Task No.

II

DETERMINATION OF Al2O3 POINT OF ZERO

CHARGE

I. Goal of the Task

The goal of the task is determination of the point of zero charge (PZC) of alumina
oxide that is the value of pH when charge density is equal to zero. PZC is determined by
the Ahmed’s method.

II. Theoretical problems

1. Mechanism of charge formation of interface.

2. Electric double-layer.
3. Point of zero charge (PZC).
4. Isoelectric point (IEP).

References:
1. G. Kortüm, Elektrochemia, PWN Warszawa, 1966, str. 346–350, 462–464.
2. W. Janusz, Koloidy”, Wydział Chemii UMCS, Lublin, Ćwiczenie nr 4, „Wyznacza-
nie ładunku i punktu zerowego PZC trudnorozpuszczalnych tlenków metodą mia-
reczkowania potencjometrycznego”.
3. H. Sonntag, Koloidy, PWN Warszawa, 1982, str. 74–88.
4. Praca zbiorowa, Chemia fizyczna, PWN Warszawa, 1966, str. 764–766.
5. E. T. Dutkiewicz, Fizykochemia powierzchni, WNT Warszawa, 1998, str. 45–51.
Electrical phenomena at the interface
III. Theoretical part

III. 1. Chemical potential

On the homogeneous surface of solid or liquid phase (even pure metal in vacuum)
usually change in electric charge distribution (density) proceed. This is the case of for-
mation of the areas with changeable potential whose thickness is a few particles size. In
the case of two contacting layers (especially solid-solution or metal-solution) there ex-
ists the tendency for attraction of charged molecules (electrons, ions) in different ways
by two layers. Dipole particles will preferentially adsorb at the interface. The formed
electric field can cause polarization effects in the neighbouring molecules. All these
effects lead to formation of potential differences between the inside of both phases. It is
the Galvani potential . Generally, the Galvani potential difference is measurable only
when the two phases have an identical chemical composition. This potential is equal to
the total work done in charge transfer from one phase to another. This value is difficult
to determine because it depends on the character of unit load charge. The smallest
charge is electron, which shows some chemical properties and can react with a phase
component. Change of system energy caused by passing actual electric charge from one
phase to another can not be a measure of electrochemical potential changes. Theoretical
charge in the electrostatic theory is infinitesimal, and its introducing to the environment
does not cause changes in charge distribution and dipole orientation.
The electrostatic potential near the phase is easy to measure because it is the work
done in transfer of point of electric charge from infinity to the point near the surface
limiting the phase. This point is located outside the phase in a distance where the poten-
tial obtains the largest value. Its value decreases with the distance in accordance with
the Coulomb law. The difference of exterior potentials is called the contact potential or
the Volta potential difference.
It is worth mentioning about the surface potential  result from polarization effects
and the dipole layers existing on the surface which can form for different reasons. For
instance in metals displacement of positive and negative (electrons) charges takes place
but in solutions stable dipoles accumulate on the boundary surface. As a result. the elec-
trical double-layer is formed. The potential jump formed by the dipole layer on the sur-
face is called the surface potential . The sum of Volta and surface potentials is the
electric interior phase potential and it is called the inner potential or the Galvani po-
tential (macro potential)

 =  +  (1)
The first term is usually considered as connected with the total transition of charges
from one phase to another, whereas the second term is connected with other microscop-
ic changes at the interface.
Zeta potential  is the electric potential in the interfacial double layer at the location
of the slipping plane versus a point in the bulk fluid away from the interface. In other
words, zeta potential is the potential difference between the dispersion medium and the sta-
tionary layer of fluid attached to the dispersed particle.

