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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: stewartgilligan on February 22, 2006, 10:34:35 PM

Title: How to calculate the formal potential?
Post by: stewartgilligan on February 22, 2006, 10:34:35 PM
Hi everyone...

I have a question regarding a potentiometric lab I have done.  I have been trying to figure out how to calculate the formal potential of each half reaction.  The lab was a titration of Fe(II) with Ce(IV), where a 25 mL aliquot of an unknown iron solution was titrated with standard cesium.  I understand that the nernst equation should be used here, and that the measured potential (from the potentiometer) is put into the equation, however, what about the concentrations?  I do not understand what the concentrations needed for completion of the equation are.  As well, 10 mL of mercury(II) chloride was added to the iron just before titrating with the cesium, what role does that play?  I have been at this for quite a bit, and it has grown incredibly frustrating..

Any help is greatly appreciated,

thanks :)
Title: Re:How to calculate the formal potential?
Post by: Borek on February 23, 2006, 04:52:29 AM
Cesium or cerium?

You don't need to know concentrations. Ratio of concentrations is what counts here - and this can be easily calculated. From the titration curve you should know the end-point. At 50% titration concentrations of both Fe2+ and Fe3+ are identical.

HgCl2 is used to remove excess SnCl2 which is in turn used to reduce Fe3+ to Fe2+. If you have not added SnCl2 solution by yourself, my bet is that the Fe solution you were given already contained some tin (II) chloride to prevent iron oxidation to Fe3+ with atmospheric oxygen.
Title: Re:How to calculate the formal potential?
Post by: stewartgilligan on February 23, 2006, 10:17:35 AM
Hi again,

thanks for the reply.  I understand that the concentrations are identical, however, wouldn't this make the formal potential equal to the measured potential as the log of 1 is zero?  (From the nernst Equation)

E = E? - 0.0592/1 * log [Fe2+]/[Fe3+]. Where E? is the formal potential
Title: Re:How to calculate the formal potential?
Post by: Borek on February 23, 2006, 11:22:15 AM
wouldn't this make the formal potential equal to the measured potential as the log of 1 is zero?

You have just determined formal potential of Fe2+/Fe3+ reaction in your solution.
Title: Re:How to calculate the formal potential?
Post by: stewartgilligan on February 23, 2006, 01:17:55 PM
Hi,

thank you very much. :)  So if the iron concentrations are equal at 1/2 the titration point, then the concentrations of Ce3+ and Ce4+ would be equal at 2x the titration point?  Does that mean that the average of these two values would give the equivalence point potential?

:)
Title: Re:How to calculate the formal potential?
Post by: Borek on February 23, 2006, 01:46:07 PM
So if the iron concentrations are equal at 1/2 the titration point, then the concentrations of Ce3+ and Ce4+ would be equal at 2x the titration point?

Sounds logical :)

Quote
Does that mean that the average of these two values would give the equivalence point potential?

Not always. In general it will be weighted average (weighted by numbers of electrons from both half-reactions). In this particular case both reactions use 1 electron, so normal average will be OK.
Title: Re:How to calculate the formal potential?
Post by: stewartgilligan on February 23, 2006, 06:57:44 PM
Hello,

thanks very much for all of your assistance.  I feel as if I have a handle on the concepts of this lab....

:)
Title: Re:How to calculate the formal potential?
Post by: CGriffins on March 02, 2006, 09:37:15 PM
Hi,

I do not know if anyone will check an older thread, however, can someone please explain why the equivalence potential is equal to the average of the two formal potentials?  I am trying to understand that one, but it does not seem to be working

Thanks
Title: Re:How to calculate the formal potential?
Post by: Borek on March 03, 2006, 03:27:37 AM
I do not know if anyone will check an older thread, however, can someone please explain why the equivalence potential is equal to the average of the two formal potentials?  I am trying to understand that one, but it does not seem to be working

Try to derive it by yourself. You have two halfreactions and two Nernst equations. Both potentials must be identical. It is endpoint so you have added equivalent amount of titrant to titrated substance (don't forget to multiply them by numbers of electrons!). This gives you two additional equations for concentrations of Red and Ox forms of both titrant and titrated substance. You have a set of equations - solve it.
Title: Re: How to calculate the formal potential?
Post by: sistermonica on April 18, 2006, 07:51:02 PM
How would I calculate 10% titrated with the same setup?
Title: Re: How to calculate the formal potential?
Post by: Borek on April 18, 2006, 07:53:38 PM
How would I calculate 10% titrated with the same setup?

Assume there is only one redox system in the solution (titrated one) and calculate concentrations of red and ox forms from stoichiometry (assuming it reacted 100%). Then use Nernst equation.
Title: Re: How to calculate the formal potential?
Post by: sistermonica on April 18, 2006, 07:58:14 PM
so I would use .68-.0592log((100-10)/10)?
Title: Re: How to calculate the formal potential?
Post by: sistermonica on April 18, 2006, 08:19:56 PM
I'm actually pretty sure that what I put isn't right. I'm not sure where the 10% comes in the stochiometry. I have 3.921g of Fe(NH4)2(SO4)2 6H20 in 25 mL and 0.1 M Ce(NH4)2(NO3)6. After I find the molarity of the Fe solution, what would I do?
Title: Re: How to calculate the formal potential?
Post by: lemonoman on April 18, 2006, 11:24:55 PM
Assume there is only one redox system in the solution (titrated one) and calculate concentrations of red and ox forms from stoichiometry (assuming it reacted 100%). Then use Nernst equation.

This means that you have an initial concentration of Fe, [Fe2+]0, but what you have at the 10% titration point is a concentration of [Fe2+] = (1 - 0.1)[Fe2+]0 = 0.9[Fe2+]0.  Hope that helps.

And by the way, I notice you revived an old, inactive thread.  That means you probably searched the chemicalforums database for similar questions...and not enough people do that.  Three cheers for you!   :D :D :D
Title: Re: How to calculate the formal potential?
Post by: sistermonica on April 18, 2006, 11:42:18 PM
Thank you so much! I started out with that yesterday but other people in my lab said otherwise.

I always try to research as much as I can so people don't have to keep repeating themselves  :)

Thank you again!
Title: Re: How to calculate the formal potential?
Post by: Borek on April 19, 2006, 03:33:00 AM
so I would use .68-.0592log((100-10)/10)?

General idea is OK, details may depend on your system (like number of electrons and whether you are trying to oxidize or reduce something - sign can change).  Assuming you are titrating Fe2+ to Fe3+ equation seems correct.