The Household Chemistry of Cleaning Pennies

Although it is well known that solutions of vinegar and table salt are effective in removing tarnish from pennies, chemical explanations of this behavior are not included in sources of this "household chemistry" and erroneous explanations have crept into popular literature on this subject. This article provides some relevant empirical information on the system, such as the fact that chloride ion (as opposed to any ionic compound) seems to be necessary for the effect, and presents a simplified treatment of equilibria that occur in the solutions. Equilibrium calculations indicate that formation of the copper(I) complex ion CuCl₂^- is favored, which may explain the role of the sodium chloride. The presence of copper(I) in the resulting solutions can be demonstrated colorimetrically with the reagent bicinchoninic acid.