Lecture 12
- Liquid Junction Potential &
Determination of the pH
Dr. Tharindu Senapathi
Department of Chemistry
University of Sri Jayewardenepura
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Liquid Junction Potential
• In a cell with two different electrolyte solutions in contact, as in the Daniell
cell, there is an additional source of potential difference across the interface
of the two electrolytes.
This potential is called the liquid junction potential, Elj.
• Another example of a junction potential is
that between different concentrations of
hydrochloric acid. At the junction, the
mobile H+ ions diffuse into the more dilute
solution. The bulkier Cl− ions follow, but
initially do so more slowly, which results in
a potential difference at the junction.
• The potential then settles down to a value
such that, after that brief initial period, the
ions diffuse at the same rates.
• The contribution of the liquid junction to the potential can be reduced (to
about 1 to 2 mV) by joining the electrolyte compartments through a salt
bridge.
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Liquid Junction Potential
The reason for the success of the
salt bridge is that the liquid
junction potentials at either end
are largely independent of the
concentrations of the two dilute
solutions, and so nearly cancel.
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Liquid Junction Potential
For example, a porous glass frit may separate two solutions of NaOH:
NaOH (0.01 M) NaOH (0.001 M)
OH- moves approximately 5X faster than Na+
…developing of a potential at the interface, or boundary, between the two
solutions.
For an electrochemical cell containing a salt bridge, the cell
potential is:
Ecell= Ecathode-Eanode +Ejunction
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Liquid Junction Potential
• The negative sign means
the left side of the junction
becomes positive.
• In the first case, K has higher
mobility, and therefore it
travels pass the junction to
the other side, making it
polarized.
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Cells With Liquid Junction Potential
• Consider two half-cells having identical electrodes dipping into
their respective solutions containing the same electrolyte but at
different mean ion activities.
• Electrical contact between the half-cells is made by the two
solutions meeting at a junction.
Porous Frit
• At the right-hand electrode, for the passage of 1 faraday, 1
equivalent of M+ ions will be deposited.
• Similarly, at the left-hand electrode, although 1 equivalent of M
dissolves as M+ ions, t+ equivalents migrate out of the region
towards the cathode.
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Cells With Liquid Junction Potential
This behaviour is summarized below.
Amout that came to the solution
Amout that is diposited
The overall process involves the transfer of material from the higher
to the lower activity,
Note: we only have 𝑡− here (to statisfy the material balance)
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Cells With Liquid Junction Potential
For this process, the free energy change per faraday is
The mean square activity is often used due to practical importance
Because there both
cations and anions
present
Therefore,
and,
The transport number which appears in the equation for the cell
e.m.f. is that of the ionic species with respect to which the
electrodes are not reversible.
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Cells With Eliminated Liquid Junction Potential
Consider now the same half-cells as used in the previous section but
joined via a salt bridge. This cell is represented by
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Cells With Eliminated Liquid Junction Potential
Now, since individual ion activity coefficients are inaccessible to
measurement, the cell e.m.f. must be related to determinable mean
ion activities.
By definition,
It can be assumed that
So,
So, this is the cell potential for a cell without liquid junction potential.
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Calculation of Liquid Junction Potential
• The difference between the e.m.f.'s of the cells with and without (eliminated)
liquid junction potential gives the liquid junction potential involved in the cell
that has a liquid junction potential.
Factoring out,
We already know that
This last equation makes clear the function of a salt bridge when eliminating a
liquid junction potential. If the electrolyte is chosen, such that t- = t+ then E1.j = 0.
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Determination of pH of a Solution
The Hydrogen Electrode
𝑝𝐻 = − log 𝑎𝐻
𝑎𝐻 = 𝐻 + ∙ 𝛾,
𝑤ℎ𝑒𝑟𝑒 𝛾 𝑖𝑠 𝑎𝑐𝑡𝑖𝑣𝑖𝑡𝑦 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡
At sufficiently diluted solutions, 𝛾 = 1 𝑎𝑛𝑑 𝑝𝐻 = − log 𝐻 +
The equilibrium at the electrode is
Combination of the Nernst equation and
the equation for free energy will give
Since E0 = 0 by definition for this
electrode as the primary standard.
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Determination of pH of a Solution
If the partial pressure of hydrogen is one atmosphere.
Exact potentiometric determination of pH using the hydrogen electrode is not
as easy as it might at first appear. In principle, it could be coupled with a
suitable reference electrode to form the cell.
The voltage difference in this cell, which is simply Ec - EA is
Assuming that the liquid junction potential is eliminated.
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Determination of pH of a Solution
Measuring With Respect to Calomel Electrode
A hydrogen electrode (Ecell) dipping into a solution of unknown pH is
combined with a reference electrode like the saturated calomel
electrode (SCE) and its emf is measured.
Pt | H2( 1 atm) | [H+] = x || KCl | Hg2Cl2( s) | Hg
𝐸𝑐𝑒𝑙𝑙 = 0.244 − 𝐸𝐻
𝐸𝑐𝑒𝑙𝑙 = 0.244 + 0.0591𝑝𝐻
This equation is valid if liquid junction
potential is eliminated.
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Determination of pH of a Solution
Glass pH Electrode
The glass electrode is the most widely used indicator electrode for pH
determinations used in the laboratory.