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 Task No. II – Point of zero charge determination
III. 2. Mechanism of charge formation at the interface
The presence of electric charge in the system is connected with potential differ-
ences. The most important mechanisms of charge formation at the interface are:
1. Difference in electron affinity (two metals or metal/conductor) In the solid-liquid,
liquid-liquid systems this mechanism is meaningless except for dispersed mercury
and metal salts.
2. Difference in ions affinity of positive or negative charge for these two phases:
– ions division between two phases (oil and water),
– ions adsorption from an electrolyte on the solid surface,
– predominance of one ion type dissolving on the other one in crystal lattice. Every
time the equilibrium is formed when electrochemical potentials in the case of
ions, which can spontaneously pass by interface, equalize in both phases; e.g.
AgI- electrolyte Ag+ or I- can change the proportion Ag+/I- on the deposit sur-
face. With the determined concentration the ions Ag+ and I–number will be the
same, there is no charge excess- point of zero charge
3. Surface groups ionization.
Surface groups ionization takes place in chemical compounds possessing on the
surface some groups e.g. carboxyl, amine or on the surface of insoluble metal ox-
ides. In these systems size and sign of the potential depend on solution pH. Potential
formation ions, on the solid surface are H+ and OH–. It is because OH– ions can be
considered as equivalent to the component ions in the crystal lattice (oxygen), while
H+ can replace metal ions, although actually they are amphoteric dissociation reac-
tions. Metal oxide crystals have ionic structure, on isolated from the environment
their surface uncompressed local electric charges occur from originating incom-
pletely coordinated ions Mn+ and O2–. As a result oxides are capable of to surface re-
actions. In the reaction with water the M–OH groups are created, their hydrogen
ions are active and can diffuse inside the solution. This causes formation of nega-
tive charge which counteracts ions diffusion into the solution. In this way, the equi-
librium is established. This state corresponds with the defined values of charge and
surface potential and pH of solution. H+ ions adsorption from solution can occur too,
because of that only OH– ions remain in the solution.

Therefore:
–MOH  –MO– + H+ (2)

–MOH + H2O  –MOH2+ + OH– (3)

The surface groups –M–OH have amphoteric character and dissociate. The process

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Electrical phenomena at the interface
of charge formation on the surface can be schematically shown:

-
M OH M O
+
O O + 2H
-
M OH M O

+
M OH M OH2
+
O + 2H O
+
M OH M OH2

As a result of this reaction positive or negative charges accumulate on the oxide sur-
face. Oxygen ions role as potential creating ions is explained too for the metal ox-
ide/electrolyte solution systems. When in solution only H  and OH  are present
and the surface possesses zero charge then:
-
M O
O
+
M OH2

but the number of positively or negatively charged surface groups occurring near
undissociated –M–OH groups is different for oxides and hydroxides. pH corre-
sponding to this state of the surface is called the point of zero charge (PZC)

4. Adsorption of immobile ions in one of the phases.

III.3. The Electrical Double Layer

An electrode becomes positively charged relative to the solution nearby if electrons
leave it and decrease the local cation concentration in the solution. The most primitive
model of the interface is that it is an electric double layer consisting of a sheet of posi-
tive charge at the surface of electrode and a sheet of negative charge next to it in the
solution. A more detailed picture of the interface can be constructed by speculating
about the arrangement of ions and electric dipoles in the solution. The Helmholtz mod-
el of the double layer assumes that the hydrated ions range themselves along the surface
of the electrode but are held away from it by the molecules in their hydration spheres.
The location of the sheet of ionic charge, which is called the outer Helmholtz plane, is
identified as the plane running through the hydrated ions.

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 Task No. II – Point of zero charge determination

Fig. 1. Electric double layer scheme by Helmholtz:

a) charge distribution; b) distribution of potential in the distance function.

The Helmholtz model ignores the disrupting effect of thermal motion which tends
to break up and disperse the rigid wall of charge. In the Gouy-Chapman model of the
diffuse double layer the stirring effect of thermal motion is taken into account in much
the same way as in the Debye-Hückel model of the ionic atmosphere of an ion with the
latter’s single central ion replaced by an infinite, plane electrode. Fig. 1 shows the local
concentrations in the Goy-Chapman model. As expected, ions of opposite charge cluster
close to the electrode, and ions of the same charge are repelled from it. The modifica-
tion of the local concentration near an electrode implies that it might be misleading to
use activity coefficients characteristic of the bulk to discuss the thermodynamic proper-
ties of ions near the interface. Neither the Helmholtz nor the Goy-Chapman model is
a very good representation of the structure of double layer. The former overemphasis
the rigidity of the local solution, the latter underemphasizes its structure. The two are
combined in the Stern model, in which the ions closest to the electrode are constrained
into a rigid Helmholtz plane while outside that plane the ions are dispersed as in the
Gouy-Chapman model.

