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Author Topic: Why did lead nitrate and copper sulphate precipitate with sodium hydroxide?  (Read 7485 times)

Offline jbperl

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Why CuSO4 and Pb(NO3)2 solution sedimented when adjusted  to pH 7 with NaOH
« Last Edit: 16/06/2010 21:37:48 by chris »


 

Offline Bill.D.Katt.

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Your Pb(NO3)2 probably reacted with the NaOH to get NaNO3 and Pb(OH)2. The SO4 ion might have reacted with the Pb ion, but I doubt it, if this happened you might have seen a slight color change.
 

Offline lightarrow

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Your Pb(NO3)2 probably reacted with the NaOH to get NaNO3 and Pb(OH)2. The SO4 ion might have reacted with the Pb ion, but I doubt it, if this happened you might have seen a slight color change.
Why a colour change? PbSO4 should be white, if I remember well. The hydroxide is more insoluble than the sulphate, but the sulphate conc. is much greater. I'm not sure it have to be necessarily the hydroxide, if I'll have time I'll try to make a computation.
« Last Edit: 17/06/2010 20:29:40 by lightarrow »
 

Offline Bill.D.Katt.

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I mentioned a color change if your Pb(NO3)2 reacted with the CuSO4, I don't believe this would happen, the CuSO4 is fairly stable. I think the blue color of CuSO4 is slightly different than the hue of Cu(NO3)2 plus of course the PbSO4 as a precipitate. Sorry about that, I'll have to be more clear next time
 

Offline lightarrow

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But in water solution the blue colour is given by water and Cu2+ only, that is by the coomplex Cu(H2O)62+.
 

Offline daveshorts

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In an ionic solution you don't have CuSO4 and Pb(NO3)2 you have Cu2+, SO42-, Pb2+ and NO3- ions floating around freely. If you add a load of OH- ions these will also float around.

However hydroxides are generally not as soluble as sulphates so depending on the concentration of OH the lead and copper ions will form a precipitate with the hydroxide ions. Which drops out first and in what proportion will depend on their relative solubilities.
 

Offline lightarrow

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In an ionic solution you don't have CuSO4 and Pb(NO3)2 you have Cu2+, SO42-, Pb2+ and NO3- ions floating around freely. If you add a load of OH- ions these will also float around.

However hydroxides are generally not as soluble as sulphates so depending on the concentration of OH the lead and copper ions will form a precipitate with the hydroxide ions. Which drops out first and in what proportion will depend on their relative solubilities.
Exactly. According to the computations I made (don't know if correct, in case I'll write them next time, but it's long) the sulphate precipitation is much more probable than hydroxide precipitation, at pH = 7.
 

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