Hanna Instruments HI 4013 - Manual (Page 14)

14
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 Known addition Known addition Known addition Known addition
An unknown concentration can be determined by adding a
known volume and concentration of NO
3
- standard to the
sample. mV values are noted before and after the addition
of standard (
∆
E). An ideal sensor slope can be used in the
equation but actual determined slopes at the temperature
of measurement should be used if known (S). This method
is preprogrammed in the Hanna HI 4222 pH/ISE/mV meter,
which simplifies the method greatly.
Example:
Nitrate ion determination with known addition.
1. A 50 mL sample of unknown (V
SAMPLE
) is placed in a
clean plastic beaker with an electrode (s). Add 50 mL
of ISISA* to sample. Mix. mV 1 is recorded.
2. 5 mL (V
STANDARD
) of 10 -1 M (C
STANDARD
) standard is added
to the beaker and the mV value decreases. (Note: for
other concentration samples, add a known volume
and concentration of standard to produce approxi-
mately 30 mV change).
The unknown Nitrate concentration in the original
sample (C
SAMPLE
) can then be determined by using the
equation that follows.
3. The procedure can be repeated with second standard
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’
*Note: ISISA is Interferent suppressent ISA see Section IV.