Hach DR/700 Procedures Manual page 594

MOLYBDENUM, MOLYBDATE, LR
continued
SAMPLING AND STORAGE
Collect samples in glass or plastic bottles.
ACCURACY CHECK
Standard Additions Method
a) Snap the neck off a Molybdenum Voluette Ampule Standard Solution,
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500 mg/L Mo .
b) Use the TenSette Pipet to add 0.1, 0.2 and 0.3 mL of standard,
respectively, to three 250-mL samples. Mix thoroughly.
c) Analyze 20 mL of each sample according to the above procedure.
The molybdenum concentration reading should increase by 0.2 mg/L for
each 0.1 mL addition of standard.
d) If these increases do not occur, see Standard Additions (Section 1) for
more information.
Standard Solution Method
To assure the accuracy of the test, use a 2-mg/L Molybdenum Standard
Solution by first pipetting 10 mL of a 10-mg/L Molybdenum Standard
Solution into a 50-mL graduated mixing cylinder. Next, dilute to a final
volume of 50 mL using demineralized water. Mix thoroughly. Analyze
20 mL of the standard according to the above procedure.
STATISTICAL EVALUATION
A single operator repetitively tested samples of two laboratory prepared
solutions, using one DR/700, matched sample cells and two
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representative lots of testing reagents. Testing 1.000 mg/L Mo
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concentration samples, the standard deviation was ±0.008 mg/L Mo .
Testing zero concentration samples, the limit of detection was
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0.035 mg/L Mo . The limit of detection was calculated as three times the
standard deviation when testing zero concentration samples (adapted from
Analytical Chemistry, 1980, 52, 2242-2249).
INTERFERENCES
Interference studies were conducted by preparing a molybdenum standard
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solution (2 mg/L Mo ) as well as a solution of the potential interfering
ion. When the standard solution concentration changed by ±5% with a
given ion concentration, the ion was considered an interference.
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