by a tubular reactor contain Cu(NO ), in , I.S per cent agar agar gel. Nonlinear plots of the band loca • tion x versus at various filed intensities (0-0. 16 v n n cml) were obtained and (ii) The two solutions were separated by a tulaular reactor containing I.S per cent agar agar gel which did not contain any electrolyte. Experi .. ment was performed at the field intensity 0. 16 v cml. In In Flow Rellctor this case, a single band was formed and propagated lin early. In a batch reactor the concentration of electro Figure 6-Comparison of precipitation patternsof copper chromate in gel media, obtained in a tubular batch lytes in the electrode chambers decreased with time due and !low reactors in the presence of an electric lick!. to diffusion and Chemical reaction. The concentrations In both the cases the electrode chambers were at the two boundaries were maintained by continuous separated by a tubular reactor containing 1.5 per feed of electrolytes using a continuous flow reactor . cent agar-Jgar gel. Conditions: [Cu(NO,hl = O.OIM (RI) [K,Cr04l = O. IM (R2), !low rate � 2mllh, lield ' o In the presence of electric field, cations and anions intensity = 0. 16 v/cm , at 40.0±0. l C move in opposite directions and meet in the tubular re actor. Patterns of CuCrO obtained in a continuous flow 4 (c) Sol coagulation model ."AI reactor were compared with those obtained in a batch reactor, keeping other experimental conditions the same. According to the model (a), the two reactants A, Results are shown in Figure 6. Results indicated that and B, diffuse from opposite directions and coexist in the gel until the solubility product reaches a critical only one band (width = 0.26 cm) was observed in a batch reactor, whereas many equally spaced bands were de value above which the nucleation occurs according to veloped at approximately the same position in a con the reaction tinuous flow reactor. It fu rther indicated that such con A + B � AB (solid) (Model l) ...( I) ditions for periodicity were satisfied due to continuous Once the nucleation has started, a depletion of con influx of electrolytes. centration of A and B occurs in the surroundings. Hence, at this stage, the formation of precipitate stops nucle Formulation of Mechanism ation in the neighbourhood. Precipitation is also stopped The mechanism of irregular deposition of precipi as diffusion continues further. This is fo llowed by fo r tate (ring or band fonnation) has been discussed from mation of clear spaces. Diffusion of ons persists and af time to time. The theories proposed earlier were based ter a certain stage, the critical supersaturation exceeds on the hypothesis of supersaturation , coagulation of and another band is formed in a similar manner. The colloid and generally postulating that the nucleation of process is repeated until the supply of one of the electro particles is discontinuous Recent theories postulate that lytes is exhausted. In spite of the fact that the theories colloid formation and crystal growth of the reacting sys based on the model are not able to explain all the experi tem when coupled with diffu sion leads to periodic pre mental facts, the computer simulations based on the cipitation". theory predicts the fo llowing three laws :

The symmetry-breaking instability leading to peri (i) Centre position x" of the nth band is related odic growth of precipitates has been classified into two to the time t of its formation through the time main types : extrinsic length scaling in which character law x II _"\JtII ." (ii) The rat io x ex of the positions two istic length of developing spatial patterns is determined Pll = / ,,) by the dimensions of the system: and intrinsic length consecutive bands approaches a constant scaling in which that characteristic length is determined value. It is known as the lablczynski law or by the reaction rates and transport relations of the sys the spacing law. tem, i.e., the dynamics of the system and not by its di (iii) Width w" of the nth band is an increasing mensions. Three different approaches have been used function of n. involving three models: In a more recent version of the theory the pic 42, ture is modified as fo llows. The two species A and B are (a) Supersaturation theory ,7 (b) Competitive particle growth models,' assumed to react to produce a new species C which also DAS & BAJPAI: LIGHT I DUCED L1ESEGANG TYPE PATTERNS 15

