Source: http://www.make-biodiesel.org/Quality-Testing/testing-methanol-for-purity.html

Over time methanol will absorb water from the air. Each time your container of methanol is opened, it will absorb a little water. If you have a drum of methanol with a pump installed in the bung, normal heating and cooling will force air in and out of the drum. That air of course contains water and the water is absorbed by the methanol. Of course water in our methanol can cause problems for us when we make biodiesel. That's why it's important to have a test that checks the purity of our methanol.

We don't use this test very often. It is useful to check your methanol purity to determine the cause of a bad batch. If all of a sudden you made a lot more soap than normal, then check your methanol for purity. Less than 95% purity can cause you problems. Remember your oil also has water in it and these two combine to make your water contamination worse. Very dry WVO can handle less pure methanol better than wet WVO.

The simplest way to check for purity is to measure its specific gravity. Specific gravity is the ratio of a substance's density to the density of water. If a liquid is lighter than water it will have a specific gravity of less than 1.0. Substances with a specific gravity greater than 1.0 will be heavier than water. Density is the mass of a substance divided by its volume. As we know things expand when heated and shrink when cooled, so density, and therefore specific gravity are temperature sensitive and will always be given at a specific temperature.

The tool for checking specific gravity is called a hydrometer. It's a weighted glass tube that floats in the methanol. Specific gravity is determined by how deep the glass tube sinks in the methanol. The specific gravity is read directly off a scale in the glass tube. There are three scales commonly used for checking purity of methanol.

The specific gravity scale reads directly in specific gravity. The specific gravity of pure methanol at 20°C is: .7913 . As our methanol absorbs water it will increase its specific gravity reaching a maximum of 1.000 at 20°C. If we have a specific gravity of .8957 then we have a solution of half water and half methanol since .8957 is half way between .7913 and 1.000. That leads to the formula of:

Methanol Purity = (1 - specific gravity) divided by 0.2087

To adjust for temperature we need to use the chart below. Find your measured specific gravity on the left hand column and the measured temperature in °F along the top. Find the spot in the chart that is under your temp and to the right of your specific gravity and read the number as percent methanol.

Hydrometers are designed to operate over a limited range of specific gravities. The narrower the range, the more accurate the reading. Wines and beers are measured for alcohol using special "tax certified" hydrometers that have very small ranges. General purpose hydrometers like one that ranges from 0.7000 to 1.000 will work for determining the purity of methanol, but they are not very precise particularly near 100% pure. It is possible to end up with a reading of 110% pure, particularly if temperature is not factored in.

Another scale used in hydrometers is the Tralles scale. It is a scale used when measuring the purity of Ethanol. Methanol and Ethanol both have very similar densities letting us use a hydrometer marked in the Tralles scale. It is actually reading percent methanol content directly. A reading of 100 is 100% pure methanol. A reading of 0 is 100% pure water. Hydrometers using the Tralles scale are available in most wine making hobby shops with a range from 0 to 100.

Another Alcohol scale is the Proof Scale. It is exactly two times the tralles scale. 200 proof would be 100% alcohol. This is the scale used to measure drinking alcohol.

If you don't have a hydrometer you can still measure the purity of your methanol. You will need some way to accurately measure 1 liter and a good scale. A glass graduated cylinder or a volumetric flask are good ways to measure out exactly one liter. Be sure you subtract out the weight of your container. Just like with a hydrometer you have to adjust for temperature before determining your percent methanol. Find the mass in kg of one liter of methanol and use that for specific gravity in the chart above.