1

Potentiometric Titration of

HCl and Na2CO3

Collect

One 250 mL volumetric flask

One 50 mL buret

pH 7.00 and pH 10.00 standard buffer solution

(shared by groups)

Two 125 mL Erlenmeyer flasks (check if broken)

One pipet filler (check for gas leak)

One magnetic stir bar (from GTA)

(2016/03/12 revised)

2

Objective & Skills I. Objective:

To prepare and standardize 0.1 M HCl solution

To determine the equivalence point of HCl/Na2CO3

titration by using the electric potential method

To determine the dissociation constants, Ka, of

carbonic acid

II. Skills:

To use analytic balance for weighing chemicals

To prepare solutions by volumetric flask

To operate graduated pipet and titration

To calibrate and operate pH-meter

To determine the equivalence point by using titration

curves

3

Standardization of Hydrochloric Acid

Primary standard: larger molar mass and high purity

Common primary

- standard acid, potassium hydrogen phthalate (KHP)

- standard base, sodium carbonate (Na2CO3)

Standardize HCl(aq) with Na2CO3

HCl(aq) + CO32-

(aq) → HCO3-(aq) + NaCl(aq)

HCl(aq) + HCO3-(aq) → H2CO3(aq) + NaCl(aq)

2 CONa of mole

HCl

32

HClMV

5.4

3.8

4

Determining the Equivalence

Point • The pH value of the reacting solution changes significantly near the

equivalence point

• Base on the color change of the acid-base indicator or monitoring

the change in pH values to determine the equivalence point

Indicator Acid form

pH range

Basic form

Methyl

orange Red 3~4 Orange

Bromothy-

mol blue Yellow 6~7 Blue

Phenol-

phthalein Colorless 8~10

Pink red 0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

0.00 10.00 20.00 30.00 40.00 50.00 60.00

NaOH滴定體積

pH

Weak acid / strong base titration curve

Equivalence point

Methyl orange

Bromothymol blue

Phenolphthalein

5

Choosing a Suitable Acid-Base

Indicator In a Titration Acid-base indicator is a weak organic acid or base

Weak acid (HIn) and its conjugate base (In－) with different colors

pKa － 1 pKa + 1

Acidic Color

HIn Basic Color

In－ Color Change

Range

pH increases

Predict the pH range of the equivalence point

Strong acid/weak base titration: pH < 7

Weak acid/strong base titration: pH > 7

Strong acid/strong base titration: pH = 7

Choose the appropriate indicator to match the end-point with the

equivalence point

HIn + H2O ⇌ H3O

+ + In－

Equivalence Point

1. Acid-base titration curve

The equivalence point is the point

on the curve with the maximum

slope

2. First derivative of titration curve

The maximum point is the equivalence point

3. Second derivative of the titration

curve

0 crossing is the equivalence point

2

4

6

8

10

20 22 24 26

pH

NaOH(aq) V (mL)

0

5

10

15

20

25

20 22 24 26

△p

H/△

V

V1 (mL)

6 -150

-100

-50

0

50

100

150

20 22 24 26

△2pH

/△V

2

V2 (mL)

Equivalence

point

A

B

Equivalence

point

Equivalence

point

7

Acid Dissociation Constant of a

Weak Acid

At Half-Equivalence point

[HA] ＝ [A-]

[H3O+] = Ka

Therefore, pH = pKa

For example

Equivalence volume = 37.50 mL

Half-equivalence volume = 18.75 mL

V = 18.00, pH = 4.60

V = 19.10, pH = 4.65

pH of the half-equivalence volume = 4.63

pKa = pH = 4.63

Ka = 2.3 × 10-5

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

0.00 10.00 20.00 30.00 40.00 50.00 60.00

V (NaOH, mL)

pHHalf-equivalence volume

pH = pKa

Weak acid / strong base

titration curve

HA

OHAKa

3

8

Components of pH Meter

pH meter consists of three parts:

