1. Define zeta potential.
The voltage difference between the bulk solution and the particle including
the layers of ions that stick so tightly that they move with the particle.
2. Explain the hump in the curve for repulsive force as particles approach
each other.
Electrostatic repulsion and van der Waal's attraction are opposing forces for
particles. As random motion brings particles toward each other, the
resultant force passes through a maximum repulsion and reaches a very strong
attraction. There must be enough energy to get over the hump. If very few
particles have sufficient energy, coagulation will be very slow.
3. Why can there be a "sediment trap" as a river flows toward the ocean ?
Salt water and fresh water meet to develop sufficient ionic strength to
encourage coagulation.
4. Explain why the microscope method for measuring zeta potential uses dark
field illumination.
Particles too small to be visible even at high magnification are easily seen
as bright spots on a black background when the microscope light path has an
obstruction for direct light rays. Only rays from an angle get through, and
some of these bounce off the particles.
5. Select any recent newsletter story and present the opposite point of view.
No generality possible.
2. What is orthokinetic flocculation ?
In contrast to perikinetic that is driven by molecular vibrations,
orthokinetic flocculation depends on mixing and velocity gradients.
3. Point out two ways that people often go wrong with the jar test.
a) Taking big increments in concentration and missing the narrow range when
coagulation works. In other words, the particles are still stable at the low
doses tried and are restabilized at the higher concentrations; turbid for
either case.
b) Failing to adjust pH when dosing with Al or Fe salts that change pH.
4. Explain how the Laser-Doppler method works for measuring zeta potential.
The laser beam is directed transverse to the viewing angle. Particles
migrating in the electric field have their velocity and the velocity of ligh
creating a new frequency because of the Doppler effect. The number of
flashes and the light frequency tell the number of particles and their
velocities.
5. Select a recent newsletter article and state the author's bias.
No general answer.
2. A brand new text for Unit Ops shows the repulsive force on particles
versus separation distance as almost a straight line. Please comment.
Bad thinking. Electrostatic repulsion depends on square of distance. The
relationship cannot be a straight line.
3. What do you actually measure with a jar test ?
Final clarity and character of the precipitate as functions of pH and dosage.
4. Why is the Laser-Doppler method displacing the microscope method for zeta
potential ?
The old Zetameter with a microscope could measure relatively few particles in
a given time. The Laser-Doppler method measures thousands of particles in 2
seconds. There is no operator bias.
5. Select any recent newsletter article and state something that you think
the author overlooked.
No general answer.