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human
Can a homogeneous mixture of two miscible liquids become non-homogeneous over time?

I work in the chemistry laboratory of sanitizer/soap manufacturing facility.  On occasion, we have noticed some weird results when testing samples from the top of a tank compared to a sample from the bottom of the tank.  

Mainly we noticed this with a product that is primarily water and isopropanol.  I know that water and IPA are miscible.  Would the difference in testing of a top and bottom sample be a result of improper mixing or do the two liquids form a gradient concentration mixture throughout the tank after a certain period of time?
gpt
How tall is your tank? It’s possible that a gradient will form based on pressure differences regardless of prior mixing.  The two fluids can’t be perfectly miscible, so over and extended period of time, and with a large enough pressure difference, a gradient can form.