|
 Turbidity
Turbidity is defined as an "expression of the optical property
that causes light to be scattered and absorbed rather than transmitted
in straight lines through the sample."1 Simply stated,
turbidity is the measure of relative sample clarity. It is not
color. Figure 1 shows the interaction of a light beam and undissolved,
finely distributed particles known as suspended solids. When
the light beam passes through the sample of fluid, the suspended
solids scatter the light in all directions (360°spherically).
Reduction in the intensity of the light beam is primarily caused
by the suspended solids scattering the light. However, absorption
(color) from dissolved substances can also reduce the intensity
and should be taken into consideration by manually or automatically
subtracting its effect. This can be accomplished optically if
a turbidimeter is used (refer to the "Modulated Four-Beam
Method" later in this article for details) or electronically
if a separate absorption photometer is used in combination with
a turbidimeter and their outputs are subtracted. Low quantities
of suspended solids are normally monitored by measuring the
scattered light effect rather than the absorption effect because
with scattered light, the photocell detects small changes in
light intensity with respect to a dark background. The disadvantage
occurs at higher suspended solids levels where multiple scattering
limits the amount of side-scattered light received by the photocell.
This condition results in lower-than-actual turbidity readings.
At suspended solids concentrations above 2000 ppm, alternate
measurement methods such as absorption must be used in place
of the turbidity measurement. Turbidity measurements provide
a reading of the amount of scattered light and cannot be directly
related to a gravimetric equivalent unless a working curve for
the specific sample is created. The intensity of scattered light
is affected by many variables including wavelength, particle
size, color, and shape.
|
