Keep It Simple: Measuring Methanol Concentration in Salty Water

After performing numerous in depth tests during a rigorous session last night I can tell you that you cannot determine the concentration of ethanol in salty water by taste. Decided to pass on the methanol.

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PRECISION LASER-BASED CONCENTRATION AND REFRACTIVE INDEX ...

Here's a case where a portable Raman would be handy, sample the liquid through glass. All you'd have to do, take a series of samples in which you alter the concentration of methanol from 5 to 80%. Put it in the library and use the Raman for your tests. That's assuming that the TDS factor is fairly constant. You can subtract the methanol profile and get a TDS fingerprint if you want, so you can monitor that as well if it varies.

The intial cost is anywhere from 10 grand afaik for a bare bones model but of course, it depends how many tests you have to do, whether that's cheap or not in the long run.

IF the salt content and TDS are essentially constant so only the methanol changes, then the boiling point (corrected for atmospheric pressure) could be used. As I recall, the methanol has a much greater effect on BP than dissolved solids, so it might even work in the case of changing TDS, depending on the precision required.

Density should be an easy way to estimate the methanol concentration.

In a pilot plant situation, you could use a titration. It is pretty straightforward once you get the hang of it. I have done thousands of very similar titrations to measure the surface area of activated carbon as iodine number.

You could adapt the ethanol titration shown in this site to do methanol, noting that you don't need to suspend the sample as they have had to.

http://www.outreach.canterbury.ac.nz/chemistry/ethanol.shtml