ANALYTICAL
METHOD
FOR
TURBIDITY
MEASUREMENT
STANDARD
METHODS
2130
A
AND
B
JUNE
2003
DRAFT
1
Draft
June
2003
STANDARD
METHODS
2130:
TURBIDITY
Reprinted
by
Permission
from
the
20th
Edition
2130
A.
INTRODUCTION
1.
Sources
and
Significance
Clarity
of
water
is
important
in
producing
products
destined
for
human
consumption
and
in
many
manufacturing
operations.
Beverage
producers,
food
processors,
and
potable
water
treatment
plants
drawing
from
a
surface
water
source
commonly
rely
on
fluid­
particle
separation
processes
such
as
sedimentation
and
filtration
to
increase
clarity
and
insure
an
acceptable
product.
The
clarity
of
a
natural
body
of
water
is
an
important
determinant
of
its
condition
and
productivity.

Turbidity
in
water
is
caused
by
suspended
and
colloidal
matter
such
as
clay,
silt,
finely
divided
organic
and
inorganic
matter,
and
plankton
and
other
microscopic
organisms.
Turbidity
is
an
expression
of
the
optical
property
that
causes
light
to
be
scattered
and
absorbed
rather
than
transmitted
with
no
change
in
direction
or
flux
level
through
the
sample.
Correlation
of
turbidity
with
the
weight
or
particle
number
concentration
of
suspended
matter
is
difficult
because
the
size,
shape,
and
refractive
index
of
the
particles
affect
the
lightscattering
properties
of
the
suspension.
When
present
in
significant
concentrations,
particles
consisting
of
light­
absorbing
materials
such
as
activated
carbon
cause
a
negative
interference.
In
low
concentrations
these
particles
tend
to
have
a
positive
influence
because
they
contribute
to
turbidity.
The
presence
of
dissolved,
color­
causing
substances
that
absorb
light
may
cause
a
negative
interference.
Some
commercial
instruments
may
have
the
capability
of
either
correcting
for
a
slight
color
interference
or
optically
blanking
out
the
color
effect.

2.
Selection
of
Method
Historically,
the
standard
method
for
determination
of
turbidity
has
been
based
on
the
Jackson
candle
turbidimeter;
however,
the
lowest
turbidity
value
that
can
be
measured
directly
on
this
device
is
25
Jackson
Turbidity
Units
(
JTU).
Because
turbidities
of
water
treated
by
conventional
fluid­
particle
separation
processes
usually
fall
within
the
range
of
0
to
1
unit,
indirect
secondary
methods
were
developed
to
estimate
turbidity.
Electronic
nephelometers
are
the
preferred
instruments
for
turbidity
measurement.

Most
commercial
turbidimeters
designed
for
measuring
low
turbidities
give
comparatively
good
indications
of
the
intensity
of
light
scattered
in
one
particular
direction,
predominantly
at
right
angles
to
the
incident
light.
Turbidimeters
with
scattered­
light
detectors
located
at
90
°
to
the
incident
beam
are
called
nephelometers.
Nephelometers
are
relatively
unaffected
by
small
differences
in
design
parameters
and
therefore
are
specified
as
the
standard
instrument
for
measurement
of
low
turbidities.
Instruments
of
different
make
and
model
may
vary
in
response.*
However,
interinstrument
variation
may
be
effectively
negligible
if
good
measurement
techniques
are
used
and
the
characteristics
of
the
particles
in
the
measured
suspensions
are
similar.
Poor
measurement
technique
can
have
a
greater
effect
on
measurement
error
than
small
differences
in
instrument
design.
Turbidimeters
of
non­
standard
design,
such
as
forwardscattering
devices,
may
be
more
sensitive
than
nephelometers
to
the
presence
of
larger
particles.
While
it
may
not
be
appropriate
to
compare
their
output
with
that
of
instruments
of
standard
design,
they
still
may
be
useful
for
process
monitoring.
2
Draft
June
2003
An
additional
cause
of
discrepancies
in
turbidity
analysis
is
the
use
of
suspensions
of
different
types
of
particulate
matter
for
instrument
calibration.
Like
water
samples,
prepared
suspensions
have
different
optical
properties
depending
on
the
particle
size
distributions,
shapes,
and
refractive
indices.
A
standard
reference
suspension
having
reproducible
light­
scattering
properties
is
specified
for
nephelometer
calibration.

