MadSci Network: Physics

Re: can you desalinate water in a centrifuge?

Date: Wed Dec 21 03:31:53 2005
Posted By: Werner Sieber, Research Scientist,
Area of science: Physics
ID: 1135119425.Ph

Dear Andrew,
I suppose that the idea of your son is to make the salt in a solution 
settle to the bottom of a tube by centrifugal force. Well, IN PRINCIPLE  
this is possible, and the principle is exploited in density-gradient 
centrifugation, using cesium chloride as the salt. But even in this case, 
you just get slight differences in salt concentration between the top and 
the bottom of the centrifuge tube at high spinning rates. The reason for 
this inefficient "desalination" is the tendency of the dissolved salt ions 
to redistribute themselves in the space where their concentration is 
reduced. This entropy-driven tendency is pitched against the centrifugal 
force. Entropy thus plays a much more important role if we are dealing 
with a true solution (like that of sodium chloride in water), as opposed 
to a suspension or a colloid.
There are much cheaper and more energy-efficient methods of desalination, 
e.g. by ion exchange or vacuum distillation.
Best Regards
Werner Sieber

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