Periodic precipitation patterns in the presence of concentration gradients. 1. Dependence on ion product and concentration difference
Periodic precipitation patterns in the presence of concentration gradients. 1. Dependence on ion product and concentration difference
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存在浓度梯度时的周期性降水模式。
DOI：
10.1021/j100217a038
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发表时间：
1982
期刊：
影响因子：
--
通讯作者：
J. Ross
中科院分区：
文献类型：
--
作者：
S. C. Mueller;S. Kai;J. Ross
当浓碘化钾（约0.5 M）与浓度低得多的硝酸铅（约0.05 M）接触时，过量的碘离子扩散到铅溶液中，在那里它们发生反应形成弱溶盐——碘化铅。随后，Pbl2在空间中不连续地析出，在数小时至数天的时间内，平行于扩散试剂的前表面依次出现分离良好的窄析出带。这种周期性降水最早是在1861年观测到的，此后被许多作者研究，通常被称为列色冈现象。当扩散从一个扁平的圆形容器（如皮氏培养皿）的中心发生时，图案由一组同心圆组成。当两种反应物放在试管中时，扩散的方向基本上被限制在一个空间坐标内；以及由众多平行的降水带组成的“一维”集（见图1，a和b）。在相互扩散过程中，许多电解质组合会导致类似的有节奏的沉淀物沉积。2’6正常情况下，相邻环之间的间距随着距离施加梯度原点的增加而增加，但有相反的实验证据（恢复间距）。7,8在大多数实验中，在溶液中加入凝胶形成材料（如琼脂或明胶），以有效地防止固相的对流和沉降。凝胶可能影响图案的一些结构细节，但对结构形成不是必需的。8,9已注意到更复杂的结构特征；这些包括二级结构（将一个带分隔成几个紧密相连的更薄的带），10个同心圆分割成径向对齐的凸部分，11个螺旋而不是同心圆，12个和
When concentrated potassium iodide (about 0.5 M) is placed in contact with lead nitrate at substantially lower concentration (of the order of 0.05 M), the iodide ions being present in excess diffuse into the lead solution where they react to form the weakly soluble salt, lead iodide. Subsequently, Pbl2 precipitates discontinuously in space, in that well-separated narrow bands of precipitate appear sequentially in a period of hours to several days, parallel to the front surface of the diffusing reagent. Such a pe-riodic precipitation, first observed in 18961 and since then investigated by many authors, 2" 4 is commonly called the Liesegang phenomenon. The patterns consist of a set of concentric rings when diffusion occurs from the center of a flat, circular container such as a Petri dish. When the two reactants are placed in a test tube, the direction of diffusion is confined to essentially one spatial coordinate; and a “one-dimensional” set of numerous parallel bands of precipitateresults (see Figure 1, A andB). s Many electrolyte combinations lead, on interdiffusion, to similar rhythmic deposition of a precipitate. 2'6 Nor-mally the spacing between adjacent rings increases with increasing distance from the origin of the imposed gradient, but there is experimental evidence for the opposite (revert spacing). 7, 8 In most experiments, a gel-forming material (such as agar-agar or gelatin) is added to the solutions which effectively prevents convection and sedimentation of the solid phase. The gel may influence some structural details of the patterns but is not essential for structure formation. 8, 9 More complex structural features have been noted; these include secondary structures (the separation of one band into several closely adjacent thinner bands), 10 segmentation of concentric rings into radially aligned convex sections, 11 spirals instead of concentric rings, 12 and