How would using different salts in a salt bridge effect the voltage generated by a voltaic cell?

Question

If I use a range of different nitrate solutions (e.g. calcium nitrate, iron nitrate) for the salt bridge in a voltaic cell, what effect would it have on the flow of ions in the voltaic cells?

Will the different properties of the salts such as ionic radius effect the voltage generated by the voltaic cell?

If a certain property would most likely have an effect, what would it be and why?

(Both the cathode and the anode are kept constant with copper in copper nitrate solution and zinc in zinc nitrate solution)

Answer 1

The salt bridges used by mine (so far) were filled with aqueous electrolytes with solutions of either potassium chloride, potassium nitrate, or sodium chloride. For an electrochemical cell of $\ce{Zn | Zn^2+||Cu^2+|Cu}$ described by you, i.e. based on

$\ce{Cu^2+ + Zn \rightleftharpoons Cu + Zn^2+}$

these appear to me a resonable choice, because

• Their ions (of KCl, KNO3, NaCl) would not interfer with the redox reactions, so the potential between the two electrodes (the "voltage" you mention) were not affected. Referring to the standard potentials, $\ce{Fe^3+}$ of iron nitrate in contrast, is more likely to interfer with your system than the $\ce{Ca^2+}$ of calcium nitrate, esp. in aqueous solution; but the design of the salt brigde (diaphragm) may reduce this electrochemical interference.
• Beside electrochemical inertia in the window of investigation, an additional criterion to select electrolytes for salt bridges is their cations and their anions should possess a similar mobility in the electrolyte of tehe salt bridge (the flow you mention). The rate of the ions to move not only depends on the viscosity of the medium and the charge of the specis, but on the efficient radius of the ion (the ion plus the hydration sphere), too; yet this should not influence the potential between the electrodes, rather than the current between the electrodes if the set up is intended to transform chemical into electrical energy.

In short: within reason, keep the KCl in the salt bridge.