Its
precision,
sensitivity,
and
applicability
over
a
wide
turbidity
range
make
the
nephelometric
method
preferable
to
visual
methods.
Report
nephelometric
measurement
results
as
nephelometric
turbidity
units
(
NTU).

3.
Storage
of
Sample
Determine
turbidity
as
soon
as
possible
after
the
sample
is
taken.
Gently
agitate
all
samples
before
examination
to
ensure
a
representative
measurement.
Sample
preservation
is
not
practical;
begin
analysis
promptly.
Refrigerate
or
cool
to
4
°
C,
to
minimize
microbiological
decomposition
of
solids,
if
storage
is
required.
For
best
results,
measure
turbidity
immediately
without
altering
the
original
sample
conditions
such
as
temperature
or
pH.

2130
B.
NEPHELOMETRIC
METHOD
1.
General
Discussion
a.
Principle:
This
method
is
based
on
a
comparison
of
the
intensity
of
light
scattered
by
the
sample
under
defined
conditions
with
the
intensity
of
light
scattered
by
a
standard
reference
suspension
under
the
same
conditions.
The
higher
the
intensity
of
scattered
light,
the
higher
the
turbidity.
Formazin
polymer
is
used
as
the
primary
standard
reference
suspension.
The
turbidity
of
a
specified
concentration
of
formazin
suspension
is
defined
as
4000
NTU.

b.
Interference:
Turbidity
can
be
determined
for
any
water
sample
that
is
free
of
debris
and
rapidly
settling
coarse
sediment.
Dirty
glassware
and
the
presence
of
air
bubbles
give
false
results.
"
True
color,"
i.
e.,
water
color
due
to
dissolved
substances
that
absorb
light,
causes
measured
turbidities
to
be
low.
This
effect
usually
is
not
significant
in
treated
water.

2.
Apparatus
a.
Laboratory
or
process
nephelometer
consisting
of
a
light
source
for
illuminating
the
sample
and
one
or
more
photoelectric
detectors
with
a
readout
device
to
indicate
intensity
of
light
scattered
at
90
°
to
the
path
of
incident
light.
Use
an
instrument
designed
to
minimize
stray
light
reaching
the
detector
in
the
absence
of
turbidity
and
to
be
free
from
significant
drift
after
a
short
warmup
period.
The
sensitivity
of
the
instrument
should
permit
detecting
turbidity
differences
of
0.02
NTU
or
less
in
the
lowest
range
in
waters
having
a
turbidity
of
less
than
1
NTU.
Several
ranges
may
be
necessary
to
obtain
both
adequate
coverage
and
sufficient
sensitivity
for
low
turbidities.
Differences
in
instrument
design
will
cause
differences
in
measured
values
for
turbidity
even
though
the
same
suspension
is
used
for
calibration.
To
minimize
such
differences,
observe
the
following
design
criteria:
3
Draft
June
2003
1)
Light
source
­
Tungsten­
filament
lamp
operated
at
a
color
temperature
between
2200
and
3000
°
K.

2)
Distance
traversed
by
incident
light
and
scattered
light
within
the
sample
tube
­
Total
not
to
exceed
10
cm.

3)
Angle
of
light
acceptance
by
detector
­
Centered
at
90
°
to
the
incident
light
path
and
not
to
exceed
±
30
°
from
90
°
.
The
detector
and
filter
system,
if
used,
shall
have
a
spectral
peak
response
between
400
and
600
nm.

b.
Sample
cells:
Use
sample
cells
or
tubes
of
clear,
colorless
glass
or
plastic.
Keep
cells
scrupulously
clean,
both
inside
and
out,
and
discard
if
scratched
or
etched.
Never
handle
them
where
the
instrument's
light
beam
will
strike
them.
Use
tubes
with
sufficient
extra
length,
or
with
a
protective
case,
so
that
they
may
be
handled
properly.
Fill
cells
with
samples
and
standards
that
have
been
agitated
thoroughly
and
allow
sufficient
time
for
bubbles
to
escape.