diffuses into the gel. Subsequently, nuclei are formed ring fo rmation while these were kept in the darkness, fo llowed by crystallization according to the fo llowing but prominent rings were fo rmed upon 'rhythmic' illu scheme : mination. Dhar and Chattelji" obtained rings in darkness, but found their number increased by light and the spac A+B -7 C ...(2 ) ing between them diminished. Both, Roy·' and Kisch·" C nuclei (n) ModelII ...(3) -7 reported that their rings were more distinct when fo rmed Nuclei Crystal (D) ...(4) -7 'under light' than in darkness. Kohn and Mainzauhsen;o believed that light effects should be particularly strong in gelatin containing silver halide and subjected such When the local concentration of C reaches some systems to more extensive examination. A set of gelatin threshold value, nucleation occurs, subsequently D clus specimens was prepared containing the same amount of ters are formed which remain at rest. The corresponding potassium dichromate, but the chlorine concentration was rate equations are as fo llows : systematically varied . Half of each dish was kept in the

8 a D V' 2 a - ...(5) , = R ' darkness, and the other half was illuminated. In the ab tI ab 8 b = D V' 2 b - ...(6) sence of chlorine, there was no light effect. In the pres , b Ra , V' 2 a b ence of chlorine the shadow edge was very distinct. 8 c = D -R - ...(7) , cab n e , t = nco ...(8) ,, Lead chromate is another photosensitive material Here, a,b,c,d denote the concentration of species which invoked many workers to carry out detailed in A,B,C, and D at time at a specific position. R expresses vestigation. Light absorption and photoconductivity of t "" the production of species C due to reaction given in Eq lead chromate in the visible and very near UV region of (2). R = k , where k=rate constant, = rate of depletion the spectrum have been described by Goldman and � :,h n c of C species resulting from nucleation and aggregation Lawson" . Later on, Hatschek52 and Das al.2«.2', reported et on existing D clusters, and D , D , and D are the diffu results of the light induced periodic precipitation of " " , sion coefficients of species A, B, and C, respectively. lead chromate in agar-agar gel medium. In order to study Modifications in Model II have been made by the effect of light on pattern formation, experiments were Polezhaev and Muller43. These workers have reported done both in the presence bf sunlight and in the com results of numerical simulation using a simple model plete darkness. Homogeneous precipitation was observed which takes into account the dependence of kinetics of when the experiment was performed in the complete nucleation44 and particle growth on supersaturation45. darkness while rhythmicity was observed in an illumi Chopard al46 have also suggested that dissolution of nated condition. el D particles should also be taken into consideration, since To study this effect, eighteen tubes were taken for on account of the nucleated D particles are formed at the three sets of experiments. In each set, containing six tubes reaction front. The concentration of the reaction particles four were wrapped with different coloured transparent is depleted in the vicinity of such particles. As a conse papers (blue, green, yellow and red) and exposed to sun quence, the level of supersaturation drops dramatically light. The fifth tube was unwrapped while the last tube and nucleation and solidification processes stop. After was wrapped with black paper so as no light could pass some time the reaction front moves away and the con through it. All the tubes were kept in an air thermostat centration of the C product at the moving front reaches a maintained at 25.0 0. 10 C. The banded structures ± ob value large enough to allow the nucleation to occur again. tained by using diffe rent coloured filters are shown in As a result, separated bands occurred. Our studies for Figure 7a. Experiments were also carried out with a liquid phase reaction showed that it was a distinct possi monochromatic light. To produce monochromatic light bility. of wave length A=405 nm, a liquid fi lter such as iodine in carbon tetra chloride2" and saturated solution of so Light dependent Liesegang Phenomena dium nitrite (A=436 nm);) were used. The experimental There are several cases in which the periodicity set up as shown in Figure 4(i) consists of a Corning occurs in the darkness. Exposure to light, however, can tube containing the liquid fi lter in which another tube of induce the onset of periodicity or cause some disturbance lower diameter containing the reactants was placed. The in the periodicity already established. Such an example whole assembly was then put in an air thermostat main is found in the system of silver chromate·7 There was no tained at a constant temperature. 16 J SCI IND RES VOL.60 JANUARY 200 1

The kinetics of propagation of the precipitate front was also followed. It satisfied the relation d=ct'" where d is the thickness of the precipitate at any time and t, 111 c are slope and intercept, respectively.

