Using Redox to Recover Sunken Treasure ( also see The Mel Fisher Story )

When a hurricane sent the galleon Atocha crashing into a reef in 1622, wooden chests filled with 47 tons of copper, gold and silver became sunken treasure. Over three hundred years later, when the treasure was found, the silver, unfortunately, was coated with silver sulfide. Since the silver had been kept in wooden chests, the wood eventually decayed, and in the process H₂S was released by bacteria. Hydrogen sulfide, H₂S, is one of the sulfur compounds that characteristically tarnishes silver.

Imagine that we had found the treasure. Is there any way we can remove the layer, and even recover elemental silver from it?

If the practically insoluble Ag₂S is treated with acid, say HNO₃, it will create soluble AgNO₃. Unfortunately, we will also produce poisonous and rotten-egg smelling H₂S, but the gas can be carefully collected. Once we have Ag⁺, we can reduce it to Ag with copper.

To simulate this process, we will use another insoluble salt of silver, one that will not produce a poisonous stench, silver acetate: AgCH₃CO₂.

AgCH₃CO_2(s) = Ag⁺ _(aq) + CH₃CO_2(aq)

The above equilibrium favours the reactants, but the acid will consume acetate, CH₃CO_2(aq), to create acetic acid. This will drive the equilibrium to the right. Once we have silver ions in solution, we can add copper to create the following redox reaction:

Cu _(s) + 2Ag⁺ _(aq) --> Cu⁺² _(aq) + 2Ag _(s) The grey stuff at the bottom of the beaker is silver that has formed around a penny. Some of the copper has been oxidized, which is why the solution is turning blue.

If you leave the penny in the solution for the right amount of time, it will be plated with silver as in figure 3. If left in the solution, the penny will be the victim of a whole minestrone of side reactions; it will be badly oxidized, and most of the silver will flake off it as in figure 4. Figure 2 has not yet been victimized by either nature or an amateur chemist. Figure 1 has been exposed to enough moisture that the interesting green compound patina has formed. This occurs when copper’s oxide (black)continues to react with carbonic acid and sulfates in the air. This is usually a slow process, leading to the interesting green colours on bronze statues and copper roofs. I once accelerated the process by adding CuO to a solution of citric acid, and I obtained a bright, greenish compound in less than a day.