ADSORPTION OF A COLLOID Submztted by: Samuel Weiner, University of Wisconsin, Extension Center, Wausau, Wisronsin C h ~ c k r dhy: George Ferris, Mirhigan State University, East Lansing
PREPARATION diI'rovide a solution of Congo Red indicator (O.lyo), lute HCI, three beakers, activated charcoal, two funnels fit,ted with qualitative type filter paper and appropriate supports, and two beakers as receivers. DEMONSTRATION Add equal quantities of Congo Red indicator solution t,o each of three beakers and remind the class that an indicat,or has two color forms, one being the conjugate acid of the other. Keep the first beaker for comparison. Add dilute HCl drop by drop with stirring to the second beaker until the color is so deeply blue that no pink is visihle a t the rim of the solution. This process requires some experience, hnt one can teat. for complete conversion by dipping into the solut,ion a strip of filter paper and noting if the blue region on the paper has a pink horder. If the paper shows a pink border, add more acid to the beaker. During this operation tell the class what you are doing, and emphasize that complete con-
version of the Congo Red requires a definite amount of acid. To the third beaker add half the qnant,ity of acid which was added to the second beaker. The solution a t this point should be dark, but the rim of the solution will be pink. Explain t o the class that the conjugate acid form of Congo Red associates to a colloid and that colloidsaremoreeasily adsorbed than low-molecular-weight solutes. Add charcoal to the second and third beakers, discussing the adsorptive powers of charcoal; and filter the contents of each of the beakers through qualitat'ive type filt,er paper into the rereivers so the class can easily see the color of the filtrate. REMARKS The second beaker should give a clear filtrate but the third should show a pink filtrate, weaker in color than the solution in the first (control) beaker. Explain again that the colloidal acid form was completely adsorbed from both beakers that had been treated with charcoal. but the red form was only slightly adsorbed.
Journal oi Chemical Education
Catalytic Oxidation of Manganse(ll) Sulfate
(A
J d y 1382
@efl Ed,
Szlbmztted by: Fidel Villarreal G., Institute Tecnologico de Monterrey, Monterrey, N. L., Mexico Checked b?j: Carl H. Rruhaker, Mirhigan State University, East Lansing
PREPARATION lUat,erials reqnired: manganese(I1) sulfate, potassium persulfate, silver nitratr d u t i o n (.5O/O). two Pyrex test t,ubes, and a hurner. DEMONSTRATION Place 0.5 g of manganese(I1) sulfate into each test tube. Dissolve the powder by shaking with 15 ml of wat,er, then add a few crystals of potassium persulfate to each t,uhe. Add a few drops of silver nitrate solution to one test tube and stir both solutions. Observe. The solut,ion that, does not contain silver ions does not become rolored until it is heated.
REMARKS The solution containing silver ions will tnrn brown, and a brown precipitate of manganese dioxide mill appear in a few seconds. The ot,her solution will not show a color change until heat,ed. The reartion is indicated in the following equation: This demonstration shows easily the catalytic effect of Ag+ in the oxidation of Mn+Zhy S208-2. The catalysis is probably effected by the formation of some .\g(II) complex. whirh then oxidizes the manganese.
Journal a/ Chemicol Edvcalion
.
July 1.983