Mercury (II) Iodide System Red and yellow forms of mercuric iodide precipi (a) (b) tate when mercuric chloride reacts with potassium io dide in the agar agar gel under diffe rent experimental Figure 7-(a) Precipitation pattern of lead chromate in 1.5 conditions. The colour of the precipitate was yellow when per cent agar-agar gel illuminated with light of KI of relatively higher concentration diffused into a less different wave lengths. Tubes were illuminated 1.5 concentrated solution of HgCI2 containing per cent with: (i) white, (ii) blue, (iii) green, (iv) yellow, agar agar in the complete darkness and precipitated and (v) red light from left to right, (vi) the last tube was kept in the dark. [K2Cr041 0.0 15 M. d��nward in a Corning tube .Red form of the HgI, pre = (upper portion) [Pb(NO))21 O.OO IM (lower ctpltated when KI solution of relatively higher cOllcen D = portion) at 2S.0±O.1 C (b) precipitation patterns tration diffused into the gel containing HgCI, of lower of mercuric iodide in gel when illuminated with concentration in the complete darkness (Figure 8a). The light of different wave lengths. Tubes 1-6 (from left to right) were illuminated wi lh : while Chemical reactions may be written as, (natural), violet, blue, green, yellow, and red light 2KI(C ) + HgCI (C ) 2KCI + HgI (yellow) , 2 2 --7 2 respectively. Conditions: [HgCI21=0. 1 M (upper ...(9) portion) and [KI1=0.OO2M (lower portion) 1 .5 25 .0±O. 1 D HgCl (C) + 2KI (C) 2KCI + HgI (red), containing per cent agar-agar gel at C l --7 , ... ( I 0) (C >C ). Experiments show that when K CrO diffu ses into I 1 , . a less concentrated solution of Pb(NO) containing 1.5 The kinetics of yellow and red wave propagation l t per cent agar-agar gel, bands are formed in the presence was studied that satisfied the relation d'=k where d is of light. However, uniform precipitation occurs in the the extent of propagation from the initial junction , k darkness (Figure 7b), when the foregoing systems are and t are the rate constant and time respectively. The inverted , no evidence of any periodicity was obtained. energy of activation for yellow wave propagation was It was shown that light in the region 400-SS0nm was found to be 9.2 kcal / mol. Potential and [H+] during the essential for chemical instability and band fo rmation. propagation of the advancing fro nt were also monitored Das et al. have found the spacing between adjacent bands as a function of time. Results lead to the conclusion that decreased with increasing distance from the junction. the phenomena involved the transition from one state to Such type of spacing is termed as revert spacing. Fur another. ther experiments were carried out using an experimen tal set up shown in Figure 4 (ii) and observed that no Das et at. 32 earlier reported that : band could be formed in the portion AB when the tube (i) Ye llow wave was produced when aqueous was wrapped with black paper for 48ha and no I ight has been allowed to pass through it. Two bands were fo rmed [KI] > [Hg++] in gel and in the region BC when the tube was illuminated with (ii) Red wave is produced when aqueous light for another 48 hours. Further formation of bands in [Hg++]>[KI] in gel in the complete the region CD was ceased when the tube was wrapped darkness. with black paper (Figure 4 iii) which inhibited the light Later on, Das et al reported that a single red band to pass through the system. It is, therefore, evident that bifurcates into several revert spaced bands in the pres in the absence of light the system is in thermodynamic ence of natural light of wavelength A < 600 nm, as equilibrium and stable to all possible variations. When shown in Figure 8 b. However, the yellow band did not the system is exposed to light, it is driven out of equilib rium due to photoChemical processes which change the bifurcates under these conditions. Other remarkable observations were : composition of the system. DAS & 8AJPAI: LIGHT INDUCED LIESEGANG TYPE PATTERNS 17

(a)

