Potentiometric Determination of the Purity and Dissociation Constant of Potassium Hydrogen Phthalate E.M.M. Medrano Medrano,, J.M. Pasco Department of Mining, Metallurgical and Materials Engineering, College of Engineering University of the Philippines, Diliman, Quezon City, Philippines Date Due: 18 February 2014 Date Submitted: 18 February 2014 RESULTS AND DISCUSSION Percent purity and dissociation constant of a sample were expected to be determined by the end of the experiment using the concepts of indirect potentiometry. In the experiment, indirect potentiometry using measures of pH was utilized to determine the dissociation constant of the sample.[1] Indirect potentiometry involves measuring the change in pH of a sample while it is being titrated.
Figure 11.2 First Derivative Plot:ΔpH/ΔV against Ave. Volume In the first derivative plot, the equivalence point was the value of x at the maximum point.[3]
The endpoint was estimated using three different plots. The significant regions in these plots include those areas before, after and at the equivalence point itself. The regions before the equivalence point have low and slowly rising pH while the regions after the equivalence point have high pH. Figure 11.3 Second Derivative Plot: Δ2pH/ΔV 2 against Ave. Ave. Volume In the second derivative plot, the equivalence point was the value of x at the point where the inflection intercepted the x-axis.[4]
Figure 11.1 Regular Curve: pH against volume In the regular curve, the inflection point signaled the equivalence point. [2] This inflection point is characterized by a sudden rise in pH which gives a steep slope. Trial 2 has a steeper slope than Trial 1 which means that the half-equivalence point of Trial 2 is sharper. It should be noted that the mass of the sample in Trial 2 is closer to the prescribed mass of 0.25 g.
The amount of titrant was added in increments to provide a smoother curve. A magnetic stirrer was used to constantly stir the sample and hasten the reaction for instant results. Table 11.1 Summary of Equivalence Points from the Three Plots Volume of NaOH at the eq. pt., mL Trial Plot 1 Plot 2 Plot 3 1 19 19 18.45 2 19.9 19.8 19.75
There was a difference in values between the two trials because of the difference in the sample weighed. Trial 1 had 0.0056 grams of sample more than Trial 2 which only has 0.2505 grams of sample. Due to this, Trial 2 has sharper endpoints than Trial 1 and therefore, has more accurate results. One of the advantages of potentiometric titration is that it does not need additional reagents as indicators. This eliminates the error introduced by indicators. However, getting the equivalence point from the plot of potentiometric titrations assume that the titration curve is symmetric.[5] This is not always the case especially in lab experiments. Results showed that the sample has an average percent purity of 77% and a dissociation constant of 5.6x10-4. CONCLUSION The percent purity and dissociation constant of the sample was determined by the end of the experiment using the concepts of potentiometry. The sample's dissociation constant deviated by 41% from the dissociation constant of KHP. The huge percent difference was expected because the impure sample was compared against the almost pure sample. It is important to determine the percent purity of the sample to account for the differences in dissociation constants. REFERENCES [1] Crouch, S.; Holler, F.; Skoog, D.; West, D. Fundamentals of Analytic Chemistry, 8th ed.; Harris: Canada, 2004; p. 624 [2-4]Institute of Chemistry Analytical Chemistry Laboratory Manual, 2007 Revised edition; University of the Philippines, Diliman, Quezon City, 2007; pp. 101-102 [5] Crouch, S.; Holler, F.; Skoog, D.; West, D. Fundamentals of Analytic Chemistry, 8th ed.; Harris: Canada, 2004; p. 646
