Hach DR/700 Procedures Manual page 217

POTASSIUM, continued
used as Standard 1 or Standard 2. Complete the calibration within 7
minutes after the 3-minute period.
SAMPLING AND STORAGE
Collect samples in acid-washed plastic bottles. Adjust the pH to 2 or less
with nitric acid (about 2 mL per liter). Preserved samples may be stored at
least six months at room temperature. Before analysis, adjust the pH to 4
to 5 with 5.0 N sodium hydroxide. Do not measure pH in the sample
container with a pH electrode, as this will introduce potassium from the
filling solution. Use pH paper or pour off sample and test pH in a separate
beaker. Correct the test result for volume additions; see Sampling and
Storage, Volume Additions, (Section 1) for more information.
ACCURACY CHECK
Standard Addition Method
a) Snap the neck off a Potassium Voluette Ampule Standard Solution,
250 mg/L.
b) Use the TenSette Pipet to add 0.1, 0.2, and 0.3 mL of standard to three
25-mL samples. Mix each thoroughly.
c) Analyze each sample as described above. The potassium concentration
should increase 1.0 mg/L for each 0.1 mL of standard added.
d) If these increases do not occur, see Standard Additions (Section 1) for
more information.
INTERFERENCES
The following ions do not interfere below the concentration shown:
Ammonium Nitrogen
Calcium
Chloride
Magnesium
SUMMARY OF METHOD
Potassium in the sample combines with sodium tetraphenylborate, an
insoluble white solid. The amount of turbidity produced is proportional
to the potassium concentration.
15 mg/L as N
7000 mg/L as CaCO
15,000 mg/L
6000 mg/L as CaCO
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