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Name: Unknown
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Date: Before 1993

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.

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!


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