An alloy is a mixture or solid solution composed of metals. One common alloy is brass, a mixture of copper and zinc. Copper comprises between 60-90% of the mixture. Brass with a high amount of copper is corrosion resistant, which means it is often used in plumbing and ship fittings. Brass that contains less amounts of copper and higher amounts of zinc are stronger, and more economical. The purpose of this experiment was to determine the amount of copper in a sample of brass. The brass sample was taken from a seal. Based on prior knowledge it was predicted that the brass would contain 70-80% copper. The first step was to dissolve the brass, which gave us aqueous salts of copper and zinc. Aqueous is a term meaning solvent in water. Although copper is usually resistant to most acids, nitric acid is able to oxidize the metal. Oxidize means to be combined or become combined chemically with oxygen. Cu (s)+ 4 HNO3 (aq) -> Cu(NO3)2 (aq) + 2 NO2 (g) + 2 H2O (l) NaOH, a strong base, was then used to treat the solution. By using this base, the acid was neutralized, as well as causing the copper and zinc ions to for hydroxide precipitates. An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving the atom a net positive or negative electrical charge. A precipitate is a solid that is formed from a solution. Additional base was then added causing zinc hydroxide to re-dissolve forming Zn(OH)42- Zn(OH)2 (s) + 2 NaOH (aq) -> Na2 [Zn(OH)4] (aq) Since the only remaining solid was copper (II) hydroxide, the precipitate was filtered from the solution, dried, and weighed. This determined how much copper was in the solution. Copper (II) Hydroxide forms fine-grained crystals, which creates a problem with the particles clogging the filter paper. Some particles would also leak through the paper.
“Digesting” the sample, which involves heating the solution near the boiling point for 5 to 10 minutes, helps with filtering. This causes the crystals to increase in size, decrease in number, and clump together. In this case, the heating also caused a chemical reaction; copper (II) hydroxide was converted to copper (II) oxide. When the process of digestion is completed the precipitate can be filtered, dried, and weighed. This determines the amount of copper in the solution.
A sample of brass between 0.7 and 0.9 g was weighed and used. This specific piece used weighed 0.858 g. The sample was placed in a 100 mL beaker. While under the fume hood, 10 mL of 6M nitric acid was added to the beaker. Beaker was then placed on a hot plate and kept there until the brass was completely dissolved. When the reaction was completed, the beaker was removed from the hot plate and swirled for a minute or two to allow any remaining fumes to escape. The brown gas given off was nitrogen dioxide. 10 mL of 3M NaOH was placed in a graduated cylinder and about 1 mL at a time was added to the solution to dissolve the brass solution. After each addition the solution was stirred until the light blue precipitate re-dissolves. After a few milliliters the acid was neutralized and the precipitate no longer re-dissolved. The NaOH was no longer added after this point. This precipitate was a mixture of white Zn(OH)2 and blue Cu(OH)2. The total volume of the beaker was estimated and an equal amount of 3M NaOH solution was added. The precipitate redissolved the zinc hydroxide and precipitated the copper. The solution was then colorless with a blue precipitate. The beaker was then placed on a hot plate and stirred every 10 seconds. This was the process of digestion. When the precipitate formed a charcoal color digestion was stopped. The dark color was due to the presence of copper (II) oxide. The precipitate was allowed to settle for several minutes. Most of the liquid was then decanted off of the top. A