[PineMarten]

Greetings,

I have following problem and I would be grateful to hear some responses/followup questions to following question:

I have some leftover liquid from wood heating process. This stuff is mostly (around 99%) water, but might have some ethanol and methanol in it (somewhere around 0-1% each). I should be able to determine water fraction of this liquid. Uncertainty of determined water fraction can be rather high, it is important to know approximate magnitude of other components, i.e is there around 0.1%-1% or less ethanol and/or methanol. 

Obvious thing to do is mass spectroscopy, but being rather time consuming and expensive, I started to think something else:

I have ph/conductivity-meter in my lab, so could I somehow turn methanol and ethanol to (some) acids and by measuring Ph of solution, I could approximately calculate how much there was alcohols to begin with?

So, could I turn these alcohols into acids somehow? How could I do that? Could there be a better way to determine approximate water fraction?

Even my question might be silly, I have some experience as a physisit/engineer in lab, but unfortunately I still lack understanding in Chemistry.

-PineMarten

[Furanone]

A simpler way might be try using density since ethanol (78.9 g/100ml) and methanol (79.2 g/100ml) are similar, while water is quite different (99.97 g/100ml) at 20 C. A good-sized pycnometer (100ml) and an accurate analytical scale with 4 decimal places, might be sensitive enough to see a difference from the density of pure water. It's worth a try since quite quick and easy to perform.

[Borek]

I have ph/conductivity-meter in my lab, so could I somehow turn methanol and ethanol to (some) acids and by measuring Ph of solution, I could approximately calculate how much there was alcohols to begin with?

Nope, it won't work. You can try to convert alcohols to weak acids, but calculating their concentration from the mixture pH is not going to give any reliable result.

Not that I can think of an easy and cheap method of determining these alcohols.

[Borek]

A simpler way might be try using density since ethanol (78.9 g/100ml) and methanol (79.2 g/100ml) are similar, while water is quite different (99.97 g/100ml) at 20 C. A good pycnometer and an accurate analytical scale with 4 decimal places, might be sensitive enough to see a difference from density of pure water. It's worth a try since quite quick and easy to perform.

It is a mixture of ethanol/methanol, so the result will be ambiguous - density will give you a line in the composition space, not a point.

Besides, at 20°C difference between pure water (0.99823 g/mL) and 1% ethanol (0.99724 g/mL) is just 0.00101 - at 4 decimal places you will get 0.0010, not much.

[Furanone]

A simpler way might be try using density since ethanol (78.9 g/100ml) and methanol (79.2 g/100ml) are similar, while water is quite different (99.97 g/100ml) at 20 C. A good pycnometer and an accurate analytical scale with 4 decimal places, might be sensitive enough to see a difference from density of pure water. It's worth a try since quite quick and easy to perform.

It is a mixture of ethanol/methanol, so the result will be ambiguous - density will give you a line in the composition space, not a point.

Yes, I agree with what you are saying, but if all he needs is a rough estimate (approximate magnitude) then perhaps this will give a sufficient answer since density of methanol and ethanol are still quite close in relation to density of water.

To be more accurate, in combination with density measurement, he could measure the refractive index of the mixture (maybe using an Abbe refractometer but a Brice-Phoenix differential refractometer would be better due to very high sensitivity), and then these two measurements (density and RI) could pinpoint exact amount of methanol and of ethanol in the water (since methanol lower RI than water and ethanol higher RI than water).

[PineMarten]

Thanks for great answers! Refractometer, even being absolutely uncertain method could give some hint. And most importantly, it seems easy method.

I have to now say something I should have said in the beginning: In addition to methanol and ethanol, there is also formic and acetic acid in solution. I thought to leave them out of the discussion because I already have fairly reasonable and at least quick method to see their magnitude (or more like H+ amount) in solution: ph-meter. This is very rough method but I can live with that.

Because densities of of those acids are in range 1.05-1.22 and alcohols have 0.79, density measurements do not give enough info, but combined with ph and even refractometer could give some hint about magnitude of another components in system.

Thanks a lot! Of course I'm interested to hear more if there is still some ways

-PineMartern

[Borek]

I already have fairly reasonable and at least quick method to see their magnitude (or more like H+ amount) in solution: ph-meter

As I told before, judging the amount of a mixture of weak acids from pH is not going to work. For 0.1% formic acid 100% change in concentration gives pH change in the 0.15 pH unit range, while accuracy of a typical pH measurement in somewhere around 0.05 (often even worse).

[AWK]

The best method for this mixture is gas chromatography.