Sciencemadness Discussion Board

Electropotential Titration Using Solvent mix

Steam - 19-7-2016 at 22:32

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...

DraconicAcid - 20-7-2016 at 08:12

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?

Steam - 20-7-2016 at 08:22

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.

DraconicAcid - 20-7-2016 at 09:00

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%.