Clean
sample
cells
by
thorough
washing
with
laboratory
soap
inside
and
out
followed
by
multiple
rinses
with
distilled
or
deionized
water;
let
cells
air­
dry.
Handle
sample
cells
only
by
the
top
to
avoid
dirt
and
fingerprints
within
the
light
path.

Cells
may
be
coated
on
the
outside
with
a
thin
layer
of
silicone
oil
to
mask
minor
imperfections
and
scratches
that
may
contribute
to
stray
light.
Use
silicone
oil
with
the
same
refractive
index
as
glass.
Avoid
excess
oil
because
it
may
attract
dirt
and
contaminate
the
sample
compartment
of
the
instrument.
Using
a
soft,
lint­
free
cloth,
spread
the
oil
uniformly
and
wipe
off
excess.
The
cell
should
appear
to
be
nearly
dry
with
little
or
no
visible
oil.

Because
small
differences
between
sample
cells
significantly
impact
measurement,
use
either
matched
pairs
of
cells
or
the
same
cell
for
both
standardization
and
sample
measurement.

3.
Reagents
a.
Dilution
water:
High­
purity
water
will
cause
some
light
scattering,
which
is
detected
by
nephelometers
as
turbidity.
To
obtain
low­
turbidity
water
for
dilutions,
nominal
value
0.02
NTU,
pass
laboratory
reagent­
grade
water
through
a
filter
with
pore
size
sufficiently
small
to
remove
essentially
all
particles
larger
than
0.1
µ
m
 
the
usual
membrane
filter
used
for
bacteriological
examinations
is
not
satisfactory.
Rinse
collecting
flask
at
least
twice
with
filtered
water
and
discard
the
next
200
mL.

Some
commercial
bottled
demineralized
waters
have
a
low
turbidity.
These
may
be
used
when
filtration
is
impractical
or
a
good
grade
of
water
is
not
available
to
filter
in
the
laboratory.
Check
turbidity
of
bottled
water
to
make
sure
it
is
lower
than
the
level
that
can
be
achieved
in
the
laboratory.
b.
Stock
primary
standard
formazin
suspension:

1)
Solution
I
­
Dissolve
1.000
g
hydrazine
sulfate,
(
NH2)
2CH2SO4
in
distilled
water
and
dilute
to
100
mL
in
a
volumetric
flask.
CAUTION:
Hydrazine
sulfate
is
a
carcinogen;
avoid
inhalation,
ingestion,
and
skin
contact.
Formazin
suspensions
can
contain
residual
hydrazine
sulfate.
4
Draft
June
2003
2)
Solution
II
­
Dissolve
10.00
g
hexamethylenetetramine,
(
CH2
)
6
N4,
in
distilled
water
and
dilute
to
100
mL
in
a
volumetric
flask.

3)
In
a
flask,
mix
5.0
mL
Solution
I
and
5.0
mL
Solution
II.
Let
stand
for
24
h
at
25
±
3
°
C.
This
results
in
a
4000­
NTU
suspension.
Transfer
stock
suspension
to
an
amber
glass
or
other
UV­
light­
blocking
bottle
for
storage.
Make
dilutions
from
this
stock
suspension.
The
stock
suspension
is
stable
for
up
to
1
year
when
properly
stored.

c.
Dilute
turbidity
suspensions:
Dilute
4000
NTU
primary
standard
suspension
with
highquality
dilution
water.
Prepare
immediately
before
use
and
discard
after
use.

