SV: Dialysis speed

Dag Rune Gjellesvik dagrg at online.no
Thu Apr 12 14:39:51 EST 2001


Dima Klenchin <klenchin at REMOVE_TO_REPLY.facstaff.wisc.edu> skrev i
meldingsnyheter:9b2oh1$9fg$1 at news.doit.wisc.edu...
> ybao at noble.org ("Bao, Yiming") wrote:
> :My original question was:
> :
> ::Would the dialysis speed of 7M Urea into pure water be slower than,
equal
> :to
> ::or faster than into a 20X SSC solution?
> :
(cut - - - -)

> And I think all three gave quite solid support for an obviously correct
> answer. You see, the dialysis membrane here is completely irrelevant
> part of the issue. Presumably, you are talking about regular dialysis
> membrane with cutt-off on the order of 10 kDa. Thus, all the components
> listed (Na, citrate, urea, water) will diffuse freely through it, at a
rate
> practically the same as their diffusion in free solution. Thus, you can
> visualize it for yourself as if it were just step gradients consisting of
> SSC/urea or water/urea. Then the question you are asking is "which
> one will equlibrate (e.g. intermix completely) faster?". Clearly, things
> will diffuse slower in the solution of  higher density/viscosity, and that
> means urea will dialyze faster against pure water.

I agree that diffusion is the only factor that matters, but still I don't
quite agree with that above. As far as I remember from diffusion theory - it
is the concentration gradient of EACH SEPARATE SOLUTE that determines the
diffusion rate. Hence, urea will diffuse with an equal rate against a high
citrate as against pure water - because the urea conc. on the other side is
zero in both cases. Maybe an increased viscosity may reduce the diffusion
rate (slightly), but this shouldn't matter if you use stirring in your
vessel.

Dag Rune






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