Hanna Instruments HI 4105 - Manual (Page 9)

16 17
7. The procedure can be repeated with a second standard
addition to verify slope and operation of the method.
Note:
This method is preprogrammed in the Hanna HI
4222pH/ISE/mV meter, which simplifies the method
greatly and permits repeated determinations easily.
Example:
Carbon Dioxide determination with known addition:
1. A 50 mL sample of unknown (V
SAMPLE
) is placed in an
clean vessel with an electrode. 5 mL of ISA is added
to the sample . The sample is covered and permitted
to mix throughly and continueously. The mV is then
recorded when the sensor has stabilized.
2. 5 mL (V
STANDARD
) of 0.1 M (C
STANDARD
) standard is then
added to the vessel and is permitted to mix. The mV
value increases as the concentration increases. (Note:
for other concentration samples, add a known volume
and concentration of standard to produce a 30 mV
change or greater.
3. The unknown carbon dioxide concentration in the
original sample (C
SAMPLE
) can then be determined by
using the equation provided.
.
XIV XIV XIV XIV XIV. . . . . Other Measurement T Other Measurement T Other Measurement T Other Measurement T Other Measurement Techniques echniques echniques echniques echniques
Known Addition
An unknown concentration of carbon dioxide can be deter-
mined by adding a known amount (volume and concen-
tration) of carbon dioxide standard to a known volume of
the sample. This technique is extremely useful for carbon
dioxide because changes in the sensor calibration are cor-
rected for continuously because, the standard and sample
are measured within minutes of one another. The tech-
nique can use an ideal sensor slope, but actual slopes at
the temperature of measurement should be determined
and used if possible. This will improve accuracy.
1. The volume of the unknown sample (V
Sample
) is mea-
sured accurately and placed into the closed sample
vessel. The sensor is secured in the vessel and then the
vessel is placed on a stirrer.
2. ISA is added at 1 part per 10 parts sample.
3. When the measurement is stable the mV value is
noted.
4. A known amount, volume (V
Standard
) and concentration
(C
Standard
), of CO
2
standard is then added to the sample.
mV values are again noted when the measurement is
stable.
5. The mV change is then calculated (
∆
E).
6. Using the measured and calculated values, the
sample concentration (C
Sample
) can be determined.
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’