Hach DR2400 - Page 691

Potassium
Potassium
Potassium_PP_Other_TPB_Eng_Ody.fm Page 3 of 6
Sample Collection, Preservation, and Storage
Collect samples in acid-washed plastic bottles. Adjust the pH to 2 or less with
Nitric Acid (about 2 mL per liter) (Cat. No. 2540-49). Preserved samples may be
stored at least six months at room temperature. Before analysis, adjust the pH to
4–5 with 5.0 N Sodium Hydroxide (Cat. No. 2450-26). Do not measure pH in the
sample container with a pH electrode, as this will introduce potassium from the
filling solution. Use pH Paper (Cat. No. 391-33) or pour off sample and test pH in
a separate beaker. Correct the test result for volume additions; see Section 3.1.3
Correcting for Volume Additions on page 29.
Accuracy Check
Standard Additions Method (Sample Spike)
1. After reading test results, leave the sample cell (unspiked sample) in the
instrument.
2. Touch
Options. Touch Standard Additions. A summary of the standard
additions procedure will appear.
3. Touch
OK to accept the default values for standard concentration, sample
volume, and spike volumes. Touch
Edit to change these values. After values
are accepted, the unspiked sample reading will appear in the top row. See
Standard Additions in the DR/2400 instrument manual for more information.
4. Snap the neck off a Potassium Voluette
®
Ampule Standard, 250-mg/L K.
5. Prepare three sample spikes. Fill three mixing cylinders (Cat. No. 1896-40)
with 25 mL of sample. Use the TenSette
®
Pipet to add 0.1 mL, 0.2 mL, and
0.3 mL of standard, respectively, and mix thoroughly.
6. Analyze each sample spike as described in the procedure above, starting with
the 0.1 mL sample spike. Accept each standard additions reading by touching
Read. Each addition should reflect approximately 100% recovery.
7. After completing the sequence, touch
Graph to view the best-fit line through
the standard additions data points, accounting for matrix interferences. Touch
View: Fit, then select Ideal Line and touch OK to view the relationship between
the sample spikes and the “Ideal Line” of 100% recovery.
See Section 3.2.2 Standard Additions on page 32 for more information.
Calibration
Standard Preparation
An approximate calibration curve is preprogrammed within Program 905. For
improved accuracy, a new calibration should be performed with each new lot of
reagents. Prepare calibration standards containing 1, 2, 3, 4, 5, 6, 7, and 8 mg/L
potassium as follows:
1. Into eight different 100-mL Class A volumetric flasks, pipet 1.0, 2.0, 3.0, 4.0,
5.0, 6.0, 7.0, and 8.0 mL of the 100-mg/L Potassium Standard Solution using
class A glassware or TenSette Pipet.
2. Dilute to the mark with deionized water. Mix thoroughly.
3. Use deionized water for the 0-mg/L potassium standard.