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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: ccccc on July 17, 2008, 08:09:43 AM

Title: Equlilibrium Constant - pH dependency
Post by: ccccc on July 17, 2008, 08:09:43 AM

Hi. I am working on an experiment in which I am trying to determine the equlilibrium constant (more specifically -stability constant) of a coordination complex. The constant is to be determined using spectrophotometry at different pH. The problem is just that I will be changing the pH by varying the amount of a hard acid, and I am afraid the acid will interfere with the spectrophotometric measurements.

I hope someone can help me in this. Thank you

Title: Re: Equlilibrium Constant - pH dependency
Post by: Borek on July 17, 2008, 10:10:21 AM

More details please. What kind on interference are you afraid of?

Title: Re: Equlilibrium Constant - pH dependency
Post by: ccccc on July 17, 2008, 12:21:42 PM

The acid (eg. HCl) will also be detected on the spectrophotograph and I cannot see how to differentiate between the complex in question and the free Cl- ions etc also in solution.

Title: Re: Equlilibrium Constant - pH dependency
Post by: Borek on July 17, 2008, 01:33:36 PM

So choose other acid or other wavelength.

Besides, I am not aware of any spectroscopic method that can be used to determine Cl^- directly, so I don't think it will interfere. I would be much more afraid of Cl^- as a ligand possibility that it'll shift equilibrium.

Title: Re: Equlilibrium Constant - pH dependency
Post by: ccccc on July 17, 2008, 03:18:31 PM

ok that sound good. Yes, the likelyhood of Cl- as a ligand must also be taken into account, and i will therefore choose a polydentate ligand which prevents Cl- from bonding. Does that sound feasible?

Title: Re: Equlilibrium Constant - pH dependency
Post by: Borek on July 17, 2008, 03:48:12 PM

I thought the complex you are researching is given - are you free to select anything you want? What is the purpose of the experiment then?

Title: Re: Equlilibrium Constant - pH dependency
Post by: ccccc on July 17, 2008, 05:09:46 PM

That is a good point. Fortunately I am given the freedom of designing the experiment and the research question, so I am trying to make everything run well, and find the pH at which there is a maximum/minimum dissociation. Does this look possible from what I have said? Thank you

Title: Re: Equlilibrium Constant - pH dependency
Post by: Borek on July 17, 2008, 06:08:19 PM

Check some analytical chemistry book dealing with spectroscopic methods used for metal determination. Many methods require known pH. This way you may be able to find the system in which complex have good spectroscopic properties and there is some known optimum pH for its determination.

Title: Re: Equlilibrium Constant - pH dependency
Post by: ccccc on July 17, 2008, 07:18:14 PM

Okay but I was hoping to vary the pH as the independent var. and thereafter determine the eq constant as dependent var, so I hope the constant can be determined at theoretically any pH as the H+ will compete with the (eg. Fe(III)) ion.

Title: Re: Equlilibrium Constant - pH dependency
Post by: ccccc on July 18, 2008, 08:13:24 AM

Or am I completely wrong?