Ask A Scientist Chemistry Archive

(Created prior to 1993)

Question: In class today, my general chemistry students were discussing the 
Tyndall Effect the TE is the scattering of light by particles suspended in a 
solution.  The TE is sometimes used in defining a true solution.  That is for 
pure substance such as pure water and true solutions like salt and water 
mixture the TE is absent.  The salt water mixture is considered a true 
solution because it is homogeneous with respect to phase.  Other types of 
solutions, suspensions for instance are not and clearly show the path of the 
light thorough them.  The question came up as to why if salt ions are in fact
particles of some type they do not scatter light and display the TE.  My 
answer was that even blue light had a wavelength much larger than the size of 
the ions involved and so simply diffracted around them.  The particle size and 
incident wavelength would have to be about the same or have the particle be 
larger than the wavelength before scattering it could be detected.
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I also think that using radiation with a small enough wavelength 
might produce a TE for a salt solution by the same logic, but as you say, you 
might need to use radiation in the X-ray or even the gamma-ray spectral 
regime.  I would guess that X-rays would be adequate, and that if you had an 
X-ray laser (do not laugh; they have been developed by the Department of 
Defense) and shone it on a beaker of salt water, one might see a TE if the 
laser were tuned into the right frequency.  Practical importance?  There is 
got to be an easier way to simply test for purity.  There are lots of 
precipitation reactions which could be used to detect for the presence of 
chloride in a salt solution, and to test for sodium, the good old sodium flame 
test is fine.  Now, if you need a quantitative measurement of concentrations 
that is a different story (maybe an analytical chemist out there can make some 
suggestions?) . . . but basically I cannot imagine an analytical chemist using 
an x-ray TE to test for the presence/absence of inorganic salts in a practical 
sense.  Hope this helps!
Topper
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