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Water content of (semi) transparent liquids could be approximated via absorption spectra. What method(s) can be used to measure the water content of an opaque liquid such as milk for example?

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I would add Karl Fischer titration.

Of course for your milk example, you would have to dilute your sample quite a lot in a dry (or known-dryness) solvent before titration. Also, the strong redox solution might interact in complicated ways with other materials in your sample (proteins, etc?).

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Many different techniques would work; here are three:

  1. Weigh, evaporate water, and weigh again.

  2. Measure the dielectric constant. This requires calibration, perhaps by method 1, but can even be used on solids such as wood.

  3. Put sample in a closed container with a hygrometer... also needs calibration.

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  • $\begingroup$ (1) Not only water evaporates. (2) How is permittivity useful here? Assuming linearity? (3) How selective is a hygrometer to water? $\endgroup$ – Sparkler Jun 7 '16 at 2:56
  • $\begingroup$ 1. Correct, if there are other volatiles, then water would need to be absorbed selectively and weighed, e.g. in NaOH. 2. See ces.ncsu.edu/nreos/wood/wpn/methods_moisture.htm for measurement of wood moisture. 3. Good question... that's why calibration is needed. $\endgroup$ – DrMoishe Pippik Jun 7 '16 at 3:16
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You could put your liquid in a sealed container to let water from your sample evaporate and saturate the air inside the container with water vapor. Then you could measure the amount of water vapor in the air. This will tell you the vapor pressure of the water in your sample and is a measure of water activity, which is slightly distinct from water content.

I suspect that for milk, it is OK to approximate the solution as ideal, which mens the water activity will be proportional to the water content.

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