d.
Secondary
standards:
Secondary
standards
are
standards
that
the
manufacturer
(
or
an
independent
testing
organization)
has
certified
will
give
instrument
calibration
results
equivalent
(
within
certain
limits)
to
the
results
obtained
when
the
instrument
is
calibrated
with
the
primary
standard,
i.
e.,
user­
prepared
formazin.
Various
secondary
standards
are
available
including:
commercial
stock
suspensions
of
4000
NTU
formazin,
commercial
suspensions
of
microspheres
of
styrene­
divinylbenzene
copolymer,*
and
items
supplied
by
instrument
manufacturers,
such
as
sealed
sample
cells
filled
with
latex
suspension
or
with
metal
oxide
particles
in
a
polymer
gel.
The
U.
S.
Environmental
Protection
Agency1
designates
user­
prepared
formazin,
commercial
stock
formazin
suspensions,
and
commercial
styrene­
divinylbenzene
suspensions
as
"
primary
standards,"
and
reserves
the
term
  
secondary
standard''
for
the
sealed
standards
mentioned
above.

Secondary
standards
made
with
suspensions
of
microspheres
of
styrene­
divinylbenzene
copolymer
typically
are
as
stable
as
concentrated
formazin
and
are
much
more
stable
than
diluted
formazin.
These
suspensions
can
be
instrument­
specific;
therefore,
use
only
suspensions
formulated
for
the
type
of
nephelometer
being
used.
Secondary
standards
provided
by
the
instrument
manufacturer
(
sometimes
called
"
permanent"
standards)
may
be
necessary
to
standardize
some
instruments
before
each
reading
and
in
other
instruments
only
as
a
calibration
check
to
determine
when
calibration
with
the
primary
standard
is
necessary.

All
secondary
standards,
even
so­
called
"
permanent"
standards,
change
with
time.
Replace
them
when
their
age
exceeds
the
shelf
life.
Deterioration
can
be
detected
by
measuring
the
turbidity
of
the
standard
after
calibrating
the
instrument
with
a
fresh
formazin
or
microsphere
suspension.
If
there
is
any
doubt
about
the
integrity
or
turbidity
value
of
any
secondary
standard,
check
instrument
calibration
first
with
another
secondary
standard
and
then,
if
necessary,
with
user­
prepared
formazin.
Most
secondary
standards
have
been
carefully
prepared
by
their
manufacturer
and
should,
if
properly
used,
give
good
agreement
with
formazin.
Prepare
formazin
primary
standard
only
as
a
last
resort.
Proper
application
of
secondary
standards
is
specific
for
each
make
and
model
of
nephelometer.
Not
all
secondary
standards
have
to
be
discarded
when
comparison
with
a
primary
standard
shows
that
their
turbidity
value
has
changed.
In
some
cases,
the
secondary
standard
should
be
simply
relabeled
with
the
new
turbidity
value.
Always
follow
the
manufacturer's
directions.
5
Draft
June
2003
4.
Procedure
a.
General
measurement
techniques:
Proper
measurement
techniques
are
important
in
minimizing
the
effects
of
instrument
variables
as
well
as
stray
light
and
air
bubbles.
Regardless
of
the
instrument
used,
the
measurement
will
be
more
accurate,
precise,
and
repeatable
if
close
attention
is
paid
to
proper
measurement
techniques.

Measure
turbidity
immediately
to
prevent
temperature
changes
and
particle
flocculation
and
sedimentation
from
changing
sample
characteristics.
If
flocculation
is
apparent,
break
up
aggregates
by
agitation.
Avoid
dilution
whenever
possible.
Particles
suspended
in
the
original
sample
may
dissolve
or
otherwise
change
characteristics
when
the
temperature
changes
or
when
the
sample
is
diluted.

Remove
air
or
other
entrained
gases
in
the
sample
before
measurement.
Preferably
degas
even
if
no
bubbles
are
visible.
Degas
by
applying
a
partial
vacuum,
adding
a
nonfoamingtype
surfactant,
using
an
ultrasonic
bath,
or
applying
heat.
In
some
cases,
two
or
more
of
these
techniques
may
be
combined
for
more
effective
bubble
removal.
For
example,
it
may
be
necessary
to
combine
addition
of
a
surfactant
with
use
of
an
ultrasonic
bath
for
some
severe
conditions.
Any
of
these
techniques,
if
misapplied,
can
alter
sample
turbidity;
use
with
care.
If
degassing
cannot
be
applied,
bubble
formation
will
be
minimized
if
the
samples
are
maintained
at
the
temperature
and
pressure
of
the
water
before
sampling.

