2
$\begingroup$

Recently I conducted a lab in which the voltage of a galvanic cell between two metals was recorded. Both metals were placed in an solution of their own ions with nitrate and a salt bridge was made by dipping some paper in salt water. The only issue is the voltage recorded in the cell was lower than that calculated using standard reduction and oxidation potentials. Does anyone have any ideas as to why this might be? I known the metals were not super clean although I don't think that would have an affect on the voltage.

$\endgroup$
2
$\begingroup$

Although it seems simple to make a galvanic cell ( i.e. a battery) it is really fairly complex.

  • The theoretical voltage is given by the Nernst equation.
  • The actual measured voltage also depends on the current. The equation $V=iR$ still applies. So various restance sources in the cell cause voltage drops.
    • The current must be very low so a high impedance voltmeter must be used. (Not really a problem these days with modern semiconductor
      voltmeters as opposed to galvanometers.)
    • The salt bridge offers resistance, so it has a voltage drop.
    • Impure metals for the anode or cathode can cause a voltage change.
    • Surface coating can cause a voltage drop.
    • The acid or base concentration can cause a voltage change.
    • The ionic strength of the solution can change the voltage.
$\endgroup$
-1
$\begingroup$

There are more factors to consider when determining the true voltage of the cell such as the development of overpotentials.

'In electrochemistry, overpotential is the potential difference (voltage) between a half reactions thermodynamically determined reduction potential and the potential at which the redox event is experimentally observed.

In a galvanic cell overpotential means less energy is recovered than thermodynamics predicts. This extra/missing energy is lost as heat.

Overpotential is specific to each cell design and varies across cells and operational conditions, even for the same reaction.

Regarding the galvanic cell, two possible polarities of overpotential can occur:

  • the anode may be less negative, supplying less energy than thermodynamically possible.

  • the cathode is less positive, supplying less energy than thermodynamically possible.

The overpotential increases with growing 'current density', or rate as described by the 'Tafel equation'. Each step of the electrochemical reaction is associated with multiple forms of overpotential. The overall overpotential is the summation of many individual losses.

'Voltage efficiency' describes the fraction of energy lost through overpotential. For a galvanic cell it is the ratio of a cells experimental potential divided by the cells thermodynamic potential converted to a percentile.

Overpotentials can be grouped into three categories, activation, concentration and resistance but can be divided into many different sub categories also that are not well defined, such as 'polarization overpotential'.'

Ref:overpotential/wikipedia

For more information on 'voltage efficency','Tafel equation','current density' I suggest to 'Google' them or to look at the page I have referenced.

Other factors such as loss of electrolyte and the temperature of the system can also cause loss of voltage.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.