ISO 13100:2024

International
Standard
ISO 13100
First edition
Methods for zeta potential
2024-03
determination — Streaming
potential and streaming current
methods for porous materials
Méthodes pour la détermination du potentiel zêta — Méthodes
de potentiel d’écoulement/courant d’écoulement pour les
matériaux poreux
Reference number
© ISO 2024
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Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
3.1 Terms related to the electric double layer and the zeta potential .1
3.2 Terms related to electrokinetic and electroacoustic phenomena .2
3.3 Terms related to porous materials .4
4 Symbols . 5
5 Streaming current and streaming potential . 6
5.1 General overview .6
5.2 Streaming potential in DC mode .6
5.2.1 General .6
5.2.2 Measurement of the streaming potential coupling coefficient .7
5.2.3 Calculation of the zeta potential.8
5.3 Streaming current in AC mode .9
6 Measurement of DC streaming potential for porous materials .11
6.1 Operational procedures .11
6.2 Instrument location . 13
6.3 Sample holder . 13
6.4 Sample preparation .14
6.5 Test solution . 15
6.6 Verification . 15
6.7 Repeatability and reproducibility . 15
6.8 Sources of measurement error .16
6.8.1 Contamination of the current sample by the previous sample .16
6.8.2 Inappropriate sample preparation procedure .16
6.8.3 Inappropriate test solution .17
6.8.4 Air bubbles .17
6.8.5 Faulty electrodes .17
6.8.6 Limitation of the Smoluchowski approximation .18
7 Measurement of AC streaming current for porous materials .18
7.1 Instrument setup for particle deposits .18
7.2 Instrument setup for consolidated porous materials .19
7.3 Sample requirements.21
7.4 Calibration and verification .21
7.4.1 Reference materials .21
7.4.2 Calibration . . .21
7.4.3 Verification . 22
7.5 Repeatability and intermediate precision . 22
7.6 Sources of measurement error . 22
8 Reporting of zeta potential results .23
8.1 General information. 23
8.2 Specific information. 23
Annex A (informative) Electric double layer models in porous materials .24
Annex B (informative) Debye length .26
Annex C (informative) Porosity determination .27
Bibliography .28

iii
Foreword
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Subcommittee SC 4, Particle characterization.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
There are several different types of porous materials, such as porous monoliths, porous particles, deposits of
solid particles, etc. Each of these different types can require a special sample handling system. This document
covers only those aspects of the measurement and interpretation of electrokinetic and electroacoustic
phenomena that are common for all these types of porous materials.
The determination of the zeta potential in wetted porous materials is complicated by the fact that this
parameter is not a directly measurable quantity. It is calculated from the measured electric signal (either
current or potential) that is generated in the wetted porous material by the liquid moving under the
influence of an applied pressure gradient. The theories used in the calculation will not be discussed in detail.
There are a number of situations where electrokinetic and electroacoustic measurements, without further
interpretation, provide extremely useful and unequivocal information for technological purposes. The most
important of these situations are:
a) identification of the isoelectric point by titration of the zeta potential against a potential determining
ion (e.g. pH titration);
b) identification of the isoelectric point by titration with other reagents such as surfactants or
polyelectrolytes;
c) identification of a saturation plateau in the adsorption indicating optimum dosage for a dispersing agent;
d) relative comparison of various porous materials with regard to their electric surface properties.

v
International Standard ISO 13100:2024(en)
Methods for zeta potential determination — Streaming
potential and streaming current methods for porous
materials
1 Scope
This document specifies methods for the zeta potential determination in porous materials that are saturated
with a liquid where the pores are readily accessible. There is no restriction on the value of the zeta potential
or on the porosity of the porous material. A pore is assumed to be on the scale of hundreds of micrometres
or smaller without any restriction on pore geometry.
This document covers the applications of alternating current (AC) and direct current (DC) methods using
aqueous media as wetting liquids.
This document is restricted to linear electrokinetic effects.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1 Terms related to the electric double layer and the zeta potential
3.1.1
Debye length
–1
κ
characteristic length of the electric double layer (3.1.2) in an electrolyte solution
Note 1 to entry: The Debye length is expressed in metres.
[SOURCE: ISO 13099-1:2012, 2.1.2, modified — In the Note to entry, "nanometres" has been changed to
"metres".]
3.1.2
electric double layer
spatial distribution of electric charges that appears on and at the vicinity of the surface of an object when it
is placed in contact with a liquid

