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Gold jewelry is made in a variety of alloys, ranging from pure gold to less than half gold by weight. Pure gold is called 24 karat or 24 K; 75% gold (by weight) is 18 K; and 50% gold is 12 K.

Materials | Procedure | Examining the Reactions

Gold is often alloyed with silver and copper in jewelry to reduce the cost and make the jewelry harder and more resistant to wear. Have you ever wondered what karat gold your jewelry is? We can use chemistry to approximate the karat!

Gold alloys, despite being harder than pure gold, can be dissolved in acids more easily than pure gold. Thus, the lower the karat, the more easily the gold alloy is dissolved. See the equations below, which show the results of adding concentrated nitric acid, HNO3, or aqua regia to different karats of gold. Aqua regia is a mixture of nitric and hydrochloric acids. It takes its name from its ability to dissolve gold, which is sometimes referred to as the royal metal. “Aqua” is the Latin word for water and “regia” is Latin for royal.

As seen in the first equation, a gold alloy of 14 K or lower will dissolve when nitric acid is added. But when the karat moves to 18 or higher (the second equation) the nitric acid causes no visible reaction. Only the acid solution of aqua regia can dissolve the higher karats of gold (the third equation).

14 K gold or lower + HNO3 (aq) → Cu+ or Ag+ (aq) alloy dissolves

18 K gold or higher + HNO3 (aq) → no reaction alloy does not dissolve

18 K gold or higher + aqua regia → AuCl4- (aq) alloy dissolves

In the following procedure you will use nitric acid and aqua regia, and perhaps other testing solutions, to determine the karat of the gold samples. Consult the illustration below as you perform the tests.

Materials

The materials and testing solutions listed below can be obtained from a jewelers’ supply store.
  • Gloves, safety glasses, and protective clothing
  • Touchstone: a flat slab of basanite
  • Gold sample to be tested (Costume jewelry may be used and students may volunteer to test their own pieces; please note that the gold plating on costume jewelry is extremely thin and excess pressure will scratch into the base metal.)
  • Set of gold testing needles (These are gold standards of known alloys for comparing unknown gold samples. Recommended alloys: 10 K, 14 K, 18 K, 20 K)
  • Droppers
  • Nitric acid (HNO3), 70% solution
  • Aqua regia*
  • Other testing solutions (optional)
**Additional materials needed to prepare the aqua regia. See the supplementary information section in the appendix for further instruction. Only personnel experienced in handling acids should prepare this solution.
  • 2 acid-resistant reagent containers
  • 2 graduated cylinders
  • Deionized water
  • Nitric acid (HNO3), concentrated
  • Hydrochloric acid (HCl), concentrated
Note: The waste from this experiment must be neutralized before washing down the sink.

Procedure

Please read through the entire procedure before you begin.

WARNING: Please be sure to wear your gloves, safety glasses, and protective clothing—the acids you will be using in this activity are highly corrosive and toxic.

If any acids are accidentally spilled, pour dry sodium bicarbonate onto the acid solution to neutralize it. If an acid comes in contact with skin, immediately rinse the area thoroughly with water.

  1. Put on gloves, safety glasses, and protective clothing.
  2. Make a streak (about 2 centimeters long) on the touchstone with the sample to be tested.
  3. Using the same amount of pressure, make a streak on one side (about 2 millimeters from the first streak) with a gold testing needle of lower karat than the unknown is expected to be. (You need to guess what karat you believe the unknown to be!) Make a streak on the other side with a known gold standard of higher karat than the unknown is expected to be.
  4. On one end of the streaks, using a dropper, place a drop of the appropriate test solution for the karat of your unknown gold sample. (Determine the appropriate testing solution by referring to the equations at left. For example, if you believe that your unknown is 12 K gold, choose nitric acid.)
  5. Make sure that the drop covers the streaks as shown in the illustration. Carefully observe the reaction. You will likely notice the streaks becoming lighter in appearance and they may eventually disappear entirely; this is the alloy dissolving in the acid. Lower karat gold alloys will dissolve more quickly than higher karat alloys because they contain more copper or silver, which dissolve in nitric acid, while gold does not. For example, if you drew a streak of 10 K and 12 K gold standards with your unknown (which you believe is 12 K), the streak from the 12 K gold standard and your unknown should dissolve at the same rate, while the 10 K gold standard streak should dissolve faster. Also, after rinsing the acid from the touchstone with water, there will be more gold remaining in the 12 K streak than the 10 K streak.

    If aqua regia is applied, then no gold will remain on the touchstone at the end of the reaction. You will determine the karat by carefully observing the rate that the streaks dissolve, with the highest karat streak being the slowest to dissolve.

  6. If needed, place another drop of another test solution on the other end of the streaks. Carefully observe the results. If the results are ambiguous, it may be necessary to repeat the tests several times, making new streaks with different gold standards and using alternate acid testing solutions.

Please note: This test works well, but the results should be treated as an approximation. Two 14 K gold samples that contain different alloy metals (such as silver or copper) will react differently in this test. Likewise, a thicker and heavier streak of 18 K gold may dissolve more slowly than a thin streak of 20 K gold. Therefore, it is important that each testing needle is applied to the touchstone with the same amount of pressure. For alloys less than 10 K, the streaks will dissolve almost instantaneously in 70% nitric acid. Diluted nitric acid (35%) may be used for those alloys. Likewise, alloys less than 18 K will dissolve rapidly in aqua regia, so that the difference in reaction rates cannot be observed. Use diluted aqua regia (50% from the stock) to test these alloys.

Examining the Reactions

The two acidic testing solutions react with gold alloys by dissolving all or some of the metals within them. Nitric acid dissolves copper and silver by oxidizing them, as described by the chemical equations below.

  1. Oxidation of copper in nitric acid:
    Cu (s) + 2 NO3- (aq) + 2 H+ (aq) → Cu2+ (aq) + 2 NO2 (g) + H2O (l)
  2. Oxidation of silver in nitric acid:
    Ag (s) + 2 NO3- (aq) + 2 H+ (aq) → Ag+ (aq) + 2 NO2 (g) + H2O (l)
  3. The more copper or silver within the gold alloy, the faster the alloy will dissolve. Gold alloys greater than 14 K will still react with nitric acid (the copper or silver component of the alloy will still dissolve), but because there is less copper or silver to react, the reaction will be much harder to observe. For alloys greater than 14 K, aqua regia is used to test the gold.

    Aqua regia dissolves gold itself (as well as dissolving the silver and copper components) and for this reason it is used to test higher karat gold alloys. Aqua regia dissolves gold in two separate reactions. The first is an oxidation reaction brought about by the nitric acid, which produces small amounts of dissolved, oxidized gold, Au3+ (3). And the second is an equilibrium reaction, which produces greater amounts of dissolved gold in the form of chloraurate ions, AuCl4- (4).

  4. Oxidation of gold by nitric acid:
    Au (s) + 3 NO3- (aq) + 6 H+ (aq) → Au3+ (aq) + 3 NO2 (g) + 3 H2O (l)
  5. Dissolution of gold by formation of chloraurate:
    Au3+ (aq) + 4 Cl- (aq) → + AuCl4- (aq)

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