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31 October 2003

Make Your Own Chemicals: Recovery of Gold

by Norm Stanley

Those of us who are into dabbling in chemistry are painfully aware of the difficulties encountered  in finding suppliers who will sell chemicals and lab. equipment to the individual experimenter. In  some cases, however, industrial and household chemicals and solvents sold in hardware stores and supermarkets can be substituted for reagent grade chemicals or used as starting materials for preparing others.  The down side is that some items once commonly available seem to be disappearing from the shelves.  Nobody makes her own soap anymore, so "concentrated lye" (sodium hydroxide) is not in demand at the supermarket.  Hardware shops, once fine sources for raw materials, are increasingly dealing in "card goods".  Your friendly neighborhood pharmacy once would sell chemicals or order them for you, but don't look for your local Rite-AidTM
 to oblige you.

I am proposing "Make Your Own Chemicals" as a department in the E-Bulletin where the experimenter (that's YOU) can present tips on sources of chemicals and procedures for adapting them for laboratory use.1  This might involve purifying, if necessary, or using to prepare other chemicals.  Whether this flies will depend on your contributions.

This said, here is the method I  used to prepare the gold salt specified in this issue's "Chemistry Corner":

Recovery of Gold

Chlorauric acid, HAuCl4, can be prepared from scrap gold such as discarded jewelry (in my case a wedding band sawed off a dislocated finger!).  In addition to what might be found around the house, a jeweler, goldsmith or dental laboratory might have scrap they'd be willing to sell reasonably.  Gold plated jewelry contains too little gold to be worth working up.  Pure gold is a rather soft metal; for durability it is alloyed with other metals, principally copper, silver and zinc.  In the trade gold content is expressed in units of 1/24, called karats.  Thus 12 karat gold is 50% gold, pure gold is 24 karat.

Gold is insoluble in the common mineral acids, but it soluble in aqua regia, a mixture of 1 volume of conc. nitric acid and 3 volumes of conc. hydrochloric acid:

Eq. 1:  Au + 5H+ + 4Cl- + NO3- -----> HAuCl4 + NO + 2H2O
Due to its strong tendency to form complexes, gold in the presence of chloride is present here as the complex chloraurate anion, Au[Cl4]-, rather than the auric cation, Au+3.
Place the scrap in a beaker and cover with aqua regia.  Cover the beaker with a watch glass and heat gently on a hot plate or a gauze mat over a Bunsen flame.  Continue heating until evolution of fumes of nitrogen oxides ceases, then add more aqua regia and continue heating.  Repeat until all of the metal has gone into solution.   A small amount of nonmetallic residue may remain.  Dilute the solution with water to about ten times its volume and filter.  Wash the filter with water, adding the wash to the filtrate.

The filtrate contains the gold plus the other metals in the scrap.  To separate the gold, treat the filtrate with ferrous sulfate.  This will reduce the gold to metallic form while leaving the other metals in solution:
Eq. 2:  AuCl4- + 3Fe+2 -----> Au + 3Fe+3 +4Cl-
Prepare a stock solution of FeSO4.7H2O by dissolving 20 g in 100 mL distilled or deionized water by heating and stirring.  Add a few steel brads and two or three drops of conc. sulfuric acid to prevent oxidation.  Store in a tightly stoppered amber glass bottle.  If stored for any great length of time, open the bottle occasionally to let any hydrogen escape and add a few more drops of acid.  Dilute the stock solution with an equal volume of water for use in precipitating the gold.  Treat the filtrate with small portions of the dilute FeSO4; the gold will precipitate out as a very finely divided brownish powder.  Allow it to settle out and add more FeSO4.  Continue until no more gold is thrown down.  It is advisable to separate the gold from the solution by decantation rather than filtration, as the fine particles will lodge in the pores of the filter, or even pass through to some extent.  Decant off the supernatant and wash with several changes of water, decanting after each wash.  Check the wash water for sulfate by testing a small portion with barium chloride solution.  Continue until no more than a faint haze of BaSO4 is seen on testing.

If you have a sensitive balance you can dry and weigh your gold to see how good a yield was obtained.

Finally dissolve the gold in aqua regia and evaporate cautiously to obtain yellow crystals of chlorauric acid trihydrate.

Note:
1. Thanks to SAS member George F. V. Minnick for putting this bee in my bonnet.