Gold is one of the most valued metals, but determining its true purity (karat level) requires proper testing. One of the most effective methods is acid testing, which uses a combination of nitric acid (HNO₃) and hydrochloric acid (HCl, also known as muriatic acid) to identify the karat purity of gold.

This article covers the gold testing process, explains how to perform the test, and details the chemistry behind the reactions, including the correct acid compositions for different karat levels.

1. How Does Gold Karat Testing with Acid Work?

Gold testing with acid relies on the fact that pure gold (24K) is highly resistant to acid corrosion, while lower-karat gold, which contains more alloy metals (like copper, silver, and zinc), reacts with acids.

The test is performed by applying different acid solutions to a gold sample to observe whether the metal dissolves, fades, or remains intact. Each acid mixture is formulated to dissolve only up to a certain gold purity, allowing you to determine the karat of your sample.

💡 Key Principle:

Lower karat gold (e.g., 10K-14K) contains more reactive alloy metals, which dissolve in nitric acid-based solutions.
Higher karat gold (18K-22K) resists nitric acid but can dissolve in solutions with higher hydrochloric acid content.
Pure 24K gold is only dissolved by aqua regia (a mix of HCl and HNO₃ in a 3:1 ratio).

2. Step-by-Step Process for Testing Gold Purity

Step 1: Preparing for the Test

Materials Needed:

Gold test acids for different karat levels (pre-made kit or homemade solutions)
Black touchstone (for scratch testing)
Gloves and safety goggles (for acid handling)
Distilled water (for cleaning)
Baking soda solution (to neutralize acid after testing)

💡 Why Use a Touchstone?
A touchstone allows you to test gold without damaging the item itself. Instead of applying acid directly to the jewelry, you rub the gold onto the touchstone, leaving a thin streak that can be tested.

Step 2: Performing the Gold Acid Test

1. Scratch the gold piece on the touchstone to create a visible streak.
Apply a drop of the test acid (starting with the lowest karat level first).
Observe the reaction:
If the streak disappears quickly, the gold is lower karat than the test solution.
If the streak fades slowly, the gold is approximately the same karat as the acid strength.
If the streak remains unchanged, the gold is higher karat than the test solution.

Repeat with stronger acids until you find the level where the streak just barely fades.

Step 3: Finalizing the Test

If the gold resists all acid solutions up to 22K, test it with aqua regia (3:1 HCl to HNO₃ mixture) to confirm 24K purity.
After testing, rinse the stone with distilled water and neutralize any remaining acid with baking soda solution.
Record the karat value based on the last acid that caused a reaction.

🔹 Example Interpretation:

The 10K acid dissolves the streak, but the 14K acid does not → The gold is between 10K and 14K.
The 18K acid fades the streak slowly, while 22K does not → The gold is likely 18K.

3. The Chemistry Behind Gold Testing Acids

Gold is a noble metal, meaning it resists oxidation and corrosion. However, lower-karat gold alloys contain more reactive metals that dissolve when exposed to acid.

Here’s what happens when different acid solutions are applied:

1️⃣ Lower-Karat Gold (10K-14K) Dissolution:

Nitric acid (HNO₃) dissolves non-gold metals (copper, zinc, and silver), leaving behind only the gold:

      Cu+4HNO3→Cu(NO3)2+2NO2+2H2O

This causes the gold streak to fade or disappear.

2️⃣ Higher-Karat Gold (18K-22K) Reaction:

A stronger mix of nitric and hydrochloric acid is required to break down gold alloys.
Hydrochloric acid creates a chloride complex that weakens the structure:

      Au+3HNO3+4HCl→HAuCl4+3NO2+H2O

If the streak remains intact, the gold is above the test level.

3️⃣ 24K Gold & Aqua Regia:

Pure gold does not react to nitric acid alone, but aqua regia (3:1 HCl to HNO₃) dissolves it completely:

      Au+3HNO3+4HCl→HAuCl4+3NO2+H2O

This is the final confirmation test for pure 24K gold.

4. Acid Compositions for Different Gold Karat Tests

Each karat testing solution consists of a specific ratio of nitric acid (HNO₃) and hydrochloric acid (HCl, muriatic acid), sometimes diluted with water. Here’s the breakdown:

10K Gold Testing Solution

70% nitric acid (HNO₃)
0% hydrochloric acid (HCl)
30% distilled water
Effect: Dissolves 10K and lower gold, but 14K and higher will resist.

12K Gold Testing Solution

65% nitric acid (HNO₃)
5% hydrochloric acid (HCl)
30% distilled water
Effect: Dissolves 12K and lower gold, 14K+ fades more slowly.

14K Gold Testing Solution

50% nitric acid (HNO₃)
20% hydrochloric acid (HCl)
30% distilled water
Effect: Dissolves 10K-12K gold, 14K fades gradually, and 18K+ resists completely.

18K Gold Testing Solution

35% nitric acid (HNO₃)
40% hydrochloric acid (HCl)
25% distilled water
Effect: Dissolves 14K gold, 18K fades slowly, and 22K+ resists.

22K Gold Testing Solution

20% nitric acid (HNO₃)
60% hydrochloric acid (HCl)
20% distilled water
Effect: Dissolves 18K gold, 22K fades more slowly, and 24K resists completely.

24K Gold Testing Solution (Aqua Regia)

25% nitric acid (HNO₃)
75% hydrochloric acid (HCl)
No dilution with water
Effect: Dissolves all gold, including pure 24K gold.

5. Conclusion: Choosing the Right Acid for Accurate Gold Testing

By using the correct acid solutions and following the step-by-step process, you can accurately determine the karat purity of any gold item.

🔹 Use lower-strength nitric acid solutions for low-karat gold.
🔹 Increase HCl content as the karat level increases.
🔹 For 24K, only aqua regia will work.