Separating nickel coin into copper and nickel components
I recommend to dissolve the copper-nickel alloy in nitric acid and determine the dissolved $\ce{Cu(II)}$ by iodometric titration.


Start by weighing the coin, then dissolve it by placing it in nitric acid solution (under a fume hood). Continue to boil the solution for 20 min after complete dissolution of the coin (to remove any traces of remaining $\ce{NO2}$ in soution. The volume of the solution is about $40 \,\mathrm{mL}$.
$$\ce{Cu(s) + 4HNO3(aq)-&gt;Cu^{2+}(aq) + 2NO3^- (aq) + 2NO2(g) + 2H2O(l)}$$ 
$$\ce{Ni(s) + 2H3O^+(aq)-&gt;Ni^{2+}(aq) + H2 (g)  + 2H2O(l)}$$
Prepare a fresh solution of sodium thiosulfate of precise concentration $\approx 0.04\,\mathrm{M}$
Add $1.00 \,\mathrm{mL}$ of the diluted copper-containing solution to $15 \,\mathrm{mL}$ of $10\% (W/V)$ potassium iodide solution.
$$\ce{Cu^{2+}(aq) +  5I^- (aq)-&gt;CuI(s) + I3^- (aq) }$$ 
Titrate the yellow-orange suspension with sodium thiosulfate solution until the solution color has become yellow. To detect the end-point add about $1 \,\mathrm{mL}$ of starch solution.
$$\ce{I3^- (aq)  +  2S2O3^{2-} (aq)-&gt;S4O6^{2-} (aq) + 3I^- (aq) }$$
Repeat the titration to precisely determine the end-point.