Purification of Benzoic Acid by Sublimation and Melting Point Determination
T.H. Tsai, R.F Vila, K.Y.* Visco, R.G. Vivar, J.C. Zafra, J.C. Ziganay Abstract:
Sublimation is the process by which a substance undergoes conversion from the solid phase to the gas phase, without going through the intermediate liquid phase. This experiment aims to purify benzoic acid through sublimation, compute for its percentage recovery and compare its purified substance with a standard to determine its melting point. The impure benzoic acid is heated in a n evaporating dish covered with wi th inv invert erted ed wat watch ch gla glass ss and aft after er som some e tim time, e, the sub sublim limate ate is col collec lected ted for per percen centag tage e rec recove overy ry computation and also packed in a capillary tube for its melting point determination. A result of 7.88% of percentage recovery was achieved from the sublimation process and a result of 130 C was obtai obtained ned in the melting point determination indicating that the pure substance (standard) and the sublimate are the same. ˚
Introduction:
Sublimation is the process by which a substanc subs tance e und undergo ergoes es conv convers ersion ion from the solid phas ase e to the gas phase, without going through the liquid phase. In this phenomenon, the transition from the solid to the gaseous state requires an inte interm rmed edia iate te liqu liquid id stat state e [1]. A number of solids can simply change into vapo vapors rs;; thes these e vapo vapors rs can can subl sublim ime, e, meaning that vapors form a solid without going going throu through gh the the liquid liquid state state.. A good good example that can undergo sublimation is ice when pressure is reduced, 1/100th of the the usual usual atmosp atmosphe heric ric press pressure ure.. Sublimation Sublimation is helpful in purifying some solids like menthol menthol,, naphthale naphthalene, ne, iodine, etc. [2]. Sublimation is a technique used by chemists to purify compounds. Under this th is re redu duce ced d pressure pressure,, the solid volatilizes and condenses as a purified comp co mpou ound nd on a coo cooled led su surfa rface. ce. Onc Once e heating ceases, the purified compound may be co coll llec ecte ted d fr from om th the e co cool olin ing g surface which has a non-vola non-volatile tile residue
of imp impurit urities ies beh behind ind [1]. Benzoic Benzoic aci acid d was first obtained by sublimation from gum gu m be benz nzoi oin. n. Th This is is al also so kn know own n as “flowers of benzoin” [3]. Benzoic acid in its basic form is called impure benzoic acid. The impurities that are in the newly formed benzene are because of phenyl and benzy benzyll compo compound unds s that that seriou seriously sly affe affect ct the the prod produc uct. t. The The impu impuri riti ties es conta contain in amino amino comp compoun ounds ds that that are present in a larger extent inside impure benzene [4]. A pure benzoic acid can be obta obtain ined ed from from recr recrys ysta tall lliz izat atio ion n and and sublimation but in this test, we’ll focus on sublimation. The melting point of a substance is the temperature at which its solid and liquid phas phases es coex coexis istt in equi equili libr briu ium. m. The The normal melting point of a substance is its its melt meltin ing g poin pointt at one one atmo atmosp sphe here re press pressure ure.. Chang Changes es in press pressur ure e have have very small effects on melting points, but they have large effects on boiling points [5].
The experiment aims to purify benzoic acid by sublimation, to determine and compare the melting point of the product with a standard and to calculate its percentage recovery that is obtained.
Figure 3.2 Melting point set-up
Methodology:
Figure 3.1 Sublimation set-up
Five grams of impure benzoic acid is weighed in an evaporating dish. The evaporating dish is covered with a perforated filter paper. An inverted preweighed watch glass is placed on the top of filter paper and heated by a hot plate. Figure 3.1 is given to serve as guide. The watch glass is cooled with a tissue paper placed on the top of watch glass that is kept moist with water. The sample is heated for 10-15 minutes until the sample has vaporized. When the set-up is cooled, it is said that the preweighed glass must be inverted. The sublimate is collected and weighed in the watch glass. For melting point determination, the sublimate was ground into a fine powder and a capillary tube is pressed into the powder. The closed end of the capillary tube is dropped several times through a glass tubing until it is well-packed with approximately 1mm of the sublimate. The capillary tube is attached to a thermometer with a piece of rubber
band together with the standard pure benzoic acid that is also packed in a capillary tube. Refer to the Figure 3.2 on the set-up for the melting point determination. Results:
The results in the experiments are given here:
This means that when 5g of benzoic acid is subjected to the process sublimation, 0.3938g of sublimate will be obtained and its percentage recovery shows that 7.88% of the pure substance is produced from the starting material. The melting point determination dictated that the sublimate obtained is a pure substance having the same melting point with the standard pure benzoic acid which is 130 ˚C. References:
Percentage recovery:
weight of sublimateweight of impure benzoic acid×100% 0.3938g5g×100%=7.88%
Melting point of benzoic acid
Discussions:
In the experiment performed, the sublimate is formed at the cooling surface (inverted face) of the watch glass after heating for about 10-15 minutes. The sublimate weighed about 0.3938g and a percentage recovery of 7.88% was obtained from the ratio of the weight of the sublimate and the weight of the impure benzoic acid then multiplying it by 100%. The melting point of the sublimate as well as the pure benzoic acid that has been obtained based on the experiment is 130˚C.
1.”Sublimation (chemistry).” New World Encyclopedia. 24 Feb. 2007. 2.”Sublimation.” 3. “Benzene.” Wikipedia: The Free Encyclopedia. 23 Jul. 2011. 4.”What Is Impure Benzoic Acid?” Mike Charmaine. eHow Health. 5. Whitten, Davis, Peck and Stanley. Chemistry, Ninth Edition. Canada, 2010. p.468.
