Studies on Rhythmic Precipitates

Abstract

(1) The conditions of the formation of various examples of rhythmic precipitates have been studied. The effects were tested of the change of species and concentrations of gel, inner electrolyte and outer electrolyte, the temperature, the light intensity and the hydrogen ion concentrations of medium and outer electrolyte solution. (2) The regularity of spacing between bands has been studied. The distance between bands is expressed empirically by the quadratic equation concerning the ordinal number of it. The same equation was deduced from a theoretical standpoint, the physical meaning of the equation being considered. (3) Classification of the rhythmic precipitates by the cause of formation and by the structure has been proposed. Secondary bands were investigated and discussed. (4) New examples of rhythmic precipitate have been described, i.e. barium chromate, cobalt phosphate, lead carbonate, lead phosphate, lead iodate, silver arsenate, silver carbonate, silver iodate and silver capronate in gelatine gel, and bismuth chromate, bismuthyl chloride, bismuthyl nitrate, cobalt phosphate, cobalt sulphide, copper carbonate, copper iodate, cerium iodate, calcium phosphate, calcium oxalate, strontium oxalate, ferric ferrocyanide, lead carbonate, lead iodate, lead phosphate, nickel dimeihylglyoxime, silver arsenate, silver ferrocyanide, silver phosphate, silver iodate, uranyl phosphate and uranyl arsenate in silicic acid gel. The conditions for the formation of these rhythmic precipitates have been thoroughly investigated.

Spatial Structure Formation in Precipitation Reactions

“…If the initial concentration of the electrolyte at the center is substantially higher than that of the second electrolyte (i.e., the concentration difference, Δ, is large) and if the initial degree of supersaturation, S , is sufficiently high, then a reaction front begins to move from the center to the periphery of the dish as a result of diffusion of the added substance, leaving behind well-separated rings of precipitated salt. This was first shown and systematically studied by R. E. Liesegang in a system producing concentric rings of silver dichromate in a thin layer of gelatin (Figure B) and was afterward reported for many other slightly soluble electrolytes. , Although a gel is not essential for banded precipitation to occur, as shown soon after the discovery of this phenomenon, its presence may influence the details of the structure formation process. Bands may be produced even in the solid or gas phase and have been found on a micrometer scale in water-swollen polymer films .…”

Periodic crystallization of barium oxalate in silica hydrogel

“…Therefore, for the next ring to nucleate, the outer ions have to cross over this barren depletion region to meet with the other reactant. Since the passage is clear without any inner electrolyte, and Ba 2 ÷ ions diffuse more rapidly, it leads to increase in K. According to Isemura (1939), as the concentration of the outer electrolyte increases, the number of bands also increases and the successive interval between them decreases. Barium oxalate rings exhibit a different behaviour so that the total number of bands formed does not vary but the spacings between them increase noticeably with higher concentrations of barium chloride.…”

The Liesegang Phenomenon.

“…While this statement hardly is provable, even a cursory examination of the literature reveals that many compounds are capable of producing Liesegang rings. Isemura (36) presents a list of substances from which rings have been prepared in silicic acid gel. It includes lead carbonate, bismuth(III) chromate, cobaltous orthophosphate, cupric iodate, cupric carbonate, silver orthophosphate, silver iodate, cobaltous sulfide, calcium orthophosphate, calcium oxalate, strontium oxalate, lead iodate, and lead orthophosphate.…”

“…Kiister (42) obtained no bands of silver chromate in the dark, but Davies (12) obtained good bands. Isemura (36) believes that light influences the reaction in cases in which the inner electrolyte reacts with the gel. For example, chromate ion exhibits a tanning action on gels.…”