Do
not
remove
air
bubbles
by
letting
sample
stand
for
a
period
of
time
because
during
standing,
turbidity­
causing
particulates
may
settle
and
sample
temperature
may
change.
Both
of
these
conditions
alter
sample
turbidity,
resulting
in
a
nonrepresentative
measurement.

Condensation
may
occur
on
the
outside
surface
of
a
sample
cell
when
a
cold
sample
is
being
measured
in
a
warm,
humid
environment.
This
interferes
with
turbidity
measurement.
Remove
all
moisture
from
the
outside
of
the
sample
cell
before
placing
the
cell
in
the
instrument.
If
fogging
recurs,
let
sample
warm
slightly
by
letting
it
stand
at
room
temperature
or
by
partially
immersing
it
in
a
warm
water
bath
for
a
short
time.
Make
sure
samples
are
again
well
mixed.

b.
Nephelometer
calibration:
Follow
the
manufacturer's
operating
instructions.
Run
at
least
one
standard
in
each
instrument
range
to
be
used.
Make
certain
the
nephelometer
gives
stable
readings
in
all
sensitivity
ranges
used.
Follow
techniques
outlined
in
¶
s
2b
and
4a
for
care
and
handling
of
sample
cells,
degassing,
and
dealing
with
condensation.

c.
Measurement
of
turbidity:
Gently
agitate
sample.
Wait
until
air
bubbles
disappear
and
pour
sample
into
cell.
When
possible,
pour
well­
mixed
sample
into
cell
and
immerse
it
in
an
ultrasonic
bath
for
1
to
2
s
or
apply
vacuum
degassing,
causing
complete
bubble
release.
Read
turbidity
directly
from
instrument
display.

d.
Calibration
of
continuous
turbidity
monitors:
Calibrate
continuous
turbidity
monitors
for
low
turbidities
by
determining
turbidity
of
the
water
flowing
out
of
them,
using
a
laboratory­
model
nephelometer,
or
calibrate
the
instruments
according
to
manufacturer's
instructions
with
formazin
primary
standard
or
appropriate
secondary
standard.
6
Draft
June
2003
5.
Interpretation
of
Results
Report
turbidity
readings
as
follows:

Turbidity
Range
Report
to
the
NTU
Nearest
NTU
0­
1.0
0.05
1­
10
0.1
10­
40
1
40­
100
5
100­
400
10
400­
1000
50
>
1000
100
When
comparing
water
treatment
efficiencies,
do
not
estimate
turbidity
more
closely
than
specified
above.
Uncertainties
and
discrepancies
in
turbidity
measurements
make
it
unlikely
that
results
can
be
duplicated
to
greater
precision
than
specified.

6.
Reference
1.
U.
S.
Environmental
Protection
Agency.
1993.
Methods
for
Determination
of
Inorganic
Substances
in
Environmental
Samples.
EPA­
600/
R/
93/
100
­
Draft.
Environmental
Monitoring
Systems
Lab.,
Cincinnati,
Ohio.

7.
Bibliography
Hach,
C.
C.,
R.
D.
Vanous[
nm
&
J.
M.
[
smHeer[
nm.
1985.
Understanding
Turbidity
Measurement.
Hach
Co.,
Technical
Information
Ser.,
Booklet
11,
Loveland,
Colo.

Katz,
E.
L.
1986.
The
stability
of
turbidity
in
raw
water
and
its
relationship
to
chlorine
demand.
J.
Amer.
Water
Works
Assoc.
78:
72.

McCoy,
W.
F.
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B.
H.
Olson.
1986.
Relationship
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turbidity,
particle
counts
and
bacteriological
quality
within
water
distribution
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Water
Res.
20:
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K.
E.,
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A.
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1991.
Penetration
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municipal
drinking
water
filters
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1013.

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A.
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Crandal.
1991.
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for
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V.
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D.
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1992.
An
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level
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W.
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