General Chemistry II
Spring Semester
Bellevue College
Department of Chemistry
Determination of equivalence point using pH titration of
Potassium Hydrogen Phalate and 0.1 N Sodium Hydroxide
with phenolphthalein indicator
By Won Kim
Date submitted: Feb 26th , 2014
AbstractThe experiment described elucidates the fundamental principle of potentiometric titrations and
determination of equivalence point using pH titration of Potassium Hydrogen Phalate and 0.1 N
Sodium Hydroxide with the use of phenolphthalein indicator. The present study revealed the
equivalence point by using simple titration curve which was further confirmed by using first and
second derivative plots and pKa value of the Potassium Hydrogen Phalate.
IntroductionThe pH meter measures the pH of a solution and provides a direct method of obtaining a titration
curve which is a graph of measured pH values versus the volume of titrant added in milliliters.
The equivalence point is the point at which a stoichiometrically equivalent amount of base has
been added to the acid. It does not mean that pH will be necessarily 7.
KHP is a monoprotic acid. The neutralization with NaOH takes place in a 1:1 ratio
HOOCC6H4COOK (aq) + NaOH ( aq) ---> C6H4 ( COO)2 2- (aq) + K+ (aq) + Na+ (aq)
The equivalence point occurs in the region where there is a relatively large change in pH with a
relatively small change in volume on the titration curve. The steeper the curve the more precisely
equivalence point can be established. Once a titration curve is constructed and the equivalence
point is established , an indicator can be used to give a suitable endpoint ( point at which the
indicator changes colour ).
Source : http://www.titrations.info/img/HCl-NaOH-01-phenolphthalein.png
In this experiment , we obtained a titration curve of measured pH against the volume of
0.1N NaOH solution added. The equivalence point is established using the steepest tangent to the
smooth curve where the pH changes rapidly. The equivalence point is the mid point between the
two lines intersecting the volume axis. The equivalence point selected using this method is a
more accurate method than using an indicator in the titration. Additionally , pKa values can also
be directly read from the titration curve.
A second method used to determine the equivalence point is by using the first derivative method.
To use this method, a graph is constructed of Delta pH / Delta V vs Average volume. The
volume at the point where the graph reaches the maxima is the equivalence point of the titration.
In the second derivative of the titration curve, zero crossing is the equivalent point.
Source : http://www2.volstate.edu/chem/1120/Titration-7.gif
pH meter consists of three parts- pH electrode ( Reference and Indicator electrode )
,Thermoprobe and Voltmeter. Reference electrode is usually made up of Ag or AgCl
and has a fixed potential value while the Indicator Electrode is made up of glass and the
potential changes with the concentration of protons . Thermoprobe is used to measure the
temperature of the solution and the Voltmeter is used to measure the potential difference between
the two electrodes.
Experiment
1.5152 g of dried KHP ( Fisher Scientific Co, Pittsburgh,PA ) was weighed and transferred to a
250 mL volumetric flask to prepare a stock solution in water. 50.0 mL of the KHP stock solution
is pipetted into a 250 mL beaker with a magnetic stirring bar and 50 mL of distilled water was
added to it. Solution is titrated with 0.1005 N NaOH ( Fisher Scientific Co, Pittsburgh,PA ), in 2
mL base aliquots for the first 10 ml and then in 0.5 mL base aliquots for the next 5 ml as endpoint approached at 15 ml. Volume and pH after each addition was recorded by using the
calibrated pH meter ( Mettler Toledo , Ohio ). Base was added drop wise until pink end point
was observed . Addition of base was continued till five more 1 mL aliquots were added and
the volume and pH was recorded after each addition.
