overhead projector dernon/tration/ Electrolytic Migration of Ions
edited by DORIS KOLB Bradley University Peoria. iL 61625
Photochemical Reduction of Fe3+ by Citrate Ion
J u a n A. Llorens-Molina CEP Gandia Sant Duc, 1 Gandia 46700. Spain
J u a n A. Llorens-Molina CEP Gandia Sant Duc. 1 Gandia 46700. Spain
T h e migration of colored ions across a s t r i p of wet filter paper when a n electric field is applied has been used for showing the electric n a t u r e of matter. T h i s demonstration uses a t h i n layer of gelatin i n place of t h e filter paper so that t h e migration c a n h e viewed using a n overhead projector.
Fe3+ is reduced by citrate ion when it is exposed to light. Fe2+ is detected b y t h e formation of blue iron(I1) hexacyanoferrate(II1) complex.
Equlpment Petri dish Porous pieees of synthetic sponge 250-mL beaker Power source and electric wires with alligator clips Paper clips Procedure Mark a line across the diameter of a Petri dish (A), using a glass marking pen or a wax pencil. Attach with glue two porous pieces of synthetic sponge (B) as shown in Figure 1. Prepare gelatin by dissolving 3 gin 100 cm:' ofvery hot (90 'C) 5% KNOSsolution. Pour the gelatin onto the Petri dish forming a thin layer (1-2 mm). Before this layer gels, place a small crystal of a salt that contains colored ions (such as C r 0 . P ) over the diameter line. or. after the eelatin has set, add 1or 2 drops of a concentrated solution of ions. In the latter instance, the excess liquid should be absorbed with a piece of filter paper. To begin the demonstration, pour a few draps of the electrolyte solution (5%KNO:J onto the porous pieces. Insert two paper clips in the lateral wall of a Petri dish so that they are in contact with the porous pieces, as is shown in Figure 2, and connect to a 0-100-V, 50mA ( d . 4 electric power source. Close the circuit and raise the voltage to 40-50 V. After 3-4 min the migration of ions is appreciable. Simultaneously, a deformation is produced in the gelatin layer because the migration of ions causes an optical effect on the overhead projector. Certain ion migrations can be seen by adding suitable reagents to the eelatin solution. For example, 2-3 drops of KCNS gives a red colo; to Fe:'+ ions
FIG.l
Figure 1. Construction of Petri dish apparatus.
1090
Journal of Chemical Education
Equipment Two Petri dishes Two 10- X 20-em dark cardboard sheets Droppers Reagents Potassium hexacyanaferrate(IlI), 1% Sodium citrate, 1% Iron(ll1) chloride, 1% Procedure Place on the overhead projector the two Petri dishes. One of them will be sandwiched between sheets of dark cardboard to block out the light. Pour intoeachpetridish 30 mLofwater and afew dropsof iron(II1) chloride and potassium hexacyanoferrate(ll1). To the Petri dish sandwiched between sheets of dark cardboard add a few drops of sodium citrate. Cover a t once with the top sheet. Add the sodium citrate to the other Petri dish. After a few minutes. tnkr nnny the tuc,cnrdhltsrrl ~ h e e t si.nd ohierve the diiicrent cokrntiun. In thr presmct of hght. thp %.itrateibn redurrs Fr ' to Fr ', u , h ~ hwmts rvuh the hrxnryan~~frrratc(I1l~ ion, rausi~ip.the nrlu. tian to turn blue. The 1% sodium citrate solution can be replaced by adding 4-5 drops of lemon juice to 10 mL of water.
Figure 2. Set-up for the demonstration (below)and resultant migration of ions (above).
