[Hunt]

I used an elementary volumetric method to determine the conc of Cu(II) in a mixture containing Pb(II) and Cu(II) ions. I applied the standard method of reacting Cu(II) with KI and then titrating I₂ formed with thiosulfate. The mixture is buffered and starch and SCN- were used ... However, does anyone think that precipitating Pb(II) and seperating it from the mixture would be better or not than just taking an aliquot of the mixture and directly titrating it? It seemed at first that either method is the same. But after trying it out , there was some difference in the volumes recorded. What am I missing ?

[Arkcon]

So you precipitated the lead with an excess of HCl, and got different results with the subsequent titration?  How different?  And statistically different?

[Borek]

Coprecipitation?

[Hunt]

Borek you mean coprecipitation as in :

Pb²⁺ + 2 I^- ---> PbI₂

Cu²⁺ + 2 I^- ----> CuI + 1/2 I₂

I titrated with thiosulfate the iodine released from the 2nd reaction.
Arkcon ,

Yes I precpitated the lead with sulfate , filtrated the solution and then titrated it against thiosulfate ( after adding KI , starch ... )

For the 1st method the volume was 20.4 ml ( C = 0.1632 M ) , for the 2nd 20.8 ml ( C = 0.1648 M ).

[Borek]

What I mean is that whenever you precipitate something and remove it from solution you may loose part of other ions in the solution - they may be 'built' into the precipitate this way or another. Sometimes it is important, sometimes in can be neglected. No idea about this case. However, 20.4 vs 20.8 doesn't look like you are loosing copper, quite the contrary

Arkcon question remains - is the difference statistically important?

[Hunt]

No no ofcourse it's not statistically imp. Im not a good experimentalist anyhow so in this case I'd say a max of 1-2% error sounds excellent to me. Im just trying to compare the methods. I guess both methods are similar but the 2nd took much more time.

[Borek]

Sometimes it is important to remove substances that can be interfering, AFAICT - not in this case.