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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., 100 mg/L and
–
1000 mg/L Cl
). Select standard solutions with a concentration above and below the typical
sample concentration. Use a minimum concentration of 7 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 Standard solution volumes and concentrations
Measured sample
concentration
1 to 2 mg/L
3 to 6 mg/L
7 to 15 mg/L
15 to 30 mg/L
30 to 60 mg/L
3. Measure the concentration of the spiked sample.
4. Calculate the expected (theoretical) concentration of the spiked sample:
C
= (C
x V
/V
) + (C
E
S
S
T
Where:
• C
= expected (theoretical) concentration of the spiked sample
E
• C
= concentration of the sample (mg/L) before the standard solution was added
S
• C
= concentration of the standard solution (mg/L)
SS
• V
= sample volume (mL) before the standard solution was added
S
• V
= volume of the standard solution (mL)
SS
• V
= total volume (standard solution volume (mL) + sample volume)
T
5. Calculate the percent recovery of the standard addition. A percent recovery of 100 (±5)% is an
indication that the sample measurements are accurate.
Percent recovery = C
Where:
• C
= measured concentration of the sample after the addition of the standard solution
M
• C
= expected (theoretical) concentration of the sample after the addition of the standard
E
solution
Volume of standard to
x V
/V
)
SS
SS
T
/C
x 100
M
E
add
0.5 mL
1.0 mL
0.3 mL
0.5 mL
1.0 mL
Table
Concentration of standard
solution
–
100 mg/L Cl
–
100 mg/L Cl
–
1000 mg/L Cl
–
1000 mg/L Cl
–
1000 mg/L Cl
English 11
2.
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