What type of reaction may be occurring when a person's skin develops a green stain beneath a 14 K gold bracelet? Using HA as the chemical formula for acid in skin, write a balanced chemical equation to represent the reaction.

Answer 1

Others may have something to add. Sweat and/or body fluids are present on the skin and many, if not most, skins are acidic in nature. Small amounts of sulfur, perhaps in the form of SO2 or SO3, can react with this fluid to form an acid and that reacts with the copper in gold. Generally, pure gold is not used in jewelry for it is too soft. Gold is alloyed, usually with copper, and 14k gold is just 14/24 pure gold which leaves the other 10/14 for other metals, mostly copper. Some nickel is used also. At any rate, the copper reacts with the acid created from SO2/SO3/CO2 absorbed from the air and copper salts are green to blue in color. That salt is deposited on the skin. HA + Cu ==> CuA + H^+ in which the A of HA stands for the anion of the acid and the salt. This equation is not very realistic for there are many reactions going on; e.g., copper metal is being oxidized to Cu^+2 by oxygen. I hope this gives you some insight into the question.