NCSU – Dept. of Chemistry – Lecture Demonstrations
Electrochemistry
Electrolytic Cell (KI)
Description: An electrolytic cell is constructed using an aqueous solution of KI
and a 6-V battery as the energy source.
Materials:
1 M KI
Phenolphthalein
Petri dish
6-V battery
2 platinum wires
Procedure:
For large lecture halls, perform demonstration on overhead projector.
1. Pour 1 M KI solution in Petri dish. Add two drops of phenolphthalein
indicator.
2. Connect platinum wires to battery terminals. Immerse these Pt electrodes
into the KI solution and observe the formation of I2 at the anode and the
generation of OH1- and H2 at the cathode.
Discussion:
In this electrolytic cell, two separate half reactions occur at each electrode. The
iodide anion (I1-) is oxidized to I2 at the anode. This is confirmed by the
appearance of a yellow-brown color in solution which is due to the formation of
triiodide ions (I31-). This reaction occurs as follows: I2 (aq) + I1- (aq) → I31- (aq)
and can be separately tested using a standard “starch” test. At the cathode the
reduction of hydrogen in water to H2 gas is confirmed by the evolution of gas
coupled with the observation of a pink color in solution, indicative of OH1- in the
presence of phenolphthalein indicator. These two half-reactions and the overall
redox reaction are shown below:
Anode
Cathode
2 I → I2 + 2 e
2 H2O + 2 e1- → 2 OH1- + H2
1-
1-
Eored (V)
+ 0.54
- 0.41
Safety: Wear proper protective equipment including gloves and safety glasses
when preparing and performing this demonstration (I2 may stain the skin). Once
electrodes are connected to the battery terminal, do not hold both wires with bare
hands as this will complete the circuit and may cause burns.
NCSU – Dept. of Chemistry – Lecture Demonstrations
Electrochemistry
Disposal: Dispose of remaining solution down the drain with plenty of water.
References:
Shakhashiri, B. Z. In Chemical Demonstrations: A Handbook for Teachers of
Chemistry; The University of Wisconsin Press: 1992; Vol. 4, p 174-180.
Video:
http://www.youtube.com/watch?v=LGwPfo_eunY