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Water/Methanol density vs. freeze point

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  • Water/Methanol density vs. freeze point

    I'm currently setting up some equipment to mix and dispense windshield washer solvent into cars. The spec calls for 3 parts water to 2 parts methanol with a Specific Gravity of 0.942 nominal and a freeze point of -30 degrees Celcious (no higher). I have a digital meter that will give me the specific gravity or the density of the solution. How do I convert from SG or Kg/cu.m to a freeze point for this solution?

  • #2
    Re: Water/Methanol density vs. freeze point

    I'm sorry I don't understand the problem if you are given the ratio by the manufacturer. 3-2 should give you what is needed...

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    • #3
      Re: Water/Methanol density vs. freeze point

      You cant go directly from a density to a freeze point. They are not directly related enough.

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      • #4
        Re: Water/Methanol density vs. freeze point

        Can I directly convert between the Brix or API readings to Freeze Point? I have to simplify this as much as possible for a quality inspector to check the mix once per shift.

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        • #5
          Re: Water/Methanol density vs. freeze point

          If you use AR methanol (i.e.really pure), and pure (milli-Q or similar) water, and use the exact mix stated, then stir well at room temperature, does your digital density meter give the result accurately? - This solution is going to absorb water, so over time, if it is left sitting, the freezing point will go up. Therefore, you could also check the measurement for 3.5 parts water, and 1.5 parts methanol.

          A domestic chest freezer is about -24C, so if your sealed solution froze in that then that would possibly be a fail.

          I couldn't find any data on the internet that related freezing point to density or mix for the methanol/water system, sorry.

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          • #6
            Re: Water/Methanol density vs. freeze point

            I don't know what API readings are, and Brix is to do with sugar solutions.

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            • #7
              Re: Water/Methanol density vs. freeze point

              API is directly related to the specific gravity. Its a scale used for petroleum in the oil industry.

              Rereading your question, I see you already know the freeze point of the substances involved right? You need to know the freeze point of the combined substances? You can probably work that out algebraically, no need to bring density into the equation at all.

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              • #8
                Re: Water/Methanol density vs. freeze point

                Originally posted by Robert Fogt
                API is directly related to the specific gravity. Its a scale used for petroleum in the oil industry.

                Rereading your question, I see you already know the freeze point of the substances involved right? You need to know the freeze point of the combined substances? You can probably work that out algebraically, no need to bring density into the equation at all.
                Thanks for explaining the API values.

                This is why chemistry is so much fun! These values are almost always determined experimentally, as the freezing point of different mixes of two materials will almost never lie along a straight line between the freezing points of the two pure materials. There may also be some hysteresis involved. (differing freezing and melting points.) Hence the maximum freezing point required.

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                • #9
                  Re: Water/Methanol density vs. freeze point

                  I will jump in the "way back machine" here. The CRC Handbook of Chemistry and Physics has a set of tables called "Concentrative Properties of Aqueous Solutions" and Table 36 is methanol mixed in water. In my 70th edition, the tables start on page D-221.

                  Unfortunately, it is based on mixture by weight, so extra playing around with the densities of the original ingredients is necessary to get mixture by volume. Among the properties it gives is freezing point depression, and mixture density.

                  The following properties are given in each row
                  % methanol by weight, solution density, freezing point
                  32%, 0.9482 kg/L, -28.15 C
                  34%, 0.9450 kg/L, -30.48 C
                  36%, 0.9416 kg/L, -32.97 C
                  NOTE: The densities are for solution at 20 C and I do not have a temperature correction to other temperatures.

                  Basically, the 34% solution should be the target. You could interpolate and slightly lower could be tolerated. The variation in density is not very great given a change in performance; this may not be a very sensitive QC test.

                  1 L of the 34% (by weight) mixture weighs 945 g and requires 321.3 g methanol and 623.7 g water. Using 20 C densities of 0.7917 kg/L and 0.9982 kg/L respectively, this requires 405.8 mL methanol and 624.8 mL water to mix 1 L (note that volume is not conserved). The 34% by weight mixture is 40.58% by (mixed) volume and the ratio of ingredients is 2 parts methanol, 3.08 parts water, very close to the given 2:3.

                  This series of tables includes (each mixed with water) ethanol, ethylene glycol, propylene glycol, actually a total of 99 chemicals.

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                  • #10
                    Re: Water/Methanol density vs. freeze point

                    Can u tell what is the value for density of methanol-water mixture at 1 bar and constant temperature?

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                    • #11
                      Re: Water/Methanol density vs. freeze point

                      The reason that the ethanol/water mix tables use weight, FYI, is that water and ethanol (or in this case, water and methanol) do not mix ideally with respect to volume -- if you mix a liter of water and a liter of methanol, the resulting solution is not two liters in volume.

                      The more you know...

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                      • #12
                        Re: Water/Methanol density vs. freeze point

                        Hey guys
                        please help me

                        what is the liquid density of pure methanol at the temperature of 36degrees celcius??

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                        • #13
                          Re: Water/Methanol density vs. freeze point

                          Originally posted by Unregistered View Post
                          Hey guys
                          please help me

                          what is the liquid density of pure methanol at the temperature of 36degrees celcius??
                          See the VCF table for methanol at Measurement Canada:
                          https://strategis.gc.ca/eic/site/mc-...g/lm00132.html

                          At 36 C, the density is 776.3 kg/m (or 0.7763 kg/L).

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                          • #14
                            Re: Water/Methanol density vs. freeze point

                            how much menthanol by volume to add to water to have -10C freezing point.

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                            • #15
                              Re: Water/Methanol density vs. freeze point

                              Originally posted by Unregistered View Post
                              how much menthanol by volume to add to water to have -10C freezing point.
                              See post #9 for methodology. HoC&P says 15% mixture by weight freezes at -10.5 C (closest value that exceeds your specs.)

                              Working through the densities as in #9, a volumetric mixture of 184.5 mL of methanol and 830.1 mL of H2O will give 1 L of 15% by weight mixture. Note the volumes are not additive. Slightly more than 1 L of total ingredients are required to yield 1 L of mixture.

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