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If I have a pure substance which may be cholesterol can I test that by polarimetry? In other words the idea would be to dissolve the sample in dichloromethane or ethanol, then test the rotation using a polarimeter.

My concern is that I read that cholesterol will not actually dissolve, only form a colloid, so that may invalidate the possibility of a test.

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  • $\begingroup$ A polarimeter alone isn't a very definite test to identify a compound. Think of this from the point of view of information theory. Let's assume that the value can be +/- 360 degrees. That is 720 buckets to put billions of billions of compounds. It would like trying to identify a person from the entire earth population by their height. $\endgroup$ – MaxW Sep 20 '16 at 23:23
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Taken from the website of a chemical supplier, the following solubility data can be easily obtained. Historically benzene and chloroform are commonplace for rotation measurements, however it can be any solvent in which the sample is soluble (or mixtures of solvents). You might be best to find a literature value for cholesterol (scifinder/reaxys) and then measure in the same solvent so at least you have a reference point to compare to.

One gram dissolves in 2.8 ml of ether, in 4.5 ml of chloroform, and in 1.5 ml of pyridine. The product is soluble in acetone, dioxane, ethyl acetate, benzene, petroleum ether, oils, fats, and in aqueous solutions of bile salts. Solutions should be protected from light.

Source Sigma Aldrich

Generally speaking, optical rotation isn't a method commonly used to determine whether or not you have a given compound. Methods like NMR, IR, MS, or even just TLC comparison are far more suited to this.

Optical rotation is useful if you already know you have the right thing, and want to know what your optical purity is like. Without having already confirmed that the sample is 1. clean and 2. the right thing, you'll struggle to make sense of the data since the polarimeter will simply give you a number.

Depending on where you bought your cholesterol from, and various other things, this number may be close to the literature value (maybe some confidence in it being the right thing), however even then you cant be sure its cholesterol as many other things will have similar values.

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  • $\begingroup$ I know the substance is pure to begin with because it is crystalline $\endgroup$ – Shaka Boom Jul 22 '16 at 21:43
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    $\begingroup$ Just because its a crystal doesn't mean its cholesterol, it just means its a crystal. You might try taking the melting point too, at least then you have two pieces of data (the standard amount for known compounds) $\endgroup$ – NotEvans. Jul 22 '16 at 21:44
  • $\begingroup$ I said I know it is pure because it is crystal. I don't know what it is composed of. $\endgroup$ – Shaka Boom Jul 22 '16 at 22:05
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    $\begingroup$ Again, just because its a crystal, doesn't mean its pure. Many things co-crystallise etc. $\endgroup$ – NotEvans. Jul 24 '16 at 13:19
  • $\begingroup$ The proton NMR should be unambiguous and published. $\endgroup$ – Zhe Oct 21 '16 at 13:49

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