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 Br
-
)
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. The volume
and concentration of the added standard must cause a mV
change of at least 30 mV. This method is preprogrammed
in the Hanna HI 4222 pH/ISE/mV meter, which simplifies
the method greatly.
Example: Bromide ion determination with known addition.
1. A 50 mL sample of unknown (Vsample) is placed in
a clean plastic beaker with a bromide sensor. Add 2
mL of HI 4000-00 ISA (V
ISA
), mix well and record the
stable mV value (mV 1).
2. 5 mL (Vstd) of 10
-1
M (Cstd) standard is added to the
beaker and the mV value decreases. The unknown
bromide 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
’
