Convert AG/AgCl to RHE: A Comprehensive Guide
To understand how to convert the reversible hydrogen electrode (RHE) scale to the-agligan potential (AG/AgCl), we must first delve into the fundamentals of both scales and the Nernst equation, which facilitates the conversion.
The Reversible Hydrogen Electrode (RHE)
RHE is one of the most frequently used reference electrodes in electrochemistry. It operates at Standard Hydrogen Electrode (SHE) conditions, which are set at 1 atmosphere of hydrogen gas at 25°C, with a pH of 0.
The potential of RHE is zero by definition at these standard conditions. Any deviation from these conditions, however, necessitates a conversion to RHE.
RHE under non-standard conditions
When the electrochemistry experiment is conducted under non-standard conditions (temperature, pressure, or pH), the potential of RHE varies. To maintain consistency and comparability, we need to convert these potentials to the RHE scale.
For instance, at room temperature (25°C) and atmospheric pressure, the RHE potential at standard pH (7) can be calculated as:
E(RHE) = E(SCE) + 0.242V - 0.059xpH
Converting to the Ag/AgCl Reference Scale
A silver/silver chloride (Ag/AgCl) electrode is commonly used in real experimental conditions due to its stability and closeness to the standard hydrogen electrode (SHE). However, its potential is usually expressed in the Ag/AgCl scale, necessitating a conversion when RHE scale is desired.
The first step is to measure or find the E(Ag/AgCl) under your experimental conditions (temperature and pH).
Conversion at room temperature
At 25°C, the conversion formula is:
E(RHE) = E(Ag/AgCl) + 0.197V - 0.059xpH
Here, the terms 0.197V and 0.059xpH represent the junction potential and the change in pH from the standard pH of 0, respectively.
Conversion at other temperatures
At temperatures other than 25°C, the Nernst equation comes into play.
The Nernst equation, ∆E = -(RT/nF)Ln(q), where R is the universal gas constant, T is the temperature in Kelvin, n is the number of electrons, and F is the Faraday constant, q is the activity quotient, can be rearranged to find ∆E.
Substituting the values and simplifying, we get the formula:
∆E = (0.059x273.15/T) x ∆pH
Therefore, the conversion formula becomes:
E(RHE) = E(Ag/AgCl) + 0.205 - ∆E
With this comprehensive approach, you should now be able to accurately convert the AG/AgCl to the RHE scale, ensuring consistency and comparability in your electrochemistry experimentation. Keep furthering your understanding, and happy experimenting!