Alloys can be found in many objects we come across in our daily life. Due to their particular charac — Physical Chemistry — Thermodynamics Chemistry Question

Alloys can be found in many objects we come across in our daily life. Due to their particular characteristics (i.e., conductivity, mechanical or corrosion resistance), alloys are successfully applied in many advanced fields such as aeronautics, construction, electronics devices, and jewelry. That is why developing reliable methods of alloys analysis is of extreme importance.

Brass is an alloy of copper and zinc which is familiar to most students. In this experiment, a brass alloy containing Cu²⁺ and Zn²⁺ ions will be analyzed by complexometric titration with Na₂H₂EDTA. Since the stability constants of the complexes of these metals with EDTA are close, masking of the Cu²⁺ ions by a complexing agent (thiosulfate) is used. In the first titration, copper and zinc are titrated together with Na₂H₂EDTA. In the second titration, sodium thiosulfate is added to bind the Cu²⁺ ions, thus allowing titration of solely zinc ions with Na₂H₂EDTA.

Chemicals and reagents
* Brass sample, ~250 mg per student, or
Test solution (a standard solution containing ions Cu²⁺ (about 1.5 g dm^-3) and Zn²⁺ (about 1 g dm^-3) simulating a digested sample of brass)
* Nitric acid, HNO₃, concentrated (~70% w/v)
* Na₂H₂EDTA standard solution, c = 0.0500 mol dm^-3
* Acetate buffer solution, pH = 5.5 – 6.0, 0.1 mol dm^-3 in acetate
* Sodium thiosulfate solution, Na₂S₂O₃, ~10% (w/v)
* Metallochromic indicator 4-(2-pyridylazo)resorcinol (PAR), 0.1% aqueous solution (w/v)
(0.1% solution of Xylenol orange indicator may be used instead of PAR).

Apparatus and glassware
* Analytical balance (± 0.0001 g)
* Beaker, 10 cm₃
* Hotplate
* Volumetric flask, 100 cm₃
* Burette, 25 or 50 cm₃
* Volumetric pipettes, 2, 5 and 10 cm₃
* Erlenmeyer flask, 100 cm₃ (3 ea.)
* Graduated cylinders, 10 and 25 cm₃

Procedure
A. Brass digestion
a) Take a precise weight of the brass sample (~250 mg) and place it in a beaker.
Note. If no certified brass samples are available, you can use a test solution simulating the digested alloy.
b) Carefully add 5 cm₃ of concentrated nitric acid (the experiment should be done under a fume hood, as NO₂ gas evolves).
c) Heat the beaker slightly on a hotplate to provide for an effective dissolution.
d) When the digestion of the sample is complete, evaporate the solution to near dryness to remove the most part of the acid (avoid evaporating to dry salts, as hydrolysis may occur. If still so, add a minimal amount of HCl to dissolve the residue). Allow the beaker cooling down to room temperature.
e) Dissolve the contents of the beaker in distilled water, transfer it to a 100.00 cm₃ volumetric flask and make it up to the mark.

B. Determination of the total amount of Cu 2+ and Zn 2+
f) Transfer 10.00 cm₃ of the test solution into a 100 cm₃ Erlenmeyer flask, add 20 cm₃ of water, 5 cm₃ of acetate buffer solution and 3 drops of PAR solution, mix thoroughly.
g) Titrate the content of the flask with a standard Na₂H₂EDTA solution (c = 0.0500 mol dm^-3) until the color of PAR indicator changes from bluish-violet to blue or greenish-yellow (for Xylenol orange indicator, the color changes from red to green). Repeat the titration when necessary.

C. Determination of Zn 2+
h) Transfer 10.00 cm₃ of the test solution into a 100 cm₃ Erlenmeyer flask, add 10 cm₃ of water, 5 cm₃ of acetate buffer solution, 2 cm₃ of Na₂S₂O₃ solution and 3 drops of PAR solution, mix thoroughly.
i) Titrate the content of the flask with 0.0500 mol dm^-3 standard Na₂H₂EDTA solution until the color changes from red to yellow (for Xylenol orange, the colors are the same).

D. Calculation of Cu 2+ concentration
j) The volume of Na₂H₂EDTA which is necessary for Cu²⁺ titration is calculated as the difference of the titrant volumes in titrations B and C.