Iron can be determined gravimetrically by precipitating as Fe(OH)3 and igniting to Fe2O3. The sample to be analyzed is weighed and transferred to a 400-mL beaker where it is dissolved in 50 mL of H2O and 10 mL of 6 M HCl. Any Fe2+ that is present is oxidized to Fe3+ with 1â€“2 mL of concentrated HNO3. After boiling to remove the oxides of nitrogen, the solution is diluted to 200 mL, brought to boiling, and Fe(OH)3 is precipitated by slowly adding 1:1 NH3 until the odour of NH3 is detected. The solution is boiled for an additional minute, and the precipitate is allowed to settle to the bottom of the beaker. The precipitate is then filtered and washed with several portions of hot 1% w/v NH4NO3 until no Clâ€“ is found in the wash water. Finally, the precipitate is ignited to constant weight at 500â€“550 Â°C, and weighed as Fe2O3. (a) If the ignition is not carried out under oxidizing conditions (plenty of O2 present), the final product will contain some Fe3O4. What effect would this have on the reported %w/w Fe? (b) The precipitate is washed with a dilute solution of NH4NO3. Why is NH4NO3 added to the wash water? (c) Why does the

Question

Iron can be determined gravimetrically by precipitating as Fe(OH)3 and igniting to Fe2O3. The sample to be analyzed is weighed and transferred to a 400-mL beaker where it is dissolved in 50 mL of H2O and 10 mL of 6 M HCl. Any Fe2+ that is present is oxidized to Fe3+ with 1â€“2 mL of concentrated HNO3. After boiling to remove the oxides of nitrogen, the solution is diluted to 200 mL, brought to boiling, and Fe(OH)3 is precipitated by slowly adding 1:1 NH3 until the odour of NH3 is detected. The solution is boiled for an additional minute, and the precipitate is allowed to settle to the bottom of the beaker. The precipitate is then filtered and washed with several portions of hot 1% w/v NH4NO3 until no Clâ€“ is found in the wash water. Finally, the precipitate is ignited to constant weight at 500â€“550 Â°C, and weighed as Fe2O3. (a) If the ignition is not carried out under oxidizing conditions (plenty of O2 present), the final product will contain some Fe3O4. What effect would this have on the reported %w/w Fe? (b) The precipitate is washed with a dilute solution of NH4NO3. Why is NH4NO3 added to the wash water? (c) Why does the

Answer 1

(a) If the ignition is not carried out under oxidizing conditions and some Fe3O4 is present in the final product, it would affect the reported %w/w Fe. This is because Fe3O4 contains both Fe2+ and Fe3+ ions, while Fe2O3 only contains Fe3+ ions. Therefore, the presence of Fe3O4 would overestimate the amount of Fe present in the sample, leading to an incorrect reported %w/w Fe.

(b) NH4NO3 is added to the wash water for several reasons. Firstly, NH4NO3 is a soluble salt, so it can easily wash away any remaining impurities and chloride ions (Cl-) that may be present on the precipitate. This ensures that the final product of Fe2O3 is as pure as possible. Additionally, NH4NO3 is non-reactive with Fe(OH)3 or Fe2O3, so it does not interfere with the analysis or alter the composition of the precipitate.

(c) The solution is boiled for an additional minute after the NH3 is added and Fe(OH)3 is precipitated to ensure complete reactions and the removal of any remaining impurities. Boiling helps to remove volatile compounds and gases, such as excess NH3 and oxides of nitrogen (formed during the oxidation step with HNO3) from the solution. It also allows any small particles or impurities to settle to the bottom of the beaker, making it easier to filter the precipitate and obtain a cleaner sample for further analysis.