6 Potentiometric measurement of pH
Theory:
see Electrochemical cells and electrodes, p. 6-29
Potentiometric measurement of pH using hydrogen electrode, p. 7-36
Potentiometric measurement of pH using glass electrode, p. 8-41, in
Kopecký F., Kaclík P., Fazekaš T.:
Laboratory manual for physical chemistry,
Department of Physical Chemistry, FaF UK, Bratislava, 1996

Task:
Measure pH of 7 different aqueous solutions using:
A: hydrogen electrode
B: glass electrode.
C: quinhydrone electrode
Equipment and chemicals:
Chemicals: 7 aqueous solutions with different pH, distilled water
Equipment for task A:
The potentiometer for the measurement of the electromotoric force, cell with the hydrogen and calomel electrodes, hydrogen gas in the steel cylinder with valve and manometers, thermometer, beakers, filtering paper

An electrochemical cell (Fig. 7-1, p. 7-38) includes: hydrogen electrode with the platinized platinum (1) placed inside of tube connected with the hydrogen gass valve
(1). Saturated calomel electrode (3) is used as the reference electrode. Electrodes are immersed in the measured solution (2). For storage, hydrogen electrode should be kept in distilled water, and the calomel electrode in the saturated KCl solution. It is necessary to prevent the sensitive parts of electrodes from drying out.
Procedure:
Manipulation with the hydrogen gas valve is allowed to the instructor only, or under his supervision!
1. Take off electrodes from the storage solution (shift them vertically up without leaving the holder). Rinse them with distilled water, and dry gently with filtering paper.
2. Fill the beaker with measured solution.
3. Put the electrodes to the solution in the beaker. At least one half of the platinum plate of the hydrogen electrode must be immersed in the measured solution. 1

4. Connect the electrodes with the potentiometer. Remember, the hydrogen electrode is the negative terminal, and the calomel electrode is the positive terminal. 5. Ask the teacher to check the set up, and to set a flow of the hydrogen. The hydrogen gass now bubbles gently through the measured solution.
6. To saturate the solution with the hydrogen takes about 10-15 minutes.
7. Read the electromotoric force (E in mV) from the display of potentiometer when its value is stable within 0.5 mV.
8. Stop the flow of hydrogen with the valve (1).
9. Write the E value down to the Tab. 1.
10. Use part of the solution for experiment B. Give back the rest of the solution to the bottle with the original solution.
11. Repeat the procedure for each solution.

Data treatment:
Express the pH of the solutions using data from the Tab. 1 according to the relation: pH =

E − Ekal
2.303( RT / F )

(1)

where E is the measured electromotoric force (in V), Ekal is the potential of the calomel electrode at given temperature (see Tab.2). R=8.314 Jkmol-1 is the gas constant, F =96485 Cmol-1 is the Faraday constant, T is the temperature. The values of 2.303(RT/F) as a function of temperature are given in the Tab.2.

Equipment for task B:
The pH meter with the pH probe, beakers

The pH probe consists from:

1. a sensing part of electrode, a bulb made from a specific glass
2. sometimes the electrode contains a small amount of AgCl precipitate inside the glass electrode
3. internal solution, usually 0.1M HCl for pH electrodes
4. internal electrode, usually silver chloride electrode or calomel electrode 5. body of electrode, made from non-conductive glass or plastic.
6. reference electrode, usually the same type as 4
7. junction with studied solution, usually made from ceramics or capillary with asbestos or quartz fiber.