Fig. 2. Potential distribution in the electrical double layer by

Helmholtz, Gouy and Chapman models.

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Electrical phenomena at the interface

Fig. 3. Structure of electric double layer by Stern

and potential decreasing as the distance function from the interface

III. 5. Electrokinetic effects

When the relative movement of both phases is caused: charged surface and bulk
phase, there can be observed the electrokinetic effects:
1. electrophoresis,
2. electroosmosis,
3. streaming potential,
4. sedimentation potential.
For determination of  potential one of the electrokinetic effects can be used. In
electrophoresis and electroosmosis methods a specified electric field is introduced and
mechanical properties are measured.

III. 6. Parameters characterising electrical double layer.

For electrical double layer characterization the following parameters are used based
on the classical theories:
– surface charge and potential,
– external (diffusion) layer charge and potential,
– capacity of interior (C1) and (C2) exterior areas of electrical double layer – com-
pact layer.
– For electrical double layer characterization two parameters are important:
– point zero of charge (PZC),
– isoelectric point (IEP).

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 Task No. II – Point of zero charge determination
According to the IUPAC definition:

PZC is the value of the negative decimal logarithm of the activity of the poten-
tial-determining ion in the bulk fluid (  o  0 i  o  0 ).

For example, the charge on the surface of silver iodide crystals may be deter-
mined by the concentration of iodide ions in the solution above the crystals. Then, the
PZC value of the AgI surface will be described by the concentration of I- in the solution
(or negative decimal logarithm of this concentration, pI-).

IEP is the pH at which a particular molecule or surface carries no net electrical

charge (   0 ).

III. 7. Point of zero charge

The point of zero charge (PZC) in physical chemistry, is a concept related to the phe-
nomenon of adsorption, and it describes the condition when the electrical charge density
on a surface is zero. It is usually determined in relation to an electrolyte's pH, and the
PZC value is assigned to a given substrate or colloidal particle. In other words, PZC is
the pH value at which a solid submerged in an electrolyte exhibits zero net electrical
charge on the surface.
The value of pH is used to describe PZC only for the systems in which H+/OH- are the
potential-determining ions. Generally, PZC is the value of the negative decimal loga-
rithm of the activity of the potential-determining ion in the bulk fluid.

Table I. Extreme pHpzc values for some oxides.

[G.A. Parks, Chem. Rev. 65, 177 (1965), P. Ney, Zeta Potentiale und Flotierbarkeit von
Mineralen, Springer Berlin 1973]

Oxide pHpzc Oxide pHpzc

WO3 0.2 UO2 5.8–6.6
Sb2O3 0.2 CdO 10.4
SiO2 1.8 Cr2O3 7.0
TiO2 4.0 (<7) ZnO 9.2
MnO2 4.2 ZrO2 10–11
HgO 7.3 Al2O3 5.0–9.2
CuO 9.5 BeO 10.2
Sn2O2 7.3 (6.6) ThO2 9.3
Fe2O3 6.6; 8.2 (9.04) MgO 12.4
NiO 10.3 La2O3 10.4 (12.4)

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Electrical phenomena at the interface
III. 8. Isoelectric point versus the point of zero charge
The terms isoelectric point (IEP) and point of zero charge (PZC) are often used inter-
changeably, although under certain circumstances, it may be productive to make the
distinction.
In the systems in which H+/OH- are the interface potential-determining ions, the point
of zero charge is given in terms of pH. The pH at which the surface exhibits a neutral
net electrical charge is the point of zero charge at the surface. Electrokinetic phenomena
generally measure zeta potential, and a zero zeta potential is interpreted as the point of
zero net charge at the shear plane. This is termed the isoelectric point. Thus, the isoelec-
tric point is the value of pH at which the colloidal particle remains stationary in an elec-
trical field. The isoelectric point is expected to be somewhat different from the point of
zero charge on the particle surface, but this difference is often ignored in practice for so-
called pristine surfaces, i.e., surfaces with no specifically adsorbed positive or negative
charges. In this context, specific adsorption is understood as adsorption occurring in a
Stern layer or chemisorption. Thus, point of zero charge at the surface is taken as equal
to the isoelectric point in the absence of specific adsorption on that surface.
According to Jolivet 1, in the absence of positive or negative charges, the surface is best
described by the point of zero charge. If positive and negative charges are both present
in equal amounts, then this is the isoelectric point. Thus, the PZC refers to the absence
of any type of surface charge, while the IEP refers to a state of neutral net surface
charge. The difference between the two, therefore, is the quantity of charged sites at the
point of net zero charge. Jolivet uses the intrinsic surface equilibrium constants, pK- and
pK+ to define the two conditions in terms of the relative number of charged sites:

For large ΔpK (>4 according to Jolivet), the predominate species is MOH while there
are relatively few charged species - so the PZC is relevant. For small values of ΔpK,
there are many charged species in approximately equal numbers, which constitutes IEP.

