Chemistry (including Biochemistry)/Chemistry

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Question
Hi Dr. Raymond,

    If I had the chance to perform decantation or centrifugation, which process is more efficient?
    Also, is there a special property that a solid needs to have when using decnatation?

John

Answer
Hello John:

Often the two are used together, with the requirements often being the same as well.  Typically, centrifugation or sedimentation occur first, followed by decantation.  Also, when used alone, centrifugation is almost always more efficient. Below are some properties of both processes.

With centrifugation alone there are two restrictions that should be taken into account:

1) The precipitate you wish to isolate needs to be something that will not be destroyed or altered by the forces applied.  This is particularly important when you are extracting living cells (which can be killed by violent centrifugation) or particles which are capable of changing in nature when pressed together, like colloids or micelles.
2) If your mixture has multiple components capable of settling, and you only want the largest one to settle, then centrifugation may cause all components to fall out.

With just decantation, the following concerns exist:

1) Time to settlement - frankly, it can take a long time for this process to occur.
2) If during that time, the solid can react with solvent, then decantation alone may not be an option.
3) Efficiency - the settlement rate of different particle sizes can vary quite a bit.  It is often difficult to ensure a high efficiency process when this is the case.  A good example would be mud with gravel in it.  In short order, rocks and stones settle out, followed by sand grains... but silt and clay may take hours or days to fully settle.
4) Finally, decantation is not great for complex mixtures where the solvent system may separate and generate turbulent flow when decanting - which will perturb the solids.

These lists should not be taken as conclusive: for a given system there can be any number of additional considerations.  However, as general guidelines this should aid in selecting a method/methods.

Take care!

Chemistry (including Biochemistry)

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Dr. Jeffery Raymond

Expertise

Materials chemistry. Materials science. Spectroscopy. Polymer science. Physical Chemistry. General Physics. Technical writing. General Applied Mathematics. Nanomaterials. Optoelectronic Behavior. Science Policy.

Experience

Teaching: General Inorganic Chemistry I & II, Organic Chemistry I & II, Physical Chemistry I, Polymeric Materials, General Physics I, Calculus I & II
My prior experience includes the United States Army and three years as a development chemist in industry. Currently I am the Assistant Director of the Laboratory for Synthetic Biological Interactions. All told, 13 years of experience in research, development and science education.

Organizations
Texas A&M University, American Chemical Society, POLY-ACS, SPIE

Publications
Journal of the American Chemical Society, Nanoletters, Journal of Physical Chemistry C, Journal of Physical Chemistry Letters, Ultramicroscopy Proceedings of SPIE, Proceedings of MRS, Polymer News, Chemical and Engineering News, Nano Letters, Small, Chemistry.org, Angewandte

Education/Credentials
PhD Macromolecular Science and Engineering (Photophysics/Nanomaterials Concentration), MS Materials Science, BS Chemistry and Physics, Graduate Certificate in Science Policy, AAS Chemical Technology, AAS Engineering Technology

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