(b) (c) 8 (A)- Precipitation of yellow and red mercuric iodide in Figure ° gel media at 35 C conditions: (a) [KI] = 0.2 M (upper -- portion) and [HgCll]=O.O 1M containing 1.5 per cent agar-agar gel (b) [HgCI ] = O. IM (upper portion) Figure 9- Precipitation patterns of red mercuric iodide in l agar- agar gel at various fieldint ensities at and [KI]=0.002M containing 1.5 per cent agar-agar 1l 30.0±0.1 C. Conditions: Tubular reactor contains gel (8) precipitation patterns of mercuric iodide in aqueous 1.5 per cent agar-agar containing gel media (a) in the complete darkness and (b) when 0.OO2MKI. Reservoirs contain aqueous HgCI2 illuminated with natural light. Conditions: (0.1 M) and KI (0.OO2M); tield intensity = 0.0 v [HgCI ]=O. 1 M (upper portion); [KI]=0.002M I I l cm- (a) and 0. 106 vcm- (b) Direction of containing 1.5 per cent agar-agar gel (lower portion); movement of precipitate is indicated by an temperature=35.0±0. l oC (C) precipitation patterns of arrow[KI]=0.002M containing 1.5 per cent agar yellow mercuric iodide in agar-agar gel in the agar gel (lower portion); tmperature=35.0±0.1 "c presence of natural light (pH 2.72) .pH values in the (C) precipitation patterns of yellow mercuric brackets indicate pH of 1.5 per cent agar-agar gel iodide in agar-agar gel in the presence of natural containing O.OIM mercuric chloride.[KI]=0.2M, at light (pH 2.72) .pH values in the brackets indicate ° 40.0±0.I C pH of 1.5 per cent agar-agar gel containing O.OIM mercuric chloride.[KI]=0.2M, at 40.0±0.1 "c (i) The light induced spatial bifurcation of the yellow d 2 versus t yield straight line indicating that the relation wave into a number of alternate red and yellow d2 =k was obeyed. Results also indicated that in the t + c bands at low pH of the gel containing Hg2+ (pH absence of an electric field the velocity of propagation 2.72) was relatively faster than those observed in the presence (ii) Dependence of precipitation patternon the orienta of electric field. tion of tubes, and In addition to these observations, following con (iii) Influence of electric field on the kinetics of yellow clusions could also be drawn : and red wave propagation. (i) In the absence of an electric field, I· ions diffuse It was observed that the yellow wave propagated into the gel on account of concentration gradi downward in the presence or absence of natural light at ent. When an electric field was applied Hg2+ ions pH 4.68 of the lower content. However, a remarkable tried to move with greater velocity and diffu change in its characteristics could be noticed when it T- sion would be reduced . As a result the velocity pH was reduced by adding a small amount of acetic acid. of propagation decreased on applying the volt At pH 2.72, the yellow wave bifurcates into many < age. alternate red and yellow bands at the lower end of the (ii) At a fixed field intensity, velocity of propaga tube in the presence of natural light. tion increased with an increase in [1-]. Plots of Experiments on the yellow wave propagation at d2 vs t yielded straight line obeying the relation different tube orientations were carried out by Das d2 = k t + A similar trend for the dependence et c. ai • Jil Results revealed that the location of bands in the of [Hg2+] on the velocity of red wave propaga horizontal tube (H) was approximately half - way be tion has been observed in the presence of elec J, tween those in the g i and g directions. tric field. The influence of externalelec tric field on the propa (iii) Another interesting feature of the system was gation of yellow wave has been studied in agar agar gel. the propagation of continuous precipitation of Field intensity was varied in the range 0 - 0.383 v cm' red Hg 1 at low [KI] in the presence of an elec 2 employing the experimental set up shown in Figure 5 a. tric field intensity greater than 0.084 v cm' and Non linear plots of location of bands as a fu nction of the revert spaced bands at field intensity in the time at various field intensities were obtained. Plots of range 0-0.84 cm' ,as shown in Figure 9. 18 J SCI IND RES VOL.60 JANUARY 200 1

(a) (c) (e) (g)

h) (b) (d) (0 (

- Photographs showing initial and final stages of precipitation patterns of mercuric iodide in agar FigurelO agar gel: (i) at zero field intensity (plates a-d) and (ii) in the presence of external electric field. (field intensity 0.567 v em·I) (plates e-h) . Conditions : [KI] 0.2M; [HgCi,j=O.OIM I per cent (plates a, g) and [HgCl 1=0. IM; [KI)=O.OIM in I per cent a agar-agar c, c, 1 gar-agar gel (plates b, d, ,h) Q = QHgCi = 2ml/h. t KI 1

