Why Sometimes Gold Is Present but Does Not Precipitate
When ore or soil is dissolved in aqua regia (a mixture of nitric acid and hydrochloric acid), and a positive result is obtained using stannous chloride (SnCl₂) — indicated by purple, brown, or black coloration — it confirms the presence of gold ions in the solution.
However, in some cases, adding common reducing agents such as zinc (Zn) or sodium metabisulfite (Na₂SO₃) fails to produce a visible gold precipitate. This issue often arises because gold forms a stable complex ion in aqua regia, known as tetrachloroaurate: [AuCl₄]⁻. If excess nitric acid remains in the solution or the pH is not properly adjusted, this complex remains highly stable and does not reduce easily to metallic gold.
In such cases, a white, gray, or sometimes purple haze may appear in the solution. This is not a failed reaction but rather the formation of colloidal gold — extremely fine particles of gold suspended in the liquid phase. These particles are too small to settle and remain dispersed, making the gold appear as a cloudy solution instead of forming solid precipitate.
To resolve this and achieve visible gold precipitation, the following steps are recommended:
Completely neutralize the remaining nitric acid using a neutralizing agent such as urea or by gentle heating.
Add a small amount of hydrochloric acid to maintain chloride ion concentration and stabilize the gold complex.
Add sodium chloride (NaCl) or ethanol to destabilize the colloid and promote coagulation (flocculation) of the gold particles.
Then proceed with reduction using zinc or sodium sulfite to obtain metallic gold as a visible powder.
In conclusion, if the gold test is positive but no metal precipitates, it does not necessarily mean that gold is absent. It may be present in colloidal form, requiring proper treatment to be recovered successfully.
Golden Science
Why Sometimes Gold Is Present but Does Not Precipitate
When ore or soil is dissolved in aqua regia (a mixture of nitric acid and hydrochloric acid), and a positive result is obtained using stannous chloride (SnCl₂) — indicated by purple, brown, or black coloration — it confirms the presence of gold ions in the solution.
However, in some cases, adding common reducing agents such as zinc (Zn) or sodium metabisulfite (Na₂SO₃) fails to produce a visible gold precipitate. This issue often arises because gold forms a stable complex ion in aqua regia, known as tetrachloroaurate: [AuCl₄]⁻. If excess nitric acid remains in the solution or the pH is not properly adjusted, this complex remains highly stable and does not reduce easily to metallic gold.
In such cases, a white, gray, or sometimes purple haze may appear in the solution. This is not a failed reaction but rather the formation of colloidal gold — extremely fine particles of gold suspended in the liquid phase. These particles are too small to settle and remain dispersed, making the gold appear as a cloudy solution instead of forming solid precipitate.
Chemical Example: H[AuCl₄] + weak reducing agent → Au⁰ (colloid) + HCl + byproducts
To resolve this and achieve visible gold precipitation, the following steps are recommended:
Completely neutralize the remaining nitric acid using a neutralizing agent such as urea or by gentle heating.
Add a small amount of hydrochloric acid to maintain chloride ion concentration and stabilize the gold complex.
Add sodium chloride (NaCl) or ethanol to destabilize the colloid and promote coagulation (flocculation) of the gold particles.
Then proceed with reduction using zinc or sodium sulfite to obtain metallic gold as a visible powder.
In conclusion, if the gold test is positive but no metal precipitates, it does not necessarily mean that gold is absent. It may be present in colloidal form, requiring proper treatment to be recovered successfully.
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3 months ago | [YT] | 60