Analytical Chem istry - DePauw University

Chapter 8 Gravimetric Methods397(a) Cite two ways in which this procedure encourages the formation oflarger particles of precipitate.(b) The ignition step must be carried out carefully to ensure the quantitativeconversion of Al(OH) 3 to Al 2 O 3 . What is the effect of anincomplete conversion on the %w/w Al?(c) What is the purpose of adding NH 4 Cl and methyl red indicator?(d) An alternative procedure involves isolating and weighing the precipitateas the 8-hydroxyquinolate, Al(C 9 H 6 ON) 3 . Why might thisbe a more advantageous form of Al for a gravimetric analysis? Arethere any disadvantages?9. Calcium is determined gravimetrically by precipitating CaC 2 O 4 •H 2 Oand isolating CaCO 3 . After dissolving a sample in 10 mL of water and15 mL of 6 M HCl, the resulting solution is heated to boiling and awarm solution of excess ammonium oxalate is added. The solution ismaintained at 80 o C and 6 M NH 3 is added dropwise, with stirring,until the solution is faintly alkaline. The resulting precipitate and solutionare removed from the heat and allowed to stand for at least onehour. After testing the solution for completeness of precipitation, thesample is filtered, rinsed with 0.1% w/v ammonium oxalate, and driedat 100–120 o C for 1 hour. The precipitate is transferred to a mufflefurnace where it is converted to CaCO 3 by drying at 500 ± 25 o C untilconstant weight.(a) Why is the precipitate of CaC 2 O 4 •H 2 O converted to CaCO 3 ?(b) In the final step, if the sample is heated at too high of a temperaturesome CaCO 3 may be converted to CaO. What effect would thishave on the reported %w/w Ca?(c) Why is the precipitant, (NH 4 ) 2 C 2 O 4 , added to a hot, acidic solutioninstead of a cold, alkaline solution?10. Iron is determined gravimetrically by precipitating as Fe(OH) 3 andigniting to Fe 2 O 3 . After dissolving a sample in 50 mL of H 2 O and 10mL of 6 M HCl, any Fe 2+ is converted Fe 3+ by oxidizing with 1–2 mLof concentrated HNO 3 . The sample is heated to remove the oxides ofnitrogen and the solution is diluted to 200 mL. After bringing the solutionto a boil, Fe(OH) 3 is precipitated by slowly adding 1:1 NH 3 untilthe odor of NH 3 is detected. The solution is boiled for an additionalminute and the precipitate is allowed to settler. The precipitate is thenfiltered and rinsed with several portions of hot 1% w/v NH 4 NO 3 untilno Cl – is found in the wash water. Finally, the precipitate is ignited toconstant weight at 500–550 o C and weighed as Fe 2 O 3 .