13-10. Hg 2+ can be measured in the presence of many other metal ions by selecti

Question

13-10. Hg 2+ can be measured in the presence of many other metal ions by selective masking with I-7. A 100- mL unknown solution containing Hg2+ , Ca2+ , Al3+, Mg2+, Co2+ , Ni2+, Cu 2+ , Zn2+ , Cd2+ , Pb2+ , Ba2+ , Cr3+ , Fe3+ , and Bi3+    was adjusted to pH 5.5 with acetate buffer and treated with more than enough EDTA (10.00 mL of 0.0400 M EDTA ) to bind all metal ions. Xylenol orange indicator was added, and the excess EDTA rewuired 5.05 mL of 0.0262 M ZnSO4 for back titration of EDTA. This procedure measures all metal ions in the unknown. To measure Hg2+, the solution at the end point of the titration was treated with 100 mg of solid KI to form HgI42- and liberate EDTA from Hg2+ , but not form any other metal. Liberated EDTA required an additional 5.81 mL of 0.0262M ZnSO4 to reach the end point.

a) From the first titration, find the total millimoles of metal ions in the unknown

b) From the second titration find mmol Hg2+ in the unknown.

c)From Table 13-2, determine what color you would expect to observe before Zn2+ is added in the first titration? At the first end point? After adding excess KI? At the second end point? (The observed end point is dark orange.)

Explanation / Answer

From the experiment.

a) total moles of all the metal ions in solution = 0.04 M x 10 ml - 0.0262 M x 5.05 ml = 0.26769 mmol

b) moles of Hg2+ in unknown = 0.0262 M x 5.81 ml = 0.152222 mmol

c) Color after all metals reacted with EDTA = Dark brown or black

After ZnSO4 reaction = reddish-black

after adding excess KI = dark yellow

second end point = pale yellow

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