13
XII.
XII.
XII.
XII.
XII. Other Measurement T
Other Measurement T
Other Measurement T
Other Measurement T
Other Measurement Techniques
echniques
echniques
echniques
echniques
Known Addition (for Cl
-
)
An unknown concentration can be determined by adding a
known amount (volume and concentration) of measured
ion to a known volume of the sample. This technique is
called Known Addition. The method can use an ideal
sensor slope, but actual determined slopes at the tempera-
ture of measurement should be used if known. This method
is preprogrammed in the Hanna HI 4222 pH/ISE/mV meter,
which simplifies the method greatly.
Example: Chloride ion determination with known addition.
1. A 50 mL sample of unknown (Vsample) is placed in
a clean plastic beaker with a chloride sensor. Add 2
mL of acetate pH buffer or HI 4000-00 ISA (V
ISA)
OR if
chloride l electrode interferences are known to exist,
add 50 mL oxidizing reagent (V
ISA
). Mix well. and
record the stable mV value. (mV 1)
2. 5 mL (Vstd) of 10
-3
M (Cstd) standard is added to the
beaker and the mV value decreases. The unknown
chloride concentration in the original sample
(Csample) can then be determined by the following
equation.
3. The procedure can be repeated with a second stan-
dard addition to verify slope and operation of the
method.
C
sample
=
(
V
T
)10
∆
E/S
- (
V
S
’
)
C
standard
V
standard
V
sample
V
S
’
(
V
sample
+
V
standard
+
V
ISA
)=
V
T
(
V
sample
+
V
ISA
)=
V
S
’