2 (iv) Spacing between two consecutive bands measured as a fu nction of time. Plots of d vs t (�s) as a fu nction of (n - n ) + c was yielded straight lines obeying the equation 1+ I I plotted. A relation (fu)4 = - (n - n,) + C m ,.1 was obeyed. J2=kt+c

(ii) The kinetics of yellow wave propagation at different Light Induced Spatial Bifu rcation of HgI, and [KI] were studied in the DPL reactor results showed Electric Field Experiments in Two Dimensional that the velocity of propagation increased with in Reactors crease in [KI]. In the earlier communication, Das al. 32 reported et iii) The velocity of yellow wave propagation also de results of the detailed investigation on the one dimen pended on temperature. The velocity increased with sional propagation of yellow/red mercuric iodide. In the temperature. later communication , Das al. 64 reported new results et (iv) Das al.3o also observed the light induced spatial on the two dimensional propagation of a single red/yel et low wave of mercuric iodide, in gel media in batch, bifurcation of a yellow wave into alternate red and DPL, and gel-ring reactors . Salient features of the in yellow bands at low pH in the DPL reactor. vestigation are as follows : (v) Due to interest in the growth of complex structures under non-equilibrium conditions, pattern formation of HgI on thin film of agar agar gel was studied . (i) Precipitation was carried out in a DPL continuous 2 Results showed that : flow reactor, as described earlier, in which the con centration and level of the entering reagent in the (a) Rhythmic or complex pattern of HgL, are formed de s empty space were always kept constant. Experiments pending on experimental condition , (b) Rhythmic were performed in the DPL reactor for the propaga ity was observed during crystallization of red mer curic iodide at low [KI] (0.00 M), whereas at rela tion of yellow and red waves of HgI2 .The location I of the precipitate front from the initial fu nction was tively high [KI] (0.05M)rh ythmicity disappeared and DAS & BAJPAI: LIGHT INDUCED L1ESEGANG TYPE PATTERNS 19