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A typical modern pH probe is a combination electrode, which combines both the glass and reference electrodes into one body. The bottom of a pH electrode balloons out into a round thin glass bulb. The pH electrode is best thought of as a tube within a tube. The inside most tube (the inner tube) contains an unchanging saturated KCl and a 0.1M HCl solution. Also inside the inner tube is the cathode terminus of the reference probe. The anodic terminus wraps itself around the outside of the inner tube and ends with the same sort of reference probe as was on the inside of the inner tube.
Both the inner tube and the outer tube contain a reference solution but only the outer tube has contact with the solution on the outside of the pH probe by way of a porous plug that serves as a salt bridge.
This device is essentially a galvanic cell. The reference end is essentially the inner tube of the pH meter, which for obvious reasons cannot lose ions to the surrounding environment (as a reference is good only so long as it stays static through the duration of the measurement). The outer tube contains the medium, which is allowed to mix with the outside environment (and as a consequence this tube must be replenished with a solution of KCl due to ion loss and evaporation).
The measuring part of the electrode, the glass bulb on the bottom, is coated both inside and outside with a ~10nm layer of a hydrated gel. These two layers are separated by a layer of dry glass. The silica glass structure is shaped in such a way that it allows Na+ ions some mobility. The metal cations (Na+) in the hydrated gel diffuse out of the glass and into solution while H+ from solution can diffuse into the hydrated gel. It is the hydrated gel, which makes the pH electrode an ion selective electrode. H+ does not cross through the glass membrane of the pH electrode, it is the Na+ which crosses and allows for a change in free energy. When an ion diffuses from a region of activity to another region of activity, there is a free energy change and this is what the pH meter actually measures. The hydrated gel membrane is connected by
Na+ transport and thus the concentration of H+ on the outside of the membrane is
'relayed' to the inside of the membrane by Na+.
Procedure:
The fragile glass electrode must be handled with care, it should be rinsed carefully before and after the use. Between experiments, electrode should be kept in the storage solution. 1.

2.
3.
4.
5.
6.
7.

Take off the electrode from the storage solution (shift it vertically up without leaving the holder). Rinse it with distilled water, and dry gently with filtering paper.
Fill the beaker with measured solution.
Put the electrode back to the beaker. The bulb and also the junction of the reference electrode (7) must be immersed in the solution.
Read the pH from the display of pH meter when the value is stable.
Write the pH value down to the Tab.1
Repeat the procedure for each solution.
Do not forget to write down the laboratory temperature.

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C: Measurement of pH using quinhydrone eletrode
The quinhydrone electrode is a type of redox electrode which can be used to measure the hydrogen ion concentration (pH) of a solution in a chemical experiment.
It provides an alternative to the commonly used glass electrode in a pH meter.
Redox reaction
It is familiar from introductory chemistry that oxidation is the removal of electrons from a species, a reduction is the addition of electrons to a species, and a redox reaction is a reaction in which there is a transfer of electrons from one species to another. The electron transfer may be accompanied by other events, such as atom or ion transfer, but the net effect is electron transfer and hence a change in oxidation number of an element. The reducting agent (or reductant) is the electron donor, and oxidizing agent (or oxidant) is the electron acceptor. It should be also familiar that any redox reaction may be expressed as the difference of two reduction half-reactions, which are conceptual reactions showing the gain of electrons. The reduced and oxidized species in a half-reaction form a redox couple. In general we write a couple as Ox/Red:
Reduction: Oxz + ne- → Redz-n (oxidation number decreases)
Oxidation: Redz-n → Oxz + ne- (oxidation number increases)
What can be written as:
Ox z + ne − ← Re d z − n
→

(2)

Redox electrodes
A redox electrode consists of an inert material like platinum or gold dipping into a solution containing a chemical species in two different oxidation states. The transfer of electrons between the species takes place through the inert material. The electrode potential is given by

E = E0 +

[a ]
RT
ln ox nF [aRe d ]

(3)

where E0 is the standard potential of electrode, R the gas constant, T (K) the temperature, F the Faraday constant, n is a number of electrons in the electrode reaction, aox and aRed are acivities of the oxidant and reductant, respectively.

Quinhydrone electrode
The quinhydrone electrode consists from a platinum dips into a solution saturated with quinhydrone. Quinhydrone (HQ) is a slightly soluble compound formed by the combination of one mole of quinone (Q) and one mole of hydroquinone (H2Q):

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The electrode reaction is

C 6 H 4 O2 + 2 H + + 2e − ← C 6 H 4 (OH ) 2
→
Quinone (Q)

(4)

Hydroquinone (H2Q)

Quinone is oxidant, and hydroquinone is reductant in this reaction. This electrode is very easy to prepare and handle. Pure solid quinhydrone is dissolved in the solution to be measured until the solution is saturated and excess is present. A platinum wire is dipped in this solution.
The electrode potential is given by
EQ / H 2 Q = E

0
Q / H 2Q

[ ]
[ ]

aQ ⋅ [a H ]
RT
+ ln 2F a H 2Q

2

(5)

Quinone (Q) and hydroquinone (H2Q) are obtained by dissolving quinhydrone in solution, therefore aH2Q=aQ. Applying the following substitution ln[a H ] = 2.303 ⋅ 2 ⋅ log[a H ] = 2.303 ⋅ (−2). pH
2

