You should use AAS or ICP-MS.

regards

G

Dear Grzegorz Boczkaj,

AAS or ICP-MS would not allow speciation of vanadium. I am interested in investigating the stability and dynamic processes within vanadium(IV) and vanadium(V) complexes in aqueous solutions.

Do you have any input as per the usage of spectrophotometry or HPLC?

Thanks,

Check this:

Article Method for vanadium speciation in aqueous samples by HPLC-ICP-OES

Article Application of HPLC to measure vanadium in environmental, bi...

regards,

G

There was a procedure for manual SPE treatment of water to speciate As III and As V. Forget what the SPE column was but the article was online. I believe there is a similar method out there for vanadium allowing you to do manual speciation and running the extracts on an ICP-OES, ICP-MS, or AA depending on your needs and equipment availability.

Hi Joseph,

Thanks for ur reply.

I am not looking at identifying species as I have done that. I am looking now at investigating the dynamic processes within these complexes. In the lab we have HPLC and spectrophotometer, any idea on how to use what we have to do that?

Thanks,

Hi Iman Boukhobza,

I'm working with molybdenium complexes and the the way we elucidate the oxidation state and concentration, is UV-spectroskopy and Anion-chromatography, because these complexes have a negative charge in aqueous solution.

Thanks,

What is about lability? Did you look at dynamics in your solutions?

Hey Iman Boukhobza,

yes, this was the target of the projekt.

But further informations about the complexes I'm not allowed to public.

I understand.

I am interested in using UV-Vis spectroscopy to do that. If you can suggest any literature that reported some thing relevant that will be great. Thanks

No, I've no litarature about that. One importent thing, if you use only UV-spectroscopy is, that you should know the UV-spectra of all possible compounds in the different oxidation states. In the past we calculated compound decay and production of converted compounds via UV-spectroscopy and fitted the spectra via least square-function. The limit was 3 compounds with different UV-Spectra. In UV you often have overlap of bands of different compounds. If you have no pure reference compounds I think it will not be possible.

I wish you good luck.

Thanks, that is helpful.

Dear Manfred

After doing some reading and coming back to the issue of vanadium complexes lability within environmental matrices, do you think that the following is a good idea? The work is done in aqueous solution 1- Using UV-Vis spectroscopy. 2- Get the spectra for V(IV) 3- Get the spectra for L 4-Get the spectra for V(IV)-L 5- Repeat 4 as a function of time to see if there is a change of the absorbance as a function of time.

6- Follow the same step for V(V) If there is a change in the absorbance, would this reflect that the complex is labile? I have worked before with 2D NMR to investigate lability, so am not sure about UV-Vis! Your input is very much appreciated. Thanks, Iman

Dear Iman,

I think it's not predictable if the complex stability could be measured by UV-spectroskopie alone. This dependts very on the spectra of educts and products. Perhaps you have to analyse the products, after complete

conversion, with additional analytical methods. The change of UV-absorbance is a hint for conversion, to which degree the conversion took place is much more difficult. If you have a fully loss of absorption at one wave-lenght this would be easy and isosbestic points in spectra-overlay will help you to enshure your predicted reaction sheme. Furthermore I don't know which kind of vanadium comlexes you have, oxides?

Stability is not an issue, as I am using another technique. My question is mainly about lability. So you are saying that the change in the absorbance is a hint for conversion? This is what I am looking for. If I get the same UV spectra after repeating at different times, that means that the complex is inert in aqueous solutions. Correct?

Yes my V(IV) and V(V) are oxides?

If the UV-spectrum doesn't change this whould be true. If you have only two oxides in solution, the UV-determination will be easy. But beware on mixed oxidation states of vanadium-comlexes. Look in wikipedia to get an idea about oxidation states. Good luck.

Hi Manfred,

After doing some more reading, wouldn't you think that the change in the wave length overtime may reflect on the inertness of the complex?

I believe that the change in the absorbance with time while keeping the same wavelength is an indicator of stabilility rather than lability?

Would you agree?

Hi Iman,

I think the absorbance of the complex will reflect its stability if the absorbance of the products are significantely lower at the same wavelength.

Yes, this is correct. What is about if the wavelength changes. I believe that should reflect on its lability?

Hi Iman,

yes, that's right.

Great. Thanks

Hi Manfred.

Could you please have a look at the attached UV-Vis spectra and tell me what do you think. I am trying to investigate if I can use UV-Vis spectroscopy to follow up on the stability and lability of V(IV) and V(V) complexes.

So for the stability I have run the aqueous solution at different times and compare. For the lability I have run the aqueous solutions at different temperature and compare.

Hi Iman

at first, the spectra were not baseline corrected. It looks like you used a Varian Cary UV-/Vis-spectrometer and if the parameters are not changed after starting it scans from 200 - 800 nm at a resolution of 2 nm and makes a lamp change at 350 nm. The lamp change is visible and the cuvette compensation is poor. This will be much better with baseline correction.

