How Much Sodium Metabisulfite to Precipitate Gold from Aqua Regia?

To precipitate gold from a gold-containing aqua regia solution, sodium metabisulfite (Na2S2O5) is commonly used. The general guideline is as follows:

  • For every gram of dissolved gold: Use approximately 1.5 grams of sodium metabisulfite.

This ratio ensures efficient precipitation, as some excess metabisulfite ensures complete reaction, and some decomposition occurs in acidic solutions. The actual amount can vary depending on the purity of the chemicals and the completeness of the gold dissolution.

Step-by-Step Process

  • Weigh or estimate the amount of gold in solution (for example, 10 grams).
  • Multiply the amount of gold by 1.5 to find out how many grams of sodium metabisulfite to use (for 10 grams gold, use 15 grams sodium metabisulfite).
  • Dissolve the sodium metabisulfite in a small amount of water.
  • Slowly add the dissolved sodium metabisulfite to the well-stirred gold solution.
  • Gold will begin to precipitate as a brown powder.

Notes

  • Always add the chemical slowly and in a well-ventilated area, as the reaction produces sulfur dioxide gas (SO2).
  • If unsure of the gold content, add sodium metabisulfite in small increments with stirring, until precipitation ceases.
  • Sodium metabisulfite should be handled with care, wearing gloves and eye protection.

This process should only be performed by qualified individuals with a proper understanding of chemical safety. Improper procedures can be dangerous.