What prevents infinite potential with Nernst Equation

[Gama]

The nernst equation has a log Q term. The denominator of that could be very close to zero. That would make Q close to infinity and log Q close to infinity. How high can the EMF be due to concentration in the real world and why?

 

[Andy Resnick]

The Nernst equation is $E = \frac{R T}{z F} \ln\frac{[\text{ion outside cell}]}{[\text{ion inside cell}]}$.

As the concentration of ions on one side of the membrane goes down (towards zero), the driving force on ions on the other side of the membrane goes up- that's simple diffusion. Since the log function is sublinear, I'ts not clear what you are having problems with.

 

[Gama]

I wasn't thinking of membranes in particular. The log of infinity is infinity. My guess is the limiting factor is some other half reaction or diffusion of ions from one side of a battery to the other. Even if this is the case the Nernst equation makes it appear that a much higher voltage can be obtained from a concentration gradient than I think can occur.

 

[invisigo]

I would imagine the Nernst equation loses accuracy at low ion concentrations. The physics would change and the equation would no longer be correct.

Wiki (not the most authoritative source, I realize) seems to agree:


At very low concentrations of the potential determining ions, the potential predicted by Nernst equation tends to ±infinity. This is physically meaningless because, under such conditions, the exchange current density becomes very low, and then other effects tend to take control of the electrochemical behavior of the system.

 

[Shaybay92]

How would you explain a 0.1V current when the concentration of copper ions in solution is 0M ?

This is in a zinc || copper galvanic cell (with salt bridge) and the [Zn2+] is 1M?

I thought that the copper ions attract the electrons? Am I mistaken?