Describe how the jar test for coagulation works. One sample is an untreated control. The others are adjusted to selected pH and have different dosages of coagulant. After the mixing is stopped, rate of settling and final turbidity are indicies of proper pH and dose. A brand new text for Unit Ops shows the repulsive force on particles versus separation distance as almost a straight line.
Electrostatic repulsion depends on square of distance.
The relationship cannot be a straight line. Final clarity and character of the precipitate as functions of pH and dosage. 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.