366
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, 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
To check accuracy, use a 1.0 mg/L Phosphate Standard Solution
listed under Optional Reagents. Or, prepare a 1.0-mg/L PO
4
3-
standard by pipetting 2 mL of solution from a Phosphate 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.
Method Performance
Precision
In a single laboratory, using a standard solution of 5.00 mg/L
PO
4
3-
and two lots of reagent with the instrument, a single
operator obtained a standard deviation of ±0.08 mg/L PO
4
3-
.
Estimated Detection Limit (EDL)
The EDL for program 82 is 0.07 mg/L PO
4
3-
. For more
information on derivation and use of Hach’s estimated detection
limit, see Section 1.
PHOSPHORUS, REACTIVE, continued
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