No Pre-Lab to complete.
Part 1.: Purify benzoic acid by recrystallization from water. Test the purity of the recrystallized solid by measuring its melting point. Fill out the Worksheet as you carry out the experiment.
Background: See the youtube video on recrystallization.
The task of designing a separation and purification scheme for a specific compound often requires a great deal of creativity from a chemist. However, there are a number of standard techniques which have been developed over a few hundred years, and these techniques will, in most cases, provide the chemist with at least a place to start, if not the entire solution to the separation problem.
The most commonly used technique for purifying a solid compound is recrystallization. In the most basic form of this process, the solid compound is dissolved in a minimum amount of a hot solvent, and then the solution is allowed to cool until the compound precipitates, or 're-crystallizes', from solution. Impurities, which are soluble in the solvent, stay dissolved, and these impurities are removed, along with the solvent, by filtration. As you may have guessed, the success of this technique depends heavily on the solvent used: the solid compound must be only slightly soluble in the solvent at room temperature, while being freely soluble in the solvent at higher temperatures. In addition, it is vital that the solvent not react with the compound to be purified.
It is often the case that this basic form of recrystallization is not sufficient to remove all of the impurities--for instance, those impurities that are insoluble in the solvent used for recrystallization. In this case, the hot solution (the solid compound dissolved in the hot solvent) must be filtered while it is hot, without allowing it to cool more than a few degrees, before the recrystallization takes place. This "hot filtration" removes those impurities, which are not soluble in the solvent. However, the "hot filtration" is generally somewhat tricky to carry out without the solution cooling enough that most of the solid compound recrystallizes prematurely.
It is always possible that there will be impurities, which, like the compound being purified, are freely soluble in the hot solvent while being only slightly soluble in the solvent at room temperature. In these instances, a second recrystallization may have to be carried out, using a different solvent. In some cases, the impurities may be removed by adding another substance which absorbs the impurity, allowing it to be removed in the hot filtration. Generally speaking, impurities, which are both organic and have a color, can be removed by adding very finely powdered activated charcoal, called "decolorizing carbon". Unfortunately, the particles of decolorizing carbon are so small that standard filter paper will not remove all of it from the mixture, so a filtration aid, called "Celite", is also added along with the decolorizing carbon, before the hot filtration.
To review: 1) the best solvent for recrystallizing a given compound is one in which the solid is only slightly soluble at room temperature, but is freely soluble at higher temperature; 2) the solvent must not react with the compound to be purified; 3) recrystallizations always involve dissolving the solid in a minimum amount of the hot solvent, cooling the solution until the solid recrystallizes, and then filtering the mixture to separate the purified solid from the solvent and soluble impurities; 4) if necessary the recrystallization will also involve a hot filtration (filtering the hot solvent with the solid compound dissolved in it) to remove insoluble impurities; 5) if necessary, decolorizing carbon and Celite are added to the mixture before the hot filtration, in order to remove colored organic impurities.
Another important aspect of any experiment, which either produces a new compound or isolates a compound from a natural source, is the determination of the purity of the product compound. While there are many high-tech instruments which may be used to detect even very small quantities of impurities, these techniques are generally expensive, complicated, or both. The simplest, most useful way to determine the purity of a solid compound is to measure its melting point. The melting point of a pure compound is always the same temperature (provided other variables, such as pressure, are at standard values). In addition, the presence of impurities will affect the melting point of a substance in a predictable way: impurities always lower the melting point, no matter what impurities are present, and the amount of change will depend on the amount of the impurity--the less pure the substance, the greater the depression in the melting point.
Melting points will be measured using the technique developed in the melting point experiment.
Part 1.: Purification of Benzoic Acid by Recrystallization
Safety: Benzoic acid is a severe irritant and a sensitizer (exposure to sensitizers does not cause cancer, but can make you more susceptible to those substances, which do cause cancer), and is therefore classified as a harmful solid. Wear gloves while handling it, and be sure to wash your gloves and your hands after handling it.
We will follow the "General Guidelines for a Recrystallization"at the end of this experiment.
Obtain about 1.0 g of "impure" benzoic acid (this sample of benzoic acid has had a small amount of sodium chloride added to it). What kind of container should you use for the solid? (Guideline 1--however, we will be using only about 15-20 mL of solvent, so use a 50-mL container for this recrystallization). Heat about 50 mL of DI water in a 150-mL beaker (Guideline 2). Add about 15 mL of the heated water to the "impure" benzoic acid, and place the benzoic acid/hot water mixture on the hot plate. Add more hot water to the benzoic acid, as needed, to keep the volume constant while the benzoic acid is dissolving (Guideline 3). If the solid does not all dissolve within about 5 minutes, add a little more hot water. Once the solid has completely dissolved, add another 1 mL (no more!) of hot water to keep it dissolved (Guideline 4--we are using less water than the guideline says to use, because the amount of solid we are recrystallizing is small). Remove the container from the hot plate (and turn off the hot plate). We will skip Guidelines 5 and 6, because our sample has no colored impurities and no insoluble impurities.
Follow Guideline 7 in letting the benzoic acid/water mixture cool (and helping it cool), and Guideline 8 for filtration (use a small Büchner funnel and 250-mL filter flask).
Part 2.: Measruing the Melting Point of "Impure" and Recrystallized Benzoic Acid
Make sure the recrystallized benzoic acid is dry by placing it in the oven for 10 minutes and then measure the melting point of both the "impure" benzoic acid and your recrystallized benzoic acid.
Here are some general rules to follow for purifying any solid compound.