This is designed to understand the importance of re-crystallization in the purification of different compounds. The chemical used in this experiment is benzoic acid. The initial purification will take place starting with impure benzoic acid, using water as a solvent to isolate pure benzoic acid. Once the benzoic has been re-crystallized (and purified away from NaCl), a percent yield and a melt temp analysis of the purified (and dried) product will be performed. Unknown chemicals will be given to each laboratory group to determine melt temperature, and by deduction, use the melting point to identify which of four different chemical was present in their unknown samples. Finally, relying on the chemical properties of benzoic acid to function as an acid, we will neutralize the benzoic acid to produce the benzoate ion which is very soluble in water, and then convert back into the less soluble benzoic acid for subsequent purification.
Part A -- Purify benzoic acid by re-crystallization from an impure mixture (benzoic acid with NaCl) using water as solvent. Determine the yield of benzoic acid as a percent of starting material and test the purity of the re-crystallized solid by measuring its melting point.
Part B -- Determine the best solvent for re-crystallizing an unknown solid. Purify benzoic acid using appropriate chemical properties. Determine the identity of two unknown chemicals using melt temperature of the chemical and comparing to known values for the chemicals.
The task of designing a separation and purification scheme 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 a separation problem.
The most commonly used technique for purifying a solid compound is re-crystallization. In the most basic form of this process, the solid compound is dissolved in a minimal amount of a hot solvent, and then the solution is allowed to cool until the compound precipitates, or "re-crystallizes," from a super-saturated solution. Impurities, which often 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 being used. The solid compound (solute) must be only slightly soluble in the solvent at room temperature, while being virtually completely soluble in the solvent at higher temperatures. In addition, it is vital that the solvent not react with the compound which is being purified.
It is often the case that this basic form of re-crystallization is not sufficient to remove all of the impurities--for instance, those impurities that are insoluble in the solvent used for re-crystallization. In this case, the hot solution (with the solute dissolved completely in the hot solvent) must be filtered while it is hot, without allowing it to cool more than a few degrees to prevent any re-crystallization to take place. This "hot filtration" technique removes those insoluble impurities all the while keeping your solute in solution. The "hot filtration" technique can be tricky if you allow the solution to cool even a little bit. Therefore, once you are fairly certain that your desired solute is fully dissolved, quickly perform a vacuum filtration using a Büchner funnel to collect the filtrate, containing your dissolved solute, to prevent appreciable formation of crystals.
Even using a "hot filtration" it is still possible that there will be impurities, which, like the compound being purified, are freely soluble in the hot solvent, and, like your solute, is only slightly soluble in the solvent at room temperature. In these instances, a second re-crystallization 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 or reacts with 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:
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 easy to use is will be described in detail by your instructor. 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. One of the most important aspects to doing a good melt temperature is to allow for the increase in temperature to be slow. As a general rule, when you are getting close to the melting point of a compound, you should decrease the increase in temperature to about 1oC per 15 seconds. If you do not get a good melt point, let the Mel-Temp cool a bit, and do the melt again. Melt points are recorded as a range in temperature starting when you first detect liquid and ending when all the solid has melted.
| 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 re-crystallization 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
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 or a 125-mL beaker for this re-crystallization). Heat about 50 mL of DI water in a 150-mL beaker. Add about 15 mL of the heated water to the "impure" benzoic acid (in your 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 (usually at boiling conditions). If the solid does not fully dissolve within about 5 minutes, using the initial 15-mL sample of hot water, add more hot water in 5-mL increments. Once the solid has completely dissolved, add an additional 2-5 mL of hot water to keep it dissolved. Remove the container from the hot plate (and turn off the hot plate).
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?
Let the benzoic acid solution cool by placing on the bench top. After the mixture, with some crystals present, has cooled to room temperature place the beacker in an ice bath to enhance crystallization and crystal recovery, since most chemicals are less soluble at cooler temperatures. Never place the beaker directly in an ice bath from the hot plate. Let crystals for normally by sitting on the bench. If you cool the supersaturated mixture too soon, before you allow it to cool to room temperature, you may actually trap impurities in the solid material. Letting nature for crystals naturally is much for efficient and practical. Using vacuum filtration you should collect your crystals. Use a small Büchner funnel placed on top of a 250-mL vacuum filter flask. The vacuum assembly consists of your vacuum flask with Büchner funnel connected to a vacuum trap which is inserted into a vacuum-trap-bottle which is then connected to the vacuum line. After pouring your crystalline mixture into the Büchner funnel, wash your beaker with DI water and collect this additional crystalline material in your funnel. Wash the solid material with a little DI water to removed filtrate material and any soluble impurities. Let the vacuum run for an addition 5 minutes or so before turning off the vacuum and collecting your crystals.
You will store your crystals until the next lab period in one of the drying ovens. Be certain that you label an evaporating dish or small beaker with the required identifying information prior to drying. During the next lab period, you will recover your material from the drying oven, weigh it and determine the melting point of both the "impure" benzoic acid and your re-crystallized benzoic acid.
| Day 2 procedures: |
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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.
Using available reference sources (MSDS or other sources), determine which of the following solvents would "likely" be the best to re-crystallize benzoic acid. Take into account solubility of benzoic acid in the different solvents, and examine their potential to dissolve a little bit at a lower temperature, but more at a higher temperature.
