The Complete Guide to Aqua Regia Gold Refining
Aqua regia — Latin for “royal water” — is one of the most powerful chemical mixtures known to chemistry. It’s one of the only substances capable of dissolving gold, platinum, and palladium. For jewelers, refiners, and urban miners, mastering the aqua regia process is the key to recovering and purifying precious metals from scrap, alloys, and electronic waste.
This guide walks you through the complete process: chemistry, equipment, step-by-step procedure, and critical safety requirements.
What Is Aqua Regia?
Aqua regia is a mixture of nitric acid (HNO₃) and hydrochloric acid (HCl) in a 1:3 molar ratio (1 part nitric to 3 parts hydrochloric). The combination produces nitrosyl chloride (NOCl) and chlorine gas, which together create an extraordinarily powerful oxidizing and complexing environment that attacks noble metals.
Neither acid alone can dissolve gold — but together, they can. This is why aqua regia has been used in gold refining for centuries.
What You’ll Need
Chemicals
- Nitric Acid 67% Lab Grade — 1 part
- Hydrochloric Acid (HCl) 31–37% — 3 parts
- Sodium metabisulfite (Na₂S₂O₅) — for gold precipitation
- Distilled water — for dilution and washing
- Urea — optional, to neutralize excess nitric acid before precipitation
Equipment
- Borosilicate glass beakers (never use metal or plastic)
- Glass stirring rod
- Fume hood or outdoor workspace with strong ventilation
- Buchner funnel and filter paper for filtration
- Hotplate (optional, speeds dissolution)
- pH strips or meter
PPE (Non-Negotiable)
- Acid-resistant gloves (neoprene or thick nitrile)
- Chemical splash goggles
- Full face shield
- Acid-resistant apron or lab coat
- Respirator rated for acid vapors (minimum OV/P100)
Step-by-Step Process
Step 1: Prepare Your Workspace
Work exclusively in a fume hood or outdoors with strong airflow. Aqua regia produces toxic nitrogen dioxide (NO₂) and chlorine gas — both are immediately dangerous. Have a neutralizing agent (baking soda solution) nearby for spill response.
Step 2: Mix the Aqua Regia
In a borosilicate glass beaker, add 3 parts hydrochloric acid first, then slowly add 1 part nitric acid. Never reverse the order. The mixture will turn orange-yellow and begin fuming — this is normal. Use immediately; aqua regia degrades within hours.
Step 3: Dissolve the Gold
Add your impure gold (scrap, alloy, or PCB material) to the aqua regia. The gold will dissolve, turning the solution a deep golden-yellow (chloroauric acid, HAuCl₄). Gentle heating on a hotplate (60–80°C) speeds dissolution but increases fuming — maintain ventilation.
Allow complete dissolution before proceeding. Undissolved material may be base metals or platinum group metals.
Step 4: Filter the Solution
Once dissolution is complete, filter the solution through filter paper in a Buchner funnel to remove undissolved solids (silver chloride, base metals, ceramics). The filtered liquid contains your dissolved gold.
Step 5: Neutralize Excess Nitric Acid (Optional but Recommended)
Add urea slowly to the filtered solution until fizzing stops. This neutralizes excess nitric acid, which can interfere with gold precipitation. Stir thoroughly.
Step 6: Precipitate the Gold
Slowly add a solution of sodium metabisulfite (Na₂S₂O₅) dissolved in water to the filtered solution. Pure gold will precipitate as fine brown-black particles. Continue adding until no more precipitate forms.
Step 7: Filter & Wash the Gold
Filter the precipitate through fresh filter paper. Wash the gold powder with distilled water several times to remove residual acid and impurities.
Step 8: Dry & Melt
Allow the gold powder to dry completely (or use a hotplate at low heat). Once dry, melt in a crucible using a torch or furnace. Pour into a mold to form bars or granules. Your refined gold is now ready for assay or sale.
Troubleshooting
- Gold not dissolving: Ensure your ratio is correct (1:3 HNO₃:HCl). Try gentle heating. Very thick pieces may need to be cut or granulated first.
- White precipitate during dissolution: This is silver chloride (AgCl) — normal if your gold contains silver. Filter it out.
- Poor gold yield: Ensure urea neutralization is complete before adding sodium metabisulfite. Incomplete neutralization reduces precipitation efficiency.
- Solution stays clear after adding precipitant: The gold may already be fully precipitated, or your starting material had very low gold content.
Waste Disposal
Aqua regia waste is a hazardous material. Neutralize spent acid with baking soda (sodium bicarbonate) until pH is 6–8, then dispose of according to your local hazardous waste regulations. Never pour concentrated acid down a drain.
Get the Acids You Need
Grow It Depot stocks Nitric Acid 67% Lab Grade in sizes from quart to 55-gallon drum — all shipped in DOT-compliant FLPE bottles with a Certificate of Analysis included.
⭐ Best value tip: The 4x Quarts bundle ships without a hazmat fee, making it the most cost-effective option for refiners needing up to 1 gallon of nitric acid.