Electrochemistry: LeClanché-Element (Zinc-Chloride Battery)

Computer-Interfaced Experiments - Voltage Measurement

Electrochemistry
Model - Leclanché Element (Zinc-Chloride Battery)

Objective: Dependence of Discharge Time on the Discharge Current Strength

Peter Keusch

Datalogging using the Program CHEMEX and the Analog-Digital-Converter CHEMBOX
IBK electronic + informatic

German version

Chemicals:
20 % ammonium chloride solution
manganese dioxide (powder)
graphite powder

Apparatus and glass wares:
beaker 100 mL
beaker 250 mL
zink folio 40 mm × 75 mm × 1 mm
carbon rod d = 5 mm
extraction "thimble" 100 mm × 30 mm
ammeter
voltmeter
switch
slide rheostat 100 W
path cords
clamps

Hazards and safety precautions:

Manganese dioxide may be harmful by ingestion, inhalation or through skin contact.

Safety glasses and protective gloves should be worn. Good ventilation required.

Theoretical background (Download)

Preparation:

The zinc sheet, rolled into a cylinder shape lies close to the inner wall of a 100 mL beaker. Using a 250 mL beaker, 7 g of graphite powder are well mixed with 40 g of manganese dioxide powder with a glass rod. The mixture is mixed with 40 mL of 20% ammonium chloride solution to a paste. The moist paste is pressed firmly and tightly into a extraction "thimble" (or porous cylinder). A carbon rod is inserted into a one-holed rubber stopper which fits the extraction "thimble". The extraction "thimble" is placed into the beaker fitted with the zinc folio. The beaker is filled with 20 % ammonia chloride solution. Using a stand and a clampk, the carbon rod is placed vertically in the center of the extraction "thimble" (Fig. 1).

The zink cylinder is connected to the negative terminal of the CHEMBOX 'Input Sensor1' - the carbon rod is connected to the positive terminal. The matching of the measuring program CHEMEX is performed as described in experiment Edison accumulator.

Fig. 1: Leclanché cell     Discharging
1: Zink folio     2: Ammonium chloride solution     3: Carbon rod
4: Extraction "thimble"     5: Graphite-Manganese dioxide-paste)

Experimental procedure:

In order to discharge the cell, the electrodes are connected in a circuit arrangement. The circuit includes a switch, a slide rheostat and an ammeter (Fig. 1). The slide rheostat is adjusted such that the desired discharge current is at a constant value. The change in voltage is logged with an interval of 2 seconds.

Measurements:

· Immediately after the set-up the cell is discharged for the first time. The switch is turned and the voltage measurement using Chemex is started. The slide rheostat is adjusted such that a current of 20 mA flows through the circuit. After 8 minutes the current flow is interrupted and for approx. For 2 minutes the recovery process of the battery is followed by measuring the change in voltage.
· five days after the set-up of the cell
First, the cell is discharged within 8 minutes with a current of 10 mA. The electrical circuit is interrupted and for 2 minutes the restoring of the cell is followed again. When the voltage has increased to 1.4 V, the cell is discharged at 20 and 30 mA, respectively.

Fig. 2: Realtime graph     Discharge of a Leclanché cell at 10 mA
five days after the set-up of the cell

Data analysis using the spread sheet program Microsoft Excel:

Fig. 3: Discharge of a Leclanché Cell at 20 mA
1: immediately after the set-up     2 5 days after the set-up

Fig. 3 shows that the cell (2) has a higher voltage during the same discharge process, as the cell discharged immediately after the set-up (1). After interrupting the external electrical circuit the two cells are restored however at the same rate.

Fig. 4: Discharge of a Leclanché Cell (5 days after the set-up of the cell)
at   10 (1)     20 (2) und   30 mA (3)

The voltage of the Leclanché cell is approx. 1.4 volts. After closing the electrical circuit, the voltage decreases immediately, depending on the strength of the discharge current. After the abrupt decrease the voltage of the cell is constant. After 8 minutes the electrical circuit is interrupted. The voltage jumps up during the restoring phase (Fig. 4). The restoring of the battery is dependent on the strength of discharge current. The Leclanché cell proves as a not constant battery.

Fig. 5: Plot of discharge current versus the cell voltage measured after 8 minutes

A linear relation is evidently between discharge current and the voltage of the discharged cell. At 10 mA the discharged battery has a voltage, twice as much as during the discharge at 20 mA and three times as much as during the discharge at 30 mA (Fig. 5).

Reference:
Pioneers of Electricity - Georges Leclanché

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