Hanna Instruments HI 4011 - Manual (Page 13)

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 I
-
)
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: Iodide ion determination in samples with con-
centrations less than 5 X 10
-4
M using known addition.
1. A 50 mL sample of unknown (Vsample) is placed in
a clean plastic beaker with a iodide sensor. 2 mL of
HI 4000-00 ISA (V
ISA
) is added to the 50 mL sample
and allowed to mix. The stable mV value (mV 1) is
recorded.
2. 10 mL (Vstd) of 10 -2 M (Cstd) standard is added to the
beaker and the mV value decreases. The unknown
Iodide 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’