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Topic: pH didn't drop as expected  (Read 14322 times)

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Online Borek

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Re: pH didn't drop as expected
« Reply #15 on: September 26, 2012, 11:06:47 AM »
If the order will change does it matter for pH?

Should not.
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Offline fledarmus

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Re: pH didn't drop as expected
« Reply #16 on: September 26, 2012, 11:34:45 AM »
Definitely an error in the preparation of the solution.

As for "the solution should be close to neutral", that's true, but if my calculations are right, that is because of the buffering effect of the urea.

We'd gotten as far as 1712.3 moles of NaOH and 1729.8 moles of DDBSA in 2325.6 liters. That gives an excess of 17.5 moles of acid - 17.5/2325.6 = 0.00752M, giving pH ~ 2.

This is where the last piece comes in - urea is a weak base with pKb = 13.8, there is 1193.1 moles of urea added to 2325.6 liters of a 0.00752M strong acid, what is the calculated pH?

Offline felitix

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Re: pH didn't drop as expected
« Reply #17 on: September 26, 2012, 06:56:22 PM »
I don't see the pH will change drastically. There will be 0.0075M concentration of conjugate acid, 0.5055M concentration of base, so pOH = 13.8 +log(0.0075/0.5055) = 11.97. pH therefore will be 2.03.
How did you came up with neutral?

Offline AWK

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Re: pH didn't drop as expected
« Reply #18 on: September 27, 2012, 01:52:59 AM »
Such pH can show solution without DDBSA.
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Offline AWK

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Re: pH didn't drop as expected
« Reply #19 on: September 28, 2012, 03:02:34 AM »
the order of adding ingredients is as follows:
- soft cold water
- urea
- 50% NaOH
- DDBSA

If the order will change does it matter for pH?
It does not matter.
Probably you took to much NaOH

Quote
NaOH (50%) = 3424.6/2 moles = 1712.3 moles
OK
Quote
NaOH (50%) - 302 lbs = (40 g/mol M.W.) 302 lbs = 136985 grams = 3424.6 moles
- not divided by 2 . Which is correct?
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Offline curiouscat

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Re: pH didn't drop as expected
« Reply #20 on: September 28, 2012, 08:56:11 AM »
I've a perhaps silly suggestion. Can you repeat and replicate?

Maybe divide everything by a factor (100?)  so that it'll fit in a flask.

Let's try and eliminate any errors of procedure.

Offline felitix

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will it work as a buffer really?
« Reply #21 on: September 28, 2012, 03:41:27 PM »
In reference to my other post 'pH did not drop as expected' fledarmus suggested Urea has a buffering effect in the solution of:
1712.3 moles of NaOH
1729.8 moles of DDBSA
1193.1 moles of Urea
in 2325.6 liters of water.
May be I'm wrong, but Urea's dissociation constant is so little that the buffering effect will be minimal.
Am I right? Or is Urea/DDBSA a buffer system?

Offline Arkcon

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Re: pH didn't drop as expected
« Reply #22 on: September 28, 2012, 04:30:11 PM »
feltix:, I hope you don't mind I merged your similar topics, it avoids multiple threads with similar discussions.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline AWK

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Re: pH didn't drop as expected
« Reply #23 on: October 02, 2012, 02:12:45 AM »
Check pH of DDBSA solution itself. May beyou use salt, not acid.
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