VOL.3, No. 4
APPARATUSFOR DEMONS~TRATION pa %ATAPR~RESIS
443
A SIMPLE APPARATUS FOR DEMONSTRATION OF CATAPHORESIS R . E. GREENFIELD
AND
A. L. ELDER, UNIVERSITYOR ILLlNOIS, URBANA, ILLINOIS
In presentation of the elementary facts of colloid chemistry, a demonstration of the charge on the colloidal particles by means of a cataphoresis experiment is highly desirable. Unfortunately, most devices described
A simple apparatus for demonstration of cataphoresis,
for such experiments are either unreliable, complicated to build, often involving glass blowing which cannot readily be done in most laboratories, or have the boundary of the sol immediately above or below stopcocks where it cannot readily be seen in a class demonstration. A simple device has been developed which, on most of the ordinary sols used, gives a very quick and easy qualitative demonstration of cata. phoresis, which is visible a t a considerable distance from the apparatus.
It consists of an H-shaped piece of glass tubing, two legs of the H being longer than the other two. Glass test tubes or bottles are stoppered onto each of the longer limbs of the H and a suitable wire electrode passed through each stopper. The electrode is wrapped around the portion of the glass tubing projecting through the stopper. Each bottle and the lower halves of the longer limbs of the H are now filled with a N/1000 KC1 solution to which has been added enough sugar to make the solution of an appreciably greater density than the sol to be studied. With gold or other ordinary suspensoid sols 5-10y0 sugar was found to be suitable. The sol to be studied is carefully pipetted onto the sugar-KC1 solution to form a supernatant layer in the upper portion of the H, filling it to a point above the crossbar. If a suitable e. m. f . is impressed on the two electrodes a movement of the two boundaries of the sol is easily noted. With 110 volts d. c. a movement of a quarter of an inch in fifteen minutes was obtained with a negative gold sol and a positive ferric oxide sol. Upon reversal of the polarity, a reverse movement of equal magnitude was demonstrable. Several such devices can be connected in parallel for the comparison of various sols. Since the H-crossbar need not be fused together as shown in the illustration, but may he joined by means of a rubber tubing, such an apparatus can be assembled in a laboratory from ordinary T-tubes. Simple copper electrodes give very good qualitative results. It is true that a small amount of gas is liberated a t such electrodes, which tends to introdnce an error in the movement of the boundaries of the sols. The magnitude of this error is not great. Silver-silver chloride electrodes can be prepared which do not liberate gas. For ordinary lecture demonstration the simple copper electrodes are to be preferred.