(6)

the equation (5) is in form:
0
EQ / H 2Q = EQ / H 2Q − 2.303

RT
⋅ pH
F

(7)

The standard potential of the quinhydrone electrode is given by
0
EQ / H 2Q = −0.6994 + 7.36 ×10 −4 (t − 25)

(8)

where t is the temperature in degree Celsius.
For the potentiometric measurements we combine the quinhydrone electrode with suitable reference electrode to create electrochemical cell. We use a saturated calomel electrode as the reference, and the cell is set up:
Hg Hg 2 Cl 2 KCl (satur.solution ) H 2Q, Q, H + (measur.solution ) Pt
According to Eq.6th.1 (p.6-32), the electromotoric force (E) of the cell is given by

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E = EQ / H 2Q − E kal

(9)

where EQ/H2Q is the potential of quinhydrone electrode. Finally, from Eq. 7 and 9, we get pH of solution as pH =

0
EQ / H 2Q − E kal − E

(10)

2.303RT / F

The quinhydrone electrode cannot be used in solutions that would react with quinone or hydroquinone. Hydroquinone being a weak acid, the electrode cannot be used above pH 8.5 when the dissociation of hydroquinone becomes appreciable. Another drawback is that quinone is oxidized by air in strongly alkaline medium. In spite of all this, the quinhydrone electrode is frequently used in titration acids with bases since the end point is reached below pH 8 in most cases.

Equipment for task C:
The potentiometer for the measurement of the electromotoric force, cell with the quinhydrone and calomel electrodes, quinhydrone, thermometer, beakers, filtering paper Procedure:
1. Take off electrodes from the storage solution. Rinse them with distilled water, and dry gently with filtering paper.
2. Fill the beaker with measured solution. Approximate values pH of solutions are written on the flask.
3. Take a little bit quinhydrone (black powder) using a wood spatula, and add to the measured solution. Mix the solution carefully using a glass stick to obtain slightly yellow solution saturated with quinhydrone.
4. Put the electrodes to the solution in the beaker.
5. Connect the electrodes with the potentiometer. The quinhydrone electrode is the positive terminal, and the calomel electrode is the negative terminal.
6. Read the electromotoric force (E in mV) from the display of potentiometer when its value is stable within 0.5 mV.
7. Write the E value down to the Tab. 1.
8. Using Eq. (8) calculate the standard potential of quinhydrone electrode at laboratory temperature.
9. Calculate pH of the solution using Eq. (10) and Tab.1
10. Repeat the procedure for each solution where the use of the quinhydrone electrode is possible.

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Tab. 1. Experimental data
Solution
pH
(approx.value)

Hydrogen electrode

E (mV)

pH

Quinhydrone electrode E (mV)

Glass electrode pH pH

Tab. 2. The potential of the calomel electrode and the value 2.303(RT/F) as a function of temperature. t (oC)
15
16
17
18

Ekal
2.303(RT/F)
t
(V)
(V)
(oC)
0.2473
0.05717
19
0.2468
0.05737
20
0.2462
0.05757
21
0.2456
0.05777
22

Ekal
2.303(RT/F)
t
(V)
(oC)
(V)
0.2450
0.05797
23
0.2443
0.05817
24
0.2436
0.05837
25
0.2430
0.05856
26

Ekal
2.303(RT/F)
(V)
(V)
0.2423
0.05876
0.2417
0.05896
0.2410
0.05916
0.2404
0.05936

The report must include:
•

Theory (electrode potential, Nernst equation, pH)

•

Equipment and chemicals

•

Working procedure and measurements

•

Tables of results and calculation

•

In conclusions, compare the pH values obtained by using all methods. The pH

value given on the bottle is only approximate.
Used literature:
Kopecký F., Kaclík P., Fazekaš T.: Laboratory manual for physical chemistry,
Farmaceutical faculty of Comenius University, Bratislava, 1996
J. Oremusová, Manual for laboratory practice in physical chemistry for students of pharmacy, Department of Physical Chemistry, Faculty of Pharmacy, Comenius
University, Bratislava, 2007, in Slovak http://en.wikipedia.org/wiki/PH_glass_electrode Athawale V.D., Mathur P.: Experimental Physical Chemistry, New Age International (P)
Ltd., New Delhi, India, 2001
Manual written by Doc. RNDr. D. Uhríková, CSc.

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