THE GROWTH OF LEAD CRYSTALS IN SILICA GELS* Doubtless many chemists a t some time in their career have indulged in the recreation of growing lead ctystals in silica gels. Few, however, have taken time to publish any observations on the growth of these interesting crystals. Simon1 was apparently the first individual t o record this phenomenon of growth. Somewhat later HolmesZ pointed out that the most favorable concentration for large crystal growth was in a solution 0.02 N with respect to lead acetate. Brewington3 and Stone4 have described procedures for the growth of these crystals as a project for beginning classes. A more extensive study of the experimental factors determining the growth of these crystals has not been published as far as the authors are aware. Chiefly as a diversion we have grown some hundreds of specimens and as the results were interesting and in some cases quite instructive we present the more important ones herewith. The experimental factors which we have varied are (1) the nature of reducing agent, (2) the concentration and composition of lead salt, (3) the hydrogen-ion concentration of the gel, and (4) the viscosity of the intercellular liquid present in the gel pores. Procedures for growing these lead crystals may be found in the references listed above. Our procedure was t o mix equal volumes of a sodium silicate solution (sp. gr. 1.06) with a mixture containing acetic acid and the lead salt so that upon mixing the two solutions the desired concentration of acid and of salt was obtained. The specimens were grown in test tubes (8" X 1") which were stoppered to prevent drying of the gel. A number of the specimens are shown in the accompanying photog~aphs.~ The usual reducing agent used to cause precipitation of lead is zinc, although tin has been suggested. In addition to these we have tried such 'Read before the Kansas Academy of Science, Manhattan, Kansas, April 27. 1929. Zeit. Chem. Ind. Kolloide. 12, 171 (1913). 3. Franklin Inst.. 184. 743 (1917). . . See also Holmes, "Laboratory Manual of Colloid Chemistry," 2nd ed., John Wiley & Sons, 1928, p. 138. THEJOURNAL, 6, 2228 (Dec, 1929). Ibid., 6, 355-6 (Feb., 1929). 6 Frequent inquiries as t o our method of photographing these deposits have been made and for that reason the procedure is briefly described. The test tubes containing the crystals were placed against a large floodlight, the face of which was covered with several thicknesses of parchment paper t o obtain diffuse illumination. A 100-watt lamp was used in this flood lamp. The specimens were thus photographed against a bright background, with no other illumination, the photographs being made in a dark room. They were photographed a t 'Ir of their natural size on a cut film of par speed, using a very small stop (f 32) with long exposure. Enlarged prints of these negatives were then made by the procedure previously described by one of the authors. [THIS JOURNAL, 6, 1928 (Nov., 1929).] While the photographs do not do the crystals justice, this method yields better results than the authors have seen elsewhere. 1520
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GROWTH OF LEAD CRYSTALS
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Reductng metal (left to right): Tin, cadmium, iron. Concenlration of lead acelate: 0.1 N . Age: 3 to 5 weeks. In the ease where tin was used, the metal was added before gelation was com~ k t eand some of the metal sank to the bottom: eonseooentlv the trees are growing from both top and bottom.
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substances as cadmium, calcium, manganese, aluminum, silver, chromium, nickel, cobalt, iron, thallium, magnesium, hydrogen, copper, lead, brass, and sodium amalgams. From an electrochemical standpoint any half-reaction producing a lower potential than the ~otentialexistinc between metallic lead in cont a d with lead ions would he capable of causing some of the lead ions to deposit. Consider the case of zinc as the reducing agent. The,reactions involved may be indicated as follows:
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Zn - 26 = Znt+ Ph++ 2r = Ph, the net reaction being Pb++ = Zn++ Ph. Zn
+
+
+
For a concentration of zinc ions equal to 1 gram mole per 1000 grams of water the potential of the metallic zinc against such a solution would be approximately -0.76 volt6 a t room temperature. That of lead against a solution containing a gram mole of lead ions per 1000 grams of water is -0.12 volt. If we have a system such as indicated in Figure 2 the concentration of zinc ions is practically zero a t the start and that of the lead ions may be any value desired up to that imposed by the solubility of the lead salt used; let us assume that its concentration is such as to make the Pb/Pb++ boundary -0.12 volt. The zinc ions having a lower concentration than 1 M will have a still lower potential than -0.76 volt7 and consequently lead deposits. As lead deposits, the concentration of lead ion decreases (hence the potential Pb/ Pb++ decreases) and that of the zinc ion increases (and consequently the Zn/Zn++ potential increases). This process will continue (provided that sufficient metallic zinc is used) until the two potentials are equal and will then cease. The concentration of lead ions, however, tends to remain constant due to the diffusion of the lead ions toward the crystals already formed and the growth of the lead crystals into the regions of higher lead-ion concentration. It is exceedingly important to notice the part . played by the concentration of the . 6 Signs are used throughout in conformity with recommendations of the American Electrochemical Society. The student of electrochemistry will recall that inmeasing the concentration of a metallic ion reversible with respect to a given electrode will make
