Stefan’s Dental Gold Experiment

By Stefan aka ‘Garage Chemist’ Loosely translated from German to English by Steven Sackett 12/08/2007

The original German version is located here: Versuchschemie.de Safety Precautions Hydrochloric acid, nitric acid and gold salts are corrosive.

Aqua Regia should not be stored as it slowly evolves chlorine (explosion hazard in sealed bottles). It must be mixed up fresh when needed. Only mix as much as you will need. Chlorine and sulfur dioxide are poisonous.

Ammonium chloride is harmful.

Platinum and palladium salts are highly allergenic. Contact with skin and inhalation of the dust must be avoided.

Trial Description Some time ago my father gave me a large quantity of gold teeth (full frame, high, weighing more than 30g after the removal of the ceramic with hydrofluoric acid) with a mandate to refine. It should contain about half gold with other precious metal ingredients. Together with the remnants of his masters thesis (Permador pins, alloy remains, palladium base alloy, gold jewelry, etc.), which were not useful in dentistry anymore, resulted in a sample that weighed about 56g. Here is a picture of the complete amount (the frame was cut into two parts, so that it would fit through the neck of the flask):

The gray parts are the finest, with 75% gold and 94% precious metal. This shows the gold bridges and tooth supports. The metal, I have brought into solution by adding 40ml Aqua Regia (10ml conc. HNO3, 30ml conc. HCl) every 3 hours at 80-100 ° C as required (or until no further reaction appeared), and the resulting solution was drawn out with a pipette. This method requires less Aqua Regia, but admittedly takes more time. A picture of the early dissolution processes (left of the large flask, is the resulting solution so far):

A total of about 300ml Aqua Regia was used and it produced about the same depth of solution. In the flask there remained silver chloride and some insoluble metal parts (too much gold jewelry will prevent the gold from dissolving in either nitric acid or in aqua regia due to the silver content). The silver chloride was cleverly precipitated using dilute HCl added to the Palladium extract; the acid was poured directly into the solution. The solution was then distilled until largely concentrated (not too far, it must still remain liquid, even after cooling! Absolutely do not overheat!), three times during the evaporation add 50ml concentrated HCl (“defuming", also not too concentrated here) but to fully remove the nitrate (the gas above the solution must appear as the vapors of HCl remaining completely colorless). Nitrate would prevent the precipitation of the gold.

There was about a 250ml or more increase in volume after defuming the solution with water and dilute HCl. There was a little silver chloride formed, which was caused by the very high HCl concentration in solution. It is filtered out. We are now going to precipitate the gold from the solution. The gold, which is present in the trivalent state, is reduced by the addition of sulfur dioxide gas at 80 ° C. The sulfur dioxide gas is generated by adding drop wise a concentrated HCl to Sodium hydrogen sulfate solution (of course many other acids used) into Sodium Bisulfate and lightly heating. About 0.4 mol of SO2 are necessary, the gas generator should be able to produce at least 0.5 mol of SO2. [EDIT by lazersteve:] Sulfur dioxide gas can be generated by adding HCl to Sodium Bisulphite and gently heating.

NaHSO3(s) + HCl(aq) SO2(g) + NaCl(aq) + H2O(l) [END EDIT] The solution is magnetically stirred. The metal arises in the form of dendritic crystal leaves, partly on the surface is a growth network. The picture shows the fascinating beauty of the incomplete gold crystals.

Here you can see the solution and the resulting crystals, the picture only partially shows how fascinating this is, in fact, it looks like thousands of tiny jagged gold crystals clinging to a spongy entity, which the magnetic stirrer compresses and compacts to a clump.

Once the SO2 is no longer absorbed and the reaction is completely cooled the reduction process is terminated (test! Heat the filtrate to 80 ° C and add more SO2). The gold should be vacuum filtered and rinsed with hot water.

The reaction produced 36g of raw gold. This gold now needs to be cleaned by dissolving it in aqua regia (it dissolves very quickly this time) (a picture of the gold solution):

The solution is concentrated three times while adding HCl to remove the nitrate fumes.

