Changing the Freeze Point of Water
As with any tool, understanding how your deicer works and relates to your finished product is important for proper use and fair evaluation of results.
Understanding how your chemcial deicer works as a tool--whether used as a deicer or as an anti-icer--is critical for success. To gain such an understanding, you must define the basic elements of ice and ice control.
What is a deicer chemical?
All deicer chemicals have at least two things in common: First, they are products that change the freeze point of water and, second, they are water-soluble. Changing the freeze point of water allows a deicer to either melt ice or prevent its formation.
Ice, as we all know, is water in a solid form. Water freezes when its temperature falls to 32°F, the molecules move freely past each other. As water cools, the molecules become sluggish. With further cooling, this process continues until the molecules, for the most part, stop moving. When this happens, hydrogen bonds form and connect the water molecules--creating ice.
Removing ice or breaking the bond
You can break the hydrogen bonds creating ice in one, or more, of three ways:
- Mechanical. An example of this is using an ice bit (on a grader) to remove ice or snow. This does not cause the ice to melt; it merely breaks the bonds so you can remove ice from the readway surface. Even with the most efficient mechanical operation to remove ice from a roadway, some ice will remain. If you use enough mechanical force to remove all of the ice, you will cause damage to the roadway surface. This is costly and unreasonable.
- Thermal. