374
d) Analyze each sample as described in the procedure. Use
5.0 mL of the prepared standard additions for each test;
the concentration should increase as follows: 0.2 mg/L,
0.4 mg/L, and 0.6 mg/L PO
4
3-
, respectively.
e) If these increases do not occur, see Standard Additions in
Section 1 for more information.
Standard Solution Method
Obtain a 1.0 mg/L Phosphate Standard Solution listed under
Optional Reagents. Or, this can be prepared by pipetting 2 mL of
a Voluette Ampule Standard for Phosphate, 50 mg/L as PO
4
3-
, into
an acid washed Class A 100-mL volumetric flask. Dilute to the
mark with deionized water. Substitute this standard for the
sample and perform the procedure as described.
Interferences
The following may interfere when present in concentrations
exceeding those listed below:
Substance Interference Level and Treatment
Aluminum 200 mg/L
Arsenate Interferes at any level.
Chromium 100 mg/L
Copper 10 mg/L
Iron 100 mg/L
Nickel 300 mg/L
Silica 50 mg/L
Silicate 10 mg/L
Sulfide 9 mg/L. Sulfide interference may be removed by
oxidation with Bromine Water as follows:
1. Measure 25 mL of sample into a 50-mL beaker.
2. Swirling constantly, add Bromine Water drop-wise
until a permanent yellow color develops.
3. Swirling constantly, add Phenol Solution dropwise
until the yellow color just disappears. Proceed with
step 1.
PHOSPHORUS, ACID HYDROLYZABLE, continued
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