Namboodiri, P.N.N. (1975) Some aspects of periodic precipitation of materials. PhD thesis, Banaras Hindu University.

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Abstract

The phenomenon of the formation of periodic precipitate structure (Liesegang Ring) had attracted the attention of several research workers. The usual method of obtaining such structures is by diffusing the external electrolyte into the gel impregnated with internal electrolyte. The explanations given by earlier workers on the formation of Liesegang Rings were of qualitative nature and were based on the theories of super saturation or coagulation. They did not emphasise the role of diffusion. Mathur explained the phenomenon based on diffusion mechanism and delivered the following logarithmic relationship Log (Xn-Xn-1) = A + α Xn In the present studies, Mathur’s logarithmic equation mentioned above has been verified quantitatively with six different systems of periodic precipitations, viz, uranyl-8-quinolinol, silver-8-quinolinol, cobalt-8-quinolinol, copper-8-quinolinol, magnesium pyrophosphate and cobalt pyrophosphate in agar agar gel. Data obtained for the spacing between the rings had been plotted as Log (Xn-Xn-1) against Xn. The curves obtained are straight lines providing the validity of the equations for all the systems. The influence of concentration of the external and internal electrolytes on the spacing of rings is another aspect predicted by Mathur’s theory. From the general equation for Liesegang Rings, (Xn-Xn-1) = [P(S/CoCo’)] exn The distance between the consecutive rings should be inversely proportional to the initial concentrations of external and internal electrolytes, (Xn-Xn-1) α 1/ CoCo’ In other words, the number of rings within the same unit length should be larger, higher the concentration of the electrolyte. The detailed studies of this relationship between ring spacings and concentrations have been made in each of the six periodic precipitation systems mentioned above. The systems studied have been classified into one or other category in accordance with the general classifications. A general review on the past work on Liesegang Ring phenomena is given. The diffusion theory of licensing Ring proposed by Mathur and the problem statement are dealt with. The experimental results and critical discussions are presented. Periodic precipitations of uranyl-8-quinolinol, silver-8-quinolinol, cobalt-8-quinolinol and copper-8-quinolinol systems in each agar agar gel medium are reported. Magnesium pyrophosphate and cobalt pyrophosphate systems produced in agar agar gel medium, dealt with. As mentioned earlier, the discussions of the systems described are confined to the verification or the space law and the logarithmic equation and some common structural aspects. However certain specific studies made on one or other systems are reported. These studies relate to the factors influencing the spacing in periodic precipitation with special reference to the effects of gel concentration, acid, foreign complexing reagents etc. All the above mentioned factors influence ring spacing by increasing or decreasing the rate of diffusion of ions. The logarithmic relationship is also verified in all these cases. The correctness of Mathur’s mathematical theory of the formation of Liesegang Ring based on diffusion mechanism is proved from the various experimental studies

Item Type:	Thesis (PhD)
Uncontrolled Keywords:	Periodic precipitation
Subjects:	Electrodics and Electrocatalysis
Divisions:	UNSPECIFIED
Depositing User:	Dr. N Meyyappan
Date Deposited:	13 Jun 2012 04:57
Last Modified:	13 Jun 2012 04:57
URI:	http://cecri.csircentral.net/id/eprint/2826

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