III. 9. Methods of point of zero charge determination

III. 9. 1. Ahmed’s method
The easiest method of point of zero charge determination is the one where the
solid with strongly expanded surface is added to the solution with simultaneous pH
measurements. Then pH after the solid addition does not change, this point is the sur-
face PZC point.
Practically, in this method a series of solutions with the same composition but
with different pH values should be prepared. Differences in pH (pH) after the addition
of carefully defined amount of the studied substances of solid can be plotted on the
graph:
1
Jolivet J.P., Metal Oxide Chemistry and Synthesis. From Solution to Solid State, John Wiley &
Sons Ltd. 2000,ISBN 0-471-97056-5 (English translation of the original French text, De la So-
lution à l'Oxyde, InterEditions et CNRS Editions, Paris, 1994).
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 Task No. II – Point of zero charge determination

pH  f ( pH r )
Where: pHr is pH of dispersion in equlibrium (terminal, equilibrium value of
pH) and o the point of zero charge substance can be read from it.

Fig. 5. PZC determination from pH for Fe2O3 (electrolyte – KNO3 solution)
and ZnO (electrolyte – NaCl solution)
[A.L. Mular, R.B. Roberts, Trans. CIM 69, 438 (1966)]

III. 9. 2. Potentiometric titration method

Dispersion potentiometric titration is the most often used method for determina-
tion of point of zero charge and isoelectric point in the metal oxide/elctrolyte solution
systems. In this method adsorption of potential-forming ions on the surface of solid
phase is used.
In the first titration the dependence between the pH and the amount of the added
acid or base is determined. The second titration solution contains some amount of solid.

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Electrical phenomena at the interface
Comparison of the titration curve allows to define PZC, which is in point of curves in-
tersection.
Surface charge density is defined from the difference between the volume of ac-
id or base added to obtain a defined value of pH:
VcF
0 
mS w
Where: V  Vs  Ve  - the difference between the volume of acid or base
which is added to obtain a given value of e electrolyte and s suspension pH, F – the Far-
aday constant, c – the acid or base concentration, Sw – the oxide specific surface, m –
oxide weight.
Surface charge can be also determined from the comparison of change of hydro-
gen or hydroxyl ions concentration in a given acid or base volume. The change can be
calculated using the equation:



Ve F 10 pH e  10 pH s  1014 pH s 
S w m 
Where: pH e  the pH electrolyte solution, pH s  the pH suspension, Ve – the
electrolyte volume (suspension),   - the activity coefficient

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 Task No. II – Point of zero charge determination

Fig. 6. Carrying electrolyte and suspension (TiO2) titration curves

[W. Janusz, „Interfacial Forces and Fields, Theory and Applications”, V. 85,
Ch. 4, str. 135 (1999)].

III. 9. 3. Suspensive effect method (Palman’s effect)

This suspensive effect method is based on the phenomenon of different activity of H+
ions measured in sediment and clear liquid over the sediment layers.

pH  pH suspension  pH solution

When there is no difference between pH measured in sediment and under sediment, this
solid is in PZC. Fig. 7 shows determination of PZC for ZnO using the potentiometric
titration and measure of pH. From the titration point OH   OH   0 was obtained at
pH=9.3, whereas pH from the Palman’s effect was pH=9.8.

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Electrical phenomena at the interface

Fig. 7. Point of zero charge for ZnO measurement by different methods: 1 – the Pal-
man’s effect, 2- the potentiometric titration.
[J. Laskowski, Chemia fizyczna w procesach mechanicznej przeróbki kopalin,
Wyd. Slask, Katowice 1969, str. 139].

