Dialysis speed

Bao, Yiming ybao at noble.org
Wed Apr 11 15:06:48 EST 2001


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?

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

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).
> 
> TTFN
> 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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