Janine V. Samelo BSChem 2

EXPERIMENT NO. 1 & 2 Calibration of pH Meter, Potentiometric Titration

and Determination of Ka of Weak Acids

Introduction:

Potentiometric analysis is based on measuring the potential of electrochemical cells in

which the current does not flow. Industries nowadays use potentiometric methods for different analysis. A potentiometric titration for example, includes measurement of suitable indicator electrode as a function of the titrant’s volume. Compared to the classical titration analysis, potentiometric analysis provides more reliable data especially in turbid solutions wherein endpoints are hard to determine using chemical indicators. It is also convenient for industrial use since it is less time consuming when compared to that of the classical method. Potentiometric measurements are used in the determination thermodynamic equilibrium constants such as Ka, Kb, and Ksp. Potentiometric titration uses pH meter as an electrode which monitors pH/potential change. It does not need a chemical indicator to observe the endpoint. Since the hydrogen ion inside the electrode responds to the activity of the hydrogen ion in the analyte solution, after sometime, sudden change of pH/ potential will signal the endpoint of the titration.

A glass electrode is widely used indicator electrode for the hydrogen ion. It consists of a thin, pH- sensitive glass membrane sealed in a heavy walled glass or plastic tube. It has an internal Ag/AgCl reference electrode. pH meters are so selective that it only responds to the activity of the hydrogen ion. Many industries uses high class pH meters for the analysis and quality control of many consumer products, analysis of blood gases which indicates some of diseases, monitoring pollutants and many other more. Objectives:

1. To be able to calibrate a pH meter 2. To measure the pH of commercially prepared hair conditioners. 3. To be able to compare the measured pH of hair conditioners to the cell potential

generated by the glass electrode. 4. To be able to conduct a potentiometric titration and construct a titration curve. 5. To be able to identify the acid dissociation constant (Ka) of the given weak monoprotic

acid. 6. To be able to get the first and second derivative as a function of volume change.

Schematic Diagram:

I. Standardization of NaOH with KHP II.Calibration of pH meter III. Determination of pH/ Electric Potential (mV) of Conditioners

Prepare 0.1 M NaOH by weighing 6.55 g solid NaOH and diluting it to 1.5 L. Set aside.

Add 20 ml water and 4 drops phenolphthalein and titrate with the prepared NaOH solution until a

light pink endpoint is reached.

Compute the actual concentration of the NaOH solution. Set aside.

Obtain three replicates of 0.3 g standard KHP in separate Erlenmeyer flasks.

Turn on the pH meter on and wash with distilled water.

Repeat the procedure for the pH 7 and pH 10 buffer solutions.

Wash the electrode with distilled water and gently dry with tissue.

Immerse the electrode in pH 4 buffer solution and press ‘standardize’ button once the reading has

been stabilized.

Repeat the procedure for the pH 7 and pH 10 buffer solutions.

Wash the electrode with distilled water and gently dry with tissue.

Prepare three sachets of different brands of hair conditioners

In three separate beakers, dissolve the samples in 100 ml water.

Dip the first solution to the electrode.

IV. Potentiometric titration

In a beaker, weigh 0.3 g 40% w/w acetic acid and dilute to 100 ml. Mix thoroughly.

Set up the burette filled with standardized NaOH in a magnetic stirrer and iron stand.

Put the analyte solution right under the burette. Drop the magnetic stirrer in the solution.

Immerse the electrode in the solution. Make sure that it is stable enough and does not touch the

edge or bottom of the beaker.

Open the stopcock of the burette and start adding NaOH drop by drop

When sudden pH change occurs, the analyte and the titrant has reached or close to reaching the

endpoint. Add very minimal amount of base.

For each addition, record the volume and pH change. Allow the reading of pH meter to stabilize

before recording the data.

Record the readings. Continue reading and take up data up to pH 11.

Create a spread sheet. Compute the first derivative of the pH and plot it against volume.

Record the reading in pH and in mV

Get the pH of the result, compare it with the measured pH in the electrode and calculate the relative

error percentage.

Using the measured electrode potential, calculate the concentration of in the solution

Dip the second solution and repeat the procedure for the remaining analytes.

Wash the electrode with distilled water. Gently dry it using a tissue.

