Aluminum can be determined gravimetrically by precipitating as Al(OH)3 and isolating as Al2O3. A sample containing approximately 0.1 g of Al is dissolved in 200 mL of H2O and 5 g of NH4Cl and a few drops of methyl red indicator is added (methyl red is red at pH levels below 4 and yellow at pH levels above 6). The solution is heated to boiling, and 1:1 NH3 is added dropwise till the indicator turns yellow, precipitating Al(OH)3. The precipitate is held at the solution’s boiling point for several minutes, filtered, and washed with a hot solution of 2%, w/v NH4NO3. The precipitate is then ignited at 1000–1100 °C, forming Al2O3. (a) Cite two ways in which this procedure has been designed to encourage the formation of larger particles of precipitate. (b) The ignition step must be carried out carefully to ensure that Al(OH)3 is quantitatively converted to Al2O3. What effect would an incomplete conversion have on the reported %w/w Al? (c) What role do NH4Cl and methyl red indicator play in this procedure? (d) An alternative procedure involves isolating and weighing the precipitate as the 8-hydroxyquinolate, Al(C9H6ON)3. Why might this be a more advantageous form of Al for a gravimetric analysis?

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a. Add 1:1 NH3 slowly. Keep at boiling point for several minutes.

b. Compare the molar masses of Al(OH)3 vs Al2O3. If you leave Al(OH)3 + Al2O3 instead of all Al2O, which is heavier.

c. Methyl red indicators assures you know the right pH range. NH4Cl added to NH3 is a buffer.

d. Aluminum 8-hydroxyquinolate (also called Aluminum oxinate) is much heavier (molar mass higher) than Al2O3; therefore, precision is improved and smaller amounts of Al can be determined.

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Aluminum can be determined gravimetrically by precipitating as Al(OH)3 and isolating as Al2O3. A sample containing approximately 0.1 g of Al is dissolved in 200 mL of H2O and 5 g of NH4Cl and a few drops of methyl red indicator is added (methyl red is red at pH levels below 4 and yellow at pH levels above 6). The solution is heated to boiling, and 1:1 NH3 is added dropwise till the indicator turns yellow, precipitating Al(OH)3. The precipitate is held at the solution’s boiling point for several minutes, filtered, and washed with a hot solution of 2%, w/v NH4NO3. The precipitate is then ignited at 1000–1100 °C, forming Al2O3. (a) Cite two ways in which this procedure has been designed to encourage the formation of larger particles of precipitate. (b) The ignition step must be carried out carefully to ensure that Al(OH)3 is quantitatively converted to Al2O3. What effect would an incomplete conversion have on the reported %w/w Al? (c) What role do NH4Cl and methyl red indicator play in this procedure? (d) An alternative procedure involves isolating and weighing the precipitate as the 8-hydroxyquinolate, Al(C9H6ON)3. Why might this be a more advantageous form of Al for a gravimetric analysis?

1 answer

a. Add 1:1 NH3 slowly. Keep at boiling point for several minutes.

b. Compare the molar masses of Al(OH)3 vs Al2O3. If you leave Al(OH)3 + Al2O3 instead of all Al2O, which is heavier.

c. Methyl red indicators assures you know the right pH range. NH4Cl added to NH3 is a buffer.

d. Aluminum 8-hydroxyquinolate (also called Aluminum oxinate) is much heavier (molar mass higher) than Al2O3; therefore, precision is improved and smaller amounts of Al can be determined.

Ask a New Question or answer this question.

DrBob222 · Answer

a. Add 1:1 NH3 slowly. Keep at boiling point for several minutes.

b. Compare the molar masses of Al(OH)3 vs Al2O3. If you leave Al(OH)3 + Al2O3 instead of all Al2O, which is heavier.

c. Methyl red indicators assures you know the right pH range. NH4Cl added to NH3 is a buffer.

d. Aluminum 8-hydroxyquinolate (also called Aluminum oxinate) is much heavier (molar mass higher) than Al2O3; therefore, precision is improved and smaller amounts of Al can be determined.