Iodometric Analysis of Copper in Copper Oxide (cont\'d) Remove all the samples f

Question

Iodometric Analysis of Copper in Copper Oxide (cont'd) Remove all the samples from the hotplate and cool, one at a time, in a tap water bath in the hood. After cooling, add 1:1 NH3 (mixture of one part ammonia to one part water) with a Pasteur pipet to the sample solutions until the permanent appearance of the deep blue copper ammine complex. There should be a faint smell of ammonia over the solution. Avoid adding excess NH3. Note. From this point on, tr he samples one at a time to the end he titration then repeat from this point on for the remaining samples. g NH4HF2 to the deep sample and swirl until completely dissolved. (Note: NHHF is a toxic and corrosive chemical-avoid contact with skin and clothing. If contact occurs, rinse chemical off immediately with copious amounts of water) Add 3 g of KI, add a Teflon stir bar, and titrate immediately with the Na2S203 solution. of When the dark brown color of the iodide is nearly gone (brownish milk color), add 2 g KSCN (or 1.7g of NaSCN) and 3 mL of the starch indicator solution. The solution should be a milky blue-black color. Continue the titration until the disappearance ofthe blue- black color. Repeat for the remaining three samples.

Explanation / Answer

Iodometric titration of copper depends upon the formation of cupprous iodidide(CuI) as precipitate. But in Cu2+ solutions Fe3+ may also present. The reduction potential of Fe3+/Fe2+ couple is +0.77V where as the reduction potential of Cu2+/Cu+ couple is +0.15V. From this reduction potential valuea we can say that iron(III) will oxidise Cu(I) and as a result the main reaction will be hampered. NH4HF2 is used for complexation of Fe(III) as follows-

Fe3+ + F- ---------------> [FeF]2-

Now in the solution Fe(III) will not be there to oxidise Cu(I) and as a result iodometric titration can take place normally.

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