Using a calcium metal anode in aqueous solution?

In school, I am doing an electrochemistry lab in which I need to create a galvanic cell battery with the highest voltage possible using two beakers, a salt bridge, and electrodes.

To maximize the voltage, I have decided to use calcium metal for the oxidation half-reaction and a sodium persulfate solution for the reduction half-reaction. I understand that for the reduction reaction in the cathode, I would merely dissolve the persulfate in water. However, calcium metal reacts with water, so how would I go about connecting the calcium to the salt bridge (which is a requirement in the lab)?

The salt bridge has an aqueous salt and my chemistry teacher said that the redox reaction that would result from letting the calcium touch the water would prevent it from being effective in the battery. Also, my class vaguely touched on the Nernst equation, but are there any other revisions I can make to the battery to increase the voltage, either by adding $\ce{OH-}$, $\ce{H+}$, or increasing/decreasing concentration of the persulfate solution?

2 Answers

Calcium does react with water, but it does so slowly especially in cold water. You should be able to use it as you would a typical metal as some water will touch the metal. Just be aware that it will react and your voltage may be less than predicted. This battery of course would be of no practically use as its shelf life would be on the order of less than an hour, but should last long enough to obtain a measurement and complete the experiment. You should also expect to see a change in voltage over time as $\ce{Ca(OH)2}$ is added to solution reducing the potential.

• Thank you! Is there any way to make the battery work without having the calcium in direct contact with water? In the anode, I would be using a graphite rod as the electrode and touch it to the calcium metal. My main problem is the salt bridge. Is there a method of connecting the calcium to the bridge to maintain charge balance without sacrificing voltage from a potential reaction with water? – user27462 Mar 3 '16 at 4:28
• typically batteries use propylene carbonate for such reactive metals. access to such may be an issue. – A.K. Mar 3 '16 at 4:29

Basically, the reaction of calcium with water short-circuits your battery; the electrons you wanted to go through the external circuit end up going directly into the electrolyte instead.

Magnesium will work better for your anode; it reacts little or none with water when cold and it's anodic potential still packs a pretty good punch.

• Okay, that makes sense, thanks! I think other people will be using magnesium so we wanted to go with calcium since the standard reduction potential of calcium ions is lower than that of magnesium. Do you know of any ways to still use calcium and prevent the short-circuiting (use something in between the calcium and salt bridge so they're not in contact)? – user27462 Mar 3 '16 at 4:20