Silver reacts with royal water
|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
Experiment 1 Manufacture of colloidal gold
Colloidal gold (Au) and colloidal silver (Ag),
different stages of flocculation (coagulation)
Experiment 2 Manufacture of colloidal silver
A comparison of flocculated colloidal gold (left) and silver (right)
Experiment 3 Dyeing textiles with Cassius' gold purple
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
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