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"Gold chloride" and "gold chloride tetrahydrate" are other names for chloroauric acid. HAuCl4 • 4H2O is the chemical formula. It is soluble in water, ethanol, ether, and chloroform, and is somewhat soluble in chloroform. The ethanol solution can be used to crystallise anhydrous chloroauric acid. It will break down in the presence of heat, and at 120° C, it will decompose into gold trichloride. After adding a cold potassium hydroxide solution, gold hydroxide precipitates. Sulphur dioxide was used to convert it to gold powder.
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Key Terms: Ethanol, Methanol, Anhydrous, Aqueous Solution, corrosive, chloroform
Chloroauric Acid
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Chloroauric acid is a solid that is orange-yellow and is made up of inorganic compounds. This acid is also a precursor for other gold compounds. Furthermore, this chemical is used as a purification step for the gold. Gold chloride and gold chloride tetrahydrate are other names for this acid. Because the acid is dangerous and corrosive, it should be avoided.
Structure of Chloroauric Acid
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Below is the structure of chloroauric acid:
Formula of Chloroauric Acid
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The hydrogen bond donor has a property value of 1. Similarly, the hydrogen bond acceptor's value is one. Chloroauric acid is represented by the chemical formula HAuCl4. Moreover, chloroauric acid's linear formula is AuCl4H. Also, for chloroauric acid, the simplified molecular-input line-entry system is [H+]. Cl[Au-](Cl)(Cl)Cl.
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Properties of Chloroauric Acid
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Following are the properties of Chloroauric Acid:
Chemical Properties of Chloroauric Acid
| IUPAC name | hydro tetrachloro gold(1-) |
| Molecular formula | HAuCl4 |
| Molecular mass | 339.785 g/mol (anhydrous) 393.833 g/mol (trihydrate) 411.85 g/mol (tetrahydrate) |
| Density | 3.9 g/cm3 2.89 g/cm3 |
| Melting point | 254oC |
| Appearance | Yellow-orange needle-like crystals |
| Solubility | H2O: soluble |
| Sensitive | Light Sensitive |
| Form | Liquid |
| Stability | Stable. Hygroscopic. Light sensitive. Incompatible with strong bases, sulphides, cyanides, carbonates. |
Physical Properties of Chloroauric Acid
- Chloroauric acid is a monoprotic acid with a high molecular weight. H5O2+ is also present in the crystalline tetrahydrate. AuCl4. There are also two water molecules in it.
- Chloroauric acid is a hydrophilic (ionic) protic solute in solid form. In addition, chloroauric acid is water-soluble. This acid is soluble in a variety of oxygen-containing solvents, including ethers, esters, alcohols, and ketones, in addition to water.
- Chloroauric acid is converted to water, metal, and tetrachloroaurate when it is treated with a standard base. In all non-reacting solvents, the thallium salt, which is related, has poor solubility. The salts of quaternary ammonium cations are known to experts.
- In a two-phase redox reaction, gold nanostructures can be made from chloroauric acid. Metallic clusters are formed in this reaction by the simultaneous attachment of thiol monolayers that self-assemble on the expanding nuclei.
- Chloroauric acid produces oxonium dichloroisocyanurate(1) after partial reduction. Furthermore, reduction may result in the formation of additional gold(I) complexes, particularly when organic ligands are present.
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Uses of Chloroauric Acid
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- Hydrogen tetrachloroaurate(III) is a precursor used in the electrolytic purification of gold. Gold recovery, concentration, purification, and analytical determination are all done with it.
- Chloroauric acid has traditionally been used to refine gold.
- Gold nanoparticles with diameters ranging from 5 nm to 400 nm have been generated from chloroauric acid thanks to advances in nanotechnology.
- One of the most common uses for chloroauric acid is as a gold toner in photography. A chemical technique that alters the colour of an image is known as toning.
- Cranberry glass, often known as "Gold Ruby," is a red glass created by mixing chloroaurate or colloidal gold into molten glass.
Points to Remember
- Gold is one of the least reactive metals but gold reacts with aqua regia to yield chloroauric acid.
- The combination of HCl and HNO2 (hydrochloric and nitric acids) produces agua Regia acid, which is the only acid that dissolves gold.
- In histology, chloroauric acid is known by the name of “brown gold chloride”.
- The AuCl-4 anion has square planar molecular geometry.
- It interacts with KI in an alkaline solution to form brown gold. In the instance of tin dichloride, it produces "cinnamon purple" in an acidic solution.
- After dealing with methyl orange, its aqueous solution turns purple, then green after 1 to 3 drops of hydrochloric acid, and finally lavender after gold salt solution.
Sample Questions
Ques: How do you precipitate gold from auric chloride? (2 Marks)
Ans: The most common procedure for making auric chloride is to dissolve gold in aqua regia and then evaporate the liquid to dryness at the lowest temperature possible. As a result, a brownish-red mass is generated, containing AuCl3 mixed with varying amounts of proto chloride and hydrochloric acid.
Ques: What happens when gold chloride is heated? (2 Marks)
Ans: Aqueous AuCl3 solutions react with an aqueous base, such as sodium hydroxide, to generate an Au(OH)3 precipitate, which dissolves in excess NaOH to form sodium aurate (NaAuO2). Au(OH)3 decomposes to gold(III) oxide, Au2O3, and ultimately to gold metal when gently heated.
Ques: What is the absorption maxima for gold nanoparticles? (2 Marks)
Ans: Various sizes of gold nanoparticles have different absorption spectra. The greatest absorption peaks are located at 520-530 nm when the diameters of gold nanoparticles are between 12 and 41 nm, and there are redshifts gradually as the diameters of gold nanoparticles grow.
Ques: How does the Wohlwill process work? (2 Marks)
Ans: The Wohlwill procedure uses electrolysis to increase purity to around 99.99 percent. In this method, a cast of impure gold is mixed in a solution of electrolyte hydrochloric acid and gold chloride. The casting acts as a positively charged electrode, or anode when it is exposed to an electric current.
Ques: What is gold III chloride used for? (2 Marks)
Ans: Evaporation produces bright yellow crystals (chloroauric acid), which are heated to make dark red gold(III) chloride crystals. At 175°C, the chloride quickly decomposes into gold(I) chloride and chlorine; at higher temperatures, it decomposes into gold and chlorine. In photography, gold(III) chloride is utilised.
Ques: Can we make gold artificially? (2 Marks)
Ans: The chemical element gold has 79 protons in each of its atomic nuclei. Every atom with 79 protons is a gold atom, and all gold atoms function chemically in the same way. Nuclear reactions involve the addition or removal of protons from a nucleus. In this result, there is no chemical process which can ever produce gold.
Ques: How do you remove gold from the aqua regia solution? (2 Marks)
Ans: Boiling or a chemical reaction is employed to eliminate the nitric acid from the excess aqua-regia utilised in digestion. A reducing reagent is added to convert the gold chloride into solid gold particles while leaving the other metal chlorides unaffected and in solution, allowing the gold to be recovered as metal.
Ques: How do you neutralise aqua regia? (2 Marks)
Ans: Pour any leftover aqua regia into a big container of ice (500 grams of ice per 100 mL of aqua regia). Until the pH is neutral, neutralise the mixture with an aqueous basic solution, such as 1M or 10% sodium hydroxide (NaOH) or saturated sodium bicarbonate (NaHCO3) in water.
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