Light Induced Liesegang Type Patterns in Batch and Flow Reactors
Ishwar Das, Alpana Bajpai · 2001
Inter-diffusion of one electrolyte into another re acting electrolyte may ·lead to a rhythmic deposit ion of prec ipi tate which i s known commonly as Liesegang phenomenon' . W hen a of large concentration of K Cr 0 is al lowed to diffuse in to a less concentrated 2 2 1 of lead n i t rate conta in ing a lyophi l ic gel (agaragar) medium, periodic deposition of prec ipitate appears after several hours. S imi lar type of periodic precipita tion has been investigated and studied by many work ers2 '2. The pattern formation or Chemical i nstab i l i ty or inhomogeneity in nucleation of col loidal particles may arise due to several factors. Inter-diffusion due to con centration gradient may be one of them. Figure I shows Liesegang ring type patterns ob served during crystall i zation of ascorbic ac id from its alcoholic solution ,periodic precipitation of copper chro mate in agar agar gel ,25 agate bands, growth of the bac teria Pr vulgaris ' 40n agar p late and the fungi Necteria cinnaharina. Rings of th is k ind are observed in nature in minerals such as malachi te , l imonite, varisc i te , chal cedony and w ings of certain colourful butter fl ies . Liesegang suggested that the rings and bands of agate arose when the Liesegang phenomena operated in natu ral s i l ica gels . Geologists agree that agate patterns can be attributed to the successive deposits of layers of s i l ica gel and i mpurities. When the number of each band in the pattern of these materials is plotted against the loga rithm of the distance of the band from the origin, the result ing graph is usually a curved l ine. On the other