POTENTIOMETRY 9 th lecture

4‑ Electrodes for Neutralization Reactions:4‑ Electrodes for Neutralization Reactions:a‑ The Hydrogen gas Electrode:a‑ The Hydrogen gas Electrode: Electrode reaction: H+ + e ½ H2

Mode of action: electron transfer electrode

Nernst equation:

E25 = Eo - 0.059 log 1/ [H+]

E25oC = zero - 0.059 log 1/ [H+]

E25o

C = - 0.059 pH (E depends on [H+] or pH)

When connected to a reference electrode the

e.m.f. of the cell

Ecell = Ereference - Eindicator

Ecell = Ereference ‑ (‑0.059 pH)

Ecell = Ereference + 0.059 pH

pH = Ecell - Ereference / 0.059

Pt blackCatalyst

platinum coated with Pt black

Salt bridge

[H+] = X M

Disadvantages:

As N.H.E. in addition: 1- It can not be used, presence of oxidant or reductant will interfere with the equilibrium and function of the electrode. 2- It can not be used in presence of catalytic poisons as S2- which interfere with the catalytic activity of pt black).

3- It can not be used in reactions involving volatile constituents e.g. H2CO3, H2S because bubbling of H2 gas causes their volatilization.

b‑ Antimony electrode Sbb‑ Antimony electrode Sb00/ Sb/ Sb22OO33::

Electrode reaction:Sb2O3 + 6H+ + 6e 2Sb0 + 3H2O


It is prepared by allowing a rod of antimony to cast in air for two or three weeks.

Nernst equation:

E Sb0/ Sb2O3 = E0 - 0.059/6 log [Sb0]2 / [Sb2O3] [H+]6

E 25o

C= E0 - 0.059/6 log [Sb0]2 / [Sb2O3] - 0.059/6 log 1/ [H+]6

[Sb0] / [Sb2O3] is considered as unity, as log 1= zero

E 25o

C= E0 - 0.059/6 log 1/ [H+]6 , E 25o

C= E0 + 0.059 log [H+]

E25o

C = E0 - 0.059 pHSb0

Sb2O3

Pt wire

Advantages:

1- Easy to be used cheep and durable.

2- It can be used for determination of pH of volatile acids (H2CO3)

Disadvantages

1-Can only be used within pH range 2‑8, at lower pH Sb2O3 dissolves and at higher pH Sb0 dissolves.

2-It can not be used in presence of oxidizing, reducing agents.

3-It can not be used in presence of complexing agents e.g. taratric acid because it gives antimony tartarate complex.

3-It can not be used in presence of noble metals which will be displaced by antimony.

c-Quinhydrone Electrodec-Quinhydrone Electrode::

Is Is formed by the addition of quinone and hydroquinone in equimolar proportion to the solution to be analyzed and an inert electrode (pt) is immersed in the solution.

Electrode reaction: Q + 2H+ + 2e H2Q


Nernst equation:

E Q/ H2Q = E0 - 0.059/2 log [H2Q] / [Q] [H+]2

E 25o

C= E0 - 0.059/2 log [H2Q] / [Q] - 0.059/2 log 1/ [H+]2

[Q] / [H2Q] =1,as they are in equimolar proportions.

As log 1= zero

E 25o

C= E0 - 0.059/2 log 1/ [H+]2 , E 25o

C= E0 + 0.059 log [H+]

E25o

C = E0 - 0.059 pH

c-Quinhydrone Electrodec-Quinhydrone Electrode:: Advantages:1- Easily prepared and used.2- not affected by catalytic poisoning.3- It comes to equilibrium rapidly.2- It can be used for determination of pH of volatile acids

(CO2).

Disadvantages1-Can not be used at pH > 8 because H2Q is dissociated,

altering the pH of the solution. 2-It can not be used in presence of oxidizing and reducing

agents.3- Atmospheric oxygen slowly oxidizes H2Q, therefore, it

must be freshly prepared.

d- Glass Electrode:d- Glass Electrode:

This the mostly widely used electrode for measuring the pH. It is ion selective electrodes which is specific for hydrogen

ion. Ion selective electrodes respond to activity more than

concentration of ions:

a = [Mn+] fa where: a is the activity ,

[Mn+] is the molar concentration of ion and

fa is the activity coefficient. fa α 1/ ionic strength of the solution.

As the ionic strength ↑ the activity coefficient ↓ activity ↓ E ↓

ELECTRICAL CONNECTION

Thick walled glass

Bulb of pH sensitive glass membrane

Solution of unknown [H+]

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Theory of operation:Theory of operation:When a thin membrane of glass of special composition separates two solutions of different hydrogen ion concentration a potential is developed on this membrane depending on the concentration of hydrogen ion or more accurately on its activity in the two solutions.This potential is due to the ion exchange that takes place between hydrogen ions and one of the components ions of the glass membrane matrix (e.g Na+). Hydrogen ion is exchanged on both sides of the membrane depending on the different activities of solution inside and outside.Leading to charge imbalance, So a potential is developed.

Hydrogen ion is exchanged in the form of H3O+, thus hydration of the membrane is essential by keeping the membrane immersed in water.

The potential developed is represented by the following equation: : E = K - 0.059 pH E = K - 0.059 pH

Where: E = potential developed K is a value formed of four components: 1- asymmetry potential 2- E of two reference electrodes 3- pH of the internal solution 4- liquid junction potential

Na+Inside [H+] outside [H+]

How to measure E:How to measure E:The glass electrode is immersed in the unknown solu and is

coupled to saturated calomel electrode then connect both electrodes with a potentiometer.

