Reference electrodes are crucial tools in electrochemistry, providing a stable baseline for measuring electrode potentials. They come in various types, each with unique characteristics and applications. Understanding their construction and proper use is essential for accurate measurements.
Selecting the right reference electrode and maintaining it properly are key to obtaining reliable results. Factors like potential range, electrolyte compatibility, and temperature must be considered. Proper care ensures stability and accuracy in electrochemical experiments.
Reference Electrodes
Common reference electrodes
- Standard Hydrogen Electrode (SHE) serves as the primary reference for electrode potential measurements and is assigned a potential of 0.00 V under standard conditions (1 atm H2 pressure, 1 M H+ concentration, 25°C)
- Saturated Calomel Electrode (SCE) consists of mercury, mercury(I) chloride (calomel), and a saturated potassium chloride solution, exhibiting a stable potential of +0.244 V vs. SHE at 25°C
- Silver/Silver Chloride Electrode (Ag/AgCl) is composed of a silver wire coated with silver chloride immersed in a potassium chloride solution, with a potential of +0.197 V vs. SHE at 25°C in saturated KCl
- Mercury/Mercury Sulfate Electrode (MSE) contains mercury, mercury(I) sulfate, and a saturated potassium sulfate solution, having a potential of +0.640 V vs. SHE at 25°C
Construction of reference electrodes
- Standard Hydrogen Electrode (SHE)
- Platinum electrode immersed in an acidic solution (1 M H+)
- Hydrogen gas bubbles over the electrode at 1 atm pressure
- Hydrogen gas oxidizes at the electrode surface: $H_2 \rightarrow 2H^+ + 2e^-$
- Saturated Calomel Electrode (SCE)
- Mercury pool at the bottom
- Paste of mercury(I) chloride (calomel) above the mercury pool
- Saturated potassium chloride solution
- Electrode reaction: $Hg_2Cl_2 + 2e^- \rightarrow 2Hg + 2Cl^-$
- Silver/Silver Chloride Electrode (Ag/AgCl)
- Silver wire coated with a thin layer of silver chloride
- Immersed in a potassium chloride solution
- Electrode reaction: $AgCl + e^- \rightarrow Ag + Cl^-$
- Mercury/Mercury Sulfate Electrode (MSE)
- Mercury pool
- Paste of mercury(I) sulfate
- Saturated potassium sulfate solution
- Electrode reaction: $Hg_2SO_4 + 2e^- \rightarrow 2Hg + SO_4^{2-}$
Measurement of electrode potentials
- Connect the reference electrode and the working electrode to a high-impedance voltmeter or potentiostat to minimize current flow and maintain stable potentials
- Ensure proper electrical contact between the electrodes and the solution to allow accurate potential measurement
- The measured potential is the difference between the working electrode potential and the known reference electrode potential (E(measured) = E(working) - E(reference))
- A positive measured potential indicates the working electrode is more positive (or less negative) than the reference electrode (e.g., measuring the potential of a zinc electrode vs. SHE)
- A negative measured potential suggests the working electrode is more negative (or less positive) than the reference electrode (e.g., measuring the potential of a copper electrode vs. SHE)
Selection of reference electrodes
- Reference electrode selection depends on factors such as the potential range, electrolyte compatibility, temperature, and pH of the system under study
- For aqueous solutions, Ag/AgCl or SCE are often used due to their stability and compatibility
- For non-aqueous solvents, pseudo-reference electrodes (e.g., silver wire) may be employed
- Proper maintenance ensures the stability and reliability of the reference electrode
- Regularly check for contamination, damage, or depletion of the filling solution
- Replace the electrode or the filling solution when necessary to maintain accuracy
- Incorrect reference electrode selection or poor maintenance can lead to inaccurate potential measurements and erroneous conclusions
- Using a reference electrode with an unstable or poorly defined potential introduces significant errors
- Contamination of the reference electrode by the test solution can cause a shift in the reference potential
- Maintaining a constant temperature is crucial, as temperature fluctuations can alter the reference electrode potential (e.g., a 1°C change can shift the potential by ~1 mV)