PERIODIC PRECIPITATION I S ORDINARY AQCEOUS SOLUTIOSS BY HARRY W. X O R S E
Historical Since Liesegang’ called attention to the phenomenon which bears his name, in 1896, a considerable literature, now numbering well over three hundred titles, has accumulated on the subject. This study has extended into geology; physiology; botany; bacteriology. I t includes “rhj-tlimic“ phenomena observed in solids, liquids, and gases, and “periodic precipitates” as various as:- rings of t,rees; mother of pearl; bone structure; drops of chloroform; drops of mercury; gas bubbles; shell structure; markings on butterflies’ wings; rhythmic settling of sediments; etc., as well as precipitates more or less periodic in nature, of slightly soluble substances formed by metathesis in gels of gelatin; agar; starch jelly; fruit jellies; silicic acid; etc., and in inert powders; in plaster of Paris; in columns of glass beads, and in capillary tubes. X number of theories have been put forward to explain this periodicity. I n some of these the gel is given major importance; in others, only a secondary role. In some of the suggested explanations no account is taken of the effect of any colloid and periodicity is referred to the effect of concentration changes resulting from the diffusion of the dissolved substances involved in the reaction. In 1903:Morse and Pierce: called attention to the fact that rhythmic precipitation took place in pure water solution, and that the presence of a jelly or other colloid (other than the water solutionj was not ncccssary. Later, isolated observations on periodic precipitation in aqueous solutions w r e published by Xotboom;” Ilreaper;4 )Yo, O s t ~ a l d ;Fischer ~ and Schmidt ;6 Fricke;’ Hedges8 and others. Similar periodicity x a s observetl by TraubeQ when precipitation takes place in plaster of Paris and by Popp’O in tubes fillcd with glass beads. These observations indicated the probability that there was a considerable class of substances which could be caused to precipitate in a periodic manner in water, without any jelly, if the proper conditions were attained. Scope of the Present Paper This paper offers data on thirty-six case,: of periodic precipitation in aqueous solution; indicates the rather general nature of the phenomenon; presents a number of microphotographs of typical precipitates; considers the bearing of the data on theories which have been offered, and suggests a general explanation of the results observed in pure water solutions. This explanation takes no account of such (evidently) more complicated cases as:-
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-rhythmic effects produced by the action of one or more substances on a gel (structure formation, etc.); -the effect of colloids on precipitation (peptization; coagulation; adsorption, etc.); -the action of colloidal or semi-permeable precipitates. That factors like these are active and important when gels are present, is well known, but the aim of this paper is to restrict the phenomena to t’heir simplest expression in water alone, and to avoid the evident complications which are introduced by colloids. Experimental Procedure X hole is bored in the center of a microscope slide; a drop of dilute solution of one of the substances which is to react by metathesis is placed on the slide and covered with a cover-glass, and a drop of the other, more concentrated, reacting solution is placed over the hole, on the other side of the slide, The slide is then placed on the microscope stage for observation. Or, instead of placing a drop of the concentrated solution in the hole, a drop of the dilute solution may bP placed there, connecting with the film of solution beneath the cover, and a crystal of the substance which is to diffuse placed in the drop. Either method requires patience and, in the author’s experience, many trials, before quiet diffusion reaults. What often occurs is a flushing of the concentrated solution into the space under the cover. And even when diffusion has once been started, changes in surface tension, due to evaporation from the drop and at the edge of the cover, cause sudden disturbances which result in convection instead of the necessary pure diffusion. These disturbances can be in large measure avoided, and a true diffusion of the more concentrated substance into the other attained, by placing in the hole a plug of macerated filter paper, lightly tamped into place. This prevents convection. -1plug of gelatin or a,gar can also be used and in many cases, especially where solutions are dilute, the colloid has no effect on the precipitate. In other cases, the plug of jelly is dissolved, and even this small amount of colloid may have a distinct effect, in modifying the “habit” of the resulting ring system. .1bout half of the photographs Tvhich illustrate this paper were made with no plug of any kind; the other half with a plug of filter paper. The preparation shown in Fig. 2 4 was made with a gelatin plug, which probably had some effect on the nature of the precipitate. General Statement of Results The phenomenon of periodic precipitation in pure water solution is a fairly genernl one. About forty cases of periodicity have been noticed, out of about tiTo hundred cases rather hastily examined. I t appears to be of about the same generality as periodicity in gelatin. Periodic precipitation is two-sided. Either substance can be taken as dilute or as concentrated solution, with equally good result, as far as this preliminary study has shown.
