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Preparation of a Buffer

Essay by   •  February 25, 2012  •  Research Paper  •  1,620 Words (7 Pages)  •  1,592 Views

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ABSTRACT

Recrystallization is a process of dissolving the solid to be crystallized in a hot solvent or solvent mixture and then cooling the solution slowly. The purpose of this experiment is to purify crude acetanilide product by recrystallization. Crude acetanilide is produced by the acetylation of aniline. The crude acetanilide is brought to boil in water, the chosen recystallization solvent. Activated charcoal is added when there is change in color; then, filtration is done to collect the solution. Once the solution reaches the room temperature, it is placed in an ice bath. Finally, the crystals were collected through filtration and washings with water.

INTRODUCTION

Only solids that can pass the vapor phase without passing through the liquid phase can be purified by sublimation. Solids that do not have this property can be purified by recrystallization. Recrystallization is the most important method for purifying solid organic compounds. It is a very efficient method of purification, and an important industrial technique still relevant in the chemical world. (Williamson and Masters, 2010).

Figure 1: process of recrystallization

A pure, crystalline organic substance has a well-defined crystal lattice, a three-dimensional array held by London forces. Most organic solids melt between the range of 22℃ and 250℃. An impure organic solid does not have a well-defined lattice and needs to undergo recrystallization, in order to remove the impurities and to allow a perfect crystal lattice to grow.

The four important concepts in the process of recrystallization are solubility, saturation level, exclusion, and nucleation (Williamson and Masters, 2010). Recrystallization is a process where in the impure organic solid will be dissolved in a hot solvent (solubility). When the solution is cooled, it becomes saturated with respect to solute (saturated level). As the perfect lattice is formed, impurities are excluded (exclusion), and crystal become pure substance again.

MATERIALS AND METHODS

The materials needed to perform this experiment are 3 test tubes, Erlenmeyer flask, beaker, tripod, Bunsen burner, tripod, wire gauze, aniline, hexane, acetic anhydride, methanol, and distilled water.

The experiment is subdivided into three parts: choosing the correct recrystallization solvent, acetylation and recrystallization.

In choosing a solvent, a small amount of acetanilide is placed in three test tubes. 1 mL of water, hexane, and methanol are placed in each of the three test tubes. Afterwards, the three test tubes are placed in a water bath (37-40℃) for 1 to 5 minutes. Last, the solutions are allowed to cool. All of the observations regarding the solubility of acetanilide are recorded. . The best recrystallization solvent will dissolve the solute in a hot solution but not in a cold solution.

Acetylation is done by mixing 2 mL of aniline and 20 mL of distilled water in an Erlenmeyer flask. A dry glassware is used to ensure that the slow addition of 3 mL of acetic anhydride will not cause any reaction. The solution is cooled in an ice bath for crystallization.

The last and the major part of the experiment is the recrystallization process where in the crude acetanilide produced from acetylation is placed in an appropriate container, which is an Erlenmeyer Flask. According to Bunelle, a beaker is not appropriate. The shape of the Erlenmeyer flask allows swirling of the mixture without sloshing, and the conical walls tend to condense and return refluxing solvent. The flask is not to be overfilled - about 60% of the listed capacity is the maximum, and so the flask volume should be chosen accordingly.

20 mL of the best recrystallizing solvent is added to the crude acetanilide and the mixture is heated in a water bath until the solute completely dissolves. The mixture stirred through the use of the glass rod to promote dissolution. The mixture while heated is required to be a colorless solution. If not, use activated charcoal. Only a small amount of charcoal is required - too much and significant amounts of the desired compound will also be absorbed, and low recovery will result. After addition of the charcoal, the mixture should be heated to a gentle boil for a few minutes, to ensure that any compound which might have precipitated during cooling is brought back into solution. (Bunnelle, Meyer, Glaser)

The hot solution from the water bath should be filtered right away. A fluted filter paper is involved and is the method of choice for removing charcoal. Figure 2, shows the way in fluting a filter paper. The filtrate is placed in an ice bath to recrystallize, filtered and washed by distilled water, and press between filter papers to dry.

Figure 2: Fluting Filter Paper

RESULTS AND DISCUSSION

The chemist is guided by the dictum "like dissolves like" in finding the appropriate solvent.(Williamson, and Masters, 2010). Because it is difficult to decide which solvent should be used, experimentation is done. The requirement is that the solvents should be miscible to each other. As stated a while ago, the best recrystallization solvent will dissolve the solute in a hot solution but not in a cold solution. It will not react with the solute, and is nonflammable and nontoxic. The results of the experiment are shown in table 1.

Table 1: Solubility of pure acetanilide in different solvents

Solvent At room Temperature During Heating Upon cooling

Water Insoluble soluble Insoluble

Methanol Soluble soluble Soluble

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