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DIFFUSION OF ELECTROLYTES: Measuring diffusion potential
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DIFFUSION OF ELECTROLYTES: Measuring diffusion potential
Diffusion potential
The merging electrolytes with different concentrations (activities) may lead to the development of an electric potential gradient, the DIFFUSION POTENTIAL (or more exactly diffusion voltage)
Laboratory report (2)
- To measure the potential difference we have to know the concentrations of the electrolytes.
- Then plot a graph of Ed against logarithm of the concentrations.
Laboratory report (1)
Diffusion
Materials and method:
- Millivoltmeter
- Measuring chambers
- HCl solutions of different concentration
- Membrane pump.
Diffusion is always directed from higher to lower concentrations.
- If anion and cation mobilities are the same --> jointly diffusion --> NO voltage
- If anion and cation mobilities differ --> the more mobile ions lead the way and the less mobile ions lag behind --> order in distribution of the opposite charge carriers --> potential difference develops.
- If anions are more mobile --> more dilute side will be at - potential.
- If cations are more mobile --> more dilute side will be at + potential.
The diffusion potential persists until the concentration difference becomes 0.
Laboratory report (3)
Nernst-Plank electrodiffusion equation
- To calculate the ratio:
- According to the equation:
As ions start moving and after some time, ions establish a potential difference at the boundary.
As shown in the diagram:
- Potential difference initially increases (- will accelerate and + will decelerate).
- After some time the - and + fluxes slow down and cause the potential difference to remain constant (after reaching the maximum Ed).
- It will remain in stationary conditions briefly.
- After some time the concentrations will balance each other, so the potential difference starts to decrease until it reaches 0.
Derivation to Henderson equation
According to Einstein equation:
--> potential gradient
a --> average slope of the obtained straight line
--> concentration gradient
F --> Faraday's constant
u --> electrophoretic mobility
z --> ion valency
D --> Diffusion coefficient