Gold is nearly always determined as the metal. The reducing agents generally employed are sulphur dioxide, oxalic acid, and iron(II) sulphate. If nitric acid is present it must be removed by repeated evaporation with concentrated hydrochloric acid, and the solution diluted with water. With sulphurous acid, small amounts of the platinum metals (primarily platinum) may be carried down with the precipitate. It is therefore usually necessary to re-dissolve the solid in dilute aqua regia and to re-precipitate the gold; oxalic acid gives a better separation from the platinum metals in the second precipitation, although the precipitate is somewhat finely divided. Iron(II) sulphate gives satisfactory results for gold alone, but difficulties are introduced if the platinum metals are subsequently to be determined. Oxalic acid is slow in its action, and yields a precipitate which is difficult to filter.
The best results are obtained with quinol (1,4-dihydroxybenzene) as the reducing agent. Precipitation in hot 1.2M hydrochloric acid solution is rapid, the gold is readily filtered, and occlusion of the platinum metals is negligible. Precipitation in the cold is complete in 2 hours. Palladium in the filtrate can be precipitated directly with dimethylglyoxime, whilst platinum in the filtrate may be determined either by evaporating to dryness in order to destroy organic matter and then digesting with a little aqua regia or by reduction with sodium formate and formic acid.
Gold may also be separated from hydrochloric acid solutions of the platinum metals by extraction with diethyl ether or with ethyl acetate; except in special cases these methods do not offer any special advantages over the reduction to the metal.
The solution must be free from nitric acid and about 1.2M with respect to hydrochloric acid (ca 5 mL of concentrated hydrochloric acid in 50 mL of water), and contain up to 0.2 g of Au in 50 mL. Heat the solution to boiling, add excess of 5 per cent aqueous quinol solution (3 mL for every 25 mg of Au), and boil for 20 minutes. Allow to cool, and filter either through a weighed porcelain filtering crucible or through a Whatman No. 42 or 542 filter paper; wash thoroughly with hot water. The small particles of gold remaining in the bottom of the beaker (easily seen with a small flash lamp) are best removed with pieces of ashless filter paper. Ignite the porcelain filtering crucible to constant weight. If filter paper is used, transfer to a weighed porcelain or silica crucible, burn off the paper carefully, and ignite to constant weight. Weigh as Au.