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Determinationof Partition Coefficient via Extraction of Benzoic Acid and itsConjugate Base from Two Immiscible Phases

Abstract

Theaim of this experiment was to establish the partition coefficient ofthe compound benzoic acid in two phases, which are immiscible. Theseare the organic and the aqueous phases. The experiment will be donein two parts. The first part consists of dichloromethane-deionizedwater. The deionized water acts as the aqueous phase whiledichloromethane is the organic layer. Liquid extraction of benzoicacid will then be done at this point. The second part also entailsliquid extraction of benzoic acid from the two phases. However,instead of deionized water, sodium bicarbonate was used as theaqueous layer. The partition coefficient is then obtained by dividingthe mass of benzoic acid recovered from the organic phase by the massof benzoic acid retained in the aqueous phase. For this experiment,the partition coefficient in the first part, which consisted ofdichloromethane-deionized water, was established as 16. For thesecond part consisting of dichloromethane-sodium bicarbonate mixture,the partition coefficient was found to be 0.06.

Discussion/Conclusion

Thisexperiment involved liquid-liquid extraction, which is a processwhere a compound is extracted from a solvent in which it is insolubleto one where it dissolves. The two liquids that formed the organicand aqueous layers were immiscible. Therefore, a distinct layer ofthe two was easily formed. The layer on top usually consists of theless dense liquid. In the first part of the experiment, de-ionizedwater was the top layer since it has a density of 1.0 g/cm3.Dichloromethane, which is a highly halogenated compound, is heavierwith a density of about 1.33 g/cm3. Hydrocarbons are lighter thanwater. The layer on top was sodium bicarbonate in the second part ofthe experiment. This is because sodium bicarbonate is highly aqueouswith a 92% of water. In the first part, the recovered mass ofbenzoic acid was found to be 47mg while the mass retained in thedeionized water was 3mg. Benzoic acid is an organic compound.Therefore, this makes it highly soluble in organic solvents.Therefore in the first part of the experiment, most of the benzenedissolved in the organic phase that contained dichloromethane. In thesecond part, the mass that was recovered was only 3mg while thatretained in the aqueous phase of sodium bicarbonate was 47mg. Thisresulted from the formation of the salt of sodium benzoate, which isan ionic compound, and, therefore, dissolves easily in the aqueousphase of sodium bicarbonate and hardly dissolves in the organic phaseof dichloromethane. This sodium benzoate salt is formed due to thereaction of benzoic acid and the aqueous phase of sodium bicarbonate.The partition coefficient found in the first part of the experimentwas 16 while that for the second part of the experiment was found tobe 0.06. These values for the partition coefficient vary greatly withthe theoretical values. For example for the first part of theexperiment, the theoretical partition coefficient value is 53.5.These discrepancies are because of errors that occurred during theexperiment. An error could have occurred from the weighing wherebysome of the benzoic acid may have been lost. Another error may haveresulted from the two phases, organic and aqueous, failing toseparate properly. Also during the extraction, water could havegotten into the pipettes hence leading to error. Contamination couldalso have led to errors mainly through the improper cleansing of thepipettes or mixing the reagents.