Sodium meta bisulphite

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Sodium meta bisulfite ( SMB ) is a popular chemical for reducing gold chloride back to metallic gold. It is quite specific for gold so even contaminated solutions can give quite pure gold.

In Europe the E-number E223 is used for SMB and E224 for Potassium Meta Bisulphite (PMB).

Contents

Chemical reaction with gold chloride

Na2S2O5 + H2O = 2 NaHSO3
3 NaHSO3 + 3 H2O + 2 HAuCl4 = 3 NaHSO4 + 8 HCl + 2 Au

SMB and silver chloride

If there is any silver chloride dissolved in the gold chloride solution it can be precipitated as silver sulfite, AgSO3. This can happen if the gold chloride solution is high in chloride ions. The silver sulfite will follow the gold powder, end up in the melt and report as a silver contamination in the final gold bar.

To avoid silver contamination, concentrated gold chloride solutions should be diluted and filtered before precipitating the gold.

SMB and palladium

SMB and palladium(II) precipitate a yellowish palladium salt. It is very voluminous. The salt is approximately Na2Pd(SO3)2*2H2O. SMB does not precipitate palladium metal, pure SO2 is needed for that.

(Ref. Loewen, pg 124)

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References

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