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Molybdenum, Molybdate, HR,
continued
(positive interference). For these samples, multiply the value obtained in the final
step of the procedure 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 sample pretreatment. Adjust the sample pH to between
3 and 5 by using a pH meter or pH paper and add, drop-wise, an appropriate
amount of acid or base such as 1.0 N Sulfuric Acid Standard Solution or 1.0 N
Sodium Hydroxide Solution. If significant volumes of acid or base are used, a
volume correction should be made. Divide the total volume (sample + acid + base)
by the original volume and multiply the test result by this factor.
After several samples have been analyzed, the sample cells may exhibit a slightly
blue buildup. Eliminate buildup with a rinse of Hydrochloric Acid Solution, 1:1.
Summary of Method
Molybdate is determined with the ternary complex method by the reaction of
molybdate molybdenum with an indicator and a sensitizing agent yielding a stable
2–
blue complex. While molybdate (MoO
) is the actual chemical species involved
4
in the chemistry, the instrument displays the result as molybdenum, Mo.
1—49
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