A LECTURE OR LABORATORY DEMONSTRATION OF COLLOID CATAPHORESIS WILLARDH. MADSON AND FRANCIS C. KRAUSROPF, UNIVERSITY O F WISCONSIN, MADI-
SON, WISCONSIN When subjected to the influenceof a difference of potential, particles of certain colloids travel toward the negative pole, the particles of others toward the positive pole. This phenomenon is given as evidence that the colloid carries either a positive or negative charge and is attracted to the pole of opposite charge. As the particles of the colloid reach the oppositely charged pole their charges are neutralized with the accompanying coagulation of the sol about the electrode. This migration and coagulation is often so slow as to require hours or even days to show a marked change in appearance. While making a study of some of the properties of a beryllium oxide sol, it was noticed that this migration and coagulation was so rapid that a marked change in appearance was evident within an hour or two, and by the end of 12-24 hours the sol was almost completely coagulated around the negative pole ... TRe liquid around the positive pole became a t the same time clear and completely free from colloidal matter. Preparation of the Sol The f6llowing method can be used to prepare the beryllium b&de sol. Three to five ec. of water are added to about one gram of beryllium carbonate (Eimer & Amend C.P.) contained in a small beaker. The carbonate is then dissolved by adding the least possible amount of concentrated hydrochloric acid. After the beryllium salt has completely dissolved, the solution is carefully evaporated over a very small Bunsen flame (not over 2 cm. high). The basic chloride thus formed is heated until perfectly dry and immediately (without removing the flame) 50 cc. of rapidly boiling water is added and the whole boiled for 1-2 minutes. The boiling colloidal suspension is then quickly poured into a 250-cc. Erlenmeyer flask containing 50 cc. of boiling water and all boiled vigorously for 3-5 minutes. The sol, after this boiling, is quickly cooled by immersing the flask as completely as possible in ice water. When cold the sol is filtered or allowed to stand for a day and then decanted. A portion of the basic chloride is not changed to the colloidal oxide but remains as a residue. The sol should have a very distinct opalescent or milky appearance. In order to obtain the best sol for this demonstration the heating and drying of the basic chloride should be done very carefully according to the foregoing directions. Otherwise, a very dilute or unstable sol will result. More dilute sols can be made by using larger volumes of boiling water.
VOL. 6, No. 2 LABORATORY DEMONSTRATION OF COLLOID CATAPHORESIS
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These give similar results but the migration and coagulation are not as easily seen from a distance. Demonstration of Cataphoresis In each arm of an ordinary five inch U-tube is suspended an electrode made of platinum foil. (The photograph shows these sealed into the side arms with sealing wax but this is not necessary.) The tube is then filled with the sol until the electrodes are about one-fifth covered and the apparatuq loosely corked Lo keep out any dust. A difference of potential of twenty' volts is impressed through the sol until the desired amount of coagulation and migration is oh tained which usually takes from :3-
