Determining the purity of gold, or its karat value, is essential because gold used in jewelry and coinage is rarely pure. It is typically alloyed with base metals like copper, silver, or zinc to increase durability and hardness. Since these added metals change the physical properties of the gold, a precise chemical assessment is the most consistent way to establish its true value.
The Primary Reagent for Gold Purity
The most common acid used in the standard test for gold purity is Nitric Acid (HNO3), which is prepared in various concentrations. This acid is effective because it relies on the principle of selective dissolution. Gold is chemically noble, resisting corrosion and reaction with most single acids, while the base metals alloyed with it are highly reactive.
When nitric acid is applied to a gold alloy, it rapidly dissolves the less noble metals, such as copper and zinc, by converting them into soluble nitrates. Pure gold remains unaffected, which is the basis for determining purity. Jewelers use different concentrations of nitric acid to correspond with specific karat levels, such as 10-karat (10K) or 14-karat (14K) gold. A lower-karat alloy, like 10K, contains more base metals and will react more vigorously with a specific acid concentration than a purer 14K sample.
Performing the Touchstone Scratch Test
The practical application of the acid test uses a technique known as the touchstone scratch test. The touchstone is a small tablet, typically made of black slate or a similar finely textured ceramic material, which does not react with the acids. The object being tested is rubbed firmly against the touchstone to leave a visible streak of metal on the surface.
To accurately determine the karat value, the jeweler creates several streaks from the test item next to reference streaks made from needles of known purity. A drop of a specific acid concentration, such as a 14K testing solution, is then applied across the streaks. If the unknown sample streak remains intact with little reaction, the gold is at least 14K or higher in purity.
If the streak dissolves, fades, or turns a noticeable color, the gold is of a lower purity than the acid concentration used. The jeweler systematically applies progressively stronger acid solutions to separate streaks until a reaction is observed. This process allows them to pinpoint the approximate karat level of the gold quickly and non-destructively.
Testing High Karat Gold
The standard nitric acid test has limitations when dealing with very high-purity gold, specifically 22-karat (22K) or 24-karat (24K) gold. These higher alloys contain such a small percentage of base metals that they exhibit almost no reaction to concentrated nitric acid. To confirm the purity of these noble metals, a stronger, specialized reagent called Aqua Regia is required.
Aqua Regia is a highly corrosive mixture of concentrated nitric acid and concentrated hydrochloric acid, typically in a 1:3 ratio. This unique combination is one of the few substances capable of dissolving pure gold itself. The nitric acid acts as a powerful oxidizer, while the hydrochloric acid supplies chloride ions that combine with the oxidized gold atoms to form a soluble complex ion.
Its use differs significantly from the standard test, as it is not used to selectively dissolve base metals. Instead, Aqua Regia is often used as a final confirmatory test on a scratch streak, or in industrial settings for completely dissolving a sample for quantitative analysis. The rapid dissolution of the test streak under Aqua Regia confirms the metal is high-purity gold.
Safe Handling of Testing Acids
The acids used in gold purity testing, including nitric acid and Aqua Regia, are highly corrosive and require strict safety protocols. These chemicals should only be handled by trained professionals in a controlled environment. Adequate ventilation, typically achieved by working under a fume hood, is mandatory to capture and remove the toxic fumes and gases produced during the chemical reactions.
Personal protective equipment (PPE) must be worn at all times, including acid-resistant gloves, a laboratory coat, and safety goggles or a face shield to protect against splashes. Proper storage is essential, as Aqua Regia cannot be stored in a sealed container due to the buildup of volatile, pressurized gases that can cause an explosion. Furthermore, all used acids and neutralized waste must be disposed of according to established hazardous waste regulations.