Journal of Scientific & Industrial Research Vo 1.60, January 200 I, pp 10-22 Light Induced Liesegang Type Patterns in Batch and Flow Reactors 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) (c) (d) (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 co- workers)' " 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.