Solvents that are going to be investigated are:
So, for this experiment, weigh out five 0.100 gram samples of pure benzoic acid and place each sample into five separate test tubes. To each test tube add 5 mL of appropriate solvent. To do this, it would probably be best to use a single 10-mL graduated cylinder for for each solvent. Therefore, it is time to share as a bench of different groups. One group will label one of their 10-mL graduated cylinders with one of the solvents. Another group provides the graduated cylinder for another solvent, and so forth. This way, you do not potentially mix one solvent with another.
After you add the 5 mL of each solvent to the appropriately labeled test tube containing 0.100 g benzoic acid mix the contents of the tube and record observations. For example, if one of the solvents easily dissolves the benzoic efficiently, it would like not be the best solvent. Then, place the tubes in a boiling water bath and as the temperature rises (use a digital thermometer to monitor temperature) periodically swirl the contents of the tube and see which solvent dissolves the benzoic acid the best. The solvent that dissolves benzoic acid the best is probably not the best solvent for re-crystallization. For example, acetone has a boiling point around 56oC. So, you may want to only increase the temperature up to about 50oC for these comparisons.
Does all the the benzoic acid dissolve completely in your samples up to about 50oC?
During the first day of this experiment, you relied on some of the physical properties of benzoic acid to purify it using re-crystallization procedures. It is also possible to purify a chemical based on chemical properties. It is generally understood that organic compounds that are uncharged will most often not be soluble in water. Conversely, charged organic compounds generally are not soluble in organic solvents. Such is the case with benzoic acid. As an acid, benzoic acid is a proton donor, and when it loses its proton, the charged benzoate ion is produced. The solubility of benzoic acid in water at room temperature is small, but the solubility of the benzoate ion is very high in water.
To do this experiment, obtain about 1.00 gram of pure benzoic acid and add it to a 50-mL or 125-mL beaker. Add 10.0 mL of 1 M NaOH to your beaker, and stir its contents using a glass stirring rod. You should observe that all the benzoic acid dissolves at room temperature. You now have a solution containing sodium benzoate. You have effectively taken a non-soluble solute and made it soluble using one of its chemical properties (acids are neutralized by base). Obviously, you will not be able to collect crystals of this solution because the benzoate ion, even at cooler temperatures will not precipitate out.
Since the benzoate ion is fully soluble in aqueous solutions, you cannot for crystals. But what you can do, is now take advantage of the fact that while benzoate ion is soluble, converting this ion back into benzoic acid would produce solid benzoic acid which can be collected via filtration. To do this, you will need to add to your solution enough acid (you will be using HCl) to make the mixture acidic to a pH of about 1 (use appropriate pH paper which allows you to determine the pH). Start adding some of the 3 M HCl to your beaker. You should immediately start to observe some solid formation, but since the overall pH is quite high (but is basic due do to the NaOH), continue to add HCl, with stirring, until the pH gets down to pH=1. A large amount of solid material should be present.
Do a vacuum filtration using a Büchner funnel to collect your crystals. Once you have collected most of your crystals, use some ice-cold DI water to wash the crystals in the funnel. Transfer the crystals to a labeled evaporating dish, or beaker, and let dry until the next lab period.
During the next lab period, you will do a yield, percent yield, and melt temp on your material. Just be aware, that the melting point you observe may not be the best since there might still be some salt in the solid.
Before you do your melt temps, you will need to get some physical information about each of the chemicals listed below. Use any reference source including the internet to obtain these data values. Place the appropriate information in the table below. This will be your reference for determining the probable identity of your unknown samples.
| Compound name | Molar mass (g/mol) | Melt point temperature (oC) |
|---|---|---|
| triphenylmethanol | ||
| trans-stilbene | ||
| acetanilide | ||
| sulfanilamide |
Based your assigned unknown chemicals (each group will have two unknowns), place a small amount of each chemical into a closed capillary tube. You will perform a melt temp of these chemicals. Based on the lowest melting point value above, you should increase temperature until you are about 20oC below the lowest melt point. At this time, reduce the heating of your Mel-Temp apparatus so that the increase in temperature is about 1oC for each 15-20 seconds of time. This should allow you to obtain good melt temp data. Observe the capillary tubes carefully during the entire heating process.
Record the melt temp range for each of your unknowns. When both are melted, you can stop your analysis. Simply turn off the instrument and it will start to cool.
Compare your melt temperature for each of your unknown samples to the melt temperature of the known solids listed above. If your melt temp is not really close to one of the samples above, you will need to repeat the melt temp of that sample.
What compounds do you think were most likely present in your solid unknowns? Show also your observed melt point temperature for your unknown samples.
| Unknown Sample Letter | Probable Chemical Name | Melt point temperature (oC) |
|---|---|---|
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.
| Compound | MW | Amount Needed | mmol | mp | bp | Density | ηD | msds |
|---|---|---|---|---|---|---|---|---|
| Benzoic acid (impure) | 122.12 | 1.00 g | --- | 122.4 | 249.2 | 1.2659 | --- | msds |
| Benzoic acid (pure) | 122.12 | 1.00 g | 8.2 | 122.4 | 249.2 | 1.2659 | --- | msds |
| NaOH (1 M) | 40.00 | 15 mL | 15.0 | msds | ||||
| HCl (3 M) | 36.45 | 5-10 mL | >25.0 | msds | ||||
| Compound | g/mol | grams or mL | 10-3 mol | oC | oC | g/mL | ηD | msds |
1Toxic/Irritant
2Corrosive; (Excess solution should be disposed of properly after the lab.)
3For 15 groups, use: