Purification of Benzoic Acid by Sublimation and Melting Point Determination

Sublimation is a purification technique where a solid transitions directly to a gas, avoiding the liquid phase. It is particularly useful for heat-sensitive compounds, and it minimizes product loss compared to other purification methods like recrystallization. Melting point, on the other hand, is a crucial characteristic used to identify pure substances. The objectives of this study were to purify benzoic acid through sublimation, compare its melting point with a standard, and calculate the percentage recovery.

Experimental:

Compounds Tested:

1. Impure benzoic acid
2. Pure benzoic acid
3. Cooking oil

Procedure:

1. Sublimation of Benzoic Acid:
   • 5.00 grams of impure benzoic acid were placed in an evaporating dish covered with a perforated filter paper.
   • A pre-weighed watchglass was placed on top, and the dish was heated for 10 minutes, allowing sublimation.
   • The sublimate was collected on the inverted watchglass, weighed, and the percentage recovery was calculated.
2. Melting Point Determination:
   • The sublimate was ground into a fine powder and packed into a capillary tube.
   • A capillary tube containing pure benzoic acid (standard) underwent the same process.
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   • Both tubes were attached to a thermometer, and their melting points were determined in an oil bath.

Calculations:

1. Percentage Recovery: Percentage Recovery=(Weight of SublimateInitial Weight of Impure Benzoic Acid)×100Percentage Recovery=(Initial Weight of Impure Benzoic AcidWeight of Sublimate​)×100
   • Example: Percentage Recovery=(0.51 g5.00 g)×100=10.2%Percentage Recovery=(5.00g0.51g​)×100=10.2%
2. Melting Point Range:
   • The melting point range for the sublimate was 118-121˚C, and for pure benzoic acid, it was 113-120˚C.

Results:

Table 1: Sublimation and Melting Point Data

Discussion:

1. Sublimation:
   • The sublimation process yielded a sublimate with a percentage recovery of 10.2%, indicating successful purification.
2. Melting Point Determination:
   • The melting point range of the sublimate (118-121˚C) was consistent with the expected range for pure benzoic acid. However, the standard's melting point range (113-120˚C) showed a slight deviation.

The laboratory experiment successfully purified benzoic acid through sublimation, resulting in a sublimate with a melting point consistent with pure benzoic acid. The slight deviation in the standard's melting point range may be due to impurities or experimental error. Further analysis and optimization of the purification process could enhance the accuracy of the results.

Table 1 The initial and final melting points of the sublimate and the pure benzoic acid

Table 1 displays the melting point range for both the sublimate and pure benzoic acid. The sublimate exhibited an initial melting point at 118˚C and complete melting at 121˚C. On the other hand, the pure benzoic acid had an initial melting point of 113˚C and a complete melting point of 120˚C. For a substance to be considered pure, its melting point range should ideally have a difference of only 1-2˚C. However, in this case, the sublimate showed a 3˚C difference, indicating impurities were still present. Consequently, the standard melting point of 121-123˚C for pure benzoic acid was not achieved. The pure benzoic acid, with a melting point range of 113-120˚C, also demonstrated impurity-induced deviations, likely stemming from environmental contaminants.

Impurities play a significant role in altering the molecular structure and behavior of substances at the crystal lattice level. These contaminants disrupt the organization of the lattice, making it more susceptible to breaking down. This disruption leads to a lower and broader melting point range. Additionally, non-uniformity arises at the molecular level due to impurities, causing varying melting rates. Molecules in proximity to impurities melt more rapidly than those farther away, resulting in a melting point either at or closer to the normal range.

Sublimation, as a purification technique, has limitations. It is more effective in a vacuum, requires compounds capable of sublimation, and is applicable only to a limited number of substances with the condition that the compound should be volatile while impurities are nonvolatile.

Alternative methods for determining melting points include the Mel-Temp Melting Point Apparatus and the Fisher-Johns Melting Point Apparatus, utilizing different designs of melting-point capillaries.

Conclusion:
In summary, while sublimation proved effective for purification, its limitations, such as selectivity for certain substances, suggest that other purification techniques may offer greater efficiency. The determination of melting points for the sublimate and pure benzoic acid revealed that the observed melting points did not align with the theoretical values, indicating the persistence of impurities in both samples.