Selected Gold Salts in Water: Speciation and Treatment
Potassium and sodium tetrachloroaurate(III) are specialist gold reagents that may be relevant to industrial process streams. Their inclusion here does not imply that these salts are prevalent in natural water. This page complements the overview of the gold element and explains why a reagent formula, a measured gold concentration and the species present in a water sample are different things.
For the broader chemistry, sampling and treatment context, see gold in water.
Selected gold salts
| Reagent | Formula | Source |
|---|---|---|
| Potassium tetrachloroaurate(III) | K[AuCl4] | Manufacturer product information |
| Sodium tetrachloroaurate(III) dihydrate | Na[AuCl4]·2H2O | Manufacturer product specification |
| Potassium dicyanoaurate(I), also called potassium gold cyanide | K[Au(CN)2] | Manufacturer product information; a water-soluble gold(I) salt used in gold electroplating. |
The table identifies specific products, including the sodium salt’s two waters of crystallization. Names such as “gold(III) chloride” or “chloroauric acid” may refer to different materials in product listings. Specify the actual formula and hydrate rather than treating an acid and a neutral salt as interchangeable. A numerical solubility should not be assumed without defined conditions.
Solubility in water
The referenced potassium tetrachloroaurate(III) and potassium dicyanoaurate(I) products are described as water-soluble. These qualitative descriptions do not establish numerical saturation limits. For sodium tetrachloroaurate(III) dihydrate, Scharlab reports a water solubility of 300 g/L at 20 °C. This value refers to the named hydrated salt, not to elemental gold, and is not a guaranteed solubility in wastewater. Confirm the product identity, temperature and solution composition before using solubility data in a process assessment.
Potassium gold cyanide is an acutely toxic cyanide-containing reagent. Its manufacturer warns that contact with acids liberates very toxic gas. Cyanide-bearing streams require specialist handling and treatment assessment; gold removal alone does not establish that cyanide or other contaminants have been removed.
Speciation and analytical reporting
A solid reagent’s formula does not establish the final distribution of gold species after it enters water. Chloride concentration, pH, redox conditions and other ligands in the process matrix can affect the species present. Treatment selection therefore depends on the stream, not just the name of the starting salt.
Check whether an analytical result is expressed as mg/L as Au or as mass of a named compound. Converting between them requires the correct formula, hydrate and, where applicable, product assay. Also distinguish total gold from gold measured after filtration. These results can help assess particulate versus filter-passing gold, but filtration alone does not identify individual dissolved species.
Treatment considerations
If gold is associated with separable solids, solids removal may be evaluated. For gold present in dissolved complexes, sorption, ion exchange or membrane processes may be considered, subject to testing against the actual water matrix and the required separation objective. A measured reduction in one stream should be assessed alongside gold in collected solids, spent media, concentrate and other residual streams to establish a mass balance.
For an industrial assessment, define the reagent or source material, analytical reporting basis and relevant water chemistry before comparing treatment options.
Concentration: mg/L as Au versus mg/L of salt
The CIAAW standard atomic weight of gold is 196.96657. Therefore, 1 mmol/L of gold corresponds to approximately 196.967 mg/L as Au. For a compound of known composition, multiply its concentration in mg/L by the number of Au atoms per formula unit and by 196.96657, then divide by the compound's molar mass in g/mol.
Sodium tetrachloroaurate(III) dihydrate has a molar mass of approximately 397.80 g/mol. Its theoretical gold mass fraction is approximately 0.4951, or 49.51%. Thus, 100 mg/L of the pure dihydrate corresponds to approximately 49.51 mg/L as Au, assuming complete dissolution. Include both waters of crystallization in this calculation. Correct for the actual reagent assay; purity reported on a metals basis is not automatically the gold mass fraction of the whole reagent.
Removal and concentration for recovery
Removal from treated water transfers gold into another stream unless it is recovered as a separate material. Evaluate both the treated water and the captured material when testing solids separation, sorption, ion exchange or membranes. Gold concentration in a liquid recovery stream equals the gold mass collected in that stream divided by its volume. Co-concentrated ligands and other metals also affect whether that stream is suitable for further recovery.
As an illustrative mass balance, 1,000 L of feed at 1 mg/L as Au contains 1,000 mg of gold. If 90% of this mass enters 10 L of concentrate, that concentrate contains 900 mg, equivalent to 90 mg/L as Au and a concentration factor of 90. This is not a predicted removal efficiency or recovery yield. Actual performance, losses and the achievable concentrate volume require assessment with representative process water.