3.1.3
electric surface charge density
σ
charges on an interface per area due to specific adsorption of ions from the liquid bulk, or due to dissociation
of the surface groups
Note 1 to entry: Electric surface charge density is expressed in coulombs per square metre.
[SOURCE: ISO 13099-1:2012, 2.1.6]
3.1.4
electrokinetic charge density
σ
ek
effective charges at the slipping plane (shear plane) per area due to partial compensation of the electric
surface charge density (3.1.3) by the accumulation of oppositely charged solutes in the bulk liquid phase
Note 1 to entry: Electrokinetic charge density is expressed in coulombs per square metre.
3.1.5
isoelectric point
condition of liquid medium, usually the value of pH, that corresponds to zero zeta-potential (3.1.7)
[SOURCE: ISO 13099-1:2012, 2.1.10]
3.1.6
surface conductivity
σ
K
excess electrical conduction tangential to a charged surface
Note 1 to entry: Surface conductivity is expressed in siemens.
[SOURCE: ISO 13099-1:2012, 2.2.11]
3.1.7
electrokinetic potential
zeta-potential
ζ-potential
ζ
difference in electric potential between that at the slipping plane and that of the bulk liquid
Note 1 to entry: Electrokinetic potential is expressed in volts.
[SOURCE: ISO 13099-1:2012, 2.1.8]
3.2 Terms related to electrokinetic and electroacoustic phenomena
3.2.1
colloid vibration current
CVI
alternating current generated between two electrodes, placed in a dispersion, if the latter is subjected to an
ultrasonic field
[SOURCE: ISO 13099-1:2012, 2.3.1, modified — The admitted term I has been removed; the note has been
CVI
removed.]
3.2.2
electrokinetic phenomena
phenomena associated with tangential liquid motion adjacent to a charged surface

3.2.3
electroacoustic phenomena
phenomena arising from the coupling between the ultrasound field and electric field in a liquid that contains ions
Note 1 to entry: Either of these fields can be primary driving force. Liquid can be a simple Newtonian liquid or complex
heterogeneous dispersion, emulsion or even a porous body. There are several different electroacoustic effects,
depending on the nature of the liquid and type of the driving force.
3.2.4
seismoelectric current
SEI
I
see
non-isochoric streaming current (3.2.5) that arises in a porous body in liquid when an ultrasound wave
propagates through
Note 1 to entry: A similar effect can be observed at a non-porous surface, when sound is bounced off at an oblique angle.
Note 2 to entry: Seismoelectric effect is expressed in amperes.
3.2.5
streaming current
I
str
current through a porous body in liquid resulting from the motion of the liquid under an applied pressure
gradient
Note 1 to entry: Streaming current is expressed in amperes.
[SOURCE: ISO 13099-1:2012, 2.2.8]
3.2.6
streaming current coupling coefficient
I /ΔP
str
electrokinetic phenomenon (3.2.2) determined by the slope of the linear dependence of the measured
streaming current (3.2.5) on an applied pressure gradient
Note 1 to entry: Streaming current coupling coefficient is expressed in amperes per pascal.
3.2.7
streaming potential
U
str
potential difference at zero electric current, caused by the flow of liquid under a pressure gradient through a
capillary, plug, diaphragm or membrane
Note 1 to entry: Streaming potentials are created by charge accumulation caused by the flow of countercharges inside
capillaries or pores (3.3.5).
Note 2 to entry: Streaming potential is expressed in volts.
[SOURCE: ISO 13099-1:2012, 2.2.10]
3.2.8
streaming potential coupling coefficient
U /ΔP
str
electrokinetic phenomenon (3.2.2) determined by the slope of the linear dependence of the measured
streaming potential (3.2.7) on an applied pressure gradient
Note 1 to entry: Streaming potential coupling coefficient is expressed in volts per pascal.