A ct> = 0.00 V em·1

A ct> = 0.567 V em·1

Figure 11 - Ion movement in presence and in absence of electric field

a complex pattern comprising yellow/red crystals tion of yellow Hg If KI (0.2M) diffused into I per cent agar agar containing Hg CI (0.0 M). It was observed was observed. 2 I that a critical field intensity V c exists at 0.283 v crnlat (vi) Experiments in the presence of extemal electric field: which a transition from yellow to red wave took place. Zinc Oxinate Interaction of chemical waves with an applied elec Oxine is used to form oxinates of Fe, Co, Hg, Zn, tric fi eld is an important feature of electrochemical sys Cd and other metals. The oxinates find biological and temsl The influence of an external electric field on the •• technological applications The haemoglobin that car precipitation of yellow and red mercuric iodide in one 16.55. ries oxygen in the blood is a chelate of iron while chlo dimensional gel media was studied by Das et al.3D using rophyll the green pigment in plants is a chelate of mag an experimental set up, shown in Figure Sa. Results of nesium. Cobalt oxinate finds its application as a catalyst the experiments performed in the absence of an electric in the hydrogenation of polymers. Das et al. 5" reported field (field intensity 0.S67b cm·l ) for yellow and red new results on precipitation of different oxinates in gel mercuric iodide in two dimensional gel media are shown media . From the data authors have found that 1 .03 < p< in Figure 10. In the absence of an electric field, the colour 1.016 for different cases. Figure 12 a shows the depen of the precipitate remained unchanged. Pricipitaion pat dence of light on velocity of precipitation and band char terns of yellow and red HgI at initial andfinal stages 2 acteristics of zinc oxinate It was observed that light are shown in Figure 10. In the presence of electric field, increased the growth rate of the precipitate front. The transitions from yellow to red (e, ) and red to yellow g slope was larger in the dark (111) in comparison to that in if, h) HgI were obtained at the field intensity 0.S67 v 2 the light ( ) Zig-zag pattern of zinc oxinate cm-I. m m, mD > • were obtained2 when the2 tube was exposed with light of In the absence of an electric field, 1- diffuses into intensity in the region 300-3S0 Lux fo r zinc oxinate, the gel due to concentration gradient and a yellow pre beyond this region, parallel bands were observed. Non cipitate is formed. When an electric field is imposed, I linear plots between location of bands x and band num moves towards the anode but at the same time Hg2+ would " ber n are obtained obeying the equation x = ce"'" where also have the tendency to move towards the cathode. As " m and c are constants. The spacing law x x" was "j = p a result of counter diffusion, the movement of 1- ions verified as evident by linear plot between X "+I VS x" . would be restricted , resulting in the reduced 1- ion con centration near the anode in the gel and Hg2+ ion concen Light induced Zonation in Fungal Growth;, tration would comparatively be increased resulting in Zonation the formation offruiting structure in con the precipitation of red HgI2. A similar explanation was centric ring on an agar surface is a common phenom et al.54 given by Das for a red to yellow transition. A enon. Zonation of reproductive structures in some fu ngi schematic representation showing ion movements dur is neither dependent upon light nor upon temperature ing precipitation of : (a) yellow and (b) red HgI2 in the fluctuation presumably nutritional factors are involved. presence and absence of an electric field may be given . The occurrence of light induced zonation is all known. in Figure 11 A simple explanation is difficult because two extreme In a separate experiment, the field intensity was also types of response to an alternationof light and dark pe varied in the range 0.0 to 0.S67 vcrnl during the precipita- riods are known viz. : 20 J SCI IND RES VOL.60 JANUARY 200 1

(a) (b)

12 - Light induced precipitation patterns of zinc oxinate : (a) In light and dark (b) In light of different Figure wavelengths. Tubes from left to right were exposed to white light: ( I), wrapped with blue (2), green (3), yellow (4), and red (5), transparent papers while tube, (6) was wrapped with black paper""

(i) The fungus sporulates well in darkness, much less stimuli, (ii) The high specificity of the response,and (iii) or not at all during the light period. Examples are Whether the light simulates or inhibits sporulation. Cephalothecium rosellln and Sclerotinia fructicola. The situations in illuminated systems are slightly (ii) Certain organism sporulates poorly in the dark and different. The interaction of light radiation with one of forms a zone of spores after even a brief exposure the species becomes an important and necessary step. to light, e.g, neither Fusarium spp. Nitzan et at.' in their communication have analyzed the general illuminated reacting-diffusing system. In the lead (iii) Temperature alterations also induce the formation chromate system the band formation does not depend of zones of reproductive structures; it appears, es upon the dimensions of the system. Therefore, it is pos pecially from the data that the immediate stimulus tulated, in formulating the mechanism, that this system for sporulation is a check in growth. is intrinsic length scaling and involves more than two (iv) zonation of some fungi is dependent neither upon reacting species. The light absorption by one of the spe light nor on temperature fluctuations .Presumably, cies, nucleation of colloidal particles, and colloidal nutritional factors are involved here but they are growth which is found to be an autocatalytic step are not understood. important steps in the mechanism.. Lead chromate is al most insoluble in water , therefore the degree of ioniza (v) pH of the medium has also an effect. tion is very small. It is proposed therefore that the de gree of ionization increases in the illuminated system. Any hypothesis to explain the action of light on For an illuminated system, it was postulated that sporulation will have to take into account the several chemical instability would occur due to light absorption facts viz. : (i) The indifference of many fu ngi to light by one of the species. Das et a./,o observed that light of DAS & BAJPAI: LIGHT INDUCED LIESEGANG TYPE PATTERNS 21

