Experiment Ib
Separation of Solids: Recrystallization and Melting Points
Reading (in The Organic Chem Lab Survival Manual, by Zubrick): Chap. 12 (pp. 102-108); Chap. 13 (pp. 122-132, 136-138)
Part A -- Purify benzoic acid by recrystallization from water. Test the purity of the recrystallized solid by measuring its melting point.
Part B -- Determine the best solvent for recrystallizing an unknown solid, purify the solid, and determine the identity and relative purity of the unknown compound by measuring its melting point (optional, at instructor's discretion).
Background
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 "recrystallizes," 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.
In order to measure the melting point of a substance in a consistent manner, we will use the Mel-Temp melting point apparatus. This device is described in Zubrick, but the basic idea is that it allows you to carefully and repeatedly measure the melting point of a very small amount of a solid, with a reasonable degree of precision. It is also very simple to operate.
Procedure
Part A – Purification of Benzoic Acid by Recrystallization
| Day 1 procedures: |
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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. You may wish to wear gloves while handling it. Be sure to wash your gloves and hands after handling it.
Before you begin the recrystallization of benzoic acid, you should have determined its solubility (you should have looked up this information in Exp 1a). If you did not find this information for benzoic acid, its solubility is 0.34 g per 100 mL of cold water..
We will follow the General Guidelines for a Recrystallization"--pp123-124 in Zubrick.
Obtain about 1.0 g of "impure" benzoic acid (this sample of benzoic acid has 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 (in a beaker), and place the benzoic acid/hot water mixture on the hot plate. Add more hot water to the benzoic acid, as needed, until the benzoic acid has completely dissolved. If the solid does not all dissolve within about 5 minutes, using the initial 15-mL sample of hot water, add more hot water in 5-mL additions. Once the solid has completely dissolved, add an additional 1-2 mL of hot water to keep it dissolved. 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.
Based on the solubility of benzoic acid in water, you can estimate your recovery. For example, if 0.34 g of benzoic acid dissolves in 100 mL of cold water, then if you started with 1.0 g of benzoic acid, the maximum you could recover by crystallization would be about 0.66 g if you used 100 mL of water. If you used 50 mL of water, then only about 0.17 g would stay dissolved, and you would recover a maximum of about 0.83 g of benzoic acid. How much water did you actually use? How much benzoic acid should you recover?
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).
Make sure the recrystallized benzoic acid is dry (you will probably need to leave it in the desiccator until the next lab period), and then measure the melting point of both the "impure" benzoic acid and your recrystallized benzoic acid.
| Day 2 procedures: |
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Part B--Determining the Best Solvent to Use (optional, at instructor's discretion)
Safety: Acetone is a flammable liquid and a severe eye irritant; Methanol is a flammable liquid, an irritant, is toxic, and has harmful vapors; Petroleum ether (ligroin) is a flammable liquid, an irritant, and is toxic; Toluene is a flammable liquid, a severe irritant, is toxic, and has harmful vapors; no flames in allowed in lab, and be sure to wear gloves while handling it, and to wash both your gloves and your hands after handling it. Also, avoid breathing the vapors of any of these compounds. The solid unknowns are all toxic and irritants, and you should avoid contact with them, as well as breathing their vapors.
Obtain 1.0 g of one of the unknown compounds (your instructor will assign one to your group). Follow the section, "Finding a Good Solvent" (pp. 122-123), using the following five solvents: acetone, methanol, petroleum ether (also known as ligroin), toluene, and water.
Once you have decided which one solvent is the best recrystallization solvent for your unknown, recrystallize the remaining part of your unknown sample, following the General Guidelines (you will not need to use activated charcoal, but you my need to use a hot filtration, even if you do not see any insoluble impurities, but this is up to you.).
After you have obtained crystals, collect the crystals using a Buchner funnel (or Hirsch funnel) and then dry your crystals. Alternatively, if you are using a volatile organic solvent, you can dry your crystals by continuing to keep the vacuum on, drawing air through the sample, and evaporating the solvent. (This drying procedure does not work well for water, however.) After you have dry crystals, do a melt analysis on the recrystallized solid and determine its melt point range.
From the melting range of your recrystallized solid, determine if your unknown solid is most likely to be (fill in the chart using data obtained from a reference source):
Compound name |
Molar mass (g/mol) |
Melt point temperature (oC) |
| triphenylmethanol | ||
| trans-stilbene | ||
| acetanilide | ||
| sulfanilamide |
Compare your melt temperature to the melt temperature of the known solids listed above. What compound do you think you mostly likely have as a solid unknown?
Disposal Notes: All liquid and solid waste must be discarded in appropriate waste containers. Never pour liquid waste, especially liquid organic waste, down the drain.
Go To Experiment: VSEPR
1a 1b
1c 2
3 4
5 6
7 8
9 10
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