Results
TABLE 1
Sample Titration Data
Volume of NaOH ( ml )
0.0
2.0
4.0
6.0
8.0
10.0
10.5
11.0
11.5
12.0
12.5
13.0
13.5
14.0
14.5
15.0
15.5
16.0
16.5
17.0
18.0
19.0
20.0
21.0
pH
4.03
4.63
4.86
5.09
5.30
5.53
5.61
5.68
5.76
5.84
5.97
6.09
6.26
6.56
7.10
9.95
10.51
10.74
10.90
10.99
11.16
11.27
11.36
11.43
Table 2 Titration Curve between 0.1 N NaOH and pH
4.03
4.63
4.86
5.09
5.3
5.53
5.61
5.68
5.76
5.84
5.97
6.09
6.26
6.56
7.1
9.95
10.51
10.74
10.9
10.99
11.16
11.27
11.36
11.43
Titration Curve
14
12
0
10
8
6
8
4
10
10.5
2
11
0
0
5
Vol of 0.1 N NaOH in ml
10
15
At half equivalence point,
[HA] = [A-]
[H3O + ] = Ka
Therefore, pH= pKa
Equivalence Volume = 15 ml
Therefore Half Equivalence Volume = 7.5 ml
V = 8.0, pH = 5.30
4
6
Calculation :
V = 6.0, pH = 5.09
2
Equivalence
Point
pH
0
2
4
6
8
10
10.5
11
11.5
12
12.5
13
13.5
14
14.5
15
15.5
16
16.5
17
18
19
20
21
11.5
20
25
pH of the Half Equivalence Volume = 5.2 = pKa
Therefore, Ka = 6.30 x 10 ^ - 6
Tabls 3 First Derivative TableVol in
ml
pH
0
2
Delta
pH
6
5.09
10.5
11
11.5
12
12.5
13
13.5
14
14.5
15
15.5
Delta pH / Delta V
0.6
2
1
0.3
0.23
2
3
0.115
0.23
2
5
0.115
0.21
2
7
0.105
0.23
2
9
0.115
0.08
0.5
10.25
0.16
0.07
0.5
10.75
0.14
0.08
0.5
11.25
0.16
0.08
0.5
11.75
0.16
0.13
0.5
12.25
0.26
0.12
0.5
12.75
0.24
0.17
0.5
13.25
0.34
0.3
0.5
13.75
0.6
0.54
0.5
14.25
1.08
2.85
0.5
14.75
5.7
0.56
0.5
15.25
1.12
0.23
0.5
15.75
0.46
4.63
4.86
10
Mean Volume in
ml
4.03
4
8
Delta V
5.3
5.53
5.61
5.68
5.76
5.84
5.97
6.09
6.26
6.56
7.1
9.95
10.51
16
10.74
16.5
0.16
0.5
16.25
0.32
0.09
0.5
16.75
0.18
0.17
1
17.5
0.17
0.11
1
18.5
0.11
0.09
1
19.5
0.09
0.07
1
20.5
0.07
10.9
17
10.99
18
11.16
19
11.27
20
11.36
21
11.43
Table 4 First Derivative Plot-
6
First Derivative Plot
Delta pH / Delta V
5
4
3
Series1
2
3 per. Mov. Avg.
(Series1)
1
0
Mean Volume in ml
Table 5 Second Derivative TableMean
Vol ml
1
Delta
pH/Delta V
0.115
5
0.115
9
10.75
0.14
12.75
0.24
0.0925
2
0
4
0
2
0.01
6
0.005
2
-0.01
8
-0.005
1.25
-0.05
9.63
-0.04
0.5
0.02
10.5
0.04
0.5
-0.02
11
-0.04
0.5
0
11.5
0
0.5
-0.1
12
-0.2
0.5
0.02
12.5
0.04
0.5
-0.1
13
-0.2
0.5
-0.26
13.5
-0.52
0.5
-0.48
14
-0.96
0.5
-4.62
14.5
-9.24
0.5
4.58
15
9.16
0.5
0.66
15.5
1.32
0.5
0.14
16
0.28
0.6
1.08
14.75
5.7
15.75
2
0.34
14.25
15.25
0.185
0.16
0.26
13.75
2
0.16
12.25
13.25
Delta (Delta pH/Delta V )/
Delta ( Delta V )
0.115
0.16
11.75
V
Ave
0.105
10.25
11.25
Delta (Delta
pH/Delta V )
0.3
3
7
Delta
(Delta V)
1.12
0.46
16.25
0.32
16.75
0.5
0.14
16.5
0.28
0.75
0.01
17.1
3
0.013
1
0.06
18
0.06
1
0.02
19
0.02
1
0.02
20
0.02
0.18
17.5
0.17
18.5
0.11
19.5
0.09
20.5
0.07
Table 6 Second Derivative Plot-
Delta(DeltapH/DeltaV)/Delta(DeltaV)
15
Second Derivative Plot
10
5
0
Series1
-5
-10
-15
Volme of 0.1N NaOH ml
Discussion
At the equivalence point, the slope of the titration curve is at a maximum and the rate of change
of pH with addition of titrant is at its highest .So, we plotted the rate of change of pH with
change in volume (ΔpH/ΔV) against volume to get the first derivative spiked curve whose peak
occur at the equivalence point. The equivalence point is the maximum which occurs
at 15 ml. Plotting first derivative graph however leads to an uncertainty that can be partially
avoided by a extrapolating another graph known as second derivative plot .
The second derivative of a titration curve is the rate of change of the first derivative with respect
to the change in the average volume and pass through zero at the equivalence point which is 15
ml. The average of the two successive volumes used for the first derivative plot is also used for
the second derivative plot (Table 5). In using derivative methods, the volume increment should
not be too large . There should be sufficient points near the equivalence point. Also using
derivatives results in the increase in noise esp the second derivative.
References
1. "Quantitative Chemical Analysis" by Daniel C. Harris, 6th Edition, 2003, W. H. Freeman
and Co.
2. Laboratory Manual