wave length 400-550 nm was most effective for peri ...(22) odic precipitation of PbCr0 in agar agar. For a com 4 where and A are the rate of chemical reactions and plete reacting diffusing system of illuminated lead chro (i) ' affinity fthe ith Chemical reaction step .Results showed mate, the following sequence of events was proposed: � that equilibrium state was very unstable under illumi (a) Absorption of light by one of the species, nation. (b) Nucleation of energized particles and colloi dal growth, and (c) Distribution of colloids and band formation. Conclusions It is evident that a very interesting class of prob lem that has been experienced concernedLies egang ring CrO 2· + hv ---+(CrO 2_)* .(\ I) 4 4 type patterns.The Liesegang ring formation may be con Pb2+ + (CrO )* !:; ( PbCr0 )# ...( 12) / 4 sidered as possible example of Turing phenomena . The Pb2++ Cr0 2. + (Pb CrO ' (PbCrO . . ( 13) 4 )Il = ) n+ ' . Liesegang ring type patternsare observed both in living However, in the complete darkness, termination and non-living systems including bacterial and fungal step will take place as follows : growth . Such studies are important in almost all areas of science and technology.

2+ · Pb + CrO 2 ---+ PbCrO ...( \ 4) 4 4 Acknowledgement Thanks are due to University Grants Commission , Taking into consideration the chances of de acti New Delhi for financial help and Prof. N B Singh , Head, vation, the whole process may be symbolized as : Chemistry Department , DDU Gorakhpur University for k, ( 15) providing necessary facilities. A + h v ---+ A * (Activation) ... k 2 References A* + A ---+ 2A (Deactivation) . . . (16) # Liesegang RE, Naturewiss Wo chensclll; (1896) 353 11 2 Ross J, Hanusse P & Ortoleva article in Advances in Chemi· (Energized particle) ...( 17) p, cal Physics. Vol. 38. (John Wiley & Sons, New York) 1978 . 3 Nitzan A. Ortoleva P & Ross J, Chem Phys, 60 (1974) 3 134. k 1 !:;4 18) 4 Kai S, Muller S C & Ross.J, Phys Chem, 87 (1983)806. C # A + B (Dissociation and ...( 5 Muller S C, Kai S & Ross J, Science, 216 (1982) 635. k' recrystallization) 4 6 Dhar N R & Chatterjee A. C, Kolloid 39 (1925) 2. Z, 7 Prager S. Chem Phys, 25 (1956) 279. 1 8 Flicker M & Ross.J, Chem Phys 60 (9) (1974) 3458. k 1 5 9 Dhar N R & Chatterjee A C, Ph),s Chem 28 (1934) 41 . 1 A B + C !:; 2C (Particle growth) ...( 19) 10 Smith DA., l Chem Ph),s, 81 (7) (1984) 3102. + 'I' 'I' Stern K H, Chem Rev, 54 (1954) 79. k II -5 12 Feinn D , Ortoleva P, Scalf Schmidt S & Wolff M, l Chem k 6 Phys. 69 (10) (1978) 27. + ...(20) A B ---+ C (Precipitation in dark) 13 Muller S C, Kai S & Ross Phys Chelll. 86 (1982) 407H. 1 14 Feeney R. Schmidt S. Striekholm P, Chadam J & Ortoleva P, .I Phys Chem, 78 (3) (1983) 1293. There are three species which may diffu se through 15 Edward M, Van L & Ross J, Ph),s Chem, 91(1987) 6300. 1 gel A, B and C as C is completely insoluble (unenergized 16 Kanniah N, Gnanam F D & Ramasamy P, Colloid Interra ce 'I' 1 & PbCrO ) The diffusion flux j of ith species is given by : Science 80, (1981) 377. 4 , 17 Keller J B & Rubinow S, Phys Chem, 74 (9) (198 1) 5000. 1 18 Vaidyan V, K Ittyachan M A & Mohanan Pillai K, .I Crys! 1. =D V2x. ...(21 ) 1 1 1 Growth, 54 (1981) 239. 19 Kai S. Muller S C & Ross J. Chem Phys. 76 ( 1982) 1392. 1 20 Venzl G & Ross J, Phys Chem, 77 (3)(1 982) 1302. The excess entropy production of the equilibrium 1 21 Ve nz1 G & Ross J, Chem Phys, 77 (3) (1982) 1308. 1 state in the proposed mechanism were calculated by us 22 Henisch H K & Garcia Ruiz J M. Crys! Growth. 75 (1986) 1 ing the relationship 203. 22 J SCI IND RES VOL.60 JANUARY 2001

