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?
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.
(a) Two ways in which this procedure encourages the formation of larger particles of precipitate are:
1. The solution is heated to boiling: Heating the solution increases the kinetic energy of the particles, leading to more collisions and the formation of larger particles.
2. The addition of 1:1 NH3 dropwise: Adding ammonia dropwise helps to maintain the alkaline pH required for the precipitation of Al(OH)3. The slower addition allows for the controlled formation of larger particles.
(b) An incomplete conversion of Al(OH)3 to Al2O3 would result in a lower reported %w/w Al. This is because the unconverted Al(OH)3 would still be present in the final composition, leading to an underestimation of the actual amount of aluminum present. A complete conversion is necessary to accurately determine the gravimetric percentage of aluminum.
(c) NH4Cl and methyl red indicator play different roles in this procedure:
- NH4Cl is added to create an acidic environment, promoting the formation of Al(OH)3. Its presence helps prevent the precipitation of other metal hydroxides that might interfere with the gravimetric determination of aluminum.
- Methyl red indicator is used to monitor the pH during the addition of ammonia. It changes color at specific pH levels, indicating when the pH is in the desired range for the precipitation of Al(OH)3.
(d) Isolating and weighing the precipitate as the 8-hydroxyquinolate, Al(C9H6ON)3, might be more advantageous for gravimetric analysis because this form of aluminum is likely to be more stable and less prone to changes in composition or decomposition during ignition compared to Al(OH)3. It could provide more accurate and precise results in determining the percentage of aluminum. Additionally, the 8-hydroxyquinolate form might also exhibit better filtration properties, allowing for easier isolation of the precipitate.
(a) Two ways in which this procedure has been designed to encourage the formation of larger particles of precipitate are:
1. Heating the solution to boiling: Heating the solution promotes the formation of larger particles by increasing the rate of nucleation and growth of the Al(OH)3 precipitate. Higher temperatures provide more energy for the particles to collide and aggregate, leading to the formation of larger and more easily filterable precipitates.
2. Holding the precipitate at the boiling point for several minutes: By keeping the precipitate at the boiling point, the solution's thermal energy helps the particles grow further and coalesce. This prolonged exposure to elevated temperatures enhances the formation of larger particles and improves the filterability of the precipitate.
(b) An incomplete conversion of Al(OH)3 to Al2O3 during the ignition step would lead to an error in the determination of the %w/w Al. This is because the reported %w/w Al is based on the assumption that all the Al in the original sample has been converted to Al2O3. If there is any remaining Al(OH)3 in the precipitate, it would contribute to the reported %w/w Al, leading to an overestimation of the actual aluminum content in the sample.
(c) NH4Cl and methyl red indicator play specific roles in this procedure:
- NH4Cl: NH4Cl is used as a reaction medium to increase the ionic strength of the solution and to maintain a slightly acidic pH. This promotes the formation and precipitation of Al(OH)3 by preventing the hydrolysis of aluminum ions and suppressing the formation of other competing precipitates.
- Methyl red indicator: Methyl red serves as a pH indicator to monitor the acidity of the solution. It changes color from red (at pH levels below 4) to yellow (at pH levels above 6). By adding NH3 dropwise until the indicator turns yellow, it ensures that the solution is slightly basic, favoring the precipitation of Al(OH)3.
(d) Using the 8-hydroxyquinolate, Al(C9H6ON)3, as a form of aluminum for gravimetric analysis might be more advantageous because it forms a more stable and easily filterable precipitate compared to Al(OH)3. The Al(C9H6ON)3 precipitate has a well-defined stoichiometry and a higher molecular weight, making it easier to handle, collect, and weigh accurately. Additionally, it can be isolated and purified more effectively, avoiding potential impurities that might interfere with the gravimetric analysis.
(a) There are two ways in which this procedure encourages the formation of larger particles of precipitate:
1. By adding NH3 dropwise until the indicator turns yellow: The addition of NH3 helps to increase the pH of the solution, which promotes the formation of larger particles of Al(OH)3. At higher pH levels, the hydroxide ions (OH-) combine with aluminum ions (Al3+) to form the precipitate. This gradual addition of NH3 ensures that the pH increases slowly, allowing the precipitate to form and grow in size.
2. Holding the precipitate at the solution's boiling point for several minutes: Heating the solution to boiling and maintaining it at that temperature for a few minutes facilitates the growth of larger particles of the precipitate. This prolonged heating allows the particles to collide and aggregate, leading to the formation of larger and more easily filterable particles.
(b) An incomplete conversion of Al(OH)3 to Al2O3 during the ignition step would affect the reported %w/w Al by causing an underestimation of the actual aluminum content. If not all of the Al(OH)3 is converted to Al2O3, the remaining unconverted Al(OH)3 would be included in the final weight measurement of the precipitate. This would result in a higher measured weight for the precipitate, leading to an overestimation of the %w/w Al.
(c) NH4Cl and methyl red indicator play important roles in this procedure:
- NH4Cl: NH4Cl is added to the solution to suppress the hydrolysis of aluminum ions and prevent the formation of aluminum hydroxides at lower pH levels. By maintaining a high concentration of chloride ions (Cl-) in the solution, NH4Cl helps to stabilize the aluminum in solution and prevent premature precipitation of Al(OH)3. This ensures that the precipitation occurs only when NH3 is added dropwise.
- Methyl red indicator: Methyl red is used to monitor the pH of the solution and determine the endpoint of the reaction. It acts as a pH indicator, turning red at pH levels below 4 and yellow at pH levels above 6. During the addition of NH3, the solution gradually changes from red to yellow as the pH increases. The point at which the indicator turns yellow indicates that the pH level has reached the desired value for the formation of Al(OH)3 precipitate.
(d) Using the 8-hydroxyquinolate, Al(C9H6ON)3, as a form of aluminum for gravimetric analysis may be advantageous for several reasons:
- Selective precipitation: The use of 8-hydroxyquinolate allows for the selective precipitation of aluminum. This means that other interfering ions or compounds present in the sample would not be precipitated along with aluminum, improving the accuracy and selectivity of the analysis.
- High stability and low solubility: Al(C9H6ON)3 is a stable precipitate and has low solubility in water. This ensures that the precipitate can be easily isolated and collected, without significant losses during the washing or filtration steps.
- High molar mass: The use of Al(C9H6ON)3 allows for the formation of a complex compound with a relatively high molar mass. This facilitates the gravimetric determination of aluminum as the weight of the precipitate can be directly related to the aluminum content in the sample.
- Less interference: The alternative procedure may have fewer interfering substances than the Al(OH)3 method, which can improve the accuracy and precision of the gravimetric analysis.