pH electrode

Reference electrode (usually made of silver and silver

chloride), the potential is a fixed value

Indicator electrode (usually made of glass), the potential

changes when the concentration of H+ varies

Thermoprobe: used to measure the temperature of soln

Voltmeter: used to measure the potential difference

between the two electrodes

9

Cell Potential and pH Value

Em = K − 2.3RT(pH)/nF

Em = mT(pH) + K

Em: measured cell potential

K: constant, determined by the

type of electrode used

R: gas constant

T: absolute temperature of the

solution

pH: pH value of solution

n: number of moles of electrons

transferred through the

electrodes during a reaction

F: Faraday constant

Second standard solution

First standard solution

The Relationship Between Measured

Cell Potential and pH value

10

Outline of Procedures

(III) Calibrate the pH-

meter and titration of

unknown

(I) Prepare HCl(aq) (II) Standardization

of HCl with Na2CO3

11

Procedure I: Prepare 0.1 M HCl

(1) Take 4.17 mL of 6 M HCl

(2) Place in 250 mL volumetric flask

(3) Add water till mark to dilute

(1) Use approx. 5 mL

of 0.1 M HCl

(2) Rinse buret twice

and fill with solution

(1) Invert the flask

several times

(2) Mix thoroughly

(3) Pour into beaker

Read initial volume of

buret (Vi) to 0.01 mL

6 M

HCl

12

Procedure II: Standardize HCl

with Na2CO3

Measure ca. 0.1~0.2 g

Na2CO3 with analytical

balance

Place in a 125 mL

Erlenmeyer flask

Record accurate weight

(1) Dissolve with 30 mL distilled

water

(2) Add 3 d. of bromocresol green

(3) Titrate with 0.1 M HCl to

appear green

Boil the soln for 2-3 min. till soln

turn to blue color

(Use stir bar to avoid bumping)

After cooling, continue titrating

to soln appear yellow

Record Vi and Vf

Carry out a duplicate test

Calculate average concentration

of HCl(aq)

藍 色 綠色 黃色

13

Procedure III: Calibrate pH-Meter

Push the “POWER” button,

warm up for 10 minutes

Remove the electrode cap

by rotating it

Place thermoprobe into

solution

Use washing bottle to clean

the electrode

Blot dry with a tissue

HOLD

Press “HOLD” when

cleaning the electrodes and

the screen will freeze

NT$ 4500 !!

Setup of pH meter

14

Clean thermoprobe and electrodes

Immerse in pH 10.0 buffer solution

Adjust Slope button until meter says

‘10.00’

Immerse thermoprobe and electrodes

into pH 7.00 buffer solution

Adjust Calib button until meter says

‘7.00’

pH 7.0

Calib

button

Slope

button

pH 10.0

(1) Collect standard buffer solution

(2) Start calibrating pH meter

Procedure III: Calibrate pH-Meter

pH

10.0

15

Manipulate pH Meter

Do not take the electrode off of the holder

The end of the electrode should be fully immersed in the test solution and not touching the walls of the container

Both thermoprobe and the electrode should be placed in solution

100 mL beaker is used for testing in this experiment

Position the electrode in the soln so that the stir bar will not strike the electrode

Turn the magnetic stir on during titration

Every time the testing solution is changed, the electrode should be rinsed with distilled water and blot dry with tissues

When the electrode is not in use, it needs to be immersed in clean distilled water

When the electrode is not in use for long periods of time, it should be immersed in 3 M KCl solution

16

Procedure III: Titration of Unknown

(1) Place stir bar,

electrode, and

thermoprobe in soln

(2) Setup apparatus

Titrate with standardized 0.1 M HCl

Add ~1 mL aliquots of HCl and record Vi, Vf,

and pH value after each addition

During pH 9-7 and 5-3, add titrant in 0.2 mL

increments

When pH is lower than 2, stop titration

Observe and record the change in color of

solution during titration

Take 10.0 mL unknown

concentration of

Na2CO3 soln into a 100

mL beaker

Add 30 mL distill water

Add 3 d. of universal

indicator

NT$ 1000 !!

17

After Experiment

Clean and check pH electrodes

Place electrode in plastic-cap that containing 3 M KCl

Turn the pH meter off

Hand in magnetic stir bar to GTA

Wash buret and place it back

Waste liquids (salts) can be discarded in sin after

neutralization

Data Analysis

First derivative Second derivative

V HCl pH V1 pH/V V2 (pH/V)/V1

9.00 9.80 9.50 -0.12 10.00 -0.04

10.00 9.68 10.50 -0.16 11.00 -0.04

11.00 9.52 11.50 -0.2 12.00 -0.15

12.00 9.32 12.50 -0.35 12.80 -0.166

13.00 8.97 13.10 -0.45 13.20 -1

13.20 8.88 13.30 -0.65 13.43 0.866

13.40 8.75 13.55 -0.433 13.70 -2.555

13.70 8.62 13.85 -1.2 14.00 1.111

14.00 8.26 14.15 -0.866 14.30 -0.444

14.30 8.00 14.45 -1 14.58 1.6

14.60 7.70 14.70 -0.6 14.80 0.25

14.80 7.58 14.90 -0.55 15.20 0.283

9.502

10.00)(9.00V1

0.001

2

10.50)(9.502

V

18

Plots and Results

0.10 M HCl titrate unknown

Na2CO3 solution

Make three plots

Determine equivalence

points

Calculate the

concentration of unknown

soln

According to the half-

equivalence points

determine Ka1 and Ka2

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

00 5 10 15 20 25 30 35 40 45

First derivative

-8

-6

-4

-2

0

2

4

6

8

0 10 20 30 40 50

Second derivative

19

0

2

4

6

8

10

12

0 10 20 30 40 50

Titration curve