5
NITRATE MEASUREMENT
I. MEASUREMENT PREPARATION
The following steps should be taken prior to performing a measurement with your PT6:
1.
Perform a
calibration check with calibration solution, and if the reading is off more than 10%, calibrate the PT6. (See
CALIBRATION).
2.
Prepare test solution:
a.
Fill the Graduated Cylinder to the 10 mL line with solution to be tested.
b.
Add 200
µ
L of the Nitrate ISA Solution to the Graduated Cylinder (using the supplied 100
µ
L pipette).
NOTE
: When using the pipette to draw, depress pipette to 1
st
stop, then place tip in solution and slowly release. To
dispense, depress pipette completely to the 2
nd
stop.
NOTE
: Do not contaminate the pipette tip by touching it with your fingers or setting it in a dirty toolbox. Do not dip the
pipette tip in the calibration solution as this will cause cross-contamination between solutions.
c.
Replace the cap on the Graduated Cylinder and shake well to mix the prepared test solution.
II. NITRATE MEASUREMENT
For highest accuracy, the temperature of the calibration solution(s) should be the same as the temperature of the test
solution(s). The temperature difference should be no more than ±2
o
C/±3.6
o
F.
1.
Remove the protective cap. Rinse the sensor for about 5 seconds with clean water (preferably DI, RO, or distilled) and pat it
dry with a clean soft lint-free cloth.
2.
Press and release the push button to turn the PT6 ON.
3.
Dip the PT6 sensor in the prepared test solution and swirl it around to remove any air bubbles.
4.
Hold the PT6 sensor steady in the solution while the measurement is taking place.
The following table explains what the LED Indicator Light signals indicate and gives the duration of each signal:
LED SIGNAL
ACTION
DURATION
Rapid Flashing
Dip sensor in solution and swirl to remove bubbles.
6 sec
Slow Flashing
Measurement in process; hold the sensor steady during measurement.
In
HOLD
mode real-time readings are displayed until the LED is ON solid.
In
LIVE
mode real-time readings are displayed until the PT6 turns OFF.
90 sec in
HOLD
mode
Up to 300 sec in
LIVE
mode
Solid Light (
HOLD
mode only)
Measurement is complete. Values are displayed until the PT6 turns OFF.
6 sec
5.
Remove the sensor from the solution and rinse with clean water (preferably DI, RO, or distilled) and pat it dry with a clean
soft lint-free cloth. Use caution not to touch the ISE membrane.
6.
Fill the protective cap half full with prepared calibration solution. Carefully put the cap back on the sensor for conditioning
(using caution as excess solution may squirt out).
NOTE
: You can turn OFF the unit at any time during LIVE measurement with a simple press and release of the push button.
•
For best results when measuring solution at the extremes of the specified temperature or nitrate range allow the PT6 to
equilibrate by submerging the sensor in the sample solution for 1 minute prior to taking a measurement.
•
ALWAYS
make sure the nitrate sensor is completely submerged in the solution being tested.
•
For multiple samples, regularly perform a calibration check with calibration solution between sample measurements.
III. INTERFERENCE
Other anions in solution may interfere with nitrate measurement, depending on the species and their relative levels to nitrate. The
impact of interference ions on measurements can be expressed by the Nikolsky equation.
𝐸
=
𝐶𝑜𝑛𝑠𝑡𝑎𝑛𝑡
+
!
.
!"!
!"
!
!
!
log
(
𝑎
+
𝐾
!
𝑎
!
!
!
!
!
)
Where
𝐸
- the voltage sensor measures
𝑎
- the activity of nitrate
𝑎
!
- the activity of interference ion
𝑧
!
- the charge of the interference ion with sign
𝐾
!
–the Selectivity coefficient of the interference ion for nitrate measurement
Generally, interference ions may cause positive errors for the measurement. The higher selectivity coefficient the interference ion
has, the higher error it may cause at a given concentration. The approximate selectivity coefficients of PT6 for some anions are
SO
4
2-
Cl
-
CH
3
COO
-
K(NO
3
-
/x)
0.00016 0.007 0.0007
