Experiment 2 - Gravimetric Analysis to Determine an Unknown [Al3+]

Gravimetric Analysis

a type of quantitative chemical analysis that relies on the measurement of mass in order identify the components in a sample or to quantify the components in a sample

Analytical Chemistry

a branch of chemistry that focuses on the analysis of samples; the sample could be gas, liquid, solid, or a mixture of phases

Determine the concentration of Al3+ ions in the aqueous solution Al(NO3)3 of unknown concentration.

What is the purpose of this experiment?

Solubility and Precipitation

We converted the substance of interest as completely as possible into a solid that could be collected, dried, and weighed. Stoichiometry was used thereafter to determine the concentration of Al3+.

What is the main form of chemistry you took advantage of during this experiment? Briefly explain why.

Coordination Compound

compound that consists of one metal ion (usually a transition metal) and ligands; charge of the compound is the sum of the charges on the metal ion and the ligands bound to that ion

Example: Al(hq)3

Lewis Acids

electron pair acceptors; usually the positively charged metal ions in a coordination compound

Lewis Bases

electron pair donors; usually the ligands in a coordination compound

Monodentate Ligand

a species that forms only a single coordinate bond to a metal ion in a complex; donates only one pair of electrons to a metal ion in a complex

Examples: Cl-, CN-, H2O

Multidentate Ligand

a species that forms multiple coordinate bonds to a metal ion in a complex; donates multiple pairs of electrons to a metal ion in a complex

(also known as a chelator)

Bidentate Ligand

a species that can form two coordinate bonds to a metal ion in a complex; donates two pairs of electrons to a metal ion in a complex

Example: 8-hydroxyquinoline (hq)

Coordination Number

number of atoms bound to the center metal ion of a coordination complex

6

hq is a bidentate ligand - donates two electron pairs. The Al3+ in the center of Al(hq)3 accepts 6 electron pairs from three ligands.

What is the coordination number of Al(hq)3?

Al3+ was complexed with hq, forming the coordination compound Al(hq)3 which is insoluble in aqueous solution.

How was Al3+ precipitated in this experiment?

yellow

The precipitate Al(hq)3 was ___________ in color.

- Cannot use hq at a pH greater than 10 because side products, such as Al2O3, Al(OH)3 and Al(OH)4 are produced, which would contaminate the final precipitate collected.

- Cannot use hq at a pH lower than 5 because the cation produced (Hhq+) does not bind with Al3+ and would not form the insoluble complex we are looking for

- Cannot just use any hq form between pH 5 and 10 because hq at this pH is insoluble on its own, meaning it would precipitate out of the aqueous solution before binding to Al3+ or alongside the Al(hq)3 complexes formed (contaminating final product).

- Have to use a form of hq somewhere in between Hhq+ and hq (around pH = 5) so that it will be insoluble in aqueous solution on its own AND able to bind to Al3+

There are three protonation states of hq: Hhq+, hq, and hq-. Which one did you use and why?

*Note:

- Hhq+: predominant form below pH = 5

- hq: predominant form between pH = 5

and pH = 10

- hq-: predominant form above pH = 10

Used the buffer solution ammonium acetate (NH4CH3COO).

How did you maintain a pH of about 5 throughout the experiment?

Al3+, hq

_________ is the limiting reactant in this experiment, and ____________ is added in excess.

The addition of too much ligand will cause it to precipitate out of the solution. This precipitate will be collected along with the Al(hq)3 solid and weighed. This would increase the mass of Al, which would increase the calculated concentration of the Al3+ species.

If you were to add to much hq, how would this affect your final results (final mass of product)?

increases

Solubility _______________ (increases/decreases) with temperature.

Dessicator

a sealed chamber in which samples can be stored in a dry atmosphere completely free of moisture

Moisture adds excess weight to the crucibles, which could lead to an artificially high mass of Al(hq)3 and then to an artificially high concentration of Al3+.

Why did you heat the crucibles (with the filter paper) in the oven?

Hot air rises. By weighing warm crucibles, you will get an artificially low weight because the air around them will have risen. This would lead to an artificially high mass of Al(hq)3 and an artificially high concentration of Al3+.

Why do you have to let the crucibles cool down before you weigh them?

Washing the volumetric pipet

Why should you always wash your volumetric pipet with the solution you want to transfer before you actually do so?

- Minimizes the possibility that precipitation of hq will occur prior to the adjustment of the pH to 5

- Speeds up the chemical reaction

Why did you heat the original solution of unknown Al3+ concentration before adding the hq?

acidic

The original solution of unknown Al3+ concentration before adding the buffer was _____________ (acidic/basic).

precipitate, redissolve, 5

NH4CH3COO should be added drop wise until _____________ forms and does not _____________, upon which it should be added in excess to ensure the pH of the solution is _______.

not be

Before adding the buffer solution, there should _________ (be/not be) precipitate formed by the addition of hq.

clear

If the supernatant is pale yellow in color, precipitate has been lost during vacuum filtration.

The supernatant should be ________ in color after filtration. Why might it be pale yellow?

- Convert g Al(hq)3 to mol Al(hq)3

- Use stoichiometry to convert mol Al(hq)3 to mol Al3+

- Concentration = (mol Al3+)/(volume of original solution)

How do you calculate the concentration of Al3+ once you have the mass of Al(hq)3?