Hanna Instruments HI 4003 - Manual (Page 13)

13
This technique is called Known Addition. The method can
use an ideal sensor slope, but actual determined slopes at
the temperature of measurement should be used if known.
The volume and concentration of the added standard must
cause a mV change of at least 8 mV. This method is
preprogrammed in the Hanna HI 4222 pH/ISE/mV meter,
which simplifies the method greatly. The method works
well for samples with high ionic strengths.
Example: Cadmium ion determination in samples with
concentrations less than 1 X 10
-3
M using known addition.
1. A 50 mL sample of unknown concentration (Vsample)
is placed in a clean plastic beaker with a cadmium
sensor with 1 mL of HI 4000-00 ISA (V
ISA=1 mL
) a. The
stable mV value (mV 1) is recorded after the sample
is mixed.
2. 5mL (Vstd) of 10 -1 M standard (Cstd) is added to the
beaker and the mV value increases as does the con-
centration.
∆
E is calculated as mV2-mV1. The un-
known cadmium concentration in the original sample
(Csample) can then be determined by the following
equation.
C
sample
=
(V
T
)10
∆E/S
- (V
S’
)
C
standard
V
standard
V
sample
V
S’
3. The procedure can be repeated with a second stan-
dard addition to verify slope and operation of the
method.
Titration of cadmium
A cadmium electrode may be used as an indicator to follow
the progress and detect the endpoint of a complexation
titration of cadmium
ions with EDTA standard. During the
titration the sensor follows the decreasing cadmium con-
centration while small additions of EDTA titrant are added.
The cadmium ion reacts with the EDTA and removes
(V
sample
+V
standard
+V
ISA
)= V
T
(V
sample
+V
ISA
)= V
S’