SUBLIMATION AND MELTING POINT DETERMINATION OF IMPURE BENZOIC ACID Hannah Marie G. Herrella, Isabelle Louise F. Ilagan, Myzhel S. Inumerable, Inumerable , Natasha R. Jain and Tactto B. Kobayashi Group 5 2C Pharmacy Organic Chemistry Laboratory –
ABSTRACT The experiment was conducted to determine the purity of benzoic acid through sublimation and melting point determination of the impure benzoic acid. Five grams of impure benzoic acid was heated for 10 to 15 minutes to form colorless needle-like crystals on the inverted watch glass. The crystals (sublimate) was used in calculating the percentage recovery. The crystals and the pure benzoic acid were then individually packed in capillary tubes for determining the melting point of the samples with the use of an oil bath and thermometer. A 110⁰ C to 124⁰ C was obtained indicating that there are impurities. Certain deviations on melting point have occurred maybe because of mishandlings and contaminations
INTRODUCTION Sublimation is the process process where some some solids can pass directly into vapor phase without undergoing through a liquid phase. Given the fact that the vapor can solidify, sublimation may be an effective purification process. It can be therefore defined as a technique used to purify solid mixtures. It is only limited, however, to those with non-volatile impurities or those that have lower vapor pressure than the pure compound. Melting point is often associated with sublimation when testing purity of compounds. Melting point is the temperature in which a solid phase is in equilibrium with the liquid phase. A substance normally melts at room temperature. Changing the pressure does not affect the melting point but boiling points are greatly affected. An oil bath was preferred because the melting point of benzoic acid is higher than the boiling point of water. Benzoic acid is a white crystalline organic compound that has a melting point of 122˚ C and
B. Procedure 1. Weighing of Materials Weigh the evaporating dish and watch glass in the analytical balance. Weigh 5 g of impure benzoic acid using a triple beam balance. 2. Preparation of the capillary tubes Heat one end of the two capillary tubes using a Bunsen burner. While heating one end, rotate it to make it even. Continue heating until completely closed. 3. Preparation of the hot oil bath set-up Pour unused cooking oil in a clean beaker. Place the beaker on top of a tripod with a wire-gauze and a Bunsen burner underneath. Secure the capillary tubes with a piece of thread or rubber band to the thermometer. Align the closed end of the capillary tubes with the mercury bulb of the thermometer. Attach the thermometer in a clamp and immerse in an oil bath.
a boiling point at 247˚ C. It is naturally found in many plants and resins. As a microbial agent, benzoic acid is commonly used in toothpastes, mouthwashes, cosmetics and deodorants. Large extent of impurities are present inside the crude form of benzoic acid because it contains phenyl and benzyl compounds and also amino acids. In this experiment, the group should be able to attain the following objectives: (1) purify benzoic acid through sublimation, (2) determine and compare the melting point of the product with a standard, and (3) calculate the percentage recovery.
EXPERIMENTAL A. Compounds tested 5.0 g of impure benzoic acid Pure benzoic acid
Figure 1. Oil Bath Set-Up
4. Purification of Benzoic Acid Place 5.0 g of impure benzoic acid in a tared evaporating dish. Cover the dish with a perforated filter paper. On top of the filter paper, place an inverted pre-weighed watch glass. Cool the top of the watch glass with a moistened tissue paper. Heat for 10 to 15 minutes. Stop heating and cool the set-up. On the fume hood, carefully invert the preweighed watch glass and collect all the sublimated in it. Calculate the percentage recovery. 5. M e l t i n g P o i n t
Figure 2. Sublimation Process
Determination Grind the sublimate into a fine powder. Press the open end of the capillary tube into the powder. Drop the closed end of the capillary tube down through a glass tubing several times until it is well-packed with 35 mm of the sublimate. Secure the capillary tubes with a piece of thread or rubber band to the thermometer. Align the closed end of the capillary tubes with the mercury bulb of the thermometer. Attach the thermometer in a clamp and immerse in an oil bath and heat. Stir constantly. Record the temperature at which the sublimate stars to melt and when it is completed melted.
RESULTS AND DISCUSSIONS Sublimation Through sublimation, the 5.0 g of amorphous benzoic acid was formed into colorless needle-like crystals, giving a weight of 0.57 g. Recovery of the benzoic acid was computed at 11.4% by dividing the weight of sublimate by the weight of the impure benzoic acid used, then multiplied to 100. This means that only 11.4% of the sample was recovered.
Weight of Impure Benzoic Acid Weight of the Watch Glass + Sublimate Weight of the Empty Watch Glass Weight of the Sublimate Percentage Recovery
5g 93.0689 g 92.50 g 0.57 g 11.4%
Table 1. Data obtained from Sublimation Process Initially, 5 g of impure benzoic acid was weighed and had undergone the sublimation process. A watch glass was pre-weighed and was found to be weighing 92.50 g. This watch glass served as the cover for the setup. Also, this is where the sublimate is contained after the sublimation process. After the said process, the watch glass was again weighed and was found to be weighing 93.0689 g. This means that the sublimate weighs only 0.57 g. From the data collected, the Percentage Recovery may be computed: eight of the ubliate %Recovery = eight of the Ipure enoic cid . = .g = 11.4%
Based from the data collected before and after the sublimation process, the percentage recovery was computed to find out the purity of the sublimate obtained. It is computed using the values of the weight of the sublimate over the weight of the initial sample, impure benzoic acid. It is then multiplied to 100 to indicate its percentage. Based on Table 1, the data here shows that there was 0.57 g of the sublimate and 5 g of the impure benzoic acid. Using the formula for percentage recovery, it indicates that only 11.4% was recovered. Meaning to say, ≤ 88.6% of the initial sample was made up of impurities. Melting Point Determination The crystals and the pure benzoic acid were then individually packed in capillary tubes attached to the thermometer. Immersion of the sample and a standard to an oil bath determined the melting point of the sample. The standard started melting at 108°C and completely melts at 122°C, while the sublimate began to melt at 110°C and was fully melted at 124°C. Therefore, the standard has a higher melting point compared to the sublimate. The difference of _ in melting point of the two substances indicate that the sample is not purified well.
Temperature T1
T2
Average
Pure Benzoic Acid
108°C
122°C
115°C
Sublimate
110°C
124°C
117°C
Table 2. Melting Point Data Table 2. Recorded Temperature for Melting Point Determination Table 2 notes the differences of the sublimate and the pure benzoic acid to the temperature at which they have started and completed melting. T1 indicates the temperature at which the two compounds started to melt. Pure benzoic acid started to melt at 108°C, while the collected sublimate started to melt at 110°C. The average melting point of T1 is 115°C. The T2 indicates the temperature at which the two has completely melted, pure benzoic acid melted completely at 122°C, and the sublimate completely melted at 124°C. The average melting point of T 2 is 117°C. Pure benzoic acid is known to have a melting point of 122°C. There is a difference of
approximately 7°C between the standard melting point and the average melting point of the compound in the experiment. Certain deviations on melting point have occurred maybe because of mishandlings and contaminations.
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