SUBLIMATION AND MELTING POINT DETERMINATION OF BENZOIC ACID

Dianne I. Pascua, Francesca F. Perez, Romulo Vincent S. Perez,

Lia S. Priela and Zeth Raquedan

Group 8 2C Medical Technology Organic Chemistry Laboratory

ABSTRACT

5.00 grams of impure benzoic acid was used in the sublimation and determination of melting point. The sample washeated for 10 minutes then the sublimate was collected and weighed. The sublimate weighed 0.51 grams with whichthe percentage recovery was 10.2%. The sublimate was then placed into a capillary tube together with another tubethat contained pure benzoic acid to serve as the standard. Both of the capillary tubes were heated in an oil bath wherethe initial and final temperatures of their respective melting points were noted. The sublimate had a melting pointrange of 118-121˚C and the pure benzoic acid had a melting point range of 113-120˚C. 

INTRODUCTIONSublimation is a phase transition process from

a solid to a gas without ever entering anintermediate liquid phase. It is adopted as atechnique for purification of solids because: theloss of product is typically minimal compared toother techniques; it is more appropriate for anyheat sensitive compound; and solvents are notinvoled in the process unlike recrystallization.

A melting point is the temperature at which asolid or liquid form of a pure substance can existin equilibrium. As heat is applied to a solid, itstemperature will increase until the melting pointis reached. More heat will then convert the solidinto a liquid with no temperature change. Themelting temperature of crystalline solids is acharacteristic figure and is used to identify purecompounds and elements. Most mixtures andamorphous solids melt over a range of temperatures.

The objectives of the study were: (1) to purifybenzoic acid by sublimation, (2) to determine andcompare the melting point of the product withthe standard, and (3) to calculate the percentagerecovery.

EXPERIMENTALA. Compounds Tested (or Samples used)Impure benzoic acid, pure benzoic acid, cookingoil

B. Procedure

1. Sublimation of benzoic acid5.00 grams of impure benzoic acid was placed

in an evaporating dish covered with a perforatedfilter paper. A pre-weighed watchglass was thenplaced on top of the filter paper. The evaporateddish was then heated for 10 minutes, allowingthe sample to undergo sublimation. To collect thesublimate, the pre-weighed watchglass wasinverted. The sublimate was then weighed andthe percentage recovery calculated.

2. Melting point determination of benzoic

acid The sublimate was ground into fine powder and

placed inside a capillary tube. It was ensured thatthe sublimate in the tube was well-packed bydropping the closed end of the tube downthrough a glass tubing several times. The samewas done with a capillary tube containing purebenzoic which served as the standard. Bothcapillary tubes were then attached to athermometer with a piece of rubber band. Theclosed ends of the tubes were aligned with themercury bulb of the thermometer. Thethermometer along with the two capillary tubeswere immersed in an oil bath which was stirredconstantly. The temperature at which thesublimate and the pure benzoic acid started andcompletely melted were noted.

Figure 1. The sublimate

 

 Figure 2. The melting point set-up

RESULTS AND DISCUSSIONThe weight of the sublimate collected was 0.51

grams. In order to obtain the percentagerecovery, the following equation was used

   

Of the 5.00 grams of benzoic acid sublimated,only 0.51 grams or 10.2% of the substance wasrecovered. This may be due to the limitations of the sublimation technique. For a higherpercentage recovery, sublimation must happen ina vacuum for reduced pressure. Sublimation isnot as selective as recrystallization and requiresthat the compound should be volatile while theimpurities be nonvolatile.

Table 1 The initial and final melting points of 

the sublimate and the pure benzoic acid

TI TF

Sublimate 118˚C 121˚C 

Pure benzoic acid 113˚C  120˚C 

Table 1 shows the melting point range of thesublimate and the pure benzoic acid. Thesublimate initially melted at a temperature of 118˚C and completely melted at a temperatureof 121˚C. The pure benzoic acid initially meltedat a temperature of 113˚C and completelymelted at a temperature of 120˚C. In order to beconsidered as a pure substance, the meltingpoint range of the sublimate should only have adifference of 1-2˚C. Since the difference was3˚C, the sublimate was still impure andcontained impurities. Because of the impuritiesstill found in the sublimate, the standard melting

point of 121-123˚C was not observed. The purebenzoic acid had a melting point range of 113-120˚C. It could not also be considered as a puresubstance because the melting point range has adifference of 7˚C. This was caused by theimpurities in the environment whichcontaminated the pure benzoic acid. Theimpurities cause the melting point range to lowerand to broaden. They disrupt the consistency andorganization of the crystal lattice at the molecularlevel. The contaminants do not fit correctly intowhat would be the normal pure lattice. Thisdisruption weakens the lattice making it easier tobe broken. The weakened structure melts moreeasily at reduced temperatures. The disruptionalso causes non-uniformity at the molecularlevel, the molecules closest to the impurities meltthe fastest while those further away where thecrystal lattice is relatively undisturbed melts at ornearer the normal range.

The limitations of sublimation is that it requiresa vacuum to be more effective, the compoundmust be able to undergo sublimation, only a fewcompounds undergo sublimation and it requiresthat the compound should be volatile while theimpurities are nonvolatile.

Other methods for determining the meltingpoint of substances include the Mel-Temp MeltingPoint Apparatus which uses long, thin melting-point capillaries and the Fisher-Johns MeltingPoint Apparatus which uses small round, glasscoverslips.

CONCLUSIONBased on the results, sublimation can be used

as a technique for purification. However, due toits limitations like only a few substances sublime,other purification techniques may be moreefficient.

In the determination of the melting points of the sublimate and the pure benzoic acid, thetheoretical melting point was not observedbecause the sublimate and the pure benzoic acidstill contained impurities.

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