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Simply put, why does adding acid to the cathode up the cell voltage? From what I understand, acids are proton donors. Do these positive protons attract electrons toward the cathode compartment of the electrochemical cell?

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The effect that can be observed by adding acids to a solution to increase conductivity is known as the Grotthuss mechanism. Protons have a much higher mobility in solution ($3.62\cdot10^{-3}~\mathrm{cm^2~V^{-1}~s^{-1}}$) than any other cation, basically allowing it to tunnel through a solution. (A similar effect can be noted for hydroxyl anions.) Basically this allows a much faster charge exchange through hydrogen bond networks, also resulting in a higher cell voltage.

(But then again I am not really sure I understood your question correctly.)

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I donn't know what a "detailed canonical answer" is, but intutitively, I can appreciate that it is easier to take an electron away from (i.e. oxidize) something with a negative charge than something that is electrically neutral. That is because it costs energy to separate charged particles and it produces energy to reunite them. So many examples - a thiol is stable to air in the absence of a base, but in the presence of base, air will oxidize it to a disulfide. Same with phenol - stable in air but with base present, you soon get colored condensation products.

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