No, aqua regia is not an effective solution for cleaning silver.
Aqua regia, a highly corrosive mixture of nitric acid and hydrochloric acid, is renowned for its ability to dissolve noble metals like gold and platinum. However, its chemical properties make it unsuitable for cleaning silver.
Why Aqua Regia Doesn't Clean Silver
The primary reason aqua regia cannot clean silver is straightforward: it does not dissolve silver. The provided reference explicitly states this: "Aqua regia does not dissolve silver or iridium, though it will dissolve osmium if only quite slowly. It will dissolve most other metals, including gold, platinum, mercury and iron."
This means that if you were to apply aqua regia to a silver object, it would not chemically react with or remove the silver metal itself. Therefore, it cannot dissolve away tarnishes or impurities that are chemically bonded to the silver surface or remove any surface layers of silver.
What Aqua Regia Does and Doesn't Dissolve
To illustrate its specific reactivity, consider the following table based on the provided reference:
| Metal | Dissolves in Aqua Regia? | Notes |
|---|---|---|
| Silver | No | Aqua regia does not affect the silver metal. |
| Iridium | No | A very dense and corrosion-resistant noble metal. |
| Osmium | Slowly | Another noble metal, dissolved very gradually. |
| Gold | Yes | Famous for dissolving gold, forming tetrachloroaurate(III) anions. |
| Platinum | Yes | Also dissolves platinum, forming chloroplatinic acid. |
| Mercury | Yes | Commonly dissolved by strong acids. |
| Iron | Yes | Many common metals are readily dissolved. |
Practical Implications for Cleaning Silver
Since aqua regia does not dissolve silver, it cannot:
- Remove silver tarnish: Silver tarnish is typically silver sulfide (Ag₂S), formed when silver reacts with sulfur-containing compounds in the air. Aqua regia is not designed to dissolve this compound or the underlying silver.
- Polish or brighten silver: The 'cleaning' action of aqua regia on metals it does dissolve involves removing surface layers. As it doesn't do this for silver, it offers no polishing or brightening effect.
- Remove scratches or minor imperfections: These require mechanical or abrasive cleaning methods, or in some cases, chemical processes that smooth the surface.
For effective silver cleaning, methods that either chemically convert the tarnish back to silver (e.g., using a baking soda and aluminum foil method) or mildly abrade it away (e.g., with a silver polishing cloth and cleaner) are typically employed.
