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#1
Old 11-17-2009, 10:54 PM
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pH Value Help - please!

Full disclosure - I am too many years out of 7th grade, and obviously dumber than a sack of rocks. I'm trying to help my son with science, and can't understand his textbook.

Just a straight question - what is the pH value for isopropyl alcohol (rubbing alcohol)? Or more simply, is it a base or an acid?
One part of the chapter states alcohol *has* no pH value, but another chart lists it as lower on the base chart than bleach or ammonia.

Maybe I'm just reading it wrong, but my son swears this will be on his test tomorrow.
Thanks in advance!
-Wallet-

Last edited by Wallet; 11-17-2009 at 10:58 PM.
#2
Old 11-17-2009, 11:14 PM
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It would have a neutral pH (~7), since alchohols do not typically dissociate.

I found a pKa of 16.5 for IPA, which is a little higher than water (15.7), so it would be neutral.

Been a long time since chem class, so I'll await the inevitable correction.
#3
Old 11-17-2009, 11:19 PM
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I found some online sources stating it is in the 5.5-6 range, which is odd. I'm now interested to see what the real chemists say when they show up


I should add that most MSDSs do not give a value for pH.

Last edited by jk1245; 11-17-2009 at 11:22 PM.
#4
Old 11-17-2009, 11:44 PM
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Thank you so much! My frantic son is convinced his whole test will rest on this one question! :-)

Hmm, he has a Meiosis quiz on Wednesday - I might be back!

Thanks once again,
-Wallet-
#5
Old 11-17-2009, 11:55 PM
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Alcohols in water don't dissociate to any significant degree, but in anhydrous chemistry they're more likely to donate an electron than accept one -- the Lewis definition of a base. However, that's tiny; isopropanol out of the bottle can be neutralized to phenolphthalein, litmus, and most other indicators with a few drops of very dilute acid or base (this is, in fact, one of the specifications).
#6
Old 11-18-2009, 01:16 AM
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It is anfoteric, i.e., it can be both a base and an acid, but in water it doesn't really swing much either way.

The problem is that there are many different definitions of "acid."

If you define "acid" as "something whose addition to distilled water lowers the pH" and a "base" as "something whose addition to distilled water rises the pH," then iPrOH is just lazy.

If you define an acid as "something which can donate H+" well, iPrOH can do that in anhydrous media, when matched with a strong enough base.

If you define a base as "something which can donate electron pairs," that's how most reactions involving alcohols begin, by the alcohol donating an electron pair. iPrOH is less likely to do this than other alcohols due to "steric problems," i.e., the iPr group is quite bulky and clashes with the other molecule more than nPr would (to choose an alcohol of the same size but which, being linear, reacts more readily).
#7
Old 11-18-2009, 05:13 PM
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If I were a nicer person, I'd get some 100% isopropanol that we have here and stick the pH meter in it and report what it said, but it's time to go home. Sorry.
#8
Old 11-18-2009, 08:45 PM
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Thanks again, guys. My son did have this on his test - for one part he had to put various items in rank order, from most acidic to most alkaline. Battery acid and water were no-brainers, but the teacher *did* include isopropyl alcohol. He put it just below water (thanks to the answers we received here!)

He asked the teacher before the test about bases donating electrons, but she just told him to be quiet and sit down.

So, thanks once more (and Smeghead, I think you're a very nice person! :-)
#9
Old 11-19-2009, 07:35 PM
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I don't even understand the question. pH is the measure of the concentration of hydrogen ions in an aqueous solution. Actually, the negative of the base 10 log of that concentration, so pH 7 means the concentration is one part in 10^7. So it first has to be an aqueous solution of alcohol. Being non-polar, it doesn't dissociate to any noticeable extent, so I expect it would be 7 or very close.
#10
Old 11-19-2009, 10:57 PM
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There's a difference between the pH value of a substance and the pKa value of a chemical species (a species can even have more than one pKa, since it can have more than one non-equivalent proton to lose). The pKa of water is about 15.7. The pH of pure water (hard to come by) is about 7. The pKa value of isopropanol is similar to that of water (the -OH behaves about the same in the two cases). pH is usually applied to solutions in water. The analog for pure isopropanol would probably be similar to that of water.
#11
Old 11-19-2009, 11:09 PM
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Quote:
Originally Posted by Wallet View Post
He asked the teacher before the test about bases donating electrons, but she just told him to be quiet and sit down.
What a beautiful and succinct summary of my entire K-12 education.

In college, one of my professors gave everyone a survey at the start of class. One of the questions was "What do you expect to learn from this class?" I drew a total blank... it had been years since I thought of class as something I could learn from.

Anyway, I hope he did well!
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