Advantages:Advantages:1- It can be used in presence of oxidizing, reducing and complexing

agents, catalytic poisons and noble metals. 2- used in solutions containing volatile constituents. Disadvantages:Disadvantages:1-It is fragile.2- Can not be used in presence of dehydrating agents. e.g.

concentrated sulphuric acid, ethyl alcohol.3- Can not be used above pH 12 (alkaline error) as Interference from

sodium ions occurs i.e. sodium ion exchange together with H+ (glass membrane becomes permeable to sodium).

4- It takes certain time to reach equilibrium due to glass resistance.

ApplicationsApplications

1- Direct Potentiometry:1- Direct Potentiometry:

The technique used in this method is a comparison of the potential developed by the indicator electrode when it is immersed in the test solution, with that when it is immersed in standard solution of the analyte.

The calibration curve method is applied for determination of substances by direct potentiometry:

In which we plot the potential of the cell versus a series of standard solutions of extra pure grade of the substance to be determined.

Then measure the potential produced when using the unknown solution, and from the calibration curve we can obtain its concentration.

2‑ Indirect Potentiometry (2‑ Indirect Potentiometry (Potentiometric titration, or Potentiometric determination of equivalence point))

In this method we measure the simultaneously potential (E25) developed when a sensitive electrode is immersed in the solution to be titrated after successive addition of the titrant.

Then we plot a curve representing the change in potential against ml of titrant.

Examples of potentiometric titrations: Examples of potentiometric titrations:

1-Acid-base titrations:

e.g. Titration of HCl with NaOH

Titration of CH3COOH with NaOH

We use glass electrode as indicator electrode.

We use calomel electrode or Ag/AgCl as reference electrode.

We use pH-meter which is a potentiometer and the mv scale (E) is converted to pH scale.

N.B: The stronger the acid the greater

the inflection of the curve.

pH

mls of titrant

HCl

CH3COOH


2-Preciptimetric titrations:

e.g.1- Titration of mixture of Cl- & Br- & I- with AgNO3

We use first type electrode (Ago/Ag+) as indicator electrode.


E α [Ag+]

I- with the smallest solubility

product will react first then

Br- then Cl-

E

mls of titrant

I- Br- Cl-


2-Preciptimetric titrations:

e.g.2- Titration of Cl- or Br- or I- with AgNO3

We use second type electrode (Ag0/AgCl/Cl-) or (Ag0/AgBr/Br) or (Ag0 / AgI / I-) as indicator electrode.


E α [1/Cl-] or E α [1/Br-]

E α [1/I-]

E

mls of titrant


3-Complexometric titrations:

e.g.1- Determination of Single metal e.g. Cu2+ with EDTA

We use first type electrode (Cuo/Cu2+) as indicator electrode.


E α [Cu2+]E

mls of titrant

E.p.

Examples of potentiometric titrations: Examples of potentiometric titrations: 3-Complexometric titrations:

e.g.2- Determination of a mixture of metals

(Ca2+,Cd2+, Bi3+ by EDTA)

We use second type electrode (Hgo / Hg2y2- / y4-)

as indicator electrode.

We use calomel electrode or

Ag/AgCl as

reference electrode.

EHgo α 1/ [Y4-]E

mls of titrant

Bi3+ Cd2+Ca2+

pH = 1.2 pH = 4 pH = 8

At PH 1.5 Bi3+ reacts and the end point is detected by sharp change in potential, then adjust the pH to 4, only Cd2+ reacts, similarly at the end point distinct decrease in potential is observed. Now raise the pH to 10 with ammonia buffer where EDTA forms a stable complex with Ca2+.


4- Redox titrations:

e.g.1- Determination of Fe2+ with Ce4+

We use platinum or gold as indicator electrode.


E25o

C α [ox] / [red]

E

mls of titrant

Fe3+/Fe2+

Ce4+/Ce3+

E.p.

2‑ Electrodes for Precipitemetry and Complexometry:2‑ Electrodes for Precipitemetry and Complexometry:

These can be classified into:

a‑ Electrodes of the First order or First kind:

The best electrode used for a cation to be determined is its elemental form.

e.g. in determination of Ag+ a silver rod is the indicator electrode, its potential is directly affected by the analyte concentration as given by:

Nernst Equation

E25 = E0 + 0.059 log [Ag+]

It is used for determination of Ag+ with Cl‑, Br‑, I- and CN‑.

Copper, lead, cadmium and mercury are also suitable electrodes for their cations.

Some hard and more brittle metals such as nickel, cobalt, iron and chromium develop non reproducible potential WHY?WHY?

due to deformation of crystal structure and presence of oxide coatings.

b‑ Second order or Second kind Electrodes:b‑ Second order or Second kind Electrodes:

These are used for determination of anions. The electrode potential is indirectly affected by the analyte. The electrode is a metal electrode which is indirectly

responsive to anions which form slightly soluble salt with the cationic form of the electrode.

e.g. Ag0 / AgCl / Cl- // i.e. Ag0 is coated with a layer of silver chloride and dipped in

chloride solution.The potential of a silver electrode reflects the concentration of

chloride in saturated silver chloride solution.The electrode potential depends on the chloride concentration

in the solution and this is used for its determination. The electrode potential is:

E25 = E0 ‑ 0.059 log [Cl-]

Similar electrodes for bromide, iodide………can be used.

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POTENTIOMETRY 9 th lecture