The spectra of V(IV) and V(V) at the beginning are nearly identical. You can't destinguish between them by UV. You have a rising band at 240 nm and perhaps at 200 nm which are overlapped. You have no time dependend decrease of absoption over the whole spectra. No concentration caclulation can be done by these spectra. If your expected product ist vadanate you shoud calibrate this and look if your spectrum at the end of the reaction, perhaps many day later than 6 days, fits with it. If not, you have different compunds in solution which absorbs light.

On disadvantage is obvious. You don't have an isosbestic point in your spectra overlay so quantification will be very uncertain.

Hi Manfred

Thanks for your reply.

No, you are right, I did not do a baseline correction. As I said in the beginning, I am not an expert in using UV-Vis. So the first thing I have done was to run a solution of potassium permanganate, and it gave me a spectrum similar to what I found in the literature. So I have assumed that my parameters were ok. So, could you please advise how to do a baseline correction?

Hi Iman,

if you have a Varian Cary UV/Vis-spektrometer and the software Cary WIN-UV (I don't know) you have to set baseline correction in 'Scan', 'setup' and 'Baseline'. Please read your software manual. After you have switched the baseline on you must first make a baseline scan with your solvents and cuvettes. After this you can do your measurements. I have not so very much time to advise you. There are always manuals delivered with the spectrometer where you get information how to do that.

Is there no analytical specialist who can advise you in UV-/Vis-spectrometry in your University?

This procedure will nothing change to the band-overlab problem. So I think you must use additional analytical systems like Ion-Chromatography to seperate Vanadate.

Dear Manfred.

Thanks for your reply.

The main idea in my project is to investigate the stability and lability of vanadium metalions (After complexation)

So I believe, a qualitative study would be ok here. As I am monitoring the aqueous solution of the complex versus time and see if there is a change in the spectrum.

For the lability, I changed the temperature to see how the complex in the aqueous solution will behave.

Do you think, am I on the correct track?

Hi Iman,

how do you will prove the stability or lability of your complex, if you get only increasing UV-/Vis-absoprtion over time and temperature. There is no chance this way. The products have obviously higher (how much ? you don't know) extinktion-coefficients over the whole spectrum. How many products are produced or any decay of your complex you can't measure this way. You must seperate them, thats why I wrote before about isosbestic points in your spectra-overlay.

These are fundamental UV-spectroscopic basics.

Hi Manfred.

The spectra are for V complexes registered separately. The first for the aqueous solution of V(IV)-EDTA. The second for the aqueous solution of V(V)-EDTA

Thus they are separated.

Hi Iman,

but how you will calculate a stability or lability from these spectra. You don't know the concentration of the V-EDTA complex in solution. Also the UV-spectra show different overlayed absorptions-bands. So you have more than one compound which absorbs light in solution. For Vanadium it is the same like Molybdenium, at one oxidation state you can get different coordinated complexes in equilibrium.

Please look also in wikipedia for vanadate and an anorganic chemistry book like 'Hollemann Wiberg',

also these links would be helpful.

https://onlinelibrary.wiley.com/doi/epdf/10.1002/hlca.19650480716

https://core.ac.uk/download/pdf/34213444.pdf

Hi Manfred,

Again, I am mainly looking for qualitative results. Different vanadium -ligand complexes have been studied from a stability point of view and lability. I have worked personally with many ligands and evaluated their stability and lability with vanadium using NMR.

Now, what is in the solution is for sure, a mixture of vanadium, ligand and complex that why we see this band. so most probably there are not more than one complex. V- EDTA has been studied in the literature.

Thus, knowing that? Do you think that UV-Vis can say something about a change in the stability over time? with T?

By the way, I have tried the baseline correction. But nothing has changed in the spectrum.

Thanks.

Hi Iman,

if you use baselinekorrection, the the lamp-change will not be visible in the sprectrum at 350 nm. Therefor you must fill the watersolution (normally buffer of water, your solvent) without compounds in both cuvettes and run then baseline. After this the next spectra shouln't show the lamp-change at 350nm, normally < 0.002 Extinction. You didn't told me until now, if you have a Varian-Cary-UV-Vis-Spectromer.

If you talk about stability of a complex you can't say 'qualitative' because your target is know how stable the complex is. So if you have much experience with Vanadium and lingands by NMR so you have already a stability by NMR. The EDTA will be a concurrent reaktion in equilibrium to form V-EDTA. How stable this V-EDTA complex is, I don't know, but perhaps you have done already test-measurement with differnt Vanadium EDTA ratios to get an molar-Extinction-coefficient of V-EDTA. If your V-ligand complex is in solution and you have a molar-Exktinction-coefficient from this you can calculate the stabilty of it. Perhaps at 240 nm or at 200 nm. (You kwow, stability of complexes are often pH-dependent.) And I hope you have only one V-EDTA complex in solution.

Hi Manfred,

For the baseline, it is correct, this is what happened. And the small change around 350 nm has gone after the baseline correction.

The instrument in our lab is Agilent/Cary Series UV-Vis spectrophotometer. Cary 100 UV-Vis.

