Analysis of Copper in a Nickel Coin
Background:
US nickels consist of an alloy of nickel and copper
(called “cupronickel”). Similar cupronickel alloys are used for
the production of coins in many countries. In this experiment
you will determine the exact mass percentage of copper in a US
nickel. The accepted value is 75.0% Cu.
Discussion:
One of the special properties of nitric acid is that in
addition to being a strong acid, it is also a very powerful
oxidizing agent. These properties make it extremely reactive but
give it broad applications as well. One of its applications is
to clean and etch metal surfaces, such as stainless steel.
For this experiment, nitric acid will be used to dissolve
the US nickel. Nitric acid rapidly oxidizes copper and nickel
metals into their highest oxidation state ions while producing
brown nitrogen dioxide gas. The resulting solution can then be
analyzed for metal ions after it has been boiled off to remove
excess nitric acid. Due to the dangerous nature of this
reaction, it will be carried out prior to the experiment and
nickel solution will be provided. Given the solution, the amount
of metal must then be determined.
Thankfully, this may be easily determined by way of titration. Often,
titrations are associated with acid-base chemistry, but their applications may
extend far beyond those limits and be used in determinations involving complex ions
(complexometric methods) and redox reactions (permanganometric and potentionmetric
methods). This experiment will utilize the iodometric titration, characterized by
the unique reaction between iodine and thiosulfate. While most halogens will
oxidize thiosulfate to sulfate, iodine, in the form of the triiodide ion (I 3-), will
oxidize it to tetrathionate (S 4O62-). Furthermore, iodine will react with starch to
form a deep purple complex. These characteristic reactions make it highly useful to
use thiosulfate in determining iodine concentrations since reaction progress can be
monitored with starch indicator—the purple color disappears as all iodine is
consumed.
This method will be used to indirectly determine copper concentrations.
Copper(II), one of the rare metal ions with a positive reduction potential, will
react with iodide to form copper(I) and triiodide while nickel will not. By adding
an excess of KI, the formation of triiodide then directly corresponds to the copper
in solution, so iodometric titration can be used to indirectly calculate the amount
of copper.
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Chemicals:
Nickel solution, a.k.a Nickel juice (5.000g/L)
0.0400 M Na2S2O3
KI solution (1.0M)
Starch indicator
Procedure:
1. Fill the buret with Na2S2O3 solution.
2. Pipette 10.00 mL of nickel solution into a 125 mL Erlenmeyer flask. Add about 5 mL
of KI solution. Swirl.
3. Titrate the solution in the Erlenmeyer flask until the color has faded. Add 3-4
drops of starch indicator at that point.
4. Continue to titrate to a pale white or pink endpoint.
5. Repeat this procedure as many times as possible (at least 2 times).
Data Table:
For your analysis, show how you determined the percent copper in the nickel. Assume that
KI was added in excess to the copper(II).
Pre-Lab Study Assignment:
Analysis of Copper in a Nickel Coin
1) Give the balanced equations for the following reactions. Use the half-reaction method to balance
them:
a. The nickel coin (Ni & Cu) is dissolved in nitric acid producing the metal ions in their most common
oxidation state, nitrogen dioxide gas and water.
b. KI solution is added to the nickel solution to produce a precipitate of copper (I) iodide and iodine.
c. The resulting iodine is titrated with thiosulfate to produce tetrathionate (S4O62-) and iodide.
2) A student prepares this experiment with a random cupronickel alloy.
a. The student creates 100.0 mL of solution with 0.874g Na2S2O3●5H2O. What is the molarity of
the solution?
b. A solution of nickel juice was made by dissolving 4.5 grams of metal alloy in 150.0mL of
solution. What is the mass of metal that would be in 13.0 mL of the metal solution?
c. The student then adds excess KI to the 13.0mL sample to form copper(I) iodide and iodine,
then titrates it to an endpoint with 53.67 mL of the solution from part a. What mass of copper
is in solution?
d. What is the mass percent of copper in that alloy? What is the mass percent of nickel in that
alloy?
e. In a separate situation, if the student adds 15.0 mL of a 0.200 M KI solution with 15.0 mL of
the metal solution from part b, which reactant is the excess reagent? How much excess is left?
3) How will the following errors affect the mass percent of copper determined in 2d?
a. A student forgets to rinse the burette with thiosulfate solution after cleaning it properly.
b. A student forgot to add starch indicator.
c. A student failed to add enough KI solution.
d. The TA did not boil off all of the nitric acid before making the nickel solution.