ausron44
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why is addition of ammonia dropping PH?
dont really know if this is the correct place to post this, i was doing an experiment and thought i would try and extract some alkaloids from coca
leaf, i did the perculation and extracted alkaloids out of kerosene with 5% sulfuric acid
i had a solution of 5% sulfuric acid which had a slightly red color, now this is where it gets interesting, i need to base the solution to around
ph9.7 it was at 5, i added in 10% ammonia solution in small increments and the ph was DROPPING instead of heading towards alkaline. this had me
absolutely puzzled as to how this is even possible?
i thought it had to be my ph meter so i tried adding ammonia into some kerosene i had laying around and the ph was heading alkaline (like it should)
this suggests that there is nothing wrong with the ammonia (came from a reputable company anyway)
does anyone have any ideas? im now more interested in why this happened then the whole experiment entirely
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AJKOER
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Ammonia water is just that, mostly free NH3 and water. On adding, my guess is that the exothermic reaction caused the exit of ammonia and lots of
water vapor. In other words, the ammonia gas may be aiding the removal of water from the solution.
Further, the formation of any ammonium sulfate (and/or the hydrogen sulfate) increases the ionic nature of the acid solution, so even if it is more
dilute per the addition of your 10% aqueous ammonia, the activity coefficient may still be elevated.
Of course, my speculation is just theoretical, not experimental in your case, and assumes the absence of any possible measurement error on your part.
[Edited on 16-2-2015 by AJKOER]
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ausron44
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Quote: Originally posted by AJKOER  | Ammonia water is just that, mostly free NH3 and water. On adding, my guess is that the exothermic reaction caused the exit of ammonia and lots of
water vapor. In other words, the ammonia gas may be aiding the removal of water from the solution.
Further, the formation of any ammonium sulfate (and/or the hydrogen sulfate) increases the ionic nature of the acid solution, so even if it is more
dilute per the addition of your 10% aqueous ammonia, the activity coefficient may still be elevated.
Of course, my speculation is just theoretical, not experimental in your case, and assumes the absence of any possible measurement error on your part.
[Edited on 16-2-2015 by AJKOER] |
so if i added more NH3 would it have eventually climbed towards being alkaline after the water vapor had been removed?
the NH3 solution was originally 25% but i diluted this in fear of destroying the alkaloids of the extracted coca leaf in H2sO4 from information i had
been given from someone performing this experiment before, would it have been better just using that?
how would this problem be rectified as i want to do this experiment again
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ausron44
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where did it go wrong? i followed the exact method of someone else who was successful they added the ammonia and alkaloids precipitated out, only they
purchasd 9.5% ammonia off the shelf and i diluted 25% to 10%
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Etaoin Shrdlu
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What kind of pH meter are you using? Most aren't designed to be used in organic solvents.
EDIT: Are you measuring in kerosene, or not? First you say you extracted with sulfuric acid which implies you're trying to measure an aqueous
solution, but then you test your ammonia in kerosene...
[Edited on 2-16-2015 by Etaoin Shrdlu]
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ausron44
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| Quote: Originally posted by Etaoin Shrdlu | What kind of pH meter are you using? Most aren't designed to be used in organic solvents.
EDIT: Are you measuring in kerosene, or not? First you say you extracted with sulfuric acid which implies you're trying to measure an aqueous
solution, but then you test your ammonia in kerosene...
[Edited on 2-16-2015 by Etaoin Shrdlu] |
just a digital one used in aquariums, i used it in the aqueous solution and noticed it was going down so i cleaned it with h20 and then retested it in
another solution (kerosene) and when i added ammonia to that it inreased like it should, so why wasnt the ammonia making the aqueous solution of
sulfuric acid more alkaline?
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Etaoin Shrdlu
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Can you reproduce? I'm inclined to think there was a problem with your pH meter more than anything. Try taking the pH of sulfuric acid, then add
ammonia and see if it drops again.
Also, recheck the pH of your alkaloid/acid/ammonia solution to see if it's actually higher and was just reading wrong.
Is the meter calibrated?
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DraconicAcid
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Kerosene is not an aqueous solution, and therefore has no pH.
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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Brain&Force
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But isn't it possible to measure the acidity/basicity of something not in aqueous solution? What would it be called then?
At the end of the day, simulating atoms doesn't beat working with the real things...
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Etaoin Shrdlu
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C'mon. This is true, strictly, but one can extend the concept of pH to non-aqueous solutions (and that added ammonia had water in it). There are
probes specifically designed to work in organic solvents.
No use really of an aquarium pH meter in kerosene though.
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DraconicAcid
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| Quote: Originally posted by Etaoin Shrdlu |
C'mon. This is true, strictly, but one can extend the concept of pH to non-aqueous solutions (and that added ammonia had water in it). There are
probes specifically designed to work in organic solvents.
No use really of an aquarium pH meter in kerosene though. |
You can extend it to protic solvents, such as ethanol, acetic acid or ammonia. But the concept is inapplicable to hydrocarbons like kerosene.
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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Etaoin Shrdlu
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Yes, now that I agree with to some extent. Though I recall DMSO being favored for some applications...
EDIT: No, on further searching people have done this even for heptane. Perhaps of some interest: http://tera.chem.ut.ee/~ivo/HA_UT/Acidity_Basicity_INOR_1036...
http://tera.chem.ut.ee/~ivo/HA_UT/
Still no use for a pH meter of course.
[Edited on 2-17-2015 by Etaoin Shrdlu]
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DraconicAcid
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From what I can read of that, it's a comparison of pKa values in various solvents, rather than pH.
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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ausron44
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| Quote: Originally posted by Etaoin Shrdlu | Can you reproduce? I'm inclined to think there was a problem with your pH meter more than anything. Try taking the pH of sulfuric acid, then add
ammonia and see if it drops again.
Also, recheck the pH of your alkaloid/acid/ammonia solution to see if it's actually higher and was just reading wrong.
Is the meter calibrated? |
im going to give it another go soon when i get more leaf, i will be taking photos of everything just incase it happens again, this time i will use a
proper calibrated pH meter im leaning towards that as the problem
the ph meter was reading water at 7, ill do the test on the acid and then ammonia to see if it raises if it doesnt then the ph meter is to blame,
if this happens again with a proper calibrated ph meter then i am completely lost
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Etaoin Shrdlu
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I always understood these as directly related concepts, where generalizing greatly for acids pKa is ease of losing hydrogen, pH is how much hydrogen
lost.
EDIT: To clarify further, if one can take measurements in a solvent to determine pKa of a dissolved compound, I would think those measurements were
analogous to "pH" in that solvent.
[Edited on 2-17-2015 by Etaoin Shrdlu]
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