The chromium content of a stainless steel (a Fe-Cr alloy) sample was analyzed by the following proce — Physical Chemistry — Thermodynamics Chemistry Question
Redox titration
The chromium content of a stainless steel (a Fe-Cr alloy) sample was analyzed by the following procedure:
i) 0.1000 g of a stainless steel sample is dissolved in 20 cm3 of heated sulfuric acid (c = 1.8 mol dm–3).
ii) 4 cm3 of conc. nitric acid is added to the solution and it is then heated for 10 min till the colour of the solution changes from blue to green.
iii) 10 cm3 of 0.5 % silver nitrate aqueous solution and 6 g of ammonium persulfate ((NH4)2S2O8) are simultaneously added to the solution. It is heated for 20 min until the decomposition of ammonium persulfate is terminated, when color of the solution turns orange.
iv) 10 cm3 of 5% NaCl aqueous solution is added.
v) 20 cm3 of ammonium iron(II) sulfate (FeSO4·(NH4)2SO4) aqueous solution (c = 1.00×10–1 mol dm–3 ) is added with a volumetric pipette.
vi) Potassium permanganate (KMnO4) aqueous solution (c = 2.00×10–2 mol dm–3) is titrated using a burette. When the colour of the solution becomes light purple, it is presumed to be the final point.
Solve the problem according to the tasks 18.1 – 18.6. The relative atomic mass of Cr is assumed to be 52.00.
Describe the chemical reaction in process ii).
Model Answer
6 FeSO4 + 2 HNO3 + 3 H2SO4 → 3 Fe2(SO4)3 + 4 H2O + 2 NO (Fe2+ → Fe3+)
Trivalent chromium ion exists in the solution after process ii). How will it change after process iii)?
Model Answer
Cr2O72–
Describe the reaction which occurs in process iv). Moreover, mention the purpose of the process.
Model Answer
Ag+ + Cl– → AgCl↓
When the ammonium persulfate is not completely decomposed, the function of AgNO3 (oxidation of the solution) will be prevented.
Describe the ionic reaction which occurs between heavy metal ions in process v). What color will the solution become in the process?
Model Answer
6 Fe2+ + Cr2O72– + 14 H+ → 6 Fe3+ + 2 Cr3+ + 7 H2O
The color changes from orange to (light) blue green.
Describe the reaction which occurs in process vi).
Model Answer
MnO4– + 5 Fe2+ + 8 H+ → Mn2+ + 5 Fe3+ + 4 H2O
The titrated amount in process vi) was 12.00 cm3. Based on chemical equations and volumes of the solutions used calculate the chromium content of this stainless steel sample.
Model Answer
Based on the chemical equations in 18.1, 18.4 and 18.5 and considering the mass of the sample of the steel and volumes of the solutions used, one can calculate by simple stoichiometric calculations the content of chromium in the sample of steel.