3.3 Terms related to porous materials
3.3.1
deposit of solid particles
accumulation of individual particles (3.3.4) by sedimentation from a dispersion (3.3.2) on a solid support
3.3.2
dispersion
multi-phase system in which discontinuities of any state (solid, liquid or gas) are homogeneously distributed
in a continuous phase of a different composition or state
Note 1 to entry: This term can also refer to the act or process of producing a dispersion, but in this context the term
“dispersion process” shall be used.
Note 2 to entry: If solid particles (3.3.4) are dispersed in a liquid, the dispersion is referred to as a suspension. If the
dispersion consists of two or more immiscible liquid phases, it is termed an emulsion.
[SOURCE: ISO/TS 22107:2021, 3.7]
3.3.3
monolith
solid porous object with size on scale of a few millimetres or larger
3.3.4
particle
minute piece of matter with defined physical boundaries
Note 1 to entry: A physical boundary can also be described as an interface.
Note 2 to entry: A particle can move as a unit.
[SOURCE: ISO 26824:2013, 1.1, modified — Note 3 to entry has been removed.]
3.3.5
pore
cavity or channel which is deeper than it is wide, otherwise it is part of the material’s roughness
[SOURCE: ISO 15901-1:2016, 3.5]
3.3.6
pore size
internal pore (3.3.5) width, which is a representative value of various sizes of vacant space inside a porous
material (3.3.8)
EXAMPLE Diameter of a cylindrical pore or the distance between the opposite walls of a slit.
[SOURCE: ISO 15901-1:2016, 3.13, modified — Part of the definition has been moved to an example.]
3.3.7
porosity
ratio of the volume of the accessible pores (3.3.5) and voids to the bulk volume occupied by an amount of the solid
[SOURCE: ISO 15901-1:2016, 3.27]
3.3.8
porous material
materials with cavities or channels which are deeper than they are wide
3.3.9
powder
porous or nonporous solid composed of discrete particles (3.3.4) with maximum dimension less than
approximately 1 mm
Note 1 to entry: Powders with a particle size below approximately 1 μm are often referred to as fine powders.

[SOURCE: ISO 15901-1:2016, 3.4, modified — Part of the definition has been moved to a Note to entry.]
4 Symbols
a pore radius
a radius of particles building sediment
i
c electrolyte concentration in the bulk
c concentration of the i-th ion species
i
t time to reach first maximum of the electroacoustic signal
cr
z valence of the i-th ion species
i
z valences of cations and anions
±
D effective diffusion coefficient of the electrolyte
eff
F Faraday constant
I ionic strength
I streaming current in high frequency AC mode (seismoelectric current)
see
I streaming current in DC mode
str
K electric conductivity
K electric conductivity of the dispersion medium or liquid
m
K electric conductivity of the wetted porous material
s
P pressure
R ideal gas constant
T absolute temperature
U voltage
U streaming potential
str

