Dialysis speed

Dima Klenchin klenchin at REMOVE_TO_REPLY.facstaff.wisc.edu
Wed Apr 11 18:15:13 EST 2001

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
::or faster than into a 20X SSC solution?
:Thanks for the following three replies, which all agreed that dialysis
:against water is faster.  

:None of them though gave solid theory to support
:their claims, so I am not very convinced.  

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. 

:Then I decided to do a simple
:experiment to test this.  I put equal amount of water or 20X SSC in two
:little weighing-boats, and added to each of them a drop of 7M Urea solution
:containing some bromphenol blue as an indicator.  The result was quite
:dramatic.  The dye spread almost instantly on the surface of 20X SSC, while
:it defused slowly in water.
:So here comes more questions:  Does this test tell us what is really going
:on during dialysis?  

No. Not at all. You are comparing apples and oranges. 

Does this result indicate that dialysis against 20X SSC
:is actually faster?  If it does, could anybody explain why?

No, it does not. You notice that the dye spread
on the surface in one case. You don't mention that 
it sunk in another case, but it did happen (urea being a lot 
denser than water). So the two situations are completely 
not comparable. In the former case, you can't expect 
urea/dye solution to just sit as a drop on the surface 
of a solution with higher density - so it spreads all over. 


       - Dima

:Thanks a lot.
:Yiming Bao, Ph.D.
:Noble Foundation
:P.O. Box 2180
:Ardmore, OK 73402
:Tel: (580)221-7363
:Fax: (580)221-7380
:Email: ybao at noble.org
:> -----Original Message-----
:> From: dbell at qnis.net [mailto:dbell at qnis.net]
:> Sent: Friday, March 16, 2001 11:37 AM
:> To: methods at hgmp.mrc.ac.uk
:> Subject: Fw: Dialysis speed
:> For this question, the concentration gradient of importance 
:> is the water.  
:> A molecule in a water suspension/solution (7M Urea) will move 
:> from the area
:> of lowest diluent (7M Urea) to highest diluent (pure water) 
:> more quickly
:> than it will move into an area that has less diluent (20X SSC).
:> This is simple diffusion problem with dialysis membrane 
:> blocking molecules
:> of certain sizes right?  You need to realize that the water 
:> molecules will
:> also be moving across the membrane.  The 20X SSC solution *may* end up
:> concentrating the Urea on the wrong side of the membrane 
:> because the WATER
:> from the 7M urea solution may diffuse into the 20X SSC.  (I 
:> don't know what
:> 20X SSC is so I can't say for sure).
:> D. Bell
:> ----------
:> > From: Ole Hartvig Mortensen <ohm at NOSPAMnerd.dk>
:> > To: methods at hgmp.mrc.ac.uk
:> > Subject: Re: Dialysis speed
:> > Date: Friday, March 16, 2001 3:25 AM
:> > 
:> > 
:> > > :Here is the question:
:> > > :Would the dialysis speed of 7M Urea into pure water be 
:> slower than,
:> equal
:> > to
:> > > :or faster than into a 20X SSC solution?
:> > >
:> > > OK, I think it is simple. Higher than water viscosity of SSC
:> > > will make dialysis against SSC slower.
:> > >
:> > 
:> > Dialysis of UREA against something that has a higher salt 
:> concentration
:> than
:> > water would
:> > as far as I know be slower (if the dialysis gradient is 
:> dependent on salt
:> > concentrations of
:> > other salts besides UREA - this might not be the case, 
:> anyone knows ?)

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