Hach DR/700 Procedures Manual page 734

PHOSPHORUS, TOTAL,
SAMPLING AND STORAGE
Most reliable results are obtained when samples are analyzed
immediately. If prompt analysis is not possible, samples may be
preserved up to 24 hours by cooling to 4 °C (39 °F). Warm to room
temperature before testing.
INTERFERENCES
For turbid samples, use 50 mL of sample and double the reagent
quantities. Use 25 mL of the 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 use additional acid in Step 3 to drop the
pH of the solution below 1.
SUMMARY OF METHOD
Phosphates present in organic and condensed inorganic forms (meta-,
pyro-, or other polyphosphates) must be converted to orthophosphate
before analysis. Pretreatment of the sample with acid and heat provides
the conditions for hydrolysis of the condensed inorganic forms. Organic
phosphates are converted to orthophosphate by heating with acid and
persulfate. Organically bound phosphates are thus determined indirectly
by subtracting the result of an acid hydrolyzable phosphorus test from
the total phosphorus result.
This procedure must be followed by one of the reactive phosphorus
(orthophosphate) analysis methods for determination of the phosphorus
content of the sample. If the ascorbic acid (PhosVer 3) method is used to
measure the reactive phosphorus, this method is EPA accepted for
NPDES reporting.
The following reagents and apparatus are required beside those required
for the reactive phosphorus test.
REQUIRED REAGENTS
Description
Potassium Persulfate
Powder Pillows. . . . . . . . . . 1 pillow . . . . 50/pkg . . . . . . . . 2451-66
Sodium Hydroxide
Solution, 5.0 N . . . . . . . . . . 2 mL. . . . . . . 100 mL*MDB . . . 2450-32
continued
Quantity
Per Test Unit
81-60
Cat. No.