Steam
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Electropotential Titration Using Solvent mix
Hey Guys, I wish to do an electropotential titration on a compound which I have found very difficult to put into aqueous solution. It does dissolve
very well and methanol. I have found that I can dissolve it sparingly in a 1:1 MeOH H2O mix. After going through some old forums (see links in bottom)
I have gathered that:
1) As long as MeOH isn't too major of a constituent, the dissociation of MeOH can be neglected. (This is the one that I feel most uneasy about)
2) That having a solution of MeOH and Water will not destroy my pH probe provided that it doesn't dry out.
3) The whole pH thing doesn't really make sense when there is a solvent mix and that it also messes with the gritty calculations like activity for
instance.
There were a few things that I am wondering that my Analytical Chemistry textbook and google won't tell me.
First, Would something like this even be possible, or am I just setting myself up for a spectacular failure. 
Second, Can I use a titrant that is completely water or should I also include the same ratio of methanol in the titrant.
Third, (this pertains to the second question) How is the best way to standardize a titrant that is not 100% aqueous.
Forth, Calibrating a pH probe for something like this. Can I just use a regular buffer series?
Fifth, Is there anything I should know before attempting this?
Thanks for any help in advanced! 
http://www.sciencemadness.org/talk/viewthread.php?tid=14877
http://www.lcresources.com/discus/messages/5133/1383.html?Th...
DISCLAIMER: The information in this post is provided for general informational purposes only and may not reflect the current law in your jurisdiction.
No information contained in this post should be construed as legal advice from the individual author, nor is it intended to be a substitute for legal
counsel on any subject matter. No reader of this post should act or refrain from acting on the basis of any information included in, or accessible
through, this post without seeking the appropriate legal or other professional advice on the particular facts and circumstances at issue from a lawyer
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DraconicAcid
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Quote: Originally posted by Steam  | Hey Guys, I wish to do an electropotential titration on a compound which I have found very difficult to put into aqueous solution. It does dissolve
very well and methanol. I have found that I can dissolve it sparingly in a 1:1 MeOH H2O mix. After going through some old forums (see links in bottom)
I have gathered that:
1) As long as MeOH isn't too major of a constituent, the dissociation of MeOH can be neglected. (This is the one that I feel most uneasy about)
2) That having a solution of MeOH and Water will not destroy my pH probe provided that it doesn't dry out.
3) The whole pH thing doesn't really make sense when there is a solvent mix and that it also messes with the gritty calculations like activity for
instance.
There were a few things that I am wondering that my Analytical Chemistry textbook and google won't tell me.
First, Would something like this even be possible, or am I just setting myself up for a spectacular failure.
Second, Can I use a titrant that is completely water or should I also include the same ratio of methanol in the titrant.
Third, (this pertains to the second question) How is the best way to standardize a titrant that is not 100% aqueous.
Forth, Calibrating a pH probe for something like this. Can I just use a regular buffer series?
Fifth, Is there anything I should know before attempting this?
Thanks for any help in advanced!
http://www.sciencemadness.org/talk/viewthread.php?tid=14877
http://www.lcresources.com/discus/messages/5133/1383.html?Th... |
Methanol will not dissociate in aqueous solution. Period. Even if you had pure methanol, it would dissociate much less than water.
I'm not sure how well your pH probe will work in a mixed solvent, or if the methanol will muck it up. The methanol will muck up any calculations you
are trying to do with activity coefficients, though.
However, if you're just doing a titration to determine the quantity of material present, then you should still be able to see the endpoint, activity
coefficients be damned.
What is your material, and what are you titrating it with?
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Steam
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It is a naphthyl based hydroxamic acid. I was originally titrating with a sodium hydroxide solution. I am thinking that back titrating it with HCl
might help matters a bit in terms actually dissolving the compound, but it is very difficult to dissolve (in water) in its protonated form.
What I really want is the pKas of the polyprotic analyte. I have software which can take a second derivative of a titration to find the points of
inflection.
DISCLAIMER: The information in this post is provided for general informational purposes only and may not reflect the current law in your jurisdiction.
No information contained in this post should be construed as legal advice from the individual author, nor is it intended to be a substitute for legal
counsel on any subject matter. No reader of this post should act or refrain from acting on the basis of any information included in, or accessible
through, this post without seeking the appropriate legal or other professional advice on the particular facts and circumstances at issue from a lawyer
licensed in the recipient’s state, country or other appropriate licensing jurisdiction.
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DraconicAcid
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Posts: 4334
Registered: 1-2-2013
Location: The tiniest college campus ever....
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Mood: Semi-victorious.
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| Quote: Originally posted by Steam | It is a naphthyl based hydroxamic acid. I was originally titrating with a sodium hydroxide solution. I am thinking that back titrating it with HCl
might help matters a bit in terms actually dissolving the compound, but it is very difficult to dissolve (in water) in its protonated form.
What I really want is the pKas of the polyprotic analyte. I have software which can take a second derivative of a titration to find the points of
inflection. |
If you want a pKa, then you want an aqueous solution. Best to titrate the conjugate base with hydrochloric acid, and then you can standardize your
titrants the same way as you always do. If the stuff precipitates out too much as you titrate it, then add just enough methanol to keep most of it in
solution- try 10% or 20%.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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