Data and Computations: I. Standardization of NaOH

Replicate Mass of KHP Volume of NaOH Concentration of

NaOH

1 0.31 g 15.45 ml 0.09826 M

2 0.34 g 16.65 ml 0.10000 M

3 0.22 g 10.80 ml 0.09977 M

Mean: 0.09933 M

II. Calibration of pH meter

buffer pH reading mV reading

pH 4 (acidic) 4.02 -101.8

pH 7 (neutral) 6.99 -276.3

pH 10 (basic) 9.91 -446.6

III. pH and mV readings of Conditioners

Brand Volume of conditioner

Volume of water added

pH Calculated

pH % Error

L’Oreal 10 ml 100 ml 3.74 -84.5 3.72 0.538%

Palmolive 12 ml 100 ml 3.84 -94.3 3.89 -1.29%

Pantene 10 ml 100 ml 3.82 -89.6 3.81 0.262%

*NOTE: and values used in the calculations came from pH 4 buffer III. Titration of Acetic Acid with NaOH

vol titrant pH ph change volume change 1st der 2nd der

0.00 3.29 -0.03

1.00 3.57 0.28 1.00 0.28 #DIV/0!

2.00 3.82 0.25 1.00 0.25 #DIV/0!

3.00 3.98 0.16 1.00 0.16 #DIV/0!

4.00 4.12 0.14 1.00 0.14 0.085

5.00 4.24 0.12 1.00 0.12 #DIV/0!

8.00 4.53 0.29 3.00 0.10 #DIV/0!

11.00 4.79 0.26 3.00 0.09 #DIV/0!

14.00 5.08 0.29 3.00 0.10 #DIV/0!

17.00 5.50 0.42 3.00 0.14 0.125

20.00 7.50 2.00 3.00 0.67 13.1

20.20 9.15 1.65 0.20 8.25 3.94E+13

20.30 9.49 0.34 0.10 3.40 -1.1E+13

20.40 9.69 0.20 0.10 2.00 0.9

20.50 9.85 0.16 0.10 1.60 -0.4

20.70 10.10 0.25 0.20 1.25 #DIV/0!

21.00 10.31 0.21 0.30 0.70 #DIV/0!

21.30 10.48 0.17 0.30 0.57 1.78E-14

21.60 10.61 0.13 0.30 0.43 0.5

22.00 10.74 0.13 0.40 0.33 0.4

22.30 10.82 0.08 0.30 0.27 0.1

22.50 10.86 0.04 0.20 0.20 0.1

22.80 10.91 0.05 0.30 0.17 0.122222

23.00 10.95 0.04 0.20 0.20 -0.03

25.00 11.21 0.26 2.00 0.13 #DIV/0!

Figure 1: plot of volume of NaOH vs. pH

Figure 2: Plot of change in volume vs. change in pH

Figure 3: Plot of 2nd derivative vs. volume

Volume at equivalence point: 20.20 mL pH at equivalence point: 8.25

Computed = Percent Relative Error: 11.43%

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

0.00 10.00 20.00 30.00

chan

ge in

pH

change in volume

Series1

Linear (Series1)

-2E+13

-1E+13

0

1E+13

2E+13

3E+13

4E+13

5E+13

0 10 20 30

Axi

s Ti

tle

Axis Title

Series1

Discussion:

The calibration of the pH meter followed a stern procedure. The electrode must first be

washed free of contaminants by distilled water and dried gently by a tissue paper. The buffer

with pH of 4 is read, and then the buffer with pH of 7 and 10 followed. Readings of the electrode

must first be stabilized before recording them to obtain accurate data.

The table above shows the readings of pH meter. L’Oreal has a pH of 3.74. Palmolive

has 8.84 and Pantene has a pH of 3.82.

After the pH meter has been standardized, titration proceeded. No chemical indicator was used unlike in that of the traditional titrations. In the titration of acetic acid with NaOH, since acetic acid is a weak monoprotic acid and NaOH is a strong base, the pH at equivalence point is expected to be higher than 7.00. After plotting the first derivative, which is the change of pH over the change in volume, the endpoint is at 20.20 mL. This also shows that the pH at equivalence point is 8.25. Conclusion:

Analysis showed that the measured pH of the three conditioners has slight deviations to

the calculated pH when the potentials of the buffer solutions are considered. Percent Errors of the pH are the following: 0.538% for L’Oreal, -1,285% for Palmolive and 0.262% for Pantene conditioner. Errors may be accounted from personal errors due to the preparation of the solutions. Since the conditioners are emulsions, they are not that very soluble in water which causes unconformities in the reading of the glass electrode. After the analysis has been conducted, the volume of the titrant at endpoint was found to be 20.20 mL. It was observed when the first derivative (change in pH over change in volume) was plotted. The sudden rise on the plot indicates the endpoint.

Calculating for the acid dissociation constant,( ),the idea of titrating a weak monoprotic acid with a strong base was used. At half-equivalence point, which is 10.10 mL, the pH is equal

to -log . Considering that relationship, the computed acid dissociation constant is .

The true value of acetic acid is . With this the computed relative error is 11.43 %. Errors can be caused by personal and random errors during the preparation of the solution for the standardization of the base and during the titration. References:

Skoog, et al.,Fundamentals of Analytical Chemistry, 8th Ed.