; P wa& tkC N e w England Association of Chem
Charles B. Hurd
and Harry F. hnareaux Union College Schenectady, New York
The Growth of Lead Trees in Silkit Acid Gels
The fact that more active metals, such as zinc and cadmium, will replace lead in solutions of lead salts is well known to students in college and high school. I t is not so well known that the lead deposited will form an attractive, tree-like growth. These trees are fragile, but, if supported in a gel, they may be kept for a long time. These experiments are easily carried out and provide good projects, especially for high school students. In addition, the literature is not extensive and much remains to be discovered concerning the growth and form of these "trees." Probably the first to mention growing trees in silicic acid gels was Simon ( I ) . Holmes (9)also described the growth of trees, especially in his well known laboratory manual (5) where one experiment was described. Stone (4) has described similar trees, not grown in gels, and Brewington (5) has given a brief discussion with photographs. In the article by Taft and Starek (6),excellent photographs are given, together with a good discussion on the effects of many factors. This article should he read, if possible. The method of taking photographs is described. One can obtain a good idea of the effect of many factors on the growth of these trees from this article, but it is difficult to devise and perform experiments, guided by the article, unless one has had experience. The inverted character of these trees has led Fillinger (7) to report on a method of growing them right-side-up. King and his associates (8) have written several articles on the general subject of lead trees in gels, with emphasis on the morphology of the trees. Persons interested will find a few other articles on the shape of lead growths deposited by displacement, or electrolytically. I t seems to us that a useful function will be performed and work in this field will be stimulated if we describe several experiments which can be performed successfully by anyone interested. Our research on this whole subject has convinced us that these simple experiments offer a good introduction to the study of this subject. Brief remarks on the theory are sufficient here since previous articles have discussed it. The tree forms because a more active metal replaces lead in the form of its ions in the gel. The lead metal deposits on the more active metal, while atoms of the more active 472 / Journal o f Chemicol Education
metal go into solution as ions. The deposited lead "grows" in a crystalline form towards sources of lead ions not yet replaced. The crystalline form results because the lead deposits slowly. The gel supports the crystalline growth or tree. So long as the advancing end of the tree has metallic connection to the active metal, the point of deposition can be a t some distance from the point where the active metal is dissolving. It is only necessary that the electrons flow through the metal to the point where they are needed, i.e., the advancing end of the tree branch. Others, such as Taft and Starek, have discussed the electrochemistry involved. Obviously, the more active metal must have a greater tendency to dissolve, forming ions, than does the lead. This tendency will depend on the metal's standard potential and that of the lead, together with the concentration of the ions of each. The hydrogen ion concentration is important since it may prevent formation of basic salt or oxide layers on the metal. Ions which will produce insoluble salts with either metal interfere, although we shall describe one experiment where an insoluble salt is involved. The silicate and the gel are important. Formation of bubbles in the gel may distort or break the tree. We have had best results with gels a t pH 4-5. The silicate must be poured into the acid solution, for gels in this pH range, to give good gels free from clots. F m a t i o n of 0 Simple T ~ e e . Dilute sodium silicate (water glass) to asp gr of about 1.06 with distilled or demineralized water, although tap water will work. Our best results have been with "C" brand silicate made by the Phildelphia Quartz Compnny. If 120 ml of glacial acetic acid are diluted to 1.0 liter, the solution is approximately 2.0 N. Far a 1.0 N lead acetate solution, dissolve 190 g solid C.P. Pb(CzHa02)..3Hn0 and make up to 1.0 liter. Place 5 ml of lead acetate solution in 20 ml of the 2 N acetic acid and mix. Pour into this, 20 ml of the silicate; mix by pouring back and forth, and pour into an &in. test tube. When the gel is set, pour ErlO ml distilled water an the top, insert a clean piece of zinc, and cork the tube. A good tree, very bushy near the top in contact with the ~ i n ebut growing down through the tube, will form. Variations. By using lower concentrations composed of a smaller volume of the 1 N solution, made up to 5 ml, you a-ill observe the effect of the lead ion concentration. For lower concentrations, the tree will be less bushy, will grow more slody, but will probably form better crystals. Variation in the Active Metal. Although zinc is usually used for the active metal, an interesting project, especidy for high school students' work, is to vary the metal. Iron, even nails, tin, cadmium, magnesium, and a number of others will work.
We have noted a difference in the type of tree produced. With zinc, the tree is bushier, with iron more slender and clearly defined. With tin, we have found that the main stem and branches are planar. Variation qf Acidity of the Gd. Best results are obtained with acidic eels. Bv usine less acid in the 20 ml, makine ua the differen& with"distilcd water, one will obtain less aeydic gels. We have found that tree growth cesaed when the gel had a fiH =7. The va"ation in the type of tree is worth noting as the pH is changed. Students should note that the basic gels are more milky. This is partly due to the gel itself, partly, probably, to basic, insoluble lead salts being formed. Growth of Tree Through Precipitate. If the regular gel ia prepared hut dilute sulfuric acid, 1 N or even more dilute, is placed on top before the zinc is thrust into the gel, a curious result is produced. The sulfate ions penetrate the gel, forming a white, milky layer of lead sulfate. Through this, the lead tree will grow. Sinoe the tips of the tree are constantly pushing farther into the gel, they deplete the lead ions before sulfate ions can reach them. Thus, a white band forms at the top, but this stops while the tree grows down through the whole tube. This experiment will suggest many combinations and can serve as an interesting project. It is well not to use too strong sulfuric acid above the gel, or the tree may be destroyed. The me of salts, such aa sodium sulfate in solution, is thus suggested. Variation of the Metol Replaced. Although the title of this
article mentions "lead" trees, others are possible. Lead appears to give by far the best tree-like growth, hut a search for others should prove interesting. Tsft and Starek (6) offer some suggestions. Tubes containing trees may he kept for a long time if the solution remaining a t the top is poured off after the tree is formed and is replaced by more water or very dilute acid. More active metal ahould he nlaced on too. The cork should fit tiehtlv so paraffin or wax seal around-the that the water is' not lost. stopper will help keep the water in the tube. We have kept some trees 10 years Literature Cited
SIMON, A. L.,Z.Chcm.Ind. Kollaide, 12,171 (1913). HOLMES, H. N., J. Fmnklinlnst., 184,743 (1917). HOLMES, H. N., "Labomtory Manual of Colloid Chemistry," 3rd ed., 1934, p. 155. STONE,C. H., J. CHEM.EDUC.,6,355 (1929). BEEWINGTON, G. P., J. CHEM.EDUC., 6,2228 (1929). TAFT,R., AND STAREK, J.,J. CHEM.EDUC.,7, 1520 (1930). FILLINGEE, H. H., J. CHEM.EDUC., 12,92 (1935). KING,A,, AND STUART,N., J. Chem. Soc., 141, part 1, 642 (1938); School Sei. Rev., 22,126 (1940); Sci. J . Roy.Coll. Sci., 11, l(1941).
Volume 36, Number 9, September 1959
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