9040C
­
1
Revision
3
August
2002
METHOD
9040C
pH
ELECTROMETRIC
MEASUREMENT
1.0
SCOPE
AND
APPLICATION
1.1
Method
9040
is
used
to
measure
the
pH
of
aqueous
wastes
and
those
multiphase
wastes
where
the
aqueous
phase
constitutes
at
least
20%
of
the
total
volume
of
the
waste.

1.2
The
corrosivity
of
concentrated
acids
and
bases,
or
of
concentrated
acids
and
bases
mixed
with
inert
substances,
cannot
be
measured.
The
pH
measurement
requires
some
water
content.

2.0
SUMMARY
2.1
The
pH
of
the
sample
is
determined
electrometrically
using
either
a
glass
electrode
in
combination
with
a
reference
potential
or
a
combination
electrode.
The
measuring
device
is
calibrated
using
a
series
of
standard
solutions
of
known
pH.

3.0
INTERFERENCES
3.1
The
glass
electrode,
in
general,
is
not
subject
to
solution
interferences
from
color,
turbidity,
colloidal
matter,
oxidants,
reductants,
or
moderate
(<
0.1
molar
solution)
salinity.

3.2
Sodium
error
at
pH
levels
>
10
can
be
reduced
or
eliminated
by
using
a
low­
sodiumerror
electrode.

3.3
Coatings
of
oily
material
or
particulate
matter
can
impair
electrode
response.
These
coatings
can
usually
be
removed
by
gentle
wiping
or
detergent
washing,
followed
by
rinsing
with
distilled
water.
An
additional
treatment
with
hydrochloric
acid
(
1:
10)
may
be
necessary
to
remove
any
remaining
film.

3.4
Temperature
effects
on
the
electrometric
determination
of
pH
arise
from
two
sources.
The
first
is
caused
by
the
change
in
electrode
output
at
various
temperatures.
This
interference
should
be
controlled
with
instruments
having
temperature
compensation
or
by
calibrating
the
electrode­
instrument
system
at
the
temperature
of
the
samples.
The
second
source
of
temperature
effects
is
the
change
of
pH
due
to
changes
in
the
sample
as
the
temperature
changes.
This
error
is
sample­
dependent
and
cannot
be
controlled.
It
should,
therefore,
be
noted
by
reporting
both
the
pH
and
temperature
at
the
time
of
analysis.

4.0
APPARATUS
AND
MATERIALS
4.1
pH
meter:
Laboratory
or
field
model.
Many
instruments
are
commercially
available
with
various
specifications
and
optional
equipment.

4.2
Glass
electrode.

4.3
Reference
electrode:
A
silver­
silver
chloride
or
other
reference
electrode
of
constant
potential
may
be
used.
9040C
­
2
Revision
3
August
2002
NOTE:
Combination
electrodes
incorporating
both
measuring
and
referenced
functions
are
convenient
to
use
and
are
available
with
solid,
gel­
type
filling
materials
that
require
minimal
maintenance.

4.4
Magnetic
stirrer
and
Teflon­
coated
stirring
bar.

4.5
Thermometer
and/
or
temperature
sensor
for
automatic
compensation.

5.0
REAGENTS
5.1
Reagent
grade
chemicals
shall
be
used
in
all
tests.
Unless
otherwise
indicated,
it
is
intended
that
all
reagents
shall
conform
to
the
specifications
of
the
Committee
on
Analytical
Reagents
of
the
American
Chemical
Society,
where
such
specifications
are
available.
Other
grades
may
be
used,
provided
it
is
first
ascertained
that
the
reagent
is
of
sufficiently
high
purity
to
permit
its
use
without
lessening
the
accuracy
of
the
determination.

5.2
Primary
standard
buffer
salts
are
available
from
the
National
Institute
of
Standards
and
Technology
(
NIST)
and
should
be
used
in
situations
where
extreme
accuracy
is
necessary.
Preparation
of
reference
solutions
from
these
salts
requires
some
special
precautions
and
handling,
such
as
low­
conductivity
dilution
water,
drying
ovens,
and
carbon­
dioxide­
free
purge
gas.
These
solutions
should
be
replaced
at
least
once
each
month.