• Operates on the principle that the potential
difference between the surface of a glass
membrane and a solution is a linear function of
pH.
• A standard solution of known pH must be in
contact with the other side of the membrane to
act as a reference electrode.
• It consists of a glass bulb membrane,
which separates an internal solution
and an Ag/AgCl electrode from the
studied solution.
Known concentration
Ag, AgCl(s) I 0·1 M HCl I glass I solution
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Determination of pH of a Solution
Notation-The Glass pH Electrode
The arrangement may be represented as
When used in practice it must be coupled with
a reference electrode also dipping into the
working solution.
If the calomel electrode is used as the outer
reference electrode
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Determination of pH of a Solution
How does the membrane work?
• pH sensitive glasses are manufactured primarily from SiO2 which are connected
via a tetrahedral network with oxygen atoms bridging two silicon atoms.
• In addition, the glasses are made to contain varying amounts of other metal
oxides, like Na2O and CaO.
Na+ ions are able to diffuse slowly in the
lattice, moving from one charge pair site
to another depending on the amount of
H+ ions on both sides.
While the membrane resistance is very
high (~100 MΩ), this movement of cations
within the glass allows potential to be
measured across it.
pH known
pH unknown
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Determination of pH of a Solution
How does the membrane work?
If glasses of this type are placed in an
aqueous solution containing H+, the
glass surface in contact with solution
becomes hydrated as water enters a
short distance into the crystal lattice
and causes it to swell. The “interior” of
the glass remains dry.
• Some of the metal ions within the glass close to the solution boundary are able
to diffuse into the solution, and some H+ ions are able to charge pair with
oxygen near the glass surface. In this way, ion exchange equilibrium is
established between the fixed negative sites on the glass surface and H+.
• As only the glass closest to solution becomes hydrated, two individual
equilibria are established that are dependent upon the respective
H+ activity on either side of the layer.
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Determination of pH of a Solution
How does the membrane work?
Glass is unique in that, while it is an ionic conductor for small ions such as sodium,
hydrogen ions are involved almost exclusively in the ion-exchange process even at
high pH values and when the activity of ions such as Na+ is high.
- Some contribution can come from Na+.
In the latter condition, the membrane shows a response to these ions, as evidenced
by the alkaline error.
A difference in the H+ concentration on either side of the glass membrane leads
to a difference in the number of ion pairs that exist and an imbalance in the
surface charge between the hydrated layers.
This results in a membrane potential that is pH dependent.
𝐸𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒 = 𝐸𝑖𝑛𝑛𝑒𝑟 − 𝐸𝑜𝑢𝑡𝑒𝑟
Answer: Callibrate!
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Determination of pH of a Solution
pH Electrode-junction potentials and asymmetry potentials
• The potential measured by a pH indicator electrode includes not
only the desired membrane potential but also small contributions
from junction potentials and asymmetry potentials.
• Asymmetry potentials result from physical differences (slightly
thicker or thinner area) between the inner and outer surfaces of
the glass membrane, leading to different inner and outer
potentials for the same H+ activity.
Remember: We only meassure avarage properties.
• Corrections for these small potential errors can be made by
frequently calibrating the glass electrode in standard solutions
covering the pH range in which measurements are desired.
Answer: Callibrate!
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Quinhydrone Electrode
This electrode system is now little used for pH determination, but it is a good
example of an organic redox system that behaves reversibly.
• Quinhydrone is in fact the name given
to the molecular crystal formed
between quinone and hydroquinone.
equimolar
• When dissolved in water, the crystal
decomposes into its constituent
compounds.
The quinhydrone electrode consists of 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) – scheme (X)
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Quinhydrone Electrode
Quinone is an oxidant, and hydroquinone is a reductant in this
reaction.
The electrode potential is given by
𝐸𝑄∕𝐻2𝑄
𝑎𝑄 𝑎𝐻
𝑅𝑇
= 𝐸°𝑄/𝐻2𝑄 +
ln
2𝐹
𝑎𝐻2𝑄
2
Quinone (Q) and hydroquinone (H2Q) are obtained by dissolving
quinhydrone in solution, therefore
ln 𝑎𝐻
𝐸𝑄∕𝐻2𝑄
2
= 2.303 × 2 × log 𝑎𝐻 = 2.303 × −2 𝑝𝐻
𝑅𝑇
= 𝐸°𝑄/𝐻2𝑄 − 2.303
𝑝𝐻
𝐹
𝐸°𝑄/𝐻2𝑄 = 0.6994 𝑉
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Quinhydrone Electrode
Calomel electrode as the reference
We cant measure EMF of an haf cell – we need another half cell as referance.
We use a saturated calomel electrode as the reference, and the cell is set up:
The electromotive force (E) of the cell is given by
𝐸 = 𝐸𝐻2𝑄 − 𝐸𝑐𝑎𝑙𝑜𝑚𝑒𝑙
𝑅𝑇
𝐸 = 𝐸°𝑄/𝐻2 𝑄 − 2.303
𝑝𝐻 − 𝐸𝑐𝑎𝑙𝑜𝑚𝑒𝑙
𝐹
𝑝𝐻 =
(𝐸°𝑄
𝐻2 𝑄
− 𝐸𝑐𝑎𝑙𝑜𝑚𝑒𝑙 − 𝐸)𝐹
2.303𝑅𝑇
• 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.
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