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HARRY F.MORSE
The precipitate is always definitely crystalline. In many cases the “ring” is made up of crystals relatively widely separated. No very fine ring-structure has been observed. In all the cases here recorded, the details of st,ructure are evident under a pocket magnifier and usually evident to the unaided eye. The range of solubility of the substances forming periodic precipitates is wide. I t extends from barium sulfate and mercurous chloride, through substances “fairly slightly soluble,” to caesium alum. The crystal “habit” of the precipitated substance varies from spheres (which are rather frequent) to long spicules. The habit of the rings of precipitate varies from broad, flat, and rather diffuse bands-mere maxima-with scattered crystals throughout the entire field, to sharply defined rings, with no scattered crystals which are visible under the powers used for observation. With patience and careful manipulation, ring systems can be produced in pure water solution which are of almost the same sharpness and regularity as those formed in jellies. I n the few cases which have been measured, the rings in water follow the
hi - h = K (constant ratio of the increasing h* - hi distance between neighboring rings) as in gels. (See Jablczynski.”). This indicates a ruling influence of diffusion, and especially the influence of the diffusing (more concentrated) substance, on ring formation and on periodicity. same rule of the constancy of
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Notes on Manipulation If measurements are to be made, or if examination is to continue for more than an hour or so, care must be taken to avoid evaporation from the hanging drop and around the edge of the cover. Evaporation causes changes in concentrations; new diffusion gradients are set up; all conditions change. For long-continued observation, the slide may be enclosed in a box lined with wet blotting paper, having a hole in the top to permit passage of the microscope objective and one in the bottom to allow light to pass. With this precaution, precipitation under constant conditions can be followed to the edge of the cover-glass. The method is only applicable to cases where ring formation progresses far enough to give several precipitates in an hour or so. It can not be used in cases like the ones observed by Wo. Ostwald5 and by Hedges,8 where precipitates form a t intervals of hours or days. In some cases the preparation shows practically no sign of definite “rings” after precipitation is complete. But even in these cases it is sometimes possible to observe the periodic nature of precipitation during di$usion. In the case of silver iodate, for example, the final form of the precipitate is often large branched crystal aggregates, and the completed slide may show no periodicity. The first form is, however, small crystals, and these precipitate in a sharply periodic manner. The front of the wave of precipitate does not
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grow continuously. The first appearance of solid is as a ring, at measurable distance from the last visible front of precipitate, and there is definitely a clear space between them. In the particular case of silver iodate, as growth proceeds, the crystals grow in size and become branched; the rings increase in width with this branching, and before growth is ended, neighboring rings have merged. Except for the new ring, just forming, and a ring or two where crystal growth is not yet ended, the circular mass of precipitate appears to be continuous. Ring Formation by Mechanical Shock In the case of some very finely divided precipitates, like silver iodide, it is possible to produce by gentle shock a series of lines which have somewhat the appearance of true periodic precipitates. Undisturbed, the wave of precipitation is continuous; there is no sign of any periodicity, nor is any part of the area of precipitate denser than another. If the table is tapped, the progress of the wave of precipitation is halted and for a time a dense sharp ring of precipitate forms. The wave then starts again, passes over the ring, and goes on as before until the table is again tapped, when another sharp ring appears.
It is difficult to explain how shock can act otherwise than to increase the general agitation and movement in the wave front, unless the effect of shock is to release supersaturation. Periodicity by Clumping Rrodersen’? describes and explains this. I t is the heaping up in lines or windrows of particles, formed elsewhere and transported by convection to the place where they are trapped and held by accumulating masses of similar particles. The difference between these lines of precipitate (which may be very sharp and very regular) and the lines of true periodic precipitation, is a fundamental one. In Brodersen’s case, particles are transported, and become heaped up at a distance from their point of origin. If there is motion of particles, even in a regular wave front, there is no true periodic precipitation. I n the periodic precipitations discussed in the present paper each particle remains where it first appears, and grows in size at that point. Other Artifacts Beside the two cases cited-precipitation by shock and formation of lines of precipitate by the heaping-up of moving particles-another apparent regularity frequently appears which may closely imitate periodic precipitation. Some substances are very sensitive to conditions of the glass surface. This sensitiveness does not seem to be wholly a function of size, for some of the finest-grained precipitates are wholly insensitive and some comparatively grossly crystalline ones are remarkably sensitive. The traces left by wiping slide or cover with a (comparatively) clean cloth often determine crystallization along lines which may counterfeit actual periodicity.
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HARRY W. MORSE
Periodic Precipitates Out of about two hundred reactions which were examined in this brief survey, those giving the following slightly soluble substances exhibited unmistakable periodicity (those illustrated are starred) :Silver cyanide’ iodate chromate* bichromate tungstate phosphate * arsenate Lead chloride* iodide* iodate cyanide* hydroxide* sulfide* oxalate sulfate tungstate arsenate* Manganous hydroxide carbonate
Thallous iodide chromate’ Mercurous chloride hydroxide iodate carbonate* arsenate Mercuric oxide sulfide* carbonate* phosphate Cupric sulfide chromate* Cadmium hydroxide* Barium sulfate* carbonate*
Caesium alum* The solubility in water of most of these substances has been determined, either directly or by electrical methods but data on the degree of supersaturation to which their solutions can be carried are almost wholly lacking. There is a cert,ain amount of ”common laboratory knowledge” about many of them and it is evident from simple analytical espericnce that’ solutions of some of them do supersaturate to some extent. The analyst knows about how long he must wait aftcr mixing reagents, before a precipitate appears. Crystals formed by Diffusion Many of the substances in the list form very minute crystals when produced by the usual methods of analytical precipitation and these must be transformed (by boiling; by slow cooling; by standing; etc.1 into larger crystals or larger aggregates for efficient filtration. The crystals formed by slow diffusion are always larger and more perfect (Dreaper4). In none of the observed cases could the ring of precipitate possibly be mistaken for a “membrane.” In all cases the specific surface of the precipitate niust be classed as “small” compared to precipitates which exhibit measurable adsorption of electrolytes.
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