23 Putnis A, Fernandez D L & Prieto M, Nature,358 (1992) 743. 40 Swami S N & Kant K, Kolloid-Z Z, Polymer, 215 (I) (1967) 24 Periodicities in Chemistry Biology, Advances in Th eoretical 60. & Chemistry edited by E Erying (Academic Press, New, York), 41 Chatterjee A C & Rastogi M C, J Indian Chem Soc 28 (1951) 1978, 138. 25 Das I, Chand S & Pushkarna A, J Phys Chem, 93 (1989) 7435. 42 Zimmermann E C, Shell M & Ross J, J Chem Phys, 81 ( 1984) 26 Das I, Pushkarna A & Lall R S, J Cryst Growth, 82 (1987) 36. 1327. 27 Das Lall R S & Pushkarna A, J Phys Chern, 91 (3)(1987) 747. 43 Polezhaev A A & Muller S C, Chaos, 4 (1994) 631. r, 28 Das I., Lall R S & Pushkarna A, J Cryst Growth, 84 (1987) 44 Rastogi R P,J Chem Society (London), 5809 (1955) 3129. 23 1. 45 Rastogi R P & Chatterj ee A C, J Phys Chem, 59 (1955) I. 29 Das I & Pushkarna A J Non Equil Th ermodynamics, 13 (3) 46 Chopard B , Luthi P & Draz M, Phys Rev Lett, 72 (1994) (1988) 209. 384. 30 Das I, Pushkarna A & Bhattacharjee, J Phys Chern, 94 (1990) 47 Crystals in Gels and Liesegang Rings, edited by H K Henisch, 896. (Cambridge University Press) 1988. 31 (a) Das I,Pushkarna A, Chand S & J Colloid Intel! Sci,130 (I) 48 Roy S, Kolloid Z, 54 (1931) 190. (1989) 176; (b) Noszticzius Z, Horsthemke W, Mc Cormik W 49 Kisch B, Kolloid Z,65(1933) 316. P, Swinney H L, & Ta m W Y, Nature, 329 (1987) 169. 50 Kohn M & Mainzhausen L, Kolloid Z, 79 (1937) 316. 32 Das I,Pushkarna A & Agrawal N R, J Phys Chem, 93 (1989) II. 7265. 51 Goldman J E & Lawson A W, Phys Rev ,64(1 -2)(1943) 33 Iwamoto K, Mitomu & Seno M, J Colloid Intel!ace Science, 52 Hatschek E, Proc Roy Soc, 99 (4) (1921) 496. & 102 (1984) 477. 53 Wi llard H H, Merrit Jr L L & Dean J A. Instrumental 34 Das I, Sharma A & Singh UK, Indian J Exp Bioi, 29 (1991) methods oj analysis,4th (Affi liated East West Press Ltd., New ed 1109. Delhi) 1965. Bourret J A, Lincon R G & Carpenter B H, 35 Science, 166 54 Das I, Pushkarana A & Bhattacharjee A, J Ph),s Chelll, 95 (1969) 736. (1991) 3866. 36 Lengyel I & Epstein I R Acc. Chern.Res, 26 (5) (1993) 235. 55 R.I. Fessenden & J S Fessenden ,The Basis oj Orgwzic Ch{'/l1is 37 Ross J & Arkin P, J Phys Chem, 99 (1995) 10417. try, (Allyn & Bacon, Boston) 1978. 38 Castets V, Dulos E, Boissonade J , Kepper P. D, Phys Rev . Lett, 2nd ed. 64 (1990) 2953. 56 Das I & Rai S, Indian J Chem 38 (1999) 311. 39 Tam W Y, Horsthemke W, Noszticzius Z & Swinney H L, 57 Y.W Cochrane, Physiology oj Fungi (John Wiley & Sons Inc, J Chern Phys, 88 (5) (1988) 3395. London) 1958.