Gravimetric Analysis Essay

The purpose of experiment 1, also named Gravimetric Analysis, is to study the use of analytical chemistry. Analytical chemistry is a branch of chemistry that involves determining what matter is and how much of it exists. It is the science of separating, identifying, and determining the components that make up a compound and determining a specific amount of that compound. There are two types of analytical chemistry: qualitative and quantitative. This experiment uses quantitative analysis to determine the amount of a substance, or several species, and measure it in numerical terms. More specifically, this experiment uses gravimetric analysis, a form of quantitative analysis that uses an analytical balance, an instrument that yields accurate and precise data, to measure the masses of several substances. Experiment 1 involves the precipitation of a complex compound called aluminum 8-hydroxyquinilinate, also called Alq3. In order for this precipitation to occur, an organic precipitating agent, 8-hydroxyquinilinate (abbreviated OxH) is used to react with aluminum ions to form this complex compound. For this reaction to take place, OxH must be deprotonated, so that its anion, Ox-, will bond with the aluminum ions (the OxH molecule itself will not react with aluminum ions). As a result, the pH of the solution has a major effect on the precipitation in this experiment. If the pH is too low and there is an excess of hydrogen ions, then by Le Chatelier’s principle, the equilibrium will shift to the left, creating more OxH molecules. Ox- will no longer be available and a precipitation will not occur. If the pH is too high and there is an excess of hydroxide ions, the aluminum ions will react with the hydroxide ions. There will be no aluminum to react in the precipitation. If the pH is neutral, OxH is not very soluble and may cause it to precipitate out itself. So in order for this precipitation in this experiment to take place, it is vital that a buffer solution of ammonium acetate is used to control the pH of the solution. Also, the less aluminum ions in the solution, the more it will precipitate. In order to do this, an excess of OxH is added to shift the reaction towards the precipitate form. However, OxH has a limited solubility in water, so too much of an excess cannot be added. In order for the precipitate to work best in this experiment, we would have to allow it to digest. This lets use filter the precipitate completely. We do this by drying the precipitate in an oven and letting it age overnight. This promotes the formation of fewer, larger, and more pure particles of the precipitate. In the end, we are trying to find the concentration of aluminum in the solution (millimole of aluminum per mililiter of solution). We do this by converting the mass of the precipitate into moles and using mole-to-mole ratio and stoichiometry; we find the moles of aluminum ions. We then divide this by the volume of the solution to find the molarity, or concentration, of the aluminum ions.