Determination of Partition Coefficient

Calculations:Mass of succinic acid used = 1.1 gAqueous Layer(0.49 M NaOH used)Ether Layer(0.1231 M NaOH used)Final Burette Reading / cm311.410.57.412.8Initial Burette Reading / cm32.91.92.47.4Result / cm38.58.65.05.4Mean Titre / cm38.555.2Table 1Since succinic acid is a dibasic acid, its reacts with NaOH with the following reaction:2 NaOH(aq) + H2A(aq)  Na2A(aq) + 2 H2O(ℓ)    where H2A is succinic acid.In aqueous layer,No. of mole of OH- ions reacted = = 4.1895 × 10-3 molNo. of mole of H+ ions used = = 2.09475 × 10-3 molConcentration of acid (CE) = = 0.209475 MIn ether layer,No. of mole of OH- ions reacted = = 6.4012 × 10-4 molNo. of mole of H+ ions used = = 3.2006 × 10-4 molConcentration of acid (CW) = = 0.032006 MTherefore Kd = = 0.153Since the standard value of Kd is 0.15(1), percentage difference = = 2%.Since the Kd measured from only one set of data is unreliable, 12 sets of data are collected from other classmates as follows:Expt. No.Mass of Acid Used / g[Acid(aq)]eqm / M[Acid(ether)]eqm / MKd10.70.2090.02850.136420.2170.02920.134630.1960.02000.102040.80.2230.02950.132350.2360.02890.122460.2350.03080.131170.90.2660.03200.120480.2570.03260.126890.2650.03350.1266101.00.2720.05420.1992110.2760.03760.1361121.10.2100.03200.1527130.3150.03450.1095Table 2Some of the data are discarded as shown above and an average value is taken amongst the same group of mass of acid used. The data obtained is then plotted into a graph:Mass of Acid Used / g[Acid(aq)]eqm / M[Acid(ether)]eqm / M0.70.2130.028850.80.2300.029220.90.2620.032701.00.2760.037521.10.2620.03325Table 3Graph 1From the graph, Kd = slope of graph = 0.13With the standard value, percentage difference = = 13.3%.Conclusion:The partition coefficient of succinic acid between water and ethoxyethane is found to be 0.13.Discussion:Importance of Immiscible Solvents Used in the ExperimentSubstances are said to be immiscible if in all proportions, they do not form a solution. One of the examples is ethoxyethane and water. Ethoxyethane is fairly soluble in water, but these two solvents are immiscible as they are not soluble in any proportions. This is important as it is one of the criteria to keep the partition coefficient constant.Interpretation of Graph 1In Graph 1, it is clear that the graph is linear, which means ratio of the equilibrium concentrations of the acid in water and ether under the experimental conditions is a constant, i.e. the partition coefficient is a constant. In addition, the straight line has a constant slope implies that the coefficient is independent of the amount or concentration of the acid in both layers, as well as the amount of solvent used.Besides, the solvents used should not contain any solute that is going to be dissolved in them. Therefore, the graph should pass through the origin as there can be no distribution if there is no succinic acid present.Factors Affecting Partition CoefficientThere are some factors which affect the value of partition coefficient. Two examples of them are the respective solubilities of succinic acid in the separate solvent and the variation of temperature.Consider one another commonly used solvent, 2-methylpropan-1-ol. Succinic acid should be more soluble in 2-methylpropan-1-ol than in ether as the acid can form extensive intermolecular hydrogen bonds with the alcohol. Although ethoxyethane can also form hydrogen bonds with the acid, the extent of formation is not as great as that in alcohol. Therefore we can conclude that [Acid(alc.)]eqm > [Acid(ether)]eqm. Since Kd = , obviously the partition coefficient between water and 2-methylpropan-1-ol is greater than that of partition coefficient between water and ethoxyethane. Therefore, the partition coefficient is related to the respective solubilities of acid in separate solvents.As for temperature, when a solid is added into a solution with higher temperature, the particles are easier to move between the solution and the solid phase. The Second Law of Thermodynamics predicts that they will shift to the more disordered, more highly dispersed, and therefore, more probably solution state. As the solubility of the two solvents increase with temperature in different rates, it is apparent that the partition coefficient varies with the temperature.However, as explained above, the partition coefficient is independent of the amount of solvent. For example, when 50 cm3 of ethoxyethane is added into the mixture, the partition coefficient is not affected. On the other hand, since the volume of ethoxyethane is increased, more acid tends to dissolve in it rather than in water. Therefore the equilibrium is disturbed and concentrations of acid in both layers are altered. After some time, the equilibrium is again maintained. The speed of diffusion from one solvent to another is the same as the reverse speed. Therefore the concentrations of acid in both layers become constant and so do the partition coefficient.Reason of Some Chemicals Must Have Known Volumes but Some Do NotThe aim of titration is to find the concentration of acid in the solvent, but not only the total number of mole. Therefore, volumes of aqueous and ether solution used in titration must be known as accurately as possible in order to find accurate concentrations.In contrast, the reason of adding water and ethoxyethane is only to leave the mixture to equilibrium and provide enough solvent for the titration. Therefore the amounts of solvents need not be measured out accurately.Reason of Adding Water to Ethereal Solution before TitrationNaOH is an aqueous solution but succinic acid to be titrated is dissolved in the organic solvent, ethoxyethane. It needs time to allow the acid to diffuse out in order to react with NaOH added. Therefore a small amount of water is added to extract the acid from the ether and thus facilitates its reaction with NaOH.Identification of Solvent in Respective LayersThe layer with higher density sinks to the bottom. Since the density of ethoxyethane is 0.7134 g cm-3, which is lower than that of water with density 1 g cm-3, ethoxyethane floats on the water and thus two layers of solvents can be identified.During the experiment, this can also be tested with a drop of water. Allow the mixture stand. Since there is a layer consist of water, when a drop of water is put into the mixture, if the droplet sinks to the bottom of the mixture (a bubble will be seen), it indicates the bottom layer is water and the remaining one is ether. If there is no bubble, then the top layer should be water and vice versa.Reference:Standard Value of Partition Coefficient of Succinic Acid between Water and EthoxyethaneChemistry in Context, Graham Hill and John Holman, page 296 - End of Report -