Hanna Instruments HI 4008 - 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: Cupric ion determination in samples with con-
centrations 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 cupric 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 cupric 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 cupric
A cupric electrode may be used as an indicator to follow the
progress and detect the endpoint of a complexation titra-
tion of cupric
ions with EDTA standard. During the titration
the sensor follows the decreasing cupric concentration while
small additions of EDTA titrant are added. The Cupric ion
reacts with the EDTA and removes
(V
sample
+V
standard
+V
ISA
)= V
T
(V
sample
+V
ISA
)= V
S’