Zinc oxide ZnO, a soft, white or faintly yellowish-white is used in the vulcanization of rubber, cer — Organic Chemistry Chemistry Question

Zinc oxide ZnO, a soft, white or faintly yellowish-white is used in the vulcanization of rubber, ceramics, paints, and many other products.

Zinc oxide is produced by burning zinc metal in air as follows:
Zn(s) → Zn(l) → Zn(g)
2 Zn(g) + O₂(g) → 2 ZnO(s)
Purity of the given zinc has an influence on the quality of zinc oxide powder. There is a very wide range of commercial grades of zinc oxide depending on content of impurities. For example, zinc oxide powder -Grade 2 (having greater than 60 wt.% of Zn) produced by electro-thermal process contains less than 4 % of Ca; 0.4 % of Fe; and 8 % of Pb.

This task is to determine the contents of zinc and lead (in %) in a commercial zinc oxide powder by EDTA and dichromate solution.

Materials and reagents
* Zinc oxide powder
* Sulfuric acid solution, H₂SO₄ (aq), 4 mol dm^-3
* Nitric acid solution, HNO₃ (aq), 6 mol dm^-3
* 0.025 mol dm^-3 EDTA standard solution (from Na₂H₂Y . 2 H₂O)
* Sodium thiosulfate, 0.02 mol dm^-3 solution (from Na₂S₂O₃ · 5 H₂O)
* Fe(II) sulphate, solution, c = 0.025 mol dm^-3 (from FeSO₄ . 7 H₂O)
* Mixture of HCl and NaCl: Dissolve 320 g of NaCl in 200 cm₃ of distilled water, add 100 cm₃ of concentrated HCl (37 %) and dilute to 1.0 dm₃ with distilled water
* Solution of 5 % (w/v) K₂Cr₂O₇
* Mixture of potassium iodide and thiocyanate, KI⁺ KSCN, 10 % w/v
* Mixture of CH₃COOH (2 mol dm^-3) and NH₄CH₃COO (1 mol dm^-3)
* NH₃, solution, (c = 6 mol dm^-3)
* 200 cm₃ of buffer solution, NH₃ / NH₄Cl, pH = 10
* Eriochrome Black T (ET^-00) indicator, 1% (w/w) in NaCl(s)
* Diphenylamine sulfonate indicator, 0.2% (w/w) in water
* Starch indicator, 1% in water. This solution is prepared daily by mixing 0.5 g soluble of starch with 2 – 3 cm₃ distilled water and then pouring the starch into 50 cm₃ boiling distilled water with stirring. Continue heating the solution until the solution is nearly transparent. Cool solution to room temperature before use.

Apparatus and glassware
* Analytical balance (± 0.0001 g)
* Hotplate
* Erlenmeyer flask, 250 cm₃
* Volumetric flask, 100 cm₃;
* Volumetric pipette, 10.00 cm₃
* Burette, 25 cm₃
* Glass beaker, 250 cm₃
* Whatman Filter paper, Grade ²V, 110 mm.

Experimental procedure

Step 1
1. Place 0.50 g of a powder sample in a 100 cm₃ glass beaker. Add 10 cm₃ of H₂SO₄ solution H₂SO₄ (4 mol dm^-3) into the beaker. Place the beaker on a hot plate and begin heating the mixture in the hood. Set the hot plate at medium heat. As a portion of solid dissolves, the yellowish residue still remains in the beaker. Add slowly 3 cm₃ of HNO₃ solution into the beaker and keep heating the solution until white precipitate appears. Evaporate the solution in the beaker until white fumes of SO₃ forms, and then stop heating. Cool the solution to the room temperature (Hint: in hood).
Filter the cooled solution by using filter paper to a 100 cm₃ volumetric flask; rinse the beaker and filter paper with 1 % H₂SO₄ several times; make up the filtrate to the mark with distilled water and shake well (solution A).
2. Place the funnel together with the filter paper on a 250 cm₃ Erlenmeyer flask. Pour slowly a hot mixture of 20 cm₃ of CH₃COOH + NH₄CH₃COO through the white precipitate on the filter paper until it is dissolved. Wash the filter paper with distilled water to get solution B.

Step 2
1. Add 10.00 cm₃ of solution (A) and 5 cm₃ of NH₃ solution into a 250 cm₃ Erlenmeyer flask using pipettes and swirl to mix. Then, add 10 cm₃ of NH3/NH₄Cl buffer solution (pH = 10) to the mixture.
Add ET^-00 indicator and about 10 cm₃ of deionized water. Titrate carefully with the EDTA standard solution until the color changes from wine red to blue. Record the volume of EDTA used for this titration. (Hint: Adjust the size of the aliquot on Zn²⁺ as necessary to stay within titration range).
2. Add the solution B and 5 cm₃ of 10 % NaCH₃COO into a 250 cm₃ beaker. Heat the mixture slowly (at least 10 minutes) to 90 oC and then add gently 10 cm₃ of K₂Cr₂O₇ solution. Cool the mixture to room temperature and keep at this condition for 1 hour. Filter the precipitate through a Whatman filter paper. Wash the precipitate with warm distilled water until the filtrate is almost colorless.
Transport the funnel with filter paper to a 250 cm₃ Erlenmeyer flask. Dissolve the yellow precipitate with 15 cm₃ of warm mixture HCl +NaCl. Rinse the filter paper with distilled water to get the solution C.
Add about 5 cm₃ of 4 mol dm^-3 H₂SO₄, 5 cm₃ of 4 mol dm^-3 H₃PO₄, and 10 cm₃ of distilled water to the solution C. Drop 8 droplets of diphenylamine sulfonate indicator in the solution C. Titrate carefully with the standard ferrous solution until the color changes from violet to green. Record the volume of ferrous solution used. (Hint: Solution C can also be titrated using the iodometric titration).