Slope Check; Standard Additions Check - Hach ISENH4181 User Manual

Problem
Sample properties cause slow
stabilization or inaccurate
measurements.
Procedure problem causes slow
stabilization and prevents
accurate calibrations or
measurements.

9.1 Slope check

Use the mV value of two standard solutions to make sure the probe gives the correct slope.
1. Prepare two standard solutions that are ten times apart in concentration (e.g., 10 mg/L and
100 mg/L NH —N). Select standard solutions with a concentration above and below the typical
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sample concentration. Use a minimum concentration of 1.8 mg/L.
2. Use the measurement procedure to add the ISA and measure the mV value of each standard
solution.
3. Calculate the difference in the mV value of the two standard solutions to find the slope. If the
probe is in good condition, the slope will be 58 mV (within the ± slope limits of the method) at
25 °C (77 °F).

9.2 Standard additions check

To make sure that the sample measurement is accurate, add a small volume of a standard solution
to the sample and calculate the percent recovery. The sample with the known volume of standard
solution is known as a spiked sample.
1. Use the measurement procedure to measure the concentration of a 25-mL sample.
2. Use a pipet to add the applicable volume of standard solution to the sample. Refer to
Table 2 Troubleshooting information (continued)
Possible cause
The sample pH with ISA is
incorrect.
The sample temperature is
low, or there is a large
temperature difference
between samples.
Air bubbles are around or
below the probe tip.
The ISA was not added.
The stir speed is too slow or
too fast.
Magnetic stirrers can
become warm and increase
the solution temperature.
An incorrect standard
solution was used or the
standard solution has
contamination.
Too much time passed after
the ISA was added.
Solution
Make sure that the sample pH
after the ISA is added is less
than pH 8.5. Make sure to add
one ISA pillow to each 25 mL of
sample.
Increase the sample temperature
or adjust the temperature of
different samples to be the same
(within 2 °C (3.6 °F)).
Carefully tap or shake the probe
to remove air bubbles.
Add one ISA powder pillow to
each 25 mL of sample and
standard solution.
Try a different stir speed.
Put a piece of insulating material
between the stirrer and beaker.
Use the specified standard
solution of good quality.
Measure the sample or standard
solutions immediately after the
ISA is added.
Table 3.
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