COLLOIDAL GARDENS FROM SODIUM METASILICATE V. R. DAMERELL and H. BROCK Western Reserve University, Cleveland, Ohio
A FAMILIAR
experiment in colloid chemistry is the formation of insoluble silicate growths, brought about by dropping crystals of various metal chlorides into diluted water glass. These systems are commonly called "Colloidal Gardens" because the silicates form as tubes which superficially resemble growing plants. A difficulty sometimes encountered is cloudy water glass, preventing good vision of the growths. The authors wondered if clear solutions of sodium silicate might not be substituted to avoid this condition, and the following experiments were accordingly tried. A ten per cent solution of KazSi03.5Hz0 was prepared in distilled water. This, too, gave a faint turbidity, but the insoluble material quickly settled out. The optimum alkalinity for growth formation was not known. To find it, a series of solutions was prepared to which hydrochloric acid or sodium hydroxide was added. The middle member in this series was the original ten per cent solution without additives. Going to the right, increasing amounts of 3N hydrochloric acid were added, and going to the left, increasing amounts of 3 N sodium hydroxide solution. Then chlorides of copper, ferric iron, and cobalt were dropped into each beaker. The results proved so interesting that the authors are writing this note with the thought that the experiment may be worth while includmg in colloid laboratory work. First, 24 ml. of 3 N hydrochloric acid could be added to 200 ml. of 10% sodium silicate solution without producing a silicic acid turbidity, but a t much higher concentrations of acid such a t,urhidity did develop.
Secondly, all solutions remained clear, afFord'mg excellent vision of the growths. Finally, and most interesting, a distinct minimum in growth formation occurred in the beaker containing 10 ml. of 3 N hydrochloric acid added to 200 ml. of 10% sodium metasilicate pentahydrate solution. On the acid side of this point the growths became tall and thin. On the alkalme side they were shorter and thicker. The term "acid side" is used in a relative sense only; all of the solutions were actually strongly alkaline. Table 1 shows some characteristics of a cobalt silicate series. Table 1 Cobalt Silicate Growths i n 330-M1. Portions of 10% NaSiOa. 5&0 Solution Approximale height of grmuth 50 ml. HCl 8 om. 8 om. 41 ml. HCI 22 r" 1. HCI 1.5 om. 16.5 ml. HC1 0.2 cm. No addition 1 om. 25 ml. NaOH 1.5 om. 9 N acid or base added
wv
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Appearance of growth after three weeks Tall, thin, bluish Tall. thin. bluish Shoit, thicker. bluish Growth barely evident Short, thicker, bluish-gray Short. thicker. reddish in color. Grav on top
The authors can offer no explanation for the minimum except to suggest that the silicate membrane a t that point may have been either completely permeable or completely impermeable, since either condition would prevent osmosis and therefore inhibit formation of a growth.
