Hach DR/850, Procedures Manual, Page: 356 / 460

356
Sampling and Storage
Collect samples in plastic or glass bottles that have been acid-
washed with 1:1 HCl and rinsed with deionized water. Do not use
detergents containing phosphates for cleaning glassware used in
this test.
Analyze samples immediately after collection for best results. If
prompt analysis is impossible, preserve samples up to 28 days by
adjusting the pH to 2 or less with concentrated sulfuric acid
(about 2 mL per liter) and storing at 4 °C. Warm to room
temperature before testing. Correct results for volume additions;
see
Volume Additions (Section 1) for more information.
Interferences
For turbid samples, use 50 mL of sample and double the reagent
quantities. Use digested sample to zero the instrument in the
reactive phosphorus procedure. This compensates for any color
or turbidity destroyed by this procedure. For alkaline or highly
buffered samples it may be necessary to add additional acid in
Step 3 to drop the pH of the solution below 1.
5.
Cool the sample to
room temperature.
6.
Add 2.0 mL of
5.0 N Sodium Hydroxide
Solution. Swirl to mix.
Note: Use the
1-mL calibrated
dropper provided.
7.
Pour the sample into
a 25-mL graduated
cylinder. Return the
volume to 25 mL.
Proceed with a reactive
phosphorus test of the
expected total
phosphorus
concentration range.
Note: Use deionized water
rinsings from the flask to
adjust the volume.
Note: Results of the
reactive phosphorus test at
this point will include the
organic phosphate plus the
orthophosphate and the
acid-hydrolyzable
(condensed) phosphate.
The organic phosphate
concentration is
determined by subtracting
results of an acid
hydrolyzable phosphorus
test from this result. Make
sure that both results are in
the same units before
taking the difference.
PHOSPHORUS, TOTAL,
continued