What Is Gold Precipitation?
During aqua regia refining, gold is first dissolved into solution. Once dissolved, the gold exists in the liquid as gold chloride and is no longer visible as metal.
To recover the gold, refiners use a chemical reaction that forces the dissolved gold to fall out of solution as solid particles.
This step is called precipitation.
One of the most widely used chemicals for this process is sodium metabisulfite, often referred to as SMB or gold precipitant.
When added to a properly prepared solution, sodium metabisulfite converts dissolved gold back into solid metallic gold powder.
Why Sodium Metabisulfite Is Used
Sodium metabisulfite is popular among refiners because it is:
• reliable
• easy to use
• effective for small-scale refining
• capable of producing clean gold powder
When added to a gold-bearing solution, it reduces dissolved gold ions and causes them to form fine brown particles that settle to the bottom of the container.
Materials Needed
To precipitate gold from aqua regia you will need:
• Sodium metabisulfite (gold precipitant)
• Glass beaker or container
• Stirring rod
• Filter paper and filtration setup
• Stannous chloride testing solution
Many refiners also use a full gold recovery chemical kit that includes these chemicals together.
Step 1: Ensure the Solution Is Ready
Before precipitation, the solution must be properly prepared.
The gold should already be:
• fully dissolved in aqua regia
• filtered to remove solids
• neutralized to remove excess nitric acid
If nitric acid is still present, precipitation may fail or produce poor results.
Step 2: Confirm Gold Is Present
Before adding precipitant, refiners usually confirm that gold is still present in the solution.
Place a drop of the solution on filter paper and add a drop of stannous chloride testing solution.
A purple or dark color reaction indicates dissolved gold is present.
Step 3: Prepare the Precipitant
Sodium metabisulfite is typically dissolved in a small amount of water before being added to the refining solution.
This allows it to mix evenly and react with the dissolved gold.
Only a small amount is usually required.
Step 4: Add the Precipitant
Slowly add the sodium metabisulfite solution to the gold-bearing liquid while stirring gently.
Within moments you should begin to observe:
• brown powder forming
• dark particles settling
• the solution gradually clearing
These brown particles are fine gold powder being recovered from the solution.
Step 5: Allow Gold Powder to Settle
After the reaction is complete, allow the container to sit undisturbed.
The gold powder will slowly settle to the bottom.
This may take several hours or overnight depending on the amount of material being processed.
Once settled, the clear liquid can be carefully poured off.
Step 6: Test the Remaining Solution
Refiners often perform another stannous chloride test on the remaining liquid.
If no purple reaction appears, this confirms that all dissolved gold has been recovered.
If a reaction still occurs, additional precipitant may be required.
Step 7: Wash the Gold Powder
The recovered gold powder should be washed several times with clean water.
This removes remaining chemicals and impurities.
Proper washing improves the purity of the final metal.
Step 8: Dry and Melt the Gold
Once washed, the gold powder is allowed to dry completely.
The powder can then be melted using a crucible and borax flux to form a solid gold button or bar.
This final step converts the recovered powder into usable refined metal.
Common Problems During Gold Precipitation
Gold does not drop from solution
Excess nitric acid is usually the cause. The solution must be denoxed before precipitation.
Powder disappears after forming
Remaining nitric acid may still be dissolving the gold.
Solution remains cloudy
Allow additional time for particles to settle or refilter the solution.
Final Thoughts
Sodium metabisulfite remains one of the most reliable and accessible chemicals for recovering gold from aqua regia solutions.
By following the proper refining sequence:
Dissolve → Filter → Neutralize → Test → Precipitate → Wash → Melt
Refiners can recover high-purity gold powder ready for melting and casting.
Recommended Gold Refining Supplies
If you're refining scrap gold, jewelry, or electronic components, using the correct chemicals and fresh reagents is critical for consistent results. These supplies are commonly used in the aqua regia gold refining process.
Hydrochloric Acid for Aqua Regia