Can someone please help me understand pH levels (Meaning mainly for certain molecule)

  • Thread starter nukeman
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  • #1
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Hey,

I am trying to figure out some research for a department at school, and need some chemistry info.

Basically, I am comparing D2O (Heavy water) HDO (Semiheavy water) and H2O (Light water)

NOW... each of them have different pH levels.

What does that mean that they each have different pH levels?

I know the pH is the activity of the hydrogen ion. But can someone explain that a little more clear for me? :)

Does it mean they vibrate at different rates? Or what?


Thanks Chem guys and galls!
 

Answers and Replies

  • #2
DrDu
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Speaking of pH here makes little sense. However they differ in the pK_W values.
This is mainly due to there being more vibrational levels in case of the DO bond as compared to the HO bond which makes it entropically less favourable to break the DO bond.
 
  • #3
Borek
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As DrDu wrote, speaking of pH of a pure water doesn't make much sense. Even for normal (not heavy) water, it is not constant, but changes with the temperature (see the table at the water ion product page).
 
  • #4
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As DrDu wrote, speaking of pH of a pure water doesn't make much sense. Even for normal (not heavy) water, it is not constant, but changes with the temperature (see the table at the water ion product page).

sorry if this is a dumb question, but if I have d2o, and by changing the temperature, do I change the frequency in which it vibrates?
 
  • #5
epenguin
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It is very unlikely that in any biological experiment the pH of water matters, since the pH would be determined by whatever else is there. There are some difficulties in both calculating and measuring. Deuteroacids are less acidic than the corresponding ordinary ones (as DrDu is saying), say H3CCOOD less acid than H3CCOOH, because the dissociation rate is slower but the association rates are about the same.

If you calculate you'd have to know the pK. But if you measure you'd have to know you have to make an adjustment. http://garfield.library.upenn.edu/classics1979/A1979HZ34800001.pdf http://pubs.acs.org/doi/abs/10.1021/ac60260a013 and others.

It would tax me to be sure what pD-pH comparison really means and as frankly it is sounding to me like uninterpetrable experiments I'd tend to avoid that tax. At most I hope a fairly small pH difference somewhere is not something that greatly affects whatever you trying to compare.
 
  • #6
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Basically my goal is to have deuterium, and some permutations of it, and find a way that each sample will have a different vibrational frequency.

example, if I want to see if this imaginary door only let in deuterium that vibrated a certain way :)
 
  • #7
Borek
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I have a feeling you are talking about vibrations not knowing what you are talking about. Molecules don't vibrate with some random frequencies, frequencies that they can take are quantized and always the same for a given molecule (and often even for a given kind of bond, regardless of surrounding atoms - for example carbonyl group always vibrates at almost perfectly the same frequency, which makes its detection in IR spectrum extremely easy).

http://en.wikipedia.org/wiki/Quantum_harmonic_oscillator

http://en.wikipedia.org/wiki/Molecular_vibration
 

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