Gravimetric analysis worksheet with answers

3. Gravimetric Determination of Iron as Fe2O3 3. Quantitative Chemical Analysis Gravimetric Determination of Iron as Fe2O315 A sample containing iron can be analyzed by precipitation of the hydrous oxide from basic solution, followed by ignition to produce Fe2O3: Fe3+ + (2 + x)H2O Base FeOOH . xH2O(s) + 3H+ Hydrous ferric oxide Green Profile See Section 0 900ºC FeOOH . xH2O Fe2O3(s) The gelatinous hydrous oxide can occlude impurities. Therefore, the initial precipitate is dissolved in acid and reprecipitated. Because the concentration of impurities is lower during the second precipitation, occlusion is diminished. Solid unknowns can be prepared from reagent ferrous ammonium sulfate or purchased from Thorn Smith.2 Procedure 1. Bring three porcelain crucibles and caps to constant mass by heating to redness for 15 min over a burner (Figure 1). Cool for 30 min in a desiccator and weigh each crucible. Repeat this procedure until successive weighings agree within 0.3 mg. Be sure that all oxidizable substances on the entire surface of each crucible have burned off. Figure 1. Positioning a crucible above a burner. 15. D. A. Skoog and D. M. West, Fundamentals of Analytical Chemistry, 3d ed. (New York: Holt, Rinehart and Winston, 1976). 18 3. Gravimetric Determination of Iron as Fe2O3 Quantitative Chemical Analysis 2. Accurately weigh three samples of unknown containing enough Fe to produce ~0.3 g of Fe2O3. Dissolve each sample in 10 mL of 3 M HCl (with heating, if necessary). If there are insoluble impurities, filter through qualitative filter paper and wash the filter well with distilled water. Add 5 mL of 6 M HNO3 to the filtrate, and boil for a few minutes to ensure that all iron is oxidized to Fe(III). 3. Dilute the sample to 200 mL with distilled water and add 3 M ammonia16 with constant stirring until the solution is basic (as determined with litmus paper or pH indicator paper). Digest the precipitate by boiling for 5 min and allow the precipitate to settle. 4. Decant the supernatant liquid through coarse, ashless filter paper (Whatman 41 or Schleicher and Schuell Black Ribbon, as in Figures 2-18 and 2-19 in the textbook). Do not pour liquid higher than 1 cm from the top of the funnel. Proceed to step 5 if a reprecipitation is desired. Wash the precipitate repeatedly with hot 1 wt% NH NO until little or no Cl- is detected in 4 3 the filtered supernate. (Test for Cl- by acidifying a few milliliters of filtrate with 1 mL of dilute HNO and adding a few drops of 0.1 M AgNO . If precipitate is observed, Cl- is 3 3 present.) Finally, transfer the solid to the filter with the aid of a rubber policeman and more hot liquid. Proceed to step 6 if a reprecipitation is not used. 5. Wash the gelatinous mass twice with 30 mL of boiling 1 wt% aqueous NH4NO3, decanting the supernate through the filter. Then put the filter paper back into the beaker with the precipitate, add 5 mL of 12 M HCl to dissolve the iron, and tear the filter paper into small pieces with a glass rod. Add ammonia with stirring and reprecipitate the iron. Decant through a funnel fitted with a fresh sheet of ashless filter paper. Wash the solid repeatedly with hot 1 wt% NH4NO3 until little or no Cl- is detected in the filtered supernate. Then transfer all the solid to the filter with the aid of a rubber policeman and more hot liquid. 6. Allow the filter to drain overnight, if possible, protected from dust. Carefully lift the paper out of the funnel, fold it (Figure 2), and transfer it to a porcelain crucible that has been brought to constant mass. 16. Basic reagents should not be stored in glass bottles because they slowly dissolve glass. If ammonia from a glass bottle is used, it may contain silica particles and should be freshly filtered. 19 3. Gravimetric Determination of Iron as Fe2O3 1. Flatten paper 2. Fold in edges Quantitative Chemical Analysis Figure 2. Folding filter paper and placing it inside a crucible for ignition. Continue folding paper so entire package fits at the bottom of the crucible. Be careful not to puncture the paper. 3. Fold over top 4. Place inside crucible with point pushed against bottom 7. Dry the crucible cautiously with a small flame, as shown in Figure 1. The flame should be directed at the top of the container, and the lid should be off. Avoid spattering. After the filter paper and precipitate are dry, char the filter paper by increasing the flame temperature.