What solution can gold dissolve?

Fabrics Tetrachlorogold (III) acid solution 0.1%, silver nitrate solution 1%, tin (II) chloride dihydrate solution 1% (only freshly prepared!), Undyed cotton fabric
equipment Test tubes and test tube rack, test tube holder, burner with tripod, beaker 250 ml, tweezers, adhesive tape, laser pointer
security The substances used can cause severe irritation or burns to the skin and eyes. In all work with colloidal silver and gold, there is a potential risk of foul gold or fuzzy silver. When preparing the solutions and during all experiments, as well as when disposing of them, protective goggles and protective gloves must be worn. The colloidal solutions must not be stored, as the slow reaction with atmospheric nitrogen or with other nitrogen compounds creates explosive fused gold Au3N or Knallsilber Ag3N can form. Only the smallest amounts and the very dilute solutions may be used. The products are not allowed to be given home to the students. The highly diluted, colloidal solutions, which only contain a few milligrams of gold or silver colloids, can be disposed of in the cast (see Disposal of silver nitrate).


Making the solutions

Tetrachloroauric (III) acid 0.1%: 0.1 g of tetrachloroauric (III) acid are dissolved in 99.9 g of water.
Silver nitrate solution 1%: Dissolve 1 g of silver nitrate in 99 g of water.
Tin (II) chloride solution 1%: Dissolve 1 g of tin (II) chloride dihydrate in 99 g of water (prepare fresh, solution cannot be kept!).

Special notes: The solutions are filled into polyethene bottles with a dropper cap (no glass!). The bottle with the silver nitrate solution must be painted dark or covered with aluminum foil so that no light can enter. When preparing the tetrachloroauric (III) acid solution, only a glass spatula may not be used. The metal of a spatula can prematurely reduce the gold bond.

Tetrachloroauric acid forms a yellow solution with water

Alternatively, you could create the gold connection by dissolving a foil of gold leaf in aqua regia. The aqua regia is obtained by mixing 5 ml of 65% nitric acid with 5 ml of 37% hydrochloric acid in a test tube. The aqua regia can be diluted with plenty of water after the gold leaf has been dissolved. The yellow solution contains tetrachloroauric (III) acid. The production with aqua regia may only be carried out in the fume cupboard and under no circumstances by students. Also, aqua regia must not be stored in bottles, as chlorine is released from it, which can burst the bottle. For destruction, small amounts can be diluted with a lot of water and poured into the drain. When producing tetrachloroauric (III) acid from aqua regia, all products must be processed immediately. It is therefore recommended to purchase a maximum of 1 gram of tetrachloroauric acid (no longer for safety reasons!) From the chemical trade. One liter of the 0.1% solution can be made from this.

Experiment 1 Manufacture of colloidal gold

Before the experiment, the reagent solutions in the bottles should be shaken briefly. A test tube is then filled 2 cm high with distilled water. Then 10 drops of the 0.1% tetrachloroauric acid solution are added dropwise and shaken briefly. The addition of one or two drops of the 1% tin chloride solution creates an orange-yellow solution with colloidal gold (in the picture below: RG far left). If you shine a laser pointer through the solution, the Tyndall effect becomes apparent. If you increase the concentration in a second attempt and take 20 or 30 drops of tetrachloroauric acid solution, the colloidal solution remains only briefly after the addition of the tin chloride solution; red tinsel of red, colloidal gold flakes out of the solution (2nd row from left ). By dosing the drops and heating them with the burner flame, the formation of the colloids, the coloration and the flocculation can be varied or accelerated (3rd row from the left).

Colloidal gold (Au) and colloidal silver (Ag),
different stages of flocculation (coagulation) 

Experiment 2 Manufacture of colloidal silver
A test tube is filled 2 cm high with distilled water. Then 3 drops of the 1% silver nitrate solution are added dropwise and shaken briefly. Adding a drop or two of the 1% stannous chloride solution causes a reddish brown colloidal solution, from which brown colloidal silver will flocculate over time. This solution must not be stored for a long time, as the nitrogen in the air, Knallsilber Ag3N can form.

A comparison of flocculated colloidal gold (left) and silver (right)

Experiment 3 Dyeing textiles with Cassius' gold purple

Pickling the fabric
A strip 15 × 200 mm long is cut out of undyed cotton fabric so that it fits into a test tube. Then 100 ml of water and a spatula of tin chloride are placed in a 250 ml beaker. Then it is heated to the boil and the cotton strip is placed in the bath for a minute. The staining makes the fabric-fiber receptive to the dyeing and at the same time serves as a reducing agent in the reaction with tetrachloroauric acid. Take out the fabric with tweezers and wash it under cold water.
Dyeing the fabric
A test tube is filled 2 cm high with distilled water, 30 drops of 0.1% tetrachloroauric acid solution are added. Then you hang the pre-pickled, still damp fabric strip in the test tube and fasten it to the opening with an adhesive tape. The piece of fabric should dip about an inch into the solution. When the test tube is heated over the burner flame, the fabric turns purple. For safety reasons, large quantities of tissue must not be dyed or stored. The product is washed well with water and then disposed of.

Dyeing cotton with Cassius' gold purple

The gold purple is named after Andreas Cassius, who produced the pigment in Leiden around 1676. The red coloring was probably already known to the alchemists in the Middle Ages, as they experimented extensively with gold and produced the aqua regia to dissolve the gold from nitric acid and ammonium chloride. Other substances such as sodium citrate or tannic acid and sodium carbonate are also suitable as reducing agents. The ancient Romans used colloidal gold and colloidal silver to color glass. The secret of the red glass windows in churches and cathedrals is based on the addition of finely divided gold during glass production. Colloidal gold appears red when the particles are around 20 nanometers in size. Depending on the manufacturing conditions, larger gold particles also arise, as is the case with the textile dyeing described above. The purple gold on the piece of textile contains particles up to 100 nanometers in size.