Muriatic acid, commonly known as hydrochloric acid (HCl), is a strong and highly corrosive mineral acid. It is an aqueous solution of hydrogen chloride, recognized by its distinctive pungent smell and clear to yellowish appearance. This potent chemical finds various applications, from industrial processes like pickling steel to household uses such as cleaning concrete and balancing swimming pool pH. This article explores what happens when muriatic acid encounters gold, differentiating between pure gold and gold alloys.
Muriatic Acid and Gold: The Interaction
Pure gold is classified as a noble metal, meaning it possesses outstanding resistance to oxidation and corrosion, and is generally found in nature in its uncombined, elemental form. When pure 24-karat gold comes into contact with muriatic acid alone, there is no chemical reaction. The gold itself remains unaffected, a property historically used as a test for pure gold.
The scenario changes when dealing with gold alloys. Gold is often alloyed with other metals to enhance its hardness, durability, and alter its color, as pure gold is very soft and malleable. Common metals mixed with gold include copper, silver, zinc, nickel, palladium, and iron.
When muriatic acid is applied to a gold alloy, it will not react with the gold content. However, it readily reacts with the other, less noble metals present. For instance, if the alloy contains copper or zinc, the muriatic acid will corrode these base metals. This reaction can change the alloy’s appearance, as non-gold components are removed, leaving behind a porous or discolored gold-rich residue.
While muriatic acid alone does not dissolve gold, gold can be dissolved by aqua regia. Latin for “royal water,” aqua regia is typically a fresh mixture of concentrated nitric acid and hydrochloric acid, usually in a 1:3 molar ratio. Neither acid can dissolve gold on its own; it is the synergistic action of both that enables dissolution. Nitric acid acts as an oxidizing agent, forming gold ions, while hydrochloric acid supplies chloride ions that react with these gold ions to form soluble complexes, effectively pulling the gold into solution.
Safe Practices and Gold Care
Muriatic acid is a hazardous chemical that demands extreme caution during handling. It is a highly corrosive liquid, and its fumes are irritating to the respiratory tract, eyes, and skin. Direct contact can cause severe chemical burns and eye damage.
When handling muriatic acid, wear appropriate personal protective equipment (PPE), including chemical-resistant gloves, safety goggles or a face shield, and protective clothing. Ensure adequate ventilation to prevent inhaling its pungent fumes.
Muriatic acid should always be stored in its original, tightly sealed, corrosion-resistant container, away from incompatible materials like other metals, alkalis, and oxidizers. A cool, dry, and well-ventilated area is suitable for storage, ideally in secondary containment to prevent spills.
If dilution is necessary, always add the acid slowly to water, never the other way around, to prevent a violent, heat-generating reaction. For disposal, muriatic acid should not be poured down drains or into the trash. Local regulations often permit diluting the acid with a large amount of water and neutralizing it with baking soda until fizzing stops before disposal, or taking it to a hazardous waste facility. For cleaning gold jewelry, safer alternatives include using mild soap and warm water with a soft brush, which effectively removes dirt and grime without the risks associated with corrosive chemicals.