Now the gold solution is reduced to the metal again, this time with oxalic acid. 50 g of oxalic acid was dissolved in water, and placed into a dropping funnel, the gold solution is brought to a boil and, while stirring, the oxalic acid is added drop wise. Under CO2 development the gold becomes crystalline. After this the oxalic acid solution was still colored yellow (indicating an incomplete reduction), although there was an excess of oxalic acid used in the solution. The solution to this problem: Is the cautious addition of ammonia water until neutralized. The remaining gold precipitated as a fine powder with the vigorous production of CO2, the supernatant solution was virtually colorless. The gold was place in a vacuum filter again, washed, and dried. Image of pure gold:

Several 5 gram portions of gold mixed with borax are melted into beads in a crucible using a welding torch (Caution: the welding torch should be very slightly adjusted, otherwise there is a risk that the gold powder will be blown away). Finally, the beads are all melted into a single nugget.

Isolation of platinum and palladium Gaseous chlorine is introduced into the gold and silver foil solution at 20 ° C with stirring until a surplus of green colored chlorine gas is above the solution. The platinum and palladiumin the solution which had been reduced to the bivalent state by the SO2 bubbling is now oxidized back to its tetravalent condition being oxidized to hexachloroplatinate and palladate.

Now the solution is boiled thoroughly in direct air for an hour to reduce the Palladium back to the bivalent state (Hexachloropalladium acid upon boiling loses chlorine and decomposes into bivalent Palladium, the platinum remains tetravalent.) These different stable oxidation levels allow for the separation of platinum and palladium. We will now add 20g ammonium chloride dissolved in water to the warm solution. This forms the poorly soluble Ammoniumhexachloroplatinat. The solution is then placed in a vacuum filter and washed with cold water. Here’s how it looks:

This so-called "Platinsalmiac" in the test tube is decomposed to platinum sponge by prolonged heating to red heat (it sublimates ammonium chloride, and releases HCl). Here is the platinum sponge with water after stirring the sediment (it was previously partially stuck to the walls at the platinum level):

After cleaning with aqua regia, the solution is concentrated with HCl for defuming and ammonium chloride added. After filtering and drying there were 5.9 g of pure Ammoniumhexachloroplatinat. After an addition of chlorine to the red filtrate a deep red precipitate formed from what I initially considered only to be only palladium, but later found to also contain rhodium. In the still almost black solution left over from the platinum precipitation reaction the Palladium and chlorine is cooled to 20 ° C. The Palladium is the tetravalent condition oxidizes and precipitates with the existing excess ammonium chloride as yellowish brown to red colored Ammoniumhexachloropalladat.

It is immediately placed in a vacuum filter (otherwise it will slowly dissolve again due to nitrogen development!); it contains chlorine, and is washed with an ammonium chloride solution (not water). For me the filtrate was still dark, and after addition of chlorine gas once again formed a precipitate, which was much darker red than the previous one. It included rhodium. Thereafter, the filtrate was blue and thus palladium free. The palladium precipitates were combined and heated in dilute HCl which contained ammonium chloride until dissolved (the ammonium in tetravalent Palladium is oxidized to nitrogen by the boiling heat). To complete dissolution Sodium bisulfite was added at room temperature. Now the solution is made alkaline with ammonia. It is initially a flesh colored precipitation, produced mostly during heating (if necessary after decanting the supernatant solution and adding additional ammonia) it dissolves again, in an ideal situation a colorless solution results (for me it was green). The brown hydroxide is filtered out if necessary. The solution is placed in an ice bath and stirred with hydrochloric acid. The Palladium quantitatively precipitates out as a pure Dichlordiamminpalladium (II), which is canary yellow. It is then placed in a vacuum filter :

Note that the filtrate exhibits a red coloration! This is the Rhodium mentioned in the other thread. I still do not know how to best isolate it.

The Dichlordiamminpalladium can, if desired, be converted to a palladium sponge by slow heating. However it stores better if it is not decomposed. By heating with hydrochloric acid it dissolves into a brown solution of Tetrachloropalladium (II) acid, which can serve as a Hydrogenation catalyst (which used is identically to a solution of palladium (II) chloride in HCl). I worked many weeks on this long inorganic experiment. It will probably never quite be finished, because I have all the solution filtrates which I will keep forever to ensure that I do not lose any of the platinum metals. I will be working on the isolation of the Rhodium.