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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: mrpizzaroo on October 01, 2018, 04:43:49 PM

Title: Why does it need to be specific pH?
Post by: mrpizzaroo on October 01, 2018, 04:43:49 PM

Scenario: You are working in a forensics lab, and you need to quantify the amount of nicotine in a given sample of saliva. The standard protocol manual gives the following sample preparation instructions: “Dissolve the sample in 20 mL of aqueous media and extract with dichloromethane. Dry over sodium sulfate and remove solvent in vacuo. It is important to note that the aqueous media must have a pH greater than 6.5”

Question: Why would the pH need to be greater than 6.5? If you could include a rxn mechanism that would be great too :)

Title: Re: Why does it need to be specific pH?
Post by: Corribus on October 01, 2018, 05:00:36 PM

What are your thoughts?

Hint: what are the pKa values of nicotine?

Title: Re: Why does it need to be specific pH?
Post by: mrpizzaroo on October 01, 2018, 05:39:26 PM

pKa of nicotene is 8.5 (pubchem) so it's a basic compound. knowing that it's a base it will be dissolved into the aqueous layer. am i on the right track or no?

Title: Re: Why does it need to be specific pH?
Post by: Corribus on October 01, 2018, 06:34:00 PM

There are actually two of them, but yes this one is the most important. It's actually around 8.0.

What do you know about pKa and degree of protonation?

Check out this link, which may help: http://www.bob-r.com/ENI/Nicotine%20Titration%20Notes-HCl.pdf

Now you need to think about the procedure. You are trying to extract nicotine from saliva into an organic solvent (chloroform). What form of nicotine (protonated or not) do you think will best dissolve in chloroform?

Title: Re: Why does it need to be specific pH?
Post by: mrpizzaroo on October 01, 2018, 08:38:02 PM

after looking into it some more, pKa is measure of acid's strength; lower number, stronger acid.
don't know much about degree of protonation but
we want nicotene to be in organic phase so we should deprotonate it; the H ions give it a + charge & deprotonating it will remove the H ions
we want pKa at least 6.5 b/c the solvent is probably really acidic like pH 1 or 2 & the greater distance b/t pKa & pH means we've isolated more nicotene
but why 6.5 specifically?
also chloroform is trichloromethane, not dichloromethane, not sure if that changes much though

Title: Re: Why does it need to be specific pH?
Post by: mrpizzaroo on October 01, 2018, 09:05:10 PM

after looking at it again, we want pH to be above 6.5 not pKa (lol sorry)
but pH=pKa means theres equal amount of nicotene deprotonated & not i believe
it appears in the link you sent, at pH 8 is when pure nicotene starts to be more isolate & i guess at ph 6.5 is when the molar concentration is starting to increase?

Title: Re: Why does it need to be specific pH?
Post by: Babcock_Hall on October 01, 2018, 09:14:36 PM

Are you familiar with the Henderson-Hasselbalch equation?

Title: Re: Why does it need to be specific pH?
Post by: mrpizzaroo on October 01, 2018, 09:25:05 PM

would say i know the formula & get it for the most part yes

Title: Re: Why does it need to be specific pH?
Post by: Borek on October 02, 2018, 02:28:07 AM

For a substance with a given pKa, how does the ratio of the protonated and non-protonated form depends on pH?

Hint: you were already suggested where to look for an answer.

Title: Re: Why does it need to be specific pH?
Post by: Babcock_Hall on October 08, 2018, 01:33:39 PM

The degree of protonation of a substance depends on how strong an acid it is and what the pH of the buffered solution is.  What equation brings those two pieces of information together.