The reference electrode value of Ag/AgCl is 0.197 V with respect to the Standard Hydrogen Electrode (SHE) at 25°C.
This value is derived from the half-cell reaction involving silver chloride and silver.
The silver wire is coated with a layer of solid silver chloride and immersed in a saturated solution of KCl and AgCl.
The stability and slight temperature dependence of the potential make the Ag/AgCl electrode a popular choice in electrochemical analysis.
Key Points Explained:
Composition and Structure of Ag/AgCl Electrode
- Silver Wire Coated with Silver Chloride: The electrode consists of a silver wire coated with a layer of solid silver chloride (AgCl).
- Saturated KCl and AgCl Solution: The silver wire is immersed in a solution that is saturated with both potassium chloride (KCl) and silver chloride (AgCl).
Half-Cell Reaction
- Reaction and Potential: The pertinent half-reaction is: [ \\text{AgCl(s)} + \\text{e}^- \\rightleftharpoons \\text{Ag(s)} + \\text{Cl}^-(sat'd) ] with a potential of 0.197 V with respect to the SHE at 25°C.
- This value differs slightly from the standard potential (E0) for the couple (0.222 V) due to the contribution of both KCl and AgCl to the chloride activity.
Stability and Temperature Dependence
- Stable Half-Cell Potentials: Both the Saturated Calomel Electrode (SCE) and the Ag/AgCl reference electrodes offer stable half-cell potentials that do not change over time.
- Temperature Dependence: The potential of these electrodes changes by approximately 0.5 – 1.0 mV/°C, indicating a slight temperature dependence.
Use in Non-Aqueous Systems
- Not Recommended for Non-Aqueous Work: While it is possible to use aqueous Ag/AgCl electrodes for short periods with non-aqueous solutions, the long-term results are not trustworthy.
- Using aqueous electrodes introduces undefined, variable, and unmeasurable junction potentials due to the liquid-liquid junction and different ionic compositions.
- Direct Comparison: Potentials measured in different solvents are not directly comparable, making the use of reference electrode in non-aqueous systems unreliable.
Practical Considerations
- Vycor Frit for Isolation: To prevent interaction between the Ag wire and the analyte solution, a vycor (porous glass) frit is used to isolate the reference electrode while maintaining electrical contact.
- Storage and Maintenance: Commercially available reference electrode calomel / silver chloride / mercury sulfate should be stored in the dark and submerged in solutions identical to the solution inside the reference electrode, usually saturated KCl.
- Testing Vycor Frit Integrity: The integrity of a vycor frit can be tested by attempting to squeeze liquid through it using a pipette bulb. If fluid filters through easily, the vycor frit should be replaced.
Potential Measurement
- Versus SHE: The reference electrode calomel / silver chloride / mercury sulfate provides 0.197 V versus the Standard Hydrogen Electrode (SHE) at 25°C.
- Practical Usage: Hydrogen electrodes are used more as a theoretical standard than practically across laboratories.
- Therefore, reference electrodes are often checked against other electrodes with the same reference system or against other references with known potentials.
By understanding these key points, a lab equipment purchaser can make informed decisions about the use and maintenance of reference electrode calomel / silver chloride / mercury sulfate, ensuring accurate and reliable measurements in electrochemical analysis.
Key Benefits:
- Precision: Ag/AgCl electrodes offer stable, reliable potential values for accurate electrochemical analysis.
- Versatility: Ideal for both aqueous and certain non-aqueous systems.
- Longevity: Low temperature dependence ensures stability over time.
- Practical Storage: Easy maintenance and storage for optimal performance.
Unique Selling Points:
- KINTEK SOLUTION's expertise in laboratory equipment ensures you receive top-tier reference electrode calomel / silver chloride / mercury sulfate.
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