Cathode/Anode confusion (electric circuits vs electrophoresis)

Discussion in 'MCAT Study Question Q&A' started by unsung, May 6, 2008.

  1. unsung

    Joined:
    Mar 12, 2007
    Messages:
    1,356
    Likes Received:
    13
    Status:
    Resident [Any Field]
    Okay, for a battery, the "cathode" is the (+) terminal and the "anode" is the (-) terminal, right?

    How come for gel electrophoresis, the (+) terminal is actually the "anode" (anions move toward the anode), and the (-) terminal is actually the "cathode"?

    I mean, I get why anions would move toward the positive terminal. What I don't get is the nomenclature. Shouldn't the positive terminal be called the cathode, the same way it is for electric circuits? Somebody please explain this to me.

    Is this just a fact about electrophoresis we should just memorize, or what?
     
  2. Thread continues after this sponsor message. SDN Members do not see this ad.

  3. BerkReviewTeach

    BerkReviewTeach Company Rep & Bad Singer
    Exhibitor

    Joined:
    May 25, 2007
    Messages:
    3,821
    Likes Received:
    570
    This apparent discrepancy consummes ten minutes of lecture every session. It comes down to a fundamental idea that is overlooked. For a battery, the system is discharging, so the poles are defined as (+) cathode and (-) anode to explain the directional flow of charges. For electrophoresis, you are using the field associated with a charging capacitor, so the terminals are reversed (different from a discharging capacitor or battery). The plates are defined according to the species that migrates through the field.

    The guy who teaches our electrophoresis, electrohcemistry, and electric circuits lecture has a great trick for this, so let me ask him how he explains it. I notice students go from hating this subject to seeing it as easy, so whatever trick he does works.
     
  4. BloodySurgeon

    Moderator Emeritus Lifetime Donor Classifieds Approved

    Joined:
    Nov 19, 2006
    Messages:
    3,234
    Likes Received:
    11
    Status:
    Medical Student
    You know that anode always discharges electrons towards the cathode. Therefore if we visualize anode left and cathode right the electrons are going to the right and the current is going left...

    Now just visualize a battery above a cell, like it needs energy imput because it is nonspontaneous.
    The batteries high potential (+) is to the left and low potential to the right (-), which will shows electrons moving to the right and current moving left. The signs are the same.


    Now if the reaction is spontaneous (galvantic cell), you do not need a battery and you will not visualize a battery over it. However there is a salt bridge with cations(+) and anions(-). Cations go to cathode (+) and anions go to anodes (-).

    Maybe that will help you to remember the two.
     
  5. Zerconia2921

    Zerconia2921 Bring your A-game!

    Joined:
    Mar 24, 2008
    Messages:
    207
    Likes Received:
    0
    Status:
    Pre-Pharmacy
    Alright, remember this RED CAT meaning reduction always occurs at the cathode. Reduction ( gaining of electrons). If your talking about a galvanic cell the electrons will automatically move to the cathode because they are attracted to the postive charge and be reduced.

    If your talking about a gel your using a voltage/battery to push your e- to the cathode where they once again get reduced.
    For the electrons to travel to the cathode it must be + but just remember one thing its the site of reduction.
     
  6. BerkReviewTeach

    BerkReviewTeach Company Rep & Bad Singer
    Exhibitor

    Joined:
    May 25, 2007
    Messages:
    3,821
    Likes Received:
    570
    Zerconia, you're RED cat and OX an mnemonic is perfect. It's very useful for electrochemical cells and completely applicable for any DISCHARGING system. But please be careful when applying that logic to electrophoresis, because the plates of the CHARGED capacitor that generate the electric field which cause the charged particles to migrate through the gel are not defined according to the electrons that are done flowing through the circuit wires. The charge of the plates is defined by the charge that has built up on them. The anode of the battery forced electrons (and thus negative charge) to collect on the cathode plate of the capacitor, so the cathode of a charged capacitor is defined as being negative, NOT positive. It's a classic mistake in gel electrophoresis.
     
  7. Zerconia2921

    Zerconia2921 Bring your A-game!

    Joined:
    Mar 24, 2008
    Messages:
    207
    Likes Received:
    0
    Status:
    Pre-Pharmacy
    Thanks for the explanation ill be careful.
     

Share This Page