Period 6 element also dissolves in alkaline solutions of cyanide, which have been used in mining. Gold dissolves in mercury, forming amalgam alloys. Gold is insoluble in nitric acid, which dissolves silver and base metals, a property that has long been used to confirm the presence of gold in items, giving rise to the term "the acid test." Gold has been a valuable and highly sought-after precious metal for coinage, jewelry, and other arts since long before the beginning of recorded history. Gold standards have been a common basis for monetary policies throughout human history, later being supplanted by fiat currency starting in
Acid-base extraction pH range. The order of the step is not important and the process can be repeated to increase the separation. However, it is often convenient to have the compound dissolved in the organic phase after the last step, so that the evaporation of the solvent yields the product. The procedure works only for acids and bases with a large difference in solubility between their charged and their uncharged form. The procedure does not work for: Alternatives to acid-base extraction including: Acid-base extraction Acid-base extraction is a procedure using sequential liquid–liquid extractions to purify acids and bases from mixtures based on