Dear Nagapradeep,

This is one of those strange quirks of chemistry!

As Subhadip says it is disproportionation.

The key is to consider the standard electrode potentials (E) for the following type reactions:

Cu(II) -------------->Cu(I)

and

Cu(I)---------------->Cu(0)

If the second reaction is sufficiently reducing (ie more negative or less positive than the other) it can drive the upper reaction backwards!

If this quirk happens for a compound, although it can exist with stability in the solid form, when it hits solution 50% is reduced to copper and the other 50% is oxidised to Cu(II).

It just depends on the ligand!

Remember a good ligand can stabilise an unstable oxidation state, I kept a sample of Cr (II) hydrazine sulphate for many years! (Prepared by reduction at a lead cathode under carbon dioxide).

Hope this helps.

Regards,

Dr John

PS: For a freaky reaction see comproportionation!

I think it is because of the much larger size of the iodine atom compared to the other halogen atom sizes. The bond length would be greatly increased and the stability will obviously decrease in that case.

The most possible answer will be- Since I(-1) is a powerful reducing agent, it can capable of reducing Cu(II) to Cu(I) spontaneously. Hence, the existence of CuI2 is very difficult (in most of the cases, it is not even possible to see this compound.). So CuI is mostly preferred than CuI2 formation. But, I still do not know Why CuI is more stable in comparison with other cuprous halides? The above mentioned reason is for the existence but not for the stability. So, Please let me know the details in terms of stability.

in my view Cu(1) and I(-1)comes in the category of soft acid and soft bases while Br(-1) is borderline bases and Cl(-1) and F(-1) are hard bases so copper and iodide will follow PEARSON RULE

Thanks for the valuable comment in connection to the concept.

Ionic radius of I- = 216 pm is much bigger than size of copper cations (Do not have exact numbers). As we move from Cu+ to Cu2+ the size of cation decreases decreasing stability. So CuI2 is less stable than CuI

This is a problem of redox potential. Cu+2 is unstable because its disproponation reaction.

Dear Nagapradeep,

This is one of those strange quirks of chemistry!

As Subhadip says it is disproportionation.

The key is to consider the standard electrode potentials (E) for the following type reactions:

Cu(II) -------------->Cu(I)

and

Cu(I)---------------->Cu(0)

If the second reaction is sufficiently reducing (ie more negative or less positive than the other) it can drive the upper reaction backwards!

If this quirk happens for a compound, although it can exist with stability in the solid form, when it hits solution 50% is reduced to copper and the other 50% is oxidised to Cu(II).

It just depends on the ligand!

Remember a good ligand can stabilise an unstable oxidation state, I kept a sample of Cr (II) hydrazine sulphate for many years! (Prepared by reduction at a lead cathode under carbon dioxide).

Hope this helps.

Regards,

Dr John

PS: For a freaky reaction see comproportionation!

@Dr john, in the cases of Cu analysis in AAS instrumentation, which ligand do u think is suitable for Cu?

My colleagues prepared 100 mg/L Cu ion solution from the Cu wire source, dissolved into concentrated Nitric acid. She found that the results showed only ~75% recovery from the AAS analysis.. Perhaps, the Ligand is needed in this situation.

Recovery / Accuracy

a) application of the analytical procedure to matrix of the product components to which known quantities of the analyte to be analysed have been added;

b) in cases where it is impossible to obtain samples of all matrix , it may be acceptable either to add known quantities of the analyte to the product/ sample or to compare the results obtained from a second, well characterized procedure, the accuracy of which is stated and/or defined (independent procedure, see 1.2.);

c) accuracy may be inferred once precision, linearity and specificity have been established.

The above lines is from ICH Guidelines; you need to study the recovery as above if really you want to know the recovery of the method.

Regards

Selvan

Dear Run,

How about something that vapourises the copper more efficiently?

This problem is found in fireworks, where copper is used to produce a blue colour.

I believe the answer may be to add some hydrochloric acid as this helps vaporise the copper.

ie copper chloride.

This could be done by simply adding some conc hydrochloric acid to the solution of copper in nitric acid just prior to AAS.

You can see this effect in a bunsen burner flame: try getting a blue colour from copper sulphate powder: not good. (And irreproducible!)

Moisten the crystals in conc hydrochloric acid et voila! a beautiful blue colouration!

I believe the standard flame test calls for all crystals of all salts to be moistened with conc hydrochloric acid before being applied to the flame on a platinum loop.

Hope this helps!

Regards,

Dr John

@Dr john

I will try to add some HCl solution prior to AAS analysis.

I have actually looked though the answer from the google, one of the india guy, he met some question as my colleague found in her experimental.

He mentioned about the pH medium in recovery of Cu in AAS.

We will let u know, whether it works or not.

TQ

Regards

Run Hong TOH

@Dr John,

In fact, the experiment that my colleague did is regarding about the Cu removal by adsorption.

Thus, in her case, the ligand will have the interference / effect upon the adsorption efficiency , as far as concern, the percentage removal of Cu is low. Therefore, she still prefer , dont add any ligand source during the adsorption reaction.

Regards

Run Hong TOH.

Dear Run,

In the optimisation and development of an analytical technique there may be hundreds of factors that must be considered in order to bring a technique to a state of reliability!

It is the function of a consultant to consider all of these, and to do this he needs to see the whole picture. (And he usually charges a fortune!) (£300 GBP and hour!)

Here we can only suggest possible solutions that may help a fellow scientist overcome a seemingly impossible problem!

These ideas are based upon our experiences suffering with intractable research problems that may have taken years to solve.

The ideas may work, they may not, but they make help break an impasse and speed a fellow analytical chemist on their way!

Apologies therefore if my idea was not much use!

With best regards,

Dr John

Why CuI2 is not stable but CuI?

try the hsab concept of pearson

You can explain it by redox potential.

The outer shell configuration of Cu(+1) is 3d10, 4s0 and Cu(+2) is 3d9, 4s0. Hence Cu(+1) state is more stable and I(-) is a very weak ligand due to its bigger size therefore CuI is more stable than CuI2.

@Dr John

Greeting, and good news here.

The recovery of Cu by adding the HNO3 , to pH ~1 prior to AAS analysis showed significant recovery percentage, almost 99 % Cu recovery from 100 mg/L

Best Regards

Toh Run Hong

This can be excellently solve by Reduction potential values

I2(s) + 2 e– 2 I–(aq)                 +0.535

Cu2+(aq) + e– Cu+(aq)           +0.153

So total reaction will be I2 +2Cu+ ___>>> 2CuI