For the mole ratio, yes I have done different ratio to investigate which ratio gave the best result.

Yes, the stability is pH dependent. The project is interested in investigating the stability of vanadium complexes within the environmental matrix as is, and without any changes in the pH.

What you are saying is that I can use the Beer Lambert equation at different mole ratio to get extinction coefficient. Now, how can I get the stability constant? Because the Keq= complex/L x V. We don't have [L]eq and [V]eq?

Using NMR, I was able to measure how much complex and L and vanadate are in the aqueous solution all at ounce? But I don't know if this is possible with UV-Vis?

Any idea?

Thanks

Hi Iman,

thanks for that additional informations.

So when I understood everything correct, the V-ETDA complex you omly use for UV-detection. The extinction koefficient you can measure like you wrote before. Use a constant concentration of Vanadium-ions in solution and change the EDTA concentration stepwise until the extinction doesn't increase any more. At the maximum absorption you can calculate an extintion coefficient of the complex. If you don't reach the maximum you can calculate an extinction coefficient from equilibrium by fitting it at differnt concentrations (Perhaps you must use also differnt wavelenghts). After this you can start with your ligands and Vanadium-Ions to look where are the best wavelength, to calculate. Perhaps you get also an extinction coefficent for your V-ligand. From these results perhaps it could be possible to calculate keq = V-EDTA / V-Ligand when you put Vanadium, Ligand and ETDA together at differnent concentrations in solution.

Hi Manfred,

I am back to work on the project. I have been busy with academic and publication responsibilities.

So to wrap up our discussion about the stability of V-EDTA complexes using UV-Vis. You believe it will be possible to calculate Kequ using this technique?

I still don't know how to calculate the concentration of the ligand and the metal ion in equilibrium. The UV-Spectrum (not like NMR) does not show the metal ion, the ligand and the complex at the same time. The spectrum does not show three peaks? (previous photos)

That why I was thinking about a qualitative approach? we follow what happens to the peak with time?

Any idea?

Hi Iman,

did you understood the last answer I wrote before?

Normally there is a constant k-value for the equilibrium between Vanadate and EDTA at one pH-value (see EDTA-metal-complex-constants). And if you put Vanadate in excess in solution, step by step, you can calculate that the UV-spectrum doesn't change any more if the excess is high enough. You wrote before, you have only one oxidation state of Vanadate. So you don't have problems polymeres of Vanadate?! So if you are not shure, I would prefer aditional analytical instruments, like NMR or Ion-Chromatography, to look if your statements are ok. These are requirements that it will be possible to calculate Kequ.

Does the UV-absortion increase also at high excess of Vanadate? Than I think the mathematical calculation of the extinktion coefficient will be difficult.

Qualitative measurements doesn't tell you anything about the complex-concentrations in solution. Please look for EDTA-metal stability constants in literature. In your example it will be k = [Vanadate-EDTA-complex] / ( [Vanadate] x [EDTA]). If the k is high (like e.g. Fe3+ (k = 10^26)) you don't see any change already at slight excess of EDTA. The extinction-coefficient of the EDTA-Vanadate-complex is than the UV-absorption dividet through the molar concentration of Fe3+ for 1cm UV-cuvettes.

At different concentrations (Vanadate / EDTA) you can check your determined k-value.

Hi, Manfred,

Yes I understood.

This is what you have said last time.

"Use a constant concentration of Vanadium-ions in solution and change the EDTA concentration stepwise until the extinction doesn't increase any more.

At the maximum absorption you can calculate an extintion coefficient of the complex."

So, just to double check.

Once the ligand is considered as a limiting reactant (Since the vanadium ion will be in excess), we can assume that the reaction can be total and the concentration of the complex can be calculated based on the concentration of the ligand. Thus, once we know the concentration of the complex, and we have the absorbance, we can calculate the extinction coefficient of the complex.

Separately, we can calculate the extinction coefficient of the ligand, and that of the vanadium ion.

Therefore, we can calculate Keq

Is this what you meant?

Hi Iman,

yes, that's right. But it decence on the Keq if the extinktion is already at maximum at low EDTA-excess. Different concentration ratios will show if the Keq is in agreement with your determined extinction coefficent.

I will try soon and keep you updated.

Thanks.

Hi Manfred.

I have tried the concept of limiting reactant to calculate the coefficient of extinction for the complex. But the ratio Abs/concentration is not constant. I believe I would need to go to very small ratio of V: L. Which ratio do you normally use for this purpose?

Hey Iman,

yes you are right.

It is possible you have to change the concentration at the same ration of V:L to see if your estimations are correct.

From your extinction changes in concetration-series you can estimate how far you away from max-Extinction.

Is your extinction maximum at all concentrations at the same wavelength? If not, perhaps you have different complexes. In that case the Keq can not be measured as easy as you want.

If you have problems to estimate the Keq, check your system with known Keqs like Fe3+. The concentration depends very on Keq, so I can't tell you what I would use. For Fe3+ 10^-6M must be more than enough if EDTA is in excess (Keq 10^26).