Determination of Gold in Carbonate Geological Samples by Hydroquinone Volumetric Method with Aqua Regia Digestion

When using the traditional aqua regia decomposition aqua regia dissolving digestion-activated carbon enrichment and Hydroquinone Volumetric Method to determine gold in rock and soil geological samples, the gold in the insoluble residue on the Buchner funnel filter is less than 4% of the total gold, which is always dismissed. However, carbonate samples, dissolving by aqua regia, can form a large amount of silica, insoluble calcium and magnesium gels which lead to gold loss from inclusion and adsorption by the gels. In this study, the insoluble residues on the Buchner funnel filter were collected. The silicon in residues was removed using HF-H₂SO₄, and then dissolved by aqua regia again before measuring the gold in residues. The total gold in the sample is the sum of gold in the filtrate and insoluble residue. The gold contents in residues, source of gold and the effect of silica, calcium and magnesium gels on gold measurement were examined. Experimental results indicate that the content of gold in insoluble residue could account for approximately 18%-22% of the total gold in the carbonate sample. Standard addition recovery experiments also showed that gold in residues was from coated and adopted gold by silica gel and insoluble calcium and magnesium gel. The native gold which cannot be dissolved by aqua regia is minor. The formation of gels was directly related to the contents of Si, Ca and Mg. When the contents of CaCO₃ and MgCO₃ are more than 30% in the sample, the gold in insoluble residue must be considered in order to obtain an accurate result.