Why is My Voltage Lower than Standard Cell Potential?

[Calle]

I have set up a Galvanic cell and used filter paper that is soaked in KCl as salt bridge. The electrolyte I used is 1M and the experiment is carried out in room temperature. However, I am unable to obtain the standard cell potential. The voltage values I obtained was lower.

I was thinking maybe the salt bridge is not good enough, so I added more filter paper strips. The voltage did increase, but I thought the salt bridge should not affect the results?

Is it impossible to obtain the standard cell potential? Could it be the resistance of the voltmeter? Thanks!

 

[chemisttree]

How far off are you? What two metals are you using?

 

[JGM_14]

try this http://www.science.uwaterloo.ca/~cchieh/cact/c123/halfcell.html"

 

[Calle]

Thanks!

I am using copper and zinc in copper (II) sulphate and zinc sulphate solutions. The voltage i obtained is about 0.3 volt away from the standard 1.1 volt after i added more layers of filter papers.

I tried to decrease the zinc ions in the solution by diluting the solution and the voltage increased. Why is that so?

 

[chemisttree]

I am using copper and zinc in copper (II) sulphate and zinc sulphate solutions. The voltage i obtained is about 0.3 volt away from the standard 1.1 volt after i added more layers of filter papers.

It could be that the salt bridge is not very conductive for some reason. Adding additional papers lowers the resistance of the circut.

I tried to decrease the zinc ions in the solution by diluting the solution and the voltage increased. Why is that so?

In this redox couple which metal is being oxidized (solubilized) and which is reduced? For the metal that is oxidized, the soluble ion of that metal represents the product of the reaction. How might removing (through dilution) the product of the oxidation (soluble metal ion) affect the reaction? Review the Nernst equation.

 

[Renge Ishyo]

>>> Is it impossible to obtain the standard cell potential?

No, but if I can remember correctly this should only occur briefly. When both Copper and Zinc are both 1M in solution it should be at standard potential...as soon as the current starts running the concentration of one ion in solution increases while the other decreases...the moment that happens the observed voltage starts to change away from the standard potential (when you changed the zinc concentration in solution a similar effect occured). As chemistree said the Nernst equation describes this.

If you are far off from expectations, it might be wise to check the copper and zinc solutions you are using for the inital voltage check (check the molarity of each with titration and very carefully prepare each at 1M for the initial voltage reading...probably this would be the source of error).