ULTRAPEN
X2
™ PTBT6 Operation Manual
Nitrate Concentration and Temperature Pen for Use with your Mobile Device Running the PTBTX2™ App
PTBT6OM-X2 01-20
7
IV. INTERFERENCE
Other anions in the sample solution may interfere with nitrate measurement, depending on the species and their relative
levels of nitrate. The impact of interference ions on measurements can be expressed by the Nikolsky Equation:
𝐸 = 𝐶𝑜𝑛𝑠𝑡𝑎𝑛𝑡 +
*.,-,./
0
1
2
log (𝑎
+ 𝐾
9
𝑎
9
:; 0
1
<
)
Where:
𝐸
- the voltage sensor measures
𝑎
- the activity of nitrate
𝑎
9
- the activity of interference ion
𝑧
9
- the charge of the interference ion with sign
𝐾
9
–the selectivity coefficient of the interference ion for nitrate measurement
TABLE 1
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
PTBT6 for some anions are listed above. For example, if the activity of Cl- is ten times higher than nitrate in the sample, Cl-
may cause 7% error for the nitrate measurement, while CH
3
COO
-
and SO
4
2-
in the same ratio may only cause 0.7% and
0.05% error, respectively.
When testing samples with high interference levels, such as wastewater, chlorinated water, and soil water, the interference
ions will be counted as nitrate with the coefficient factor and cause measurement errors. Some commonly existing
interference ions, such as Cl
-
, Br
-
, I
-
, S
2-
, HCO
3
-
and NO
2
-
can be reduced by using Nitrate Interference Suppression Buffer
(NISB). Add NISB at 1:1 volume ratio to all sample/standard solutions (e.g. add 5 mL NISB to 5 mL solution) in place of
Nitrate Ionic Strength Adjuster (NISA) throughout the sensor conditioning, calibration and measurement.
NOTES:
•
Nitrate Interference Suppression Buffer (NISB) must be purchased separately (see the ACCESSORIES section below).
•
When using NISBSOL, do NOT add Nitrate Ionic Strength Adjuster (NISA). White precipitate may form in sample solution
after mixing with NISBSOL, it will not affect the measurement.
•
Strong interference ions such as ClO
4
-
and SCN
-
require their concentrations to be up to 2000x lower than nitrate in the
sample to avoid high errors.
For best results when measuring solution at the extremes of the specified temperature or nitrate range allow the PTBT6 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 reference solution between sample measurements.
V. MEASUREMENT VALIDATION METHODS
CALIBRATION CHECK:
1.
Choose one standard solution within the testing range, and prepare the solution according to the measurement
procedure.
2.
Take a measurement using the readings in the reference solution.
•
The value should be within 10% of the expected value.
•
If not, the measurement result may not be reliable.
•
The source of error should be identified and the measurement should be redone after correction.
KNOWN ADDITION RECOVERY CHECK:
This validation method works well for validating low concentration measurements (1-60 ppm-N):
•
Using concentrated standard solutions to increase the nitrate concentration in the sample by a known value, optimally
doubles the measured value.
•
Comparing the value with the increase in PTBT6 readings.
o
For about 10 mL sample, one spike of standard solution with the 100 µL pipette will increase the sample nitrate
concentration by roughly 1% of the standard solution concentration.
o
Example
~1 ppm increase by one spike of 100 ppm standard solution, ~10 ppm increase by one spike of 1000 ppm
standard solution.
o
See Table 2 below for more examples.
SO
4
2-
Cl
-
CH
3
COO
-
K(NO
3
-
/x)
0.00016
0.007
0.0007
