Dissolving and Purifying Gold

Gold is a member of the group of inert metals called the noble metals. Very few chemicals react with these metals. The first reagent found that reacts with gold was a mixture of hydrochloric and nitric acids called aqua regia, or royal water in Latin. Tiny amounts of gold react with the nitric acid on the surface of the metal, and the products of this reaction react with the hydrochloric acid and are dissolved in solution.

Au + 6 HNO3 ---> Au(NO3)3 + 3 NO + 3 H2O

And then:

Au(NO3)3 + 3 HCl ---> AuCl3 + 3 HNO3
AuCl3 + HCl ---> HAuCl4

Once the gold is all dissolved it can then be slowly evaporated and crystals of pure chlorauric acid will form (HAuCl4).

Warnings:
Aqua regia is very corrosive!
Aqua regia will also slowly release NO, NO2, NOCl and Cl2 gasses, these are all toxic and corrosive. This experiment was done outside.
Chlorauric acid is toxic and will stain skin purple.















(This was from cleaning the glassware after the experiment. A single drop which was immediatly drowned in water)

Materials:
31.45% hydrochloric acid (pH down for pools)
Potassium nitrate (Stump remover, was substituted for nitric acid)
Alaskan gold dust

August 3, 2006
6.15g of 31.45% HCl was added to a test tube containing 1.15g gold dust and 1.34g KNO3.

KNO3 + HCl ---> KCl + HNO3

The bottom of the solution turned green and then the color was lost as the solution turned yellow. It began to bubble slowly. A faint smell of chlorine was noticed coming from the test tube. The gold began to turn black. Water was added until all KNO3 dissolved.

August 4
In the morning the gold was surrounded by a gray dust. The gold had stopped bubbling, however, it was not all consumed. A similar quantity of HCl and KNO3 was readded to the gold after the solution was decanted.

~5 hours later
The gold had again stopped bubbling. The test tube was decanted and a similar quantity of reagents was again added. The test tube was heated this time in a beaker of water. The test tube was again decanted and a similar amount of reagents added. The solution stopped bubbling while it was somewhat uncolored.















Left in the test tube was a gray metallic solid and a white dust. My guess is that the solids were composed of metallic lead or silver as well as ionic lead or silver, both have insoluble chlorides. The solution was filtered and the solids discarded. 72.7 Ml clear golden liquid was recovered. The solution was set out to evaporate but since the solution contained large amounts of KCl it was annoying to separate the two crystals. The solution was redissolved in water and the gold was recovered by adding zinc powder, displacing the gold in solution. The excess zinc was dissolved with HCl and the resulting gold powder was washed, dried, and stored.

The text presented here is for informational purposes only. The author is not liable for actions taken by the reader.

Demonstraitions with Dry Ice

Frozen carbon dioxide is sold in most grocery stores under the name 'dry ice' (Because it sublimes instead of melting. Since it sublimes at a very cold temperature and releases carbon dioxide in the process, this can lead to some interesting demonstrations.

Warnings:
Dry ice is very cold, it can easily cause your skin to freeze.
Dry ice expands by a large amount when subliming, never store it in a sealed container or it may rupture.

Materials:
1 lb of dry ice
Hot water
A lit candle
A zip lock bag
A flat metal surface of some kind

February 23, 2006
Several pieces of dry ice were placed in a container and it was covered with a piece of paper. A few minutes later the sublimed gas was pour over a candle. The candle went out immediately without smoking.

Several pieces of dry ice were placed in a sealed plastic bag. The bag slowly inflated and eventually burst.

Several pieces of dry ice were put in hot water. The dry ice bubbled hard enough to completely surround the submerged pieces with carbon dioxide. The surface of the water was covered in an opaque layer of fog which overflowed the container. The surface of the fog appeared smooth except for an occasional bubble of fog which would shoot up out of it about half an inch.

A flat piece of metal was placed against a piece of dry ice. This created a screeching sound as the carbon dioxide sublimed.

August 11, 2006
Demonstrations were repeated with pictures.

The text presented here is for informational purposes only. The author is not liable for actions taken by the reader.

Dissolving the Oxide Layer of Aluminum

Aluminum metal is very reactive, and yet is resistant against a wide range of chemicals. Some chemicals that it is resistant to include acids, reductants, ionic compounds, some bases and oxidisers. This is because aluminum has a very thin oxide layer that forms very quickly in our oxygen rich atmosphere. Anodized aluminum has a even thicker layer, built up using electrochemistry. There are only a few chemicals that can quickly destroy the oxide layer, mercury and alkali hydroxides to name two. If mercury is applied to a piece of aluminum, it will corrode away very quickly due to mercury disrupting the oxide layer. This is why mercury and mercury compounds are forbidden on airplanes. There is a few more chemicals which can quickly attack aluminum, one of them is the tetrachlorocopper anion. The tetrachlorocopper anion can be formed from any soluble copper salt and any soluble chloride salt or hydrochloric acid.

