Gold(III) Chloride
Properties
| State | Solid |
| Color | Red-brown to dark red |
| Solubility | Soluble in water (hydrolyzes); soluble in ethanol and HCl |
| Melting Point | 254 °C (decomposes) |
About Gold(III) Chloride
Gold(III) chloride is a red-brown crystalline solid that doesn't actually exist as the AuCl3 monomer in any condensed phase — it dimerizes to Au2Cl6, a planar molecule with two square-planar Au(III) centers bridged by two chlorides, like the structure of Al2Cl6 but with d⁸ Au(III) demanding square-planar geometry instead of tetrahedral. That d⁸ preference for square-planar coordination is the same one that drives Pd(II) and Pt(II) chemistry, and it's the structural motif that makes Au(III) catalysis interesting in organic synthesis. AuCl3 hydrolyzes rapidly in water and dissolves in HCl to give the tetrachloroaurate anion [AuCl4]⁻, isolated commercially as the yellow-orange chloroauric acid HAuCl4·3H2O — the form that >90% of gold-on-the-shelf reagent chemistry actually uses. Reduce HAuCl4 with sodium citrate in boiling water and you get the famous wine-red colloid of 10-20 nm gold nanoparticles via the Turkevich method; citrate first reduces Au(III) to Au(0) and then coordinates the nanoparticle surface to keep them from aggregating. Those colloids are what you see as the red line on a COVID lateral-flow test, the surface-enhanced Raman scattering substrates for trace molecule detection, and the photothermal absorbers in research-stage cancer treatments where nanoparticles concentrate in tumors and release heat under near-infrared illumination. AuCl3 is also seeing growing use as a homogeneous catalyst for alkyne hydration and gold-carbene-mediated C-H functionalization, chemistry that won Hashmi and Toste a lot of attention in the late 2000s.
Where you'll encounter it
If you've ever taken a rapid antigen test (COVID, pregnancy, strep), the colored line that shows up was made of gold nanoparticles synthesized from chloroauric acid — typically 20-40 nm spheres conjugated to antibodies, which migrate up the strip with the sample and concentrate at the test line if the analyte is present. In a nanoparticle synthesis lab running the Turkevich citrate method, you bring 100 mL of 0.25 mM HAuCl4 to a vigorous boil, inject 5 mL of 1% sodium citrate, and watch the wine-red color stabilize in five to ten minutes as 10–20 nm spheres nucleate. In a homogeneous gold-catalysis lab studying alkyne hydration, AuCl3 at 1–5 mol% in wet methanol gives Markovnikov hydration without needing the heavier Hg(II) catalysis that ruled the field before Hashmi's papers.
Common Uses
- Precursor for gold nanoparticle synthesis (Turkevich citrate method)
- Source of nanoparticles for lateral-flow rapid diagnostic test lines
- Au(III) catalyst for alkyne hydration and hydroarylation reactions
- Starting material for gold electroplating bath formulations
- Photothermal nanoparticle precursor for research-stage cancer therapy
- Reagent for SERS (surface-enhanced Raman scattering) substrate fabrication
- Historical photographic toner for albumen and silver-gelatin prints
Safety Information
GHS: Acute toxicity oral Category 4 (H302, harmful if swallowed), skin corrosion/irritation Category 2 (H315), serious eye irritation Category 2A (H319). Soluble Au(III) salts cause contact dermatitis ('gold rash') in sensitized individuals — about 1-2% of dental workers develop it from gold restoration alloys. Chronic systemic exposure causes chrysiasis, a permanent blue-gray skin discoloration from sub-dermal Au(0) deposits formed by photoreduction of accumulated gold. OSHA has no specific PEL. Practical handling: wear nitrile gloves (Au(III) won't permeate latex but will stain the skin), work in a hood when manipulating powder to avoid inhalation, and keep separate from cyanides (the [Au(CN)4]⁻ complex is far more bioavailable than [AuCl4]⁻). Aqueous solutions slowly photoreduce in sunlight to colloidal gold — store HAuCl4 stocks in amber glass.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.