SUBLIMATION AND MELTING POINT DETERMINATION OF IMPURE BENZOIC ACID Marian Angelu Ramos, Rose Ann Refuela, Leomarie Duanne Sanchez , Paula Lynne Santos, Geraline Sarmiento, Jon Carlo Semana Group number Seven, 2E- Pharmacy, Faculty of Pharmacy, University of Santo Tomas
ABSTRACT Sublimation is the transition of a solid into a gas without passing the liquid phase. Melting point is the temperature at which a solid melts. The objective of the group is to purify benzoic acid through sublimation and to determine its melting point. The group placed impure benzoic acid in an evaporating dish covered with a perforated filter paper, topped with a watch glass and heated it using a hot plate. After heating, the set-up was cooled and the sublimate was collected and weighed. The sublimate was then grinded and placed in a capillary tube and the tube was attached to a thermometer. The thermometer with the capillary tube attached, was then immersed in an oil bath and heated until the sublimate melted. After the experiment, experiment, the t he group retrieved two point five grams (2.5 g) of sublimate and got a percentage recovery of 50 %. Also, from the experiment done, the determined that the melting point of pure benzoic acid is 123°C.
INTRODUCTION Sublimation Sublimation is the transition of a substance from a solid state to a gas state without passing through the intermediary liquid stage (Wolfenden, 1999-2013) . This process normally normally occurs occurs when the the vapor pressure of a substance substance is greater greater than the the atmospheric pressure. It is a process used in the purification of substances that have nonvolatile impurities or substances that have a significantly significantly lower vapor pressure than a pure compound. compound.
Melting point is the temperature at which a solid is changed into a liquid. This happens when the solid phase of a substance and its liquid phase are in a dynamic equilibrium (Mendoza, 2008). The melting point of a substance can be used to determine its purity. The melting of a pure substance is fixed while an impure substance melts over a wide
range of temperature and a lower temperature. Benzoic Acid (C6H5COOH) is a white crystalline powder and is the simplest of aromatic carboxylic acids. It has the melting point of 122°C and boiling point of 249°C. Benzoic acid is commonly used as a food preservative and a germicide or antifungal agent. It also can be used in the production of of cosmetics, cosmetics, dyes, plastics and insect repellents (Rausch, 1999-2013). One objective of the experiment is to purify impure benzoic acid through the process of sublimation. sublimation. Another is to determine the melting point of the pure benzoic acid acid obtained obtained through through the sublimation of impure benzoic acid.
MATERIALS AND METHODS
melting and melted completely was recorded by the students. students.
MATERIALS MATERIALS
RESULTS The group used an evaporating dish – where where the impure benzoic acid was placed; filter paper, watch glass, and a hot plate for sublimation. A bunsen burner, beaker, thermometer, capillary tubes, copper wire, iron ring, iron stand and clamp, wire gauze, rubber band and cooking oil were used for the determination of benzoic acid’s melting point. METHODS METHODS
First, the group placed 5 g of impure benzoic acid acid in an an evaporating evaporating dish and and covered it with a perforated filter paper. A watch glass was then weighed and was placed on top of the covered evaporating dish. The set-up was then secured with masking tape, making it an air tight container. A moist tissue paper was placed on top of the watch glass while it was heated for 15 minutes. After heating, the set-up was cooled and the sublimate was collected and weighed. Using the data acquired, the percentage percentage recovery recovery was calculated. calculated. The sublimate and pure benzoic acid were then grinded grinded into a fine powder. After, that each of the powdered substances was placed inside inside two separate capillary tubes that were heated in order to close one end. The capillary tubes were then attached with the thermometer and aligned to the mercury bulb of the thermometer. thermometer. Next, Next, the thermometer, together with the attached capillary tubes were immersed in an oil bath and heated until both the sublimate and pure benzoic acid acid melted. melted. The oil oil bath was continually being stirred with copper wire for the uniform distribution of heat. The temperature at which the sublimate started
From the experiment done, the following data were gathered.
Table 1-1: Data gathered from the experiment. Weight of impure benzoic acid acid
5g
Weight of sublimate + watch glass
95 g
Weight of empty watch glass
92.5 g
Weight of sublimate
2.5 g
The group got the temperature at which the sublimate and pure benzoic acid started to melt and completely melted. The temperatures can be seen in Table 1-2. Table 1-2: Temperature at which the substances started to melt and completely melted. Substance
Sublimate Pure Benzoic Acid
Temperature the substance started to melt 119°C 119°C
Temperature the substance melted completely 122°C 123°C
DISCUSSION At the start of the experiment, a watch glass was weighed. This was done because the sublimate sublimate was going going to be placed in the watch watch glass after after sublimation sublimation
and to get the weight of the sublimate, the weight of the watch glass it is placed on must also be known. After sublimation, the group weighed the watch glass again now with the sublimate. The weight of the empty watch glass was then subtracted to the weight of the watch glass containing the sublimate to get the actual weight of the sublimate. From this, one can see that the group gathered 2.5 g of sublimate after performing sublimation on impure benzoic acid. The figures presented in Table 1-1 were the acquired datum gathered during and/or after the experiment was conducted. The said datum was used in to calculate for the percentage recovery of the sublimate. The formula used for the calculation was: With the use of the following formula above, it was computed that the percentage percentage recovery recovery of the sublimate sublimate retrieved was 50 percent. The 50 percent means that the students recovered half of the weight of impure benzoic acid, leading the group to conclude that half was the pure benzoic acid acid obtained obtained from the the impure benzoic acid. acid. In Table 1-2, one can see the temperatures at which the pure benzoic acid and sublimate starts melting. Both of the substances started melting at a temperature of 119°C. However, the sublimate and the pure benzoic benzoic acid differed very very slightly in the temperature at which they completely melted. The sublimate had a lower melting point which which is 122°C while pure benzoic acid had a melting point of 123°C.
As stated in the introduction, the melting point of a substance can help in determining its purity. From Table 1-2, it can be seen that the melting point of the pure benzoic benzoic acid that that was collected collected was 123°C which is actually very close to the standard melting point of pure benzoic acid which is 122°C.
REFERENCES Bayquen, A. & et. al. (2009). Laboratory Manual in Organic Chemistry. Quezon City: C&E Publishing, Inc. Mendoza, E. & Religioso, T. (2008) . Chemistry. Quezon City: Phoenix Publishing House Inc. Rausch, D. (1993-2013). Physical Properties of Benzoic Acid. Retrieved July 28, 2013 from http://www.ehow.com/about_5558410_physical properties-benzoic-acid.html#ixzz2aLQ96e2Q properties-benzoic-acid.htm l#ixzz2aLQ96e2Q Wolfenden, S. (1999-2013). Hot to Set-up for the Sublimation of Solids. Retrieved July 28, 2013 from http://www.ehow.com/how_8734610_set-upsublimation-solids.html Recrystallization Recrystallization of Benzoic Acid and Determination of its Melting Point Retrieved July 28, 2013 from http://editions.sciencetechnologyaction.com/lessons/2/ 26/GSK-lesson.pdf
