cnidocyte
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pH of non polar solvent
Would I be right in assuming that H+ protons cannot be solvanted truly non polar solvents and therefore pH does not apply to them? What about solvents
like acetone. I don't really know much about acetone but I know its miscible with water as well as plenty of organics so I assume it will solvate
protons but not to the same degree that water does so how does the concept of pH apply to this solvent? I only really learned about pH of aqueous
solutions so I don't know how it applies to solvents other than water.
Also is there such thing as a truly non polar solvent? The least polar solvents I'm familiar with are hydrocarbon distillates but even low molecular
weight liquid hydrocarbons like hexane exhibit Van der Waals forces so I assume they will still solvate H+ protons to a certain degree.
[Edited on 6-1-2011 by cnidocyte]
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blogfast25
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Firstly, pH is a clear concept: the negative logarithm of the concentration of H3O+ ions in an aqueous solution:
pH = - log [H3O+]
Only in water can H3O+ ions exist and a pH be measured or calculated. Outside of water, pH is not meaningful.
Also you need to distinguish between protonation and solvation: water self-protonates: H2O(l) + H2O(l) < === > H3O+(aq) + OH-(aq). But the H3O+
and OH- ions, because of their charge, solvate, i.e. surround themselves with a cloud of water molecules attracted to them because water molecules are
polarised and polarisable.
But several other solvents are self-protonable: glacial acetic acid for instance:
2 CH3COOH(l)< === > CH3COOH2+(sol) + CH3COO-(sol)
Conductivity of the substance would be a good indicator of ion concentrations and thus degree of dissociation.
And because acetic acid is a polar solvent, solvation of these species occurs.
A strictly non-polar solvent cannot be protonated as far as I know. But a protonatable substance (say B) dissolved in a non-polar solvent with some
strong acid (say HA) dissolved in it too, would be protonated by the strong acid: B(sol) + HA(sol) < === > BH+(sol) + A-(sol). But solvation of
the formed ions would be extremely weak, because of the very weak polarity of the non-polar solvent…
[Edited on 6-1-2011 by blogfast25]
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turd
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[smartass]
the negative decimal logarithm of the H+/H3O+ activity. The activity being identical to the numerical value of the
concentration expressed in mol/l only in dilute solutions.
[/smartass]
Yeah, I know that you know this and that it's mostly irrelevant in practice. Just pointing out that things are often a little bit more complicated
than they seem at first.
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blogfast25
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Yes, activity:
pH = - log aH3O+
Sigh... ;-)
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kmno4
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The concept of pH (....) in terms of its (notional) definition
pH = -lg a<sub>H</sub>
involving as it does a single ion activity, it is immeasurable.
http://www.iupac.org/publications/pac/1985/pdf/5703x0531.pdf
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Yeah, I know that you know this and that it's mostly irrelevant in practice. Just pointing out that things are often a little bit more complicated
than they seem at first. |
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blogfast25
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I think this point about it being ‘immeasurable’ is seriously spurious: the paper then goes on to explain in great detail how pH is…
measured! It’s a relative measurement, so what??
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