Cleaning Gold and Silver Articles: The Role of Acids

Goldsmiths use specific chemical solutions, primarily acids, to clean gold and silver articles. The goal is to remove surface impurities, tarnish, and base metals without significantly damaging the precious metals themselves. Different acids have different properties and reactivities with various metals.

Understanding Acid Reactivity with Metals

The reactivity of an acid with a metal depends on the nature of the acid (oxidizing or non-oxidizing, strong or weak) and the position of the metal in the reactivity series. Gold is a very noble (unreactive) metal, while silver is less noble than gold but still relatively unreactive compared to base metals like copper, zinc, or iron, which are often present as impurities or in alloys.

Why Nitric Acid is Used by Goldsmiths

Nitric acid (\(\text{HNO}_3\)) is a strong, oxidizing acid. It is commonly used by goldsmiths for cleaning purposes because it effectively reacts with and dissolves base metals and impurities often found on the surface of gold and silver articles, such as copper, zinc, and iron oxides.

• Nitric acid reacts vigorously with less noble metals, helping to remove them from the surface. For example, it reacts with copper: \(\text{3Cu (s) + 8HNO}_3\text{ (aq, dilute) \rightarrow 3Cu(NO}_3)_2\text{ (aq) + 2NO (g) + 4H}_2\text{O (l)}\).
• While concentrated nitric acid can passivate some metals, it effectively cleans the surface by oxidizing and dissolving impurities.
• Pure gold is largely unaffected by nitric acid, even when hot and concentrated. This is why nitric acid is sometimes called "aqua fortis" (strong water) and historically used to test the purity of gold.
• Pure silver reacts with nitric acid, but the reaction is relatively slow and controlled for surface cleaning purposes on articles. The acid primarily targets the more reactive surface impurities and tarnish.

The controlled reaction of nitric acid with impurities and less noble constituents of the alloy makes it an ideal choice for brightening gold and silver articles without causing significant loss of the precious metal itself, unlike harsher methods or acids that might attack the gold or silver more aggressively.

Comparing Other Acids

Let's look at why the other options are generally not preferred for this specific task by goldsmiths:

• Sulphuric acid (\(\text{H}_2\text{SO}_4\)): Sulphuric acid is a strong acid. Dilute \(\text{H}_2\text{SO}_4\) is non-oxidizing and reacts only with metals above hydrogen in the reactivity series. Hot concentrated \(\text{H}_2\text{SO}_4\) is an oxidizing acid and can react with many metals, including silver, and even slowly with gold. It's less commonly used than nitric acid for routine cleaning of finished articles due to its aggressive nature when hot.
• Hydrochloric acid (\(\text{HCl}\)): Hydrochloric acid is a strong, non-oxidizing acid. It reacts with metals above hydrogen in the reactivity series. While it can react with some impurities, it is not effective at dissolving many others or oxides. Furthermore, a mixture of nitric acid and hydrochloric acid (aqua regia) is known to dissolve gold, which is the opposite of what is desired when cleaning an article.
• Phosphoric acid (\(\text{H}_3\text{PO}_4\)): Phosphoric acid is a moderately strong acid, weaker than nitric, sulfuric, or hydrochloric acid. It is often used for rust removal (iron oxides) or as a passivator, but it is generally not strong enough or reactive enough to efficiently clean the wide range of impurities found on gold and silver articles in a goldsmithing context.

Therefore, based on the reactivity and effectiveness in removing common impurities from gold and silver surfaces without dissolving the precious metal itself, nitric acid is the acid traditionally used by goldsmiths for cleaning these articles.

In summary, nitric acid is the preferred choice for goldsmiths to clean gold and silver articles because it effectively targets and removes common surface impurities and base metals while leaving the gold and silver largely intact.

Revision Table: Cleaning Precious Metals

Additional Information: Aqua Regia

While nitric acid alone is used for cleaning, it's important to note that gold can be dissolved by a mixture of concentrated nitric acid and concentrated hydrochloric acid, known as Aqua Regia. This mixture is highly corrosive and is used for dissolving gold, not cleaning articles where preservation of the gold is the goal. Aqua Regia's ability to dissolve gold comes from the synergistic effect where nitric acid oxidizes gold to \(\text{Au}^{3+}\) ions, and hydrochloric acid provides chloride ions (\(\text{Cl}^{-}\)) which complex with \(\text{Au}^{3+}\) to form the stable tetrachloroaurate(III) anion (\(\text{[AuCl}_4]^{-}\)). This reaction effectively removes the \(\text{Au}^{3+}\) ions from solution, driving the oxidation forward.

\(\text{Au (s) + 3HNO}_3\text{ (aq) + 4HCl (aq) \rightarrow H[AuCl}_4]\text{ (aq) + 3NO}_2\text{ (g) + 2H}_2\text{O (l)}\) (Simplified representation with \(\text{NO}_2\) as product)

This highlights the specific nature of nitric acid's use for cleaning vs. dissolving gold when mixed with \(\text{HCl}\). For cleaning, only nitric acid is used to target less noble metals and impurities.