A discussion with a friend via gmail chat earlier today sparked my mind about high purity precious metals. Sprinkled throughout the literature are methods for obtaining metals and compounds of exceptional purity.
Here is one such method that reliably produces 6N+ pure gold.
As I find methods, I will continue to post them. At the very least, some of us here may have academic interests.
Method for UHP gold:
All acids and reagents used must be of trace metal grade. Recommended is the Trace Metal line of products from J.T. Baker. Under no circumstances should any metal containing object or apparatus contact the gold or solution; only aqua regia boiled quartz and PTFE apparatus should be used.
Gold of 99,995% purity (20 g.) well rinsed in ether and then in HCl is dissolved in aqua regia in an 800-ml. acid washed quartz jointed flask, and the solution is concentrated to a thick syrup at 80 degrees Centigrade, removing the most part of the nitric acid. It is then boiled 5 times further and periodically 20 ml. of hydrochloric acid (4:1) is added to expel nitrogen oxides. The residue is taken up in 650 ml. of hot 18.2 MΩ water and digested until all soluble material is dissolved. It is then allowed to settle for eight days in a dust-free atmosphere, preferably in a closed system. The precipitate consists of AgCl containing small amounts of Au, Pd, SiO2, etc. The gold solution is vacuum filtered through a micron quartz frit via decanting (do not disturb the precipitate). This and all later precipitates are not worked up further to obtain gold.
Gold refined by use of SO2 still contains some Pd, while that precipitated with oxalic acid contains Cu, Pb and other metals. Therefore both of these procedures must be used to obtain gold of the desired purity. Ultra high purity sulfur dioxide is passed through the warm gold solution (80°C) obtained above; the gold precipitates quantitatively on careful neutralization with ammonia (1:1). The product is allowed to settle overnight and the deposit of spongy gold is washed by decantation (7 times) with 200 ml hot 18.2 MΩ water; it is then heated for four hours on a steam bath with concentrated hydrochloric acid and washed free of acid with hot 18.2 MΩ water. Then it is redissolved in aqua regia in a quartz flask, the nitrogen oxides removed, filtered, and reduced. The entire procedure above is repeated seven times in order to remove all Ag, Cu, Ni, Zn and Pb. It is no longer necessary for the gold solution to sit. The precipitated gold product is then digested for 12 hours with ammonia (1:1), washed free of ammonia with 18.2 MΩ water, heated for four hours on a steam bath with hot concentrated nitric acid, and decanted. Ammonia (1:1) is again added and later removed by washing with 18.2 MΩ water . The gold sponge is dissolved in dilute aqua regia; after addition of HCl, the solution is concentrated by evaporation, diluted with UHP H2O, decanted and filtered.
The gold is precipitated by careful addition (there is a danger of foaming over) of small portions of powdered, UHP crystalline oxalic acid (ACS grade may be resublimed twice). If the solution retains a yellow colour, it is carefully neutralized with ammonia and more oxalic acid is added; the addition of the acid is continued until the solution remains colorless. The resultant gold sponge is again dissolved and reprecipitated with oxalic acid. It is then Pd-free. Finally the gold is redissolved, precipitated with sulfur dioxide and heated with concentrated hydrochloric acid, and washed with 18.2 MΩ water. The residual traces of HCl are removed with ammonia and it is again washed with UHP water. The product is transferred to a quartz dish and dried at 110°C. Yield 75-80%. The gold prepared in this manner is spectroscopically and analytically pure (free of metallic Cu, Ag, Ni, Pd, Fe, Zn and Pt) and is above 99,9999% in quality.
Here is one such method that reliably produces 6N+ pure gold.
As I find methods, I will continue to post them. At the very least, some of us here may have academic interests.
Method for UHP gold:
All acids and reagents used must be of trace metal grade. Recommended is the Trace Metal line of products from J.T. Baker. Under no circumstances should any metal containing object or apparatus contact the gold or solution; only aqua regia boiled quartz and PTFE apparatus should be used.
Gold of 99,995% purity (20 g.) well rinsed in ether and then in HCl is dissolved in aqua regia in an 800-ml. acid washed quartz jointed flask, and the solution is concentrated to a thick syrup at 80 degrees Centigrade, removing the most part of the nitric acid. It is then boiled 5 times further and periodically 20 ml. of hydrochloric acid (4:1) is added to expel nitrogen oxides. The residue is taken up in 650 ml. of hot 18.2 MΩ water and digested until all soluble material is dissolved. It is then allowed to settle for eight days in a dust-free atmosphere, preferably in a closed system. The precipitate consists of AgCl containing small amounts of Au, Pd, SiO2, etc. The gold solution is vacuum filtered through a micron quartz frit via decanting (do not disturb the precipitate). This and all later precipitates are not worked up further to obtain gold.
Gold refined by use of SO2 still contains some Pd, while that precipitated with oxalic acid contains Cu, Pb and other metals. Therefore both of these procedures must be used to obtain gold of the desired purity. Ultra high purity sulfur dioxide is passed through the warm gold solution (80°C) obtained above; the gold precipitates quantitatively on careful neutralization with ammonia (1:1). The product is allowed to settle overnight and the deposit of spongy gold is washed by decantation (7 times) with 200 ml hot 18.2 MΩ water; it is then heated for four hours on a steam bath with concentrated hydrochloric acid and washed free of acid with hot 18.2 MΩ water. Then it is redissolved in aqua regia in a quartz flask, the nitrogen oxides removed, filtered, and reduced. The entire procedure above is repeated seven times in order to remove all Ag, Cu, Ni, Zn and Pb. It is no longer necessary for the gold solution to sit. The precipitated gold product is then digested for 12 hours with ammonia (1:1), washed free of ammonia with 18.2 MΩ water, heated for four hours on a steam bath with hot concentrated nitric acid, and decanted. Ammonia (1:1) is again added and later removed by washing with 18.2 MΩ water . The gold sponge is dissolved in dilute aqua regia; after addition of HCl, the solution is concentrated by evaporation, diluted with UHP H2O, decanted and filtered.
The gold is precipitated by careful addition (there is a danger of foaming over) of small portions of powdered, UHP crystalline oxalic acid (ACS grade may be resublimed twice). If the solution retains a yellow colour, it is carefully neutralized with ammonia and more oxalic acid is added; the addition of the acid is continued until the solution remains colorless. The resultant gold sponge is again dissolved and reprecipitated with oxalic acid. It is then Pd-free. Finally the gold is redissolved, precipitated with sulfur dioxide and heated with concentrated hydrochloric acid, and washed with 18.2 MΩ water. The residual traces of HCl are removed with ammonia and it is again washed with UHP water. The product is transferred to a quartz dish and dried at 110°C. Yield 75-80%. The gold prepared in this manner is spectroscopically and analytically pure (free of metallic Cu, Ag, Ni, Pd, Fe, Zn and Pt) and is above 99,9999% in quality.
. The machine to make that water costs (or used to) about $8k, oh and did I mention that it likes to break down and that you need to replace cartridges on it. Water that pure is a pain in the butt to make--multiply distilled deionized water that's been passed over all sorts of ion exchange resins, activated carbon, assorted membranes, and then repeated. Unfortunately, if you left such water in an untreated glass bottle or out in the open air, it would be ruined! It would be basic from reacting with terminal hydroxys on the glass surface, and if exposed long enough to the atmosphere, it will become acidic from absorbing CO2. Now you see why analytical chemistry is somewhat challenging--variables, and lots of them!