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 Task No. II – Point of zero charge determination
IV. Experimental part

A. Apparatus and reagents

1. Apparatus: pH-meter CP-501, combined electrode , magnetic stirrer.
2. Equipment:
– calibrated flask, 500 cm3 – 1 piece,
– polymer vessels volume, 100 cm3 with lids– 6 pieces,
– calibrated pipette volume, 50 cm3 – 1 piece,
– polymer pipettes – 2 pieces,
– beaker volume, 100 cm3 – 1 piece,
– spraying vessel.
3. Reagents:
– KCl stock solution, concentration 0.1 M,
– neutral alumina oxide (0.063–0.2 mm),
– KOH solution concentration 0.1 M and 0.02 M,
– HCl solution concentration 0.05 M,
– standard buffet solutions pH = 4.7 and 10,
– distilled water.

B. Tasks program
1. pH-meter calibration.
2. Preparation of KCl solutions with proper pH.
3. Alumina oxide samples preparation.
4. Measurement of pH value of alumina oxide suspensions after the equilibrium state.

C. Apparatus operation
A pH-meter CP-501 is used for accurate measurements of pH, redox potential and
temperature. With the electrode measurements of pH of water, solutions, gutters,
suspensions, soils etc. are possible. Five point calibration is possible. Buffers pH
value is automatically detected and this value can be changed by the user. Tempera-
ture corrections for pH value are automatic, and compatible with national standards.
Membrane condition is evaluated automatically.

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Electrical phenomena at the interface

Calibration of pH-meter CP-501 with automatic temperature compensation

pH
ON

CAL

Rys. 10. pH-meter CP-501.

1. Turn on pH-meter ON.

2. Insert the combined electrode and temperature sensor into the vessel with buffer,
pH 4.
3. Press button CAL and hold till symbol CAL blinks in the left side of light crys-
tal display.
4. Press for a short time the button CAL and wait for establishment of pH value.
5. Take out the electrode and temperature sensor, rinse with distilled water and
gently dry using filter paper.
6. Insert the electrode and temperature sensor into the vessel with buffer, pH 10.
7. Press shortly button CAL and wait for settlement of pH value.
8. Press the button pH to change the calibration course for the measurement course
(blinking symbol CAL will disappear)
9. The apparatus is calibrated now and ready for measurements.

D. Method of doing the task

The calibrated 500 cm3 flask prepare KCl solution of 0.01 M concentration from
standard 0.1 M solution concentration. Next measure of 50 cm3 of KCl for each of six
polymer vessels. Adding HCl or NaOH solution (with proper concentration) establish
pH of KCl solutions to about 4, 5, 6, 7, 9 and 10.
To obtain the demanded value of pH some droplets of 0.05 M HCl (pH 4 or 5) or
0.02 M KOH (pH 6 or 7) or 0.1 M KOH (pH 9 or 10) should be added to KCl solution
using a plastic pipette. After adding each droplet of acid or base, the solution should be
mixed thoroughly using a magnetic stirrer. After stirring pH should be measured. Re-
peat this procedure until you obtain the required pH value. Write down the initial values

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 Task No. II – Point of zero charge determination
of pH0, of KCl solutions.
Due to small amounts of HCl and KOH added concentrations of KCl solutions do
not change. After each pH measurement the electrode should be wiped.

pH measurement should be conducted with the magnetic stirrer turn down.

(not to damage the electrode).

Weigh 6 samples of 0.5 g neutral aluminum oxide. Add them to KCl solutions with
pH0. Cover vessels, mix them and leave for 30 min. for equilibrium fixing (mix the ves-
sels several times). Then measure pH of the obtained solutions, dipping the electrode in
the vessel under sediment. Read the pH value from the pH-meter after 1 min. after elec-
trode dipping. Repeat these measurements for other KCl solutions, writing down pHr
values.

E. Data elaboration
1. The obtained data should be given in a table:

Concentration KCl = ...

pHo pHr  pH = pHr –pHo

where: pHr – the pH value of aluminum oxide in equilibrium

(last fixed value of pH) ,
pHo – the initial pH value of KCl solution, for each pHr determined.
2. Show graphically the dependence  pH = f(pHr).
3. Find the value of pHr, where  pH = 0. On the graph this value of pH determines the
aluminum oxide point of zero charge.

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