Hach IntelliCAL ISENH3181 Series, User Manual

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Table 1 Troubleshooting information (continued)
Problem Possible cause Solution
Procedure problem causes
slow stabilization and
prevents accurate
calibrations or
measurements.
Air bubbles are around or
below the probe tip. Carefully tap or shake the probe to
remove air bubbles.
The ISA was not added. Add one ISA powder pillow to each
25 mL of sample and standard
solution.
The stir speed is too slow or
too fast. Try a different stir speed.
Magnetic stirrers can
become warm and increase
the solution temperature. Put a piece of insulating material
between the stirrer and beaker.
An incorrect standard
solution was used or the
standard solution has
contamination. Use the specified standard solution of
good quality.
Too much ammonia gas
goes into the air after the
ISA is added. Measure the sample or standard
solutions immediately after the ISA is
added to prevent the loss of ammonia
gas. Put a cover on the beaker with a
hole for the probe to decrease the
loss of ammonia.
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
3
—N). Select standard solutions with a concentration above and below the typical
sample concentration. Use a minimum concentration of 0.5 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 57 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 .
Table 2 Standard solution volumes and concentrations
Measured sample
concentration
Volume of standard to
add
Concentration of standard solution
1 to 2 mg/L 0.5 mL 100 mg/L NH
3
—N
3 to 6 mg/L 1.0 mL 100 mg/L NH
3
—N
7 to 15 mg/L 0.3 mL 1000 mg/L NH
3
—N
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