Lemon Cell Experiment

1. Aim

To construct a lemon cell and measure the EMF produced by different metal electrode pairs using one SEELab input channel.

2. Apparatus / Components Required

3. Theory & Principle

A lemon acts as an electrolyte. Two dissimilar metals inserted into the lemon form a galvanic cell.
The open-circuit voltage (EMF) depends on the electrode pair and electrolyte condition.

For this experiment, use one channel (A1 or A3) and measure with respect to GND.

Expected EMF trend follows metal reactivity difference:

Always measure voltage with respect to GND: \(V_{\text{measured}} = V(\text{Input}) - V(\text{GND})\)

4. Circuit Diagram / Setup

  1. Select one input channel (A1 recommended; A3 is also fine here).
  2. Insert two different metal electrodes into the lemon, separated by ~2-3 cm.
  3. Connect the electrode expected to be negative (e.g., zinc) to GND.
  4. Connect the other electrode (e.g., copper) to the selected input.
  5. Open DC voltage measurement in the SEELab software/app.

5. Procedure

  1. Start with the Zn-Cu pair and note the voltage.
  2. Swap leads once and confirm sign reversal.
  3. Repeat with other pairs (Fe-Cu, Al-Cu, Zn-Fe, etc.).
  4. For each pair, wait 5-10 seconds for reading to stabilize and record value.
  5. Optional: connect two lemon cells in series and verify voltage addition.
Mobile App

Mobile App

Desktop App

Desktop App

Mobile App

6. Observation Table

Metal Pair Typical EMF Range in Lemon (V) Measured Voltage (V) Remarks
Zn-Cu 0.8 to 1.1    
Al-Cu 0.6 to 1.0    
Fe-Cu 0.4 to 0.8    
Zn-Fe 0.2 to 0.5    
Al-Fe 0.1 to 0.4    

7. Reference EMF Values (Common Metals)

Approximate standard electrode potentials (vs SHE, at 25 C):

Metal / Electrode $E^\circ$ (V)
Zn / Zn$^{2+}$ -0.76
Al / Al$^{3+}$ -1.66
Fe / Fe$^{2+}$ -0.44
Cu / Cu$^{2+}$ +0.34

Approximate ideal EMF for common pairs (difference of $E^\circ$):

Pair (Anode-Cathode) Ideal $\Delta E^\circ$ (V)
Al-Cu 2.00
Zn-Cu 1.10
Fe-Cu 0.78
Al-Fe 1.22
Zn-Fe 0.32

In real lemon cells, measured values are usually lower due to internal resistance, polarization, oxide layers, and non-standard ion concentrations.

8. Advanced: Estimating Internal Resistance of Lemon Cell

Model the lemon cell as an ideal source $E$ in series with internal resistance $r$.

When connected to a voltmeter of input resistance $R_{in}$, measured terminal voltage is: \(V = E\cdot\frac{R_{in}}{r+R_{in}}\)

For this experiment:

Steps

  1. Build one lemon cell (for example Zn-Cu).
  2. Measure open terminal voltage with A1: call it $V_{A1}$.
  3. Measure same cell with A3: call it $V_{A3}$.
  4. Use the formula below to estimate $r$.

From two readings: \(r=\frac{R_{A1}R_{A3}(V_{A3}-V_{A1})}{V_{A1}R_{A3}-V_{A3}R_{A1}}\)

Then estimate EMF: \(E=V_{A1}\left(1+\frac{r}{R_{A1}}\right)\)

Worked Example

Suppose measured values are:

Using $R_{A1}=1M\Omega$, $R_{A3}=10M\Omega$: \(r=\frac{(1)(10)(0.90-0.60)}{(0.60)(10)-(0.90)(1)}M\Omega =\frac{3}{5.1}M\Omega\approx0.588M\Omega\)

So internal resistance is about: \(r\approx5.9\times10^5\Omega\)

Now EMF: \(E\approx0.60\left(1+\frac{0.588}{1}\right)\approx0.95V\)

9. Error Analysis

Possible causes of small mismatch:

10. Results and Discussion

11. Precautions

  1. Keep electrode tips clean for repeatable readings.
  2. Do not let electrodes touch each other inside the lemon.
  3. Use common GND reference and firm connections.

12. Troubleshooting

Symptom Possible Cause Corrective Action
Reading near 0V Same metal used / short / wrong wiring Use dissimilar metals; recheck GND and input wiring
Unstable reading Loose clips / dry lemon region Reinsert electrodes, moisten area, tighten contacts
Too low voltage Oxidized electrodes / weak acidity Clean electrodes and try fresh lemon
Wrong polarity sign Leads reversed Swap electrode connections

13. Viva-Voce Questions

Q1. Why does a lemon cell produce voltage?

Ans: Two dissimilar metals in an electrolyte form an electrochemical cell. Their different electrode potentials create an EMF.

Q2. Why is Zn-Cu usually higher than Zn-Fe in a lemon cell?

Ans: The electrode potential difference for Zn-Cu is larger than Zn-Fe, so EMF is generally higher.

Q3. Why are measured lemon-cell voltages lower than ideal EMF values?

Ans: Due to internal resistance, polarization, oxide layers, and non-standard chemical conditions.

Q4. If a 3V source is connected to A1 through a 1MΩ resistor, what is the displayed voltage (A1 input impedance = 1MΩ)?

Ans: Using voltage divider: $$ V_{A1}=3\times\frac{1}{1+1}=1.5V $$ Displayed value is approximately 1.5V.

Q5. For the same setup, what is displayed on A3 (input impedance = 10MΩ)?

Ans: $$ V_{A3}=3\times\frac{10}{1+10}=3\times\frac{10}{11}\approx2.73V $$ A3 shows approximately 2.73V.