Hach DR/700 Procedures Manual page 148

PHOSPHORUS, REACTIVE,
continued
SAMPLING AND STORAGE
Collect samples in clean plastic or glass bottles that have been cleaned
with 1:1 Hydrochloric Acid Solution and rinsed with demineralized
water. Do not use a commercial detergent because the phosphate
content will contaminate the sample.
If samples cannot be analyzed the same day, adjust the pH to 2 or less by
adding about 2 mL of sulfuric acid, ACS, per liter of sample. Store the
sample at 4 °C (39 °F) or below. Samples can be stored up to 24 hours.
For longer storage periods, add 4.0 mL of Mercuric Chloride Solution
for each liter of sample taken and mix. Use of mercuric chloride is
discouraged to minimize the amount of mercury released to the
environment. Sample refrigeration is still required. Sample preserved
with mercuric chloride must be spiked with 0.1 g sodium chloride per
liter of sample, to bring the sodium chloride level to 50 mg/L or more if
the sample is low in chloride. The addition of chloride prevents mercury
interference in the test.
Before analysis, adjust the acidified sample to about pH 7 by adding 5 N
Sodium Hydroxide Standard Solution. Mix thoroughly. Warm to room
temperature before analyzing.
ACCURACY CHECK
Standard Additions Method
a) Snap the neck off a Phosphate Voluette Ampule Standard Solution,
3-
500 mg/L as PO .
4
b) Use the TenSette Pipet to add 0.1 mL, 0.2 mL and 0.3 mL of standard
respectively to three 25-mL water samples. Mix well.
c) Analyze each sample as described in the procedure and compare the
results with that of the original test sample. Each 0.1-mL addition of
3-
standard should cause an increase of 2.0 mg/L PO or 0.67 mg/L P.
4
d) If these increases do not occur, see Standard Additions (Section 1) for
more information.
Standard Solution Method
A 10.0 mg/L phosphate standard can be prepared by pipetting 10.0 mL
3-
of a Phosphate Standard Solution, 50 mg/L PO , into a 50-mL
4
volumetric flask. Dilute to volume with demineralized water.
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