Diffusion of Potassium Permanganate as a Lecture Demonstration Descriptions of lecture demonstrations of diffusion in liquids are very scarce. J. H. Gardner [J. CHEM. EDUC., 17,494 (1940)l describes the diffusion of fuchsine with t,he use of gelakin to avoid convection currents. A very old classroom demonstration consistn of dropping cryst,als of copper sdfsto into a tall cylinder of water which is mounted in a conspicuous posibion in a. lecture roam. The rfystals, if reasonably large, fall through the water without appreciable solution, form a saturated solution a t the bottom of t,he cylinder and t,hen over a period of many weeks, usually s. full semester, the blue color gradua.11~rises to the tap. A variation of this experiment employs pota?sinm permangmate. The color is darker, but the rate is appraximately the same as for copper sulfate. (This is contrary t,o a statement in t,he film "Molecular Theory of Matter," EBF that diffusion takes place wmpletely in afew hours.) If the demonst,ration ir performed correctly, a full semester is required for any noticeable color to reach the top of the cylinder. For several years we have nsed hot,h copper sulfate and pot,assium permanganate, side by side. A difficulty is encountered with potassium permanganate due to the intense color of dilute sol~rt~ians coupled with the fact that small fluctations in room lemperature cause convection currents which carry the color to the top. In one instance a demonstration was prepared wilh s. sharp boundary between solution and water. With a few degrees fall in room temperat,ure a clearly defined column of colored solut,ion rose along t,he axis of the cylinder while wat,er around this colored column slowly moved downward, remaining perfectly colorless. Directions for this demonstration consist in first filling a 7 X 47-cm cylinder with a solot,ion of a salt which does not react with potrtssium permanganate but which is sufficiently soluble to permit forming a density gradient which will stop convection. Zinc sulfate, sodium nit,rate, and magnesium sulfate have been nsed. (A series of experiments using salt solutions of widely different concentrations showed no effect of concentration on the rate of diffusion of potassium permanganate.) The density gradient is established by half filling the cylinder with a concentrated solution and then adding suffioient water to fill the cylinder. To produce a fairly uniform grsdient the cylinder is tipped untilhhe boundmy between these two layers extends nearly tho full length of the cylinder. llatation of t,he cylinder in this position about its axis serves to produce the desired gradient. Vigorous agitation is to be avoided. A concentrated solution of the salt being used for the density gradient is now satwated with a. gram or two of potassium permanganste and then introduced to the bot,tom of the cylinder wit,h the aid of a long capillary tnhe made by drawing out a test tube to the proper length. With care to avoid trapping bubbles of air which, if introduced with oermanesnate solution. will cause stirrine. a sham houndarv is oroduced. The caoillarv is removed hv att,aching a of the colorless salntian b y the permanganste. The cylinder may then be hung by a wire, to avoid vibration, near the upper edge of a. blackboard a t the front of the room preferably where there is the least temperature fluctuation. I t serves as 8. reminder to a student that a hesvv ion will diffuse uoward in suite of the force of aravitv hut that this diffusion is very slow in a condensed phase. w e have found some stud& ereatlv interested in duolieating this demonstrati& when told of some of the
758
/
Journal o f Chemical Education