2 HCl + CuCl2 ---> H2(CuCl4)

2 Al + 3 H2(CuCl4) ---> 2 H(AlCl4) + 4 HCl + 3 Cu

2 Al + 6 H2O ---> 2 Al(OH)3 + 3 H2

Warnings:
All copper compounds are moderately toxic.
Hydrogen gas is formed in this reaction, which is extremely flammable and can explode.
Hydrochloric acid is corrosive and produces heat when mixed with water.

Materials:
31.45% hydrochloric acid (pH down for pools)
Copper (II) chloride (Any copper compound can be substituted)
Aluminum foil

September 21, 2005
A stoichiometric amount of copper (II) chloride and hydrochloric acid were combined, the solid turned black and the solution a dark green. Enough water was added to dissolve the reagents. A grey-white substance was left undissolved.

A few drops were put on a piece of aluminum foil and it bubbled vigorously, liberating hydrogen, steam and leaving behind a copper powder.

A graduated cylinder was filled with distilled water and inverted into a dish full of distilled water without letting any air into the cylinder. 0.12g of aluminum foil was placed in the cylinder. The prepared solution was forced into the top of the cylinder with a pipette. Water is also removed from the dish so it doesn't overflow. A few seconds after the concentrated solution was added to the cylinder, small brown hair-like growths appeared. About 15 minutes later the aluminum was covered in mossy copper deposits. Hydrogen was continuously bubbling off. This was continued for approximately 40 minutes until all aluminum was consumed. 39.6 mL of hydrogen gas was collected in the graduated cylinder.














Unanswered Questions:
By what mechanism does the tetrachlorocopper anion destroy aluminum oxide?

The text presented here is for informational purposes only. The author is not liable for actions taken by the reader.

Synthesis of Lead (II) Nitrate

Lead (II) nitrate is one of the few soluble lead compounds, thus is one of the few sources of lead (II) ions. Lead (II) nitrate is a clear, colorless ionic solid.

Since my lab is currently lacking nitric acid, lead (II) nitrate must be prepared by other methods. A possible route to lead (II) nitrate is a single replacement reaction between lead and the salt of a less reactive metal. First copper (II) nitrate must be prepared.

CuSO4 + 2 KNO3 ---> Cu(NO3)2 + K2SO4

Cu(NO3)2 + Pb ---> Pb(NO3)2 + Cu

Warnings:
Lead compounds are highly toxic. Lead will remain in a biological system for decades, doing damage. All lead compounds should be disposed of at your local hazardous waste disposal facility. Gloves and goggles were worn at all times during this experiment.
Copper compounds are moderately toxic.

Materials:
Potassium nitrate (Stump remover)
Copper (II) sulfate (Root killer for drains)
Lead metal (Lead fishing sinker)

July 27, 2005
59.3g potassium nitrate, 70g copper (II) sulfate and 200mL distilled water was added to a 300mL beaker. This is a slight excess of copper (II) sulfate so there is no potassium nitrate impurity since lead (II) sulfate will precipitate and copper will plate out. Immediately there was a precipitate, probably due to the low solubility of potassium sulfate. The beaker became slightly cool to the touch. The beaker was then heated on an hot plate and stirred until all of the reactants dissolved. When the beaker was cooled potassium sulfate began to precipitate again, this was allowed to continue until the beaker reached room temperature. The solution was decanted and filtered. 100mL of the solution was transferred to a beaker, the rest was transferred to a round bottom flask. A piece of lead metal was immersed in the solution. Very slight bubbling was observed. White fuzzy lead (II) sulfate soon appeared on the surface of the lead. This reaction was allowed to continue for several days until the solution turned colorless (No blue from copper ions).

Several days later the white fuzz was now covering the lead and the bottom of the beaker like moss. Copper formations had grown on the lead as well, which were covered in a blue dust. Water was added to replenish the water lost by evaporation. The solution was decanted and filtered and transferred to a flask. A few mL of the lead (II) nitrate solution was placed in a test tube and a few drops of weak ammonia solution was added. No noticeable change occurred but when the test tube was cleaned a white deposit was left behind on the glass. The lead contaminated glassware was cleaned thoroughly with a weak ammonia solution to precipitate the lead. All liquids used to clean the glassware were evaporated and the solids stored for safe disposal.

Unanswered Questions:

What is the blue dust and why did it form? Is it Cu(OH)₂?

The text presented here is for informational purposes only. The author is not liable for actions taken by the reader.