How to Use Sodium Metabisulfite to Precipitate Gold from Aqua Regia

Sodium metabisulfite (Na2S2O5) is commonly used to precipitate gold from dissolved solutions such as aqua regia. Follow these steps to safely and effectively precipitate gold:

Required Materials

  • Aqua regia solution containing dissolved gold
  • Sodium metabisulfite powder
  • Distilled water
  • Glass stirring rod
  • Beaker or glass container
  • Protective gloves, goggles, and mask (for safety)

Step-by-Step Instructions

  1. Neutralize Nitric Acid:
    Allow the aqua regia to sit until the reaction stops. To ensure all nitric acid is neutralized, add a small amount of urea until bubbling ceases.
  2. Dilute the Solution:
    Add distilled water to the solution. Diluting helps reduce the concentration of acids and improves precipitation.
  3. Prepare Sodium Metabisulfite Solution:
    Dissolve sodium metabisulfite in a small amount of distilled water (optional, can also add dry).
  4. Add Sodium Metabisulfite Slowly:
    Slowly add the sodium metabisulfite (or its solution) to the gold-bearing solution while stirring gently. Do this gradually to prevent excessive foaming or splattering.
  5. Observe the Reaction:
    A brown powder (precipitated gold) will begin to form and settle in the container. Continue adding sodium metabisulfite until no more gold precipitates.
  6. Allow to Settle:
    Leave the solution undisturbed for several hours or overnight to allow all gold particles to settle at the bottom.
  7. Decant or Filter:
    Carefully pour off the clear liquid or filter the solution to collect the precipitated gold.
  8. Wash the Precipitate:
    Wash the gold powder several times with distilled water to remove impurities and any remaining chemicals.

Safety Precautions

  • Always wear protective gear (gloves, goggles, mask) and work in a well-ventilated area.
  • Sodium metabisulfite and aqua regia fumes are toxic—avoid inhaling and ensure proper ventilation.
  • Dispose of waste solutions responsibly according to local regulations.

Additional Notes

The brown powder is impure gold (“gold sponge”). Further refining by melting or chemical processes is recommended for higher purity.