Testing Methanol for Purity

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Over time methanol absorbs water from the air. Each time you open a container of methanol, it absorbs a little water. If you have a drum of methanol with a pump installed in the bung, normal heating and cooling forces air in and out of the drum. That air contains water until the methanol absorbs it. Water in our methanol can cause problems for us when we make biodiesel. That is why it is important to have a test that checks the purity of our methanol.

hydrometer

We do not use this test very often unless you recover your methanol. However, it is useful to check your methanol purity to determine the cause of a bad batch. If suddenly 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.

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 has a specific gravity of less than 1.0. Substances with a specific gravity greater than 1.0 are 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 is temperature sensitive. So, to be accurate in our readings, we must record the temperature of the methanol when we measure specific gravity.

The tool for checking specific gravity is called a hydrometer. It is 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 the purity of methanol.

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  • Specific Gravity
  • Tralles
  • Proof

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, its specific gravity increases, 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 halfway 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.

methanol density chart for hydrometers calibrated at 15C

Hydrometers are designed to operate over a limited range of specific gravities. The narrower the range, the more accurate the reading. Distillers and wineries use special "tax certified" hydrometers that have very small ranges. General purpose hydrometers like one that ranges from 0.7000 to 1.000 work for determining the purity of methanol, but they are not very precise particularly near 100% pure. With general purpose hydrometers, it is possible to end up with a reading of 110% pure, particularly if the 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 reading percent methanol content directly. A reading of 100 is 100% pure ethanol. A reading of 0 is 100% pure water. Hydrometers using the Tralles scale are available in most winemaking 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. It is the scale used to measure liquor.

If you do not have a hydrometer, you can still roughly measure the purity of your methanol. You need the labware to measure 1 liter accurately, and a scale capable of measuring to a tenth of a gram. Certified glass graduated cylinders, and volumetric flasks are both good ways to measure out exactly one liter. Be sure you subtract out the weight of your container. Just like with a hydrometer you must adjust for temperature before determining your percent methanol. Find the mass in kilograms of one liter of methanol and use that for specific gravity in the chart above.

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Related Links

Infopop Thread on Hydrometer Accuracy

How To Use A Hydrometer