BTG Mutek PCD-05 Smart, Руководство по эксплуатации

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© BTG Operation Manual Mütek
TM
PCD-05, September 2017 11
II. Product description
II. Product description
2.1 Theoretical background
Whether a suspension of particles or a true solution of
large molecules, most materials dispersed in water
carry an electric charge.
In such an aqueous system charges at solid/ liquid or
liquid/ liquid interfaces play a crucial role.
They cause reaction mechanism of the type found in
dispersions, suspensions or emulsions.
Measuring the streaming potential gives information
about interactions of the components in water and is
used to assess the stability of a system.
Double layer
Once a particle is dispersed in water it develops a
charge at the particle surface which affects the distribu-
tion of the dissolved ions in the water. This results in an
increased concentration of ions of opposite charge to
that of the particle close to its surface. Closest to the
surface a stationary layer of counter-ions forms; further
away a diffuse layer surrounds the particle. Thus each
particle carries an electrical double layer. Such a double
layer can only be formed by particles at least 1 nm in
size, the so-called colloids.
Electrokinetic effect
A relative movement of a liquid to a dispersed material
generates an electrokinetic effect. If an electric field or a
pressure is applied, the solid particles are caused to
move relative to the liquid electrolyte or vice versa.
Depending on which of the phases is moving, the elec-
trokinetic effect can be identified via
- Electrophoresis
- Electroosmosis
- Sedimentation potential
- Streaming potential
Streaming potential
If a liquid is forced along a particle surface under pres-
sure, it will shear off the counter-ions of the diffuse layer
and an electric field is built up. The resulting electric cur-
rent is picked up by two electrodes and is either indica-
ted by a streaming current signal or converted to a
potential. The Mütek™ PCD-05 Particle Charge Detec-
tor displays the potential in ±mV. The sign of the measu-
red value indicates whether the sample carries a
positive (cationic) or negative (anionic) charge. The
streaming potential is a relative value and depends on
various influential factors, such as
- Dimensions of the measuring cell
- Cleanliness of the cell
- Conductivity
- Chemical properties of the sample
- Molecular weight
- Particle size
- pH
- Viscosity
- Temperature
To quantify the charges of a sample, a titration with an
oppositely charged titrant (polyelectrolyte) has to be
performed up to the point where a streaming potential of
0 mV is reached.
There is a correlation between streaming potential and
zeta potential, which latter has been identified by elec-
trophoresis. In actual practice, however, a comparison
of the two potentials is seldom possible and if at all, only
under defined ideal conditions.