Question

How would you analyze a solution known to contain Pb2+ and As3+?

Answer #1

we can perform the qualitative test to know whether these metals are present or not:

both of these metals react with the H_{2}S to give black
and yellow precipitate.

Pb^{2+} + H_{2}S ---------> PbS
(ppt) + 2H^{+} (blacK ppt)

2As^{3+} + 3H_{2}S ------->
As_{2}S_{3}(ppt) +6 H^{+} Yellow ppt)

confirmation of lead :-

Pb^{2+ }+ 2KI ------------>
PbI_{2}(yellow ppt) +2K^{+}

confirmation of arsenic :-

As_{2}S_{3} + 10 HNO_{3}
------------->2H_{3}AsO_{4} + 10
NO_{2} + 3S(soluble) +2H_{2}O

H_{3}AsO_{4} + 12
(NH_{4})_{2}MoO_{4} + 21
HNO_{3}
----------------->(NH_{4})_{3}AsO_{4}.12MoO_{3}(yellow
ppt) +21 NH_{4}NO_{3} +12
H_{2}O

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You have to test a solution with one to five possible
representative cations: Pb2+, Fe3+, Al3+, Ca2+, Cu2+. You raise the
pH of the solution to 9.5 with ammonia solution, and your unknown
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0.015 mg/L
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