5.3
Secondary
standard
buffers
may
be
prepared
from
NIST
salts
or
purchased
as
solutions
from
commercial
vendors.
These
commercially
available
solutions
have
been
validated
by
comparison
with
NIST
standards
and
are
recommended
for
routine
use.

6.0
SAMPLE
COLLECTION,
PRESERVATION,
AND
HANDLING
Samples
should
be
analyzed
as
soon
as
possible.

7.0
PROCEDURE
7.1
Calibration
7.1.1
Because
of
the
wide
variety
of
pH
meters
and
accessories,
detailed
operating
procedures
cannot
be
incorporated
into
this
method.
Each
analyst
must
be
acquainted
with
the
operation
of
each
system
and
familiar
with
all
instrument
functions.
Special
attention
to
care
of
the
electrodes
is
recommended.

7.1.2
Each
instrument/
electrode
system
must
be
calibrated
at
a
minimum
of
two
points
that
bracket
the
expected
pH
of
the
samples
and
are
approximately
three
pH
units
or
more
apart.
(
For
corrosivity
characteri­
zation,
the
calibration
of
the
pH
meter
should
include
a
buffer
of
pH
2
for
acidic
wastes
and
a
pH
12
buffer
for
caustic
wastes;
also,
for
corrosivity
characterization,
the
sample
must
be
measured
at
25
±
1
E
C
if
the
pH
of
the
waste
is
above
12.0.)
Various
instrument
designs
may
involve
use
of
a
dial
(
to
"
balance"
or
"
standardize")
or
a
slope
adjustment,
as
outlined
in
the
manufacturer's
instructions.
Repeat
adjustments
on
successive
portions
of
the
two
buffer
solutions
until
readings
are
within
0.05
pH
units
of
the
buffer
solution
value.
9040C
­
3
Revision
3
August
2002
7.2
Place
the
sample
or
buffer
solution
in
a
clean
glass
beaker
using
a
sufficient
volume
to
cover
the
sensing
elements
of
the
electrodes
and
to
give
adequate
clearance
for
the
magnetic
stirring
bar.
If
field
measurements
are
being
made,
the
electrodes
may
be
immersed
directly
into
the
sample
stream
to
an
adequate
depth
and
moved
in
a
manner
to
ensure
sufficient
sample
movement
across
the
electrode­
sensing
element
as
indicated
by
drift­
free
readings
(<
0.1
pH).

7.3
If
the
sample
temperature
differs
by
more
than
2
E
C
from
the
buffer
solution,
the
measured
pH
values
must
be
corrected.
Instruments
are
equipped
with
automatic
or
manual
compensators
that
electronically
adjust
for
temperature
differences.
Refer
to
manufacturer's
instructions.

7.4
Thoroughly
rinse
and
gently
wipe
the
electrodes
prior
to
measuring
pH
of
samples.
Immerse
the
electrodes
into
the
sample
beaker
or
sample
stream
and
gently
stir
at
a
constant
rate
to
provide
homogeneity
and
suspension
of
solids.
Note
and
record
sample
pH
and
temperature.
Repeat
measurement
on
successive
aliquots
of
sample
until
values
differ
by
<
0.1
pH
units.
Two
or
three
volume
changes
are
usually
sufficient.

8.0
QUALITY
CONTROL
8.1
Refer
to
Chapter
One
for
the
appropriate
QC
protocols.

8.2
Electrodes
must
be
thoroughly
rinsed
between
samples.

9.0
METHOD
PERFORMANCE
9.1
Forty­
four
analysts
in
twenty
laboratories
analyzed
six
synthetic
water
samples
containing
exact
increments
of
hydrogen­
hydroxyl
ions,
with
the
following
results:

Accuracy
as
Standard
Deviation
Bias
Bias
pH
Units
pH
Units
%
pH
Units
3.5
0.10
­
0.29
­
0.01
3.5
0.11
­
0.00
7.1
0.20
+
1.01
+
0.07
7.2
0.18
­
0.03
­
0.002
8.0
0.13
­
0.12
­
0.01
8.0
0.12
+
0.16
+
0.01
10.0
REFERENCES
1.
National
Bureau
of
Standards,
Standard
Reference
Material
Catalog
1986­
87,
Special
Publication
260.
9040C
­
4
Revision
3
August
2002
METHOD
9040C
pH
ELECTROMETRIC
MEASUREMENT
