CCA 3100
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4.3 CHARGE DEMAND (TITRATION)
To determine the charge demand of a sample, a coagulant of opposite charge is added until
the isoelectric (0.00) point is achieved. The coagulant is made-up to a known concentration and
referred to as the “titrant.” The CHARGE DEMAND refers to the amount of titrant, in
milliliters, that is required to bring a sample to the isoelectric point. This is demonstrated in
Figure 5, where a net anionic water sample is dosed with a coagulant until the isoelectric point is
achieved and then surpassed.
FIGURE 5
Note that in Figure 5, the SCV tangentially approaches zero. This is critical to achieving a valid
titration, and is achieved by an appropriate addition rate of titrant. A slow addition rate ensures
that each molecule is given the opportunity to adsorb onto a colloid and stabilize itself to achieve
its lowest energy level. A hasty titration will cause the SCV to overshoot the isoelectric point,
giving a false CHARGE DEMAND.
This “overshoot” is depicted in Figure 5 but is not the normal procedure for a titration.
Titration Curve
1000
0
-1000
Streaming
Current
Value
Isoelectric
Point
Milliliters of
Coagulant
Summary of Contents for CCA3100
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