United States nickels (0.05 coin) or other material made of a cupronickel alloy * Nitric acid soluti — Physical Chemistry — Thermodynamics Chemistry Question
Analysis of Copper in a Nickel Coin
United States nickels ($ 0.05 coins) consist of an alloy of nickel and copper (called “cupronickel”). Cupronickel alloys of similar composition are used for production of coins is some other countries. In this experiment you will determine the exact mass percentage of copper in a coin made of a copper-nickel alloy by dissolving the coin in nitric acid and determining the dissolved Cu(II) by iodometric titration.
Sample, Chemicals and Reagents
* US nickel ($0.05 coin) or other material made of a cupronickel alloy
* Nitric acid solution, HNO3 (aq), c = 8 mol dm–3
* Sodium thiosulfate pentahydrate, Na2S2O3 · 5 H2O
* Potassium iodide solution, KI, 10 % w/v
* Starch solution, 2 % w/v
Equipment and Glassware:
* Analytical balance (± 0.0001 g)
* Hotplate
* Erlenmeyer flasks, 250 cm3 and 125 cm3
* Volumetric flask, 100 cm3
* Volumetric pipet, 1.00 cm3
Procedure
1. Weigh the coin and then dissolve it by placing it in a 250 cm3 Erlenmeyer flask and then carefully adding 40 cm3 acid solution. Heat the flask on a hotplate while the dissolution takes place, over ~20 min (the flask should be in a fume hood, as NO2 gas is evolved). After dissolution of the coin is complete, continue to boil the solution for 20 min, then allow the flask to cool to room temperature. Dilute the solution to 100.00 cm3 with distilled water.
2. While the nickel is dissolving, make up 50 cm3 of sodium thiosulfate solution (c ≈ 0.04 mol dm–3). You will need to know the exact concentration of this solution; commercial crystalline Na2S2O5·5 H2O is sufficiently pure that the concentration can be determined accurately from its mass. The thiosulfate solution should be made up fresh on the day of the titration, as it degrades over time.
3. Into a 125 cm3 Erlenmeyer flask add 15 cm3 10% (w/v) KI solution, then 1.00 cm3 of the diluted copper-containing solution.
4. Titrate the yellow-orange slurry with the sodium thiosulfate solution until the color has faded to pale yellow. Then add 1 cm3 of the starch solution and titrate to the starch endpoint. At the endpoint, the mixture should be milky and white or very pale pink. The titration can be repeated on a fresh aliquot of the copper-containing solution, and the results averaged, for improved precision.
Give balanced chemical equations for the reactions that take place when:
i. The coin dissolves in nitric acid.
ii. The copper/nickel/nitric acid solution is added to the potassium iodide solution.
iii. The sodium thiosulfate solution is titrated into the mixture.
Model Answer
i. Cu(s) + 4 HNO3(aq) → Cu2+(aq) + 2 NO3–(aq) + 2 NO2(g) + H2O(l) (reaction to form NO is also reasonable)
Ni(s) + 2 H3O+(aq) → Ni2+(aq) + H2(g) + 2 H2O(l) (other reduction products such as NO or NO2 are also reasonable)
ii. 2 Cu2+(aq) + 5 I–(aq) → 2 CuI(s) + I3–(aq)
iii. I3–(aq) + 2 S2O3 2–(aq) → 3 I–(aq) + S4O6 2–(aq)
Calculate the mass percentage of copper in the coin.
Model Answer
Mass of nickel coin = 4.9739 g
Concentration of thiosulfate:
1.0315 g Na2S2O5 · 5 H2O / 248.17 g mol–1 = 4.156·10-3 mol S2O3 2–
4.156 · 10–3 mol S2O3 2- / 0.1000 dm3 = 0.04156 mol dm–3 S2O3 2–
Aliquot 1: 14.08 cm3 thiosulfate solution × 0.04156 mol dm–3 = 5.852·10–4 mol Cu
Aliquot 2: 14.22 cm3 thiosulfate solution = 5.910 · 10–4 mol Cu
Aliquot 3: 14.18 cm3 thiosulfate solution = 5.893 · 10–4 mol Cu
Average = (5.885 ± 0.024) · 10–4 mol Cu per aliquot
(5.885 ± 0.024) · 10–4 mol Cu/aliquot × [VISUAL] = (5.885 ± 0.024) · 10–2 mol Cu/coin × 63.546 g Cu·mol–1
= (3.740 ± 0.015) g Cu/coin ÷ 4.9739 g × 100 %
= 75.2 ± 0.3 % copper in the coin
The US Mint states that nickel coins are 75 % of Cu and 25 % of Ni by mass. That is in satisfactory agreement with this analysis.
If the coin is dissolved at room temperature, and the boiling step is omitted, then the amount of copper is overestimated, and the endpoint is not stable (the mixture turns white, but then spontaneously turns purple again within a few seconds). Explain why.
Model Answer
If the solution is not boiled, some NO2 remains dissolved in the solution and slowly oxidizes I– to I3 –, leading to more thiosulfate being required to reach the endpoint and the endpoint being unstable (as the residual NO2 slowly oxidizes the iodide even after the endpoint is reached).
The Canadian nickel coin is mostly steel, with nickel plating and a small amount of copper. Could this procedure be used to analyze the copper content of a Canadian nickel coin? Explain why or why not.
Model Answer
Steel will dissolve in nitric acid to give Fe3+, which interferes with the titration since it also oxidizes I– to I3 –.