Potentiometric Determination of the Purity and Dissociation Constant of Potassium Hydrogen Phthalate Auvrae Jherxel G. See Department of Mining, Metallurgical and Materials Engineering, College of Engineering University of the Philippines, Diliman Quezon City, Philippines Date Performed: 13 February 2014 Instructor’s Name: Ms. Kaye Carillo
RESULTS and DISCUSSION: Potentiometry is a method used in analytical chemistry to find the concentration of a solution using electrodes. In this experiment, the acid dissociation constant of potassium hydrogen phthalate or KHP is determined through this electro-analytical method. Potentiometry has two types: direct and indirect. In direct potentiometry, an electrode in a constant potential medium is compared with the potentials an electrode submerged in different analyte concentrations. In indirect potentiometry, as performed in this experiment, the pH of the solution id measured at each addiction of the titrant. Compared to volumetric titrations, potentiometric determination is more accurate. Volumetric titration needs chemical indicators which may indicate various end points depending on the observer’s judgment
of the color change of the solution, while in potentiometry the sensitive pH meter is used as the basis for the determination of an end point, or in this case, the equivalence point. As stated above, the indirect potentiometry follows the change of pH with respect to the volume of titrant added in the solution, thus making it possible to plot the titration curve and easily determining the equivalence point of the solution. The endpoint in potentiometry is indicated at the largest potential breaks, therefore the value of the exact potential is unnecessary and only the change of the cell potentials is needed. The approximate equivalence point may be determined when the pH had undergone a radical change and this may indicate that the solution is near to its endpoint. In the experiment, the pH meter’s electrode is immersed in the solution while it is being titrated w ith
NaOH solution. A magnetic stir bar was used to stir the solution constantly to distribute the titrant evenly throughout the solution containing the KHP analyte. In trial 1, the titrant is added with one mL increments.
Trial 1: Titration Curve 12 11 10 e u l a v H p
9
(20, 8.06)
8 7 6 5 0.00E+00
5.00E+00
1.00E+01
1.50E+01
2.00E+01
2.50E+01
Volume of NaOH, mL
Figure 1: Titration Curve of Trial 1
With this graph, the approximate equivalence point is determined at the corresponding volume in which a radical change of pH has occurred, in this case, at 20mL. Now, for trial 2, the increments of the added titrant in the analyte solution decrease as it approaches the approximate equivalence point, thus, giving the graph shown below.
Trial 2: Titration Curve 12 11 10 e u l a V H p
9 22, 8.66
8 7 6 5 0.00E+00
5.00E+00
1.00E+01
1.50E+01
2.00E+01
2.50E+01
Volume of NaOH, mL
Figure 2: Titration curve of trial 2
In this graph, the approximate equivalence point is 22mL. The points before the equivalence point are acidic while the points after are basic.
Another graph that can determine the equivalence point is the first derivative plot, wherein the change of the pH divided by the change of the volume is plotted with the volume of NaOH added in the solution.
Trial 2: First Derivative Plot 4.50E+00
2.20E+01, 4.16E+00
4.00E+00 3.50E+00 3.00E+00
V ∆2.50E+00 / H p2.00E+00 ∆
1.50E+00 1.00E+00 5.00E-01 0.00E+00 0.00E+00
5.00E+00
1.00E+01
1.50E+01
2.00E+01
2.50E+01
Volume of NaOH, mL
Figure 3: First derivative plot
This graph gives a more accurate value of the equivalence point. In the equivalence point, the DpH/DV has the largest value, while the pre and post-equivalence DpH/DV values have almost equal changes. Lastly, the second derivative plot was then graphed to achieve a more accurate value of the equivalence point.
Trial 2: Second Derivative Plot 8 6
(21.5, 6)
4
2 ^ V 2 ∆ / ) 0 H p0.00E+00 ∆-2 ( ∆
5.00E+00
1.00E+01
1.50E+01
2.00E+01
-4
(22, -6.32)
-6 -8
Volume of NaOH, mL
Figure 4: Second Derivative plot
2.50E+01
As shown in the graph, a line passes through the x-axis, and the value in which y=0 is determined as the equivalence point. Since the graph doesn’t show the value of x when y=0, another graph was used to
obtain the most accurate value of the equivalence point. In this case, equivalence point takes place when there are 21.7 mL of NaOH added in the analyte solution as shown in the graph below. 8.00E+00 6.00E+00 4.00E+00 2.00E+00
(21.7, 0)
0.00E+00 2.14E+01 2.16E+01 2.18E+01 2.20E+01 2.22E+01 -2.00E+00 -4.00E+00 -6.00E+00 -8.00E+00
Figure 5: Determination of the equivalence point when y=0
Potassium hydrogen phthalate has a dissociation reaction: KHP ↔KP
-
+
+ H
The acid dissociation constant of the KHP is also determined by equation: Ka =
[][] []
-
At half- equivalence, the number of moles of KP is equal to the number of moles of KHP left, therefore giving us a simple equation: +
Ka = [H ]
In half-equivalence, the value of the volume in t he equivalence point is divided into 2. The sample’s percent purity was obtained by multiplying the molarity and the volume of the NaOH with
the formula weight of KHP and was then divided by the total weight of the sample. The percent purity of the KHP in this experiment is determined to be 98.77%. The acid dissociation constant of trial 1 and 2 are different but near to each other and yielded a 92.60% of relative error with respect to the 3.9E-6 theoretical Ka value of KHP.
REFERENCES: th D.A. Skoog et al., Fundamentals of Analytical Chemistry 9 edition (Belmont: Cengage, 2013) th G.D. Christian et al., Analytical Chemistry 7 edition (New Jersey: John Wiley & Sons, Inc., 2013) th R.A. Day, A.L. Underwood, Quantitative Analysis 6 edition (New Jersey: Prentice Hall, 1 991)
