NCSU – Dept. of Chemistry – Lecture Demonstrations

Electrolytic Cell (KI)

Description: An electrolytic cell is constructed using an aqueous of KI and a 6-V battery as the energy source.

Materials:

1 M K I Petri dish Phenolphthalein 6-V battery 2 platinum wires

Procedure:

For large lecture halls, perform demonstration on overhead projector.

1. Pour 1 M K I solution in Petri dish. Add two drops of phenolphthalein indicator.

2. Connect platinum wires to battery terminals. Immerse these Pt into the K I solution and observe the formation of I2 at the and the 1- generation of OH and H 2 at the .

Discussion:

In this electrolytic cell, two separate half reactions occur at each . The 1- iodide anion ( I ) 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 1- 1- 1- triiodide ions ( I3 ). This reaction occurs as follows: I2 (aq) + I (aq) → I3 (aq) and can be separately tested using a standard “starch” test. At the cathode the reduction of in to H 2 gas is confirmed by the evolution of gas coupled with the observation of a pink color in solution, indicative of OH 1- in the presence of phenolphthalein indicator. These two half-reactions and the overall reaction are shown below:

o E red (V) 1- 1- Anode 2 I → I2 + 2 e + 0.54 1- 1- Cathode 2 H 2O + 2 e → 2 OH + H 2 - 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