volume flow rate
V
ε vacuum permittivity
ε relative permittivity of the medium
m
η dynamic viscosity
φ volume fraction of solids
φ volume fraction of solids in sediment
sed
−1
κ Debye length
ρ liquid density
m
ρ particle density
p
ρ density of the dispersion
s
σ electric surface charge density
d
σ electric charge density of the diffuse layer
ω circular frequency
ζ electrokinetic potential, zeta potential
Ω porosity
5 Streaming current and streaming potential
5.1 General overview
The phenomena of streaming current and streaming potential occur in porous materials that are wetted
with a liquid. The pore walls of such porous materials are covered with electric charges that are generated
either by the dissociation of surface functional groups or by the specific adsorption of solutes at the solid-
liquid interface. These surface and interfacial charges are screened by stationary and diffuse layers of
accumulated ions forming a structure known as the “electric double layer” (EDL). Different types and models
of EDLs applicable to porous materials are described in Annex A.
The application of a pressure gradient on wetted porous materials generates a flow of liquid passing through
the pores. This liquid flow, in turn, causes an electric response, either current or potential, depending on
the method of its measurement. This electric response occurs due to the motion of the diffuse layer that
is dragged by the liquid flow tangentially to the pore-liquid interfaces. This response is referred to as the
“streaming current” or the “streaming potential”.
The AC and DC modes of the streaming current and streaming potential require different measuring
techniques and different instrumentation. The applicability of AC and DC techniques for the calculation of
the zeta potential is determined by the pore size of the porous material. The threshold between the two
methods may be defined in terms of the pore size. This critical pore size is approximately 10 µm, where the
AC mode is applicable below and the DC mode above this threshold size. There is a certain size range around
10 µm where both AC and DC methods are applicable. Due to the different applicable size ranges, the AC and
DC methods are discussed separately below.
There is another justification for a separate presentation of AC and DC techniques. The generation of liquid
flow with a DC pressure gradient is possible only for porous materials with a sufficiently high hydrodynamic
permeability. Decreasing either pore size or porosity leads to a decrease of the permeability and eventually
blocks liquid flow. This means that a DC pressure can be applied for generating streaming current and
streaming potential only for sufficiently large pores and porosity. In contrary, an AC pressure gradient at
high frequency (MHz) can penetrate only into porous materials with small pores and limited porosity.
This differentiation indicates different application ranges for the DC and AC streaming current and streaming
potential measurements. The DC mode is applicable to materials with large pores and high porosity, the AC
mode to materials with small pores and low porosity.
5.2 Streaming potential in DC mode
5.2.1 General
The streaming potential depends strongly on the distribution of the electric potential inside the pores.
Figure A.1 illustrates possible space distributions of this potential including two extreme cases:
a) isolated thin double layers;
b) homogeneous, completely overlapped double layers.

There are analytical theories of the streaming potential that correspond to these two extreme cases.
[5]
Smoluchowski developed the theory for the streaming potential in case of isolated thin electric double
layers. This theory yields the following expression [Formula (1)] for the electric potential difference ΔU
str
generated by a pressure difference ΔP:
εε ζ
m 0
ΔΔUP= (1)
str
ηK
where
ε is the dielectric coefficient of the liquid;
m
ε is the vacuum permittivity;
ζ is the electrokinetic potential (zeta potential) of the pore surface;
η is the dynamic viscosity of the liquid;
K is the electric conductivity inside a capillary flow channel.
5.2.2 Measurement of the streaming potential coupling coefficient
The streaming potential and alternatively the streaming current are the electrical responses to the flow of
a test liquid through a capillary or a capillary network driven by a pressure gradient that is applied between
both ends of this capillary (capillary network). According to Formula (1), the streaming potential is strongly
dependent on the applied pressure gradient. For the sake of measurement reproducibility and independence
of the instrument design and operation conditions, the zeta potential is related to the streaming potential
coupling coefficient dU /dΔP. The streaming potential coupling coefficient is calculated by referring the
str
measured streaming potential to the applied pressure gradient. Figure 1 shows the result of a streaming
potential measurement in DC mode during a continuously decreasing pressure gradient.

