222
Phosphonate HEDP at concentrations up to 30 mg/L will increase
the apparent molybdenum concentration reading by
approximately 10% (positive interference). For these samples,
multiply the value obtained in step 12 by 0.9 to obtain the actual
molybdenum concentration. As the concentration of HEDP
increases above 30 mg/L, a decrease in the molybdenum
concentration reading occurs (negative interference).
Highly buffered samples or extreme sample pH may exceed the
buffering capacity of the reagent and require pretreatment.
Adjust the sample pH to 3-5 (use a pH meter or pH paper) by
adding drops of an acid or base such as 1.0 N Sulfuric Acid
Standard Solution, or 1.0 N Sodium Hydroxide Standard
Solution. If a significant volume of acid or base is used, correct
the result by dividing the total volume (sample + acid + base) by
the original volume and multiplying the test result by this factor.
Positive Interferences
Carbonate 1,325
Silica 600
Benzotriazole 210
Morpholine 6
1
Read molybdenum concentration immediately after the completion of the two-minute
reaction period.
Table 1 Negative Interferences (continued)
Ion Level above which it interferes (mg/L)
Table 2 No Interference
Ion Highest Concentration Tested (mg/L)
Zinc 400
Calcium 720
Magnesium 8,000
Manganese 1,600
Chlorine 7.5
PBTC (phosphonate) 500
Sulfate 12,800
Bisulfite 9,600
Nickel 250
MOLYBDENUM, MOLYBDATE, LR, continued
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