Key
X pressure difference (kPa)
Y streaming potential (mV)
NOTE polyether ether ketone, particle size 80 µm, 0,001 mol/l KCl, conductivity 0,014 S/m, pH 5,36, ζ = –43,76 mV.
Figure 1 — Measurement result of the streaming potential at a continuously decreasing
pressure gradient in DC mode
5.2.3 Calculation of the zeta potential
The calculation of the zeta potential at the particle-liquid or pore-liquid interfaces from the measurement of
the streaming potential coupling coefficient is derived from Formula (1).
For a sufficiently large distance between adjacent particles in the sample plug or a sufficiently large pore
size at a sufficiently high ionic strength of the test liquid, the conductivity term K in Formula (1) may be
replaced by the conductivity of the bulk liquid phase K . After re-arrangement of Formula (1), we obtain
m
Formula (2):
dU
η
str
ζ = K (2)
m
dΔP εε
m 0
for the zeta potential determined from a streaming potential measurement. The application of Formula (2)
is valid if the inter-particle distance in a plug of a particulate sample or the wall-to-wall distance of a porous
material is large in comparison with the extension of the electric double layer at the solid-liquid interface,
–1
which is a function of the ionic strength according to Formula (A.1). If the condition of a >> κ , where a is
either the average distance between particles in a plug of a particulate sample or the average pore radius of
a porous material, is not fulfilled by the properties of the corresponding sample, Formula (2) delivers a zeta
potential result that is too low in magnitude and commonly denoted an apparent zeta potential. Formula (2)
is also not applicable to determine the true zeta potential if the material bulk or the material surface are
–1
electrically conductive even if the boundary condition of a >> κ is fulfilled.
Alternatively, the zeta potential is determined from the streaming current coupling coefficient dI /dΔP
str
according to Formula (3):
dI η L
str
ζ = (3)
d”P εε A
m 0
where
L is the length of the flow channel;
A is the cross-section of the flow channel;
The application of Formula (3) requires the knowledge of the ratio L/A. For a plug of a particulate sample
or a porous material, the voids inside the particle plug and the pores of the porous material, respectively,
determine the geometry of the flow channel. The exact length and cross-section of these networks of
capillaries cannot be determined quantitatively and therefore makes Formula (3) not applicable for the zeta
potential analysis of a plug of particles or a porous material.
5.3 Streaming current in AC mode
Ultrasound at high frequency is the driving force for the generation of the streaming current in the AC
[6]
mode of measurement. Frenkel introduced the term “seismoelectric effect”. Liquid becomes compressible
at a frequency in the MHz range, and, consequently, the corresponding electrokinetic effects become non-
[7][8]
isochoric.
Frenkel used the theory of Smoluchowski for the streaming current in DC mode to derive the equation for
the seismoelectric current, which is also limited to EDLs that are isolated and thin compared to the pore
[9][10]
size. The seismoelectric current I is related to the zeta potential by Formula (4) :
see
εε ζ  ρ  K
m 0 m s
IP= 1−∇ (4)
see  
η ρ K
 
s m
where
ρ is the density of the liquid;
m
ρ is the density of the wetted porous material;
s
K is the electric conductivity of the liquid;
m
[11]
K is the electric conductivity of the porous material.
s
[12]
An alternative theory for smaller pores is also available.
The measurement of the seismoelectric current is demonstrated by an example of silica particles with a
median diameter of 1,5 µm, which were used for building up a sediment as a model of a porous body. A
dispersion of these particles was prepared at a weight/volume percentage concentration of 10 % dispersed
in distilled water at pH 10. Sonication was applied for 5 min for dispersing particles in this solution.
Each of the sedimentation experiments with the silica particles consisted of 3 000 continuous measurements
of the electroacoustic signal generated by silica particles sedimenting to the surface of the electroacoustic
probe. Each experiment took approximately 20 h to complete. All the particles settled on the probe surface
during this time.
Key
X time (min)
7 1/2
Y electroacoustic magnitude [10 mV·(s/g) ]
Figure 2 — Time dependence of the measured seismoelectric current generated by the gradual
build-up of the sediment on the face of the electroacoustic probe
Figure 2 shows the evolution of the magnitude of the electroacoustic signal with time. The particle dispersion
is thoroughly mixed before pouring into the cup on the face of the electroacoustic probe. Initially, particles
are therefore homogeneously distributed. The first measurement reflects the colloid vibration current (CVI)
signal from such homogeneously dispersed particles. As time passes by, particles start settling on the surface
of the probe. As a result, their concentration in the vicinity of the surface of the probe increases. The CVI
signal increases as well because it is proportional to the particle volume fraction. This increase continues
until particles fill a layer with the thickness of the half wavelength of the sound. After that, particles begin
filling the second half wavelength layer where the direction of the pressure gradient of the ultrasound wave
reverses. Consequently, particles start moving to the opposite direction, which leads to the reversal of the
CVI signal generated by these particles. This contribution to the CVI signal will be subtracted from the CVI
signal caused by the particles in the first wavelength layer. As a result, the total CVI signal starts dropping.
This is reflected in Figure 2 as the signal of the electroacoustic magnitude reaches the first maximum.
The growing deposit of particles becomes more and more dense due to the ongoing sedimentation. At some
point, the electroacoustic phenomenon switches from CVI mode (particles moving relative to a steady
liquid) to seismoelectric current (SEI) mode (liquid moves relative to densely packed particles). Apparently,
the first minimum in the curve in Figure 2 corresponds to this transition. A small increase in the signal
occurs due to the larger magnitude of SEI compared to CVI for the same number of particles. The decline
after the second maximum reflects the continuing filling up of the second wavelength layer. Afterwards, the
signal starts growing again when the third half wavelength layer begins to be filled with settling particles.
The amplitude of these oscillations decays because of ultrasound attenuation. It eventually reaches
saturation after the particles have filled approximately 10 wavelength layers. The thickness of each layer
depends on the ultrasound frequency. At a frequency of 3,3 MHz, which corresponds to a wavelength of
approximately 450 µm, the electroacoustic signal reaches saturation when the particles fill a roughly 4,5 mm
thick layer on top of the probe.
There are two important parameters that characterize the curve in Figure 2.

The first one is the time required for the electroacoustic signal to reach the first maximum. This number, t ,
cr
can be used to determine the porosity of the sediment Ω or, alternatively, the volume fraction of silica in the
sediment φ (see Annex C), using Formula (5):
sed
ϕΩ=−1 (5)
sed
The second parameter is the final magnitude of the electroacoustic signal after it reaches saturation at the
end of the experiment. This parameter can be used to calculate the zeta potential in the sediment using the
derived seismoelectric theory.
The final magnitude of the electroacoustic signal, which is obtained when reaching saturation at the end
of the experiment, is identical to the magnitude of the electroacoustic signal for the particle dispersions
measured through the colloid vibration current. A dispersion usually fills a layer that is thicker than 450 µm.
The calibration constant determined with a certified reference material corresponds to this saturated value.
It means that the final magnitude of the electroacoustic signal can be used to calculate the ζ-potential of
particles in a sediment by assuming the same calibration constant.
Application of Formula (4) for the calculation of the zeta potential requires information on the ratio of
conductivities of the liquid and the sediment. Instead of measuring them, a calculation can be employed
[13][14]
based on the Maxwell-Wagner theory. It gives an expression for conductivity in heterogeneous
systems, which is rather simple in the case of a negligible surface conductivity, as indicated by Formula (6):
K 1−ϕ Ω
s sed
== (6)
K 1+0,5ϕ 1,5−0,5Ω
m sed
Substituting the ratio of conductivities in Formula (4) by Formula (6) and expressing the ratio of densities of
the liquid and the sediment though the porosity Ω of the sediment and the particle density ρ , Formula (7) is
p
obtained for the seismoelectric current generated by the propagation of ultrasound through the sediment:
 
 
εε ζ 1 Ω
0 m
 
IP= 1− ∇ (7)
see
 
ρ
η 1,5−0,5Ω
p
 Ω +1()−Ω 
 
ρ
 
m
A result of the zeta potential calculated using Formula (7) is shown in Table 1. The zeta potential for the silica
particle dispersion at a mass fraction of 10 % was determined by the measurement of the colloid vibration
current as ζ = −61,2 ± 0,2 mV. There is rather close agreement between ζ potential values for dispersion and
for sediment.
Table 1 — Zeta potential for a dispersion and for a sediment of silica particles
Zeta potential
Sample Experimental condition
mV
Dispersion at 4,8 % vol. Mixing. Inertia size
Silica particle dispersion −61,2 ± 0,2
a
related correction, CVI theory
Sediment with 62 % volume fraction, porosity
Silica particle sediment −57,9 ± 0,9
b
38 %, seismoelectric theory, Formula (7)
a
Determination using the measurement of the colloid vibration current (CVI). Data sourced from Reference [15].
b
Determination using the seismoelectric current (conductivity 0,03 S/m, pH 7,3).
6 Measurement of DC streaming potential for porous materials
6.1 Operational procedures
The main components of an instrument for DC streaming potential/streaming current measurement
comprise a sample holder for the solid sample, a container for the test liquid, a set of electrodes for the
measurement of streaming potential and streaming current, a supply of pressure (either a mechanical pump

or a pressurized gas reservoir), a pressure sensor for recording the pressure difference, an electrometer
with the capability to measure streaming potential (that requires an electric circuit with a high internal
impedance of the electrometer) and streaming current (that requires an electric circuit with a low
internal impedance of the electrometer), and a conductivity probe for the measurement of the bulk electric
conductivity of the test liquid. When using an aqueous solution as the test liquid a significant dependence of
the zeta potential on the pH of the aqueous solution is commonly observed. The equipment of the instrument
for streaming potential and streaming current measurement with an integrated pH probe for a continuous
monitoring of the pH of an aqueous test solution is therefore recommended.
A granular or powder sample is prepared as a porous plug and the voids between granular particles are filled
with a test liquid. A condensed porous material provides a network of pores, which are again filled with the
test liquid. A pressure gradient is applied between the ends of the plug of the granular sample or between
the ends of the porous material, which are soaked with the test liquid. The pressure gradient provokes
liquid flow and the transport of the diffuse layer of the electric double layer at the solid-liquid interface,
which contains ions that compensate the electrokinetic charge density. The moving ions in the diffuse
layer generate an electric current (a DC current, which is called the streaming current). At a sufficiently
high impedance of the electrometer (voltmeter), the streaming current is compensated by a back-current
through the conductive pathway of the test liquid. At the equilibrium of streaming current and back-current,
the electric potential between the ends of the plug of the granular sample or between the ends of the porous
material (a DC voltage, which is called the streaming potential) is measured. Alternatively, the streaming
current is measured directly by using a low impedance of the electrometer (amperemeter).
Figure 3 shows the schematic drawing of a typical setup for the streaming potential and streaming current
measurement in DC mode.
Key
1 3-way valve 4 sample holder
2 syringe pump 5 pH probe
3 electrodes, pressure sensors 6 conductivity probe
Figure 3 — Schematic drawing of an experimental setup for the streaming potential/streaming
[16]
current measurement in DC mode
The streaming potential (streaming current) is measured by a set of electrodes, which are located at both
ends of the plug of the particle sample or of the porous material. Reversible electrodes such as silver-silver
chloride (Ag│AgCl) electrodes are recommended to suppress the effect of electrode polarization, i.e. an
offset of the measured voltage different from 0 V in the absence of flow of the test liquid (see 6.8.5).

The streaming potential coupling coefficient (streaming current coupling coefficient) may be determined by
different methods.
— A single-point measurement of the streaming potential (streaming current) at a given pressure difference
applied between the ends of the plug of the particulate sample or of the porous material enables the
calculation of the quotient ΔU /ΔP (ΔI /ΔP). This approach can be subject to a significant error if the
str str
assumption of a negligible electrode polarization does not hold (see 6.8.5).
— The measurement of streaming potential (streaming current) at a single pressure difference, where
pressure is applied alternatively on either end of the plug of the particulate sample or of the porous
material, eliminates the contribution of any electrode polarization to the calculation of the quotient
ΔU /ΔP (ΔI /ΔP).
str str
— A series of streaming potential measurements (streaming current measurements) at a series of
individual pressure differences shall present a linear dependence of the streaming potential (streaming
current) on the applied pressure difference. A fit of the set of streaming potential (streaming current)
and pressure difference data by a linear regression gives the differential quotient dΔU /dΔP (dΔI /
str str
dΔP) and eliminates the contribution of any electrode polarization.
— The measurement of streaming potential (streaming current) simultaneously to a continuously changing
pressure gradient enables the recording of a large set of streaming potential (streaming current) and
pressure difference data for a fit by a linear regression with improved measurement statistics. Preferably
the pressure difference is varied without the action of a mechanical pump in order to eliminate pressure
fluctuations.
6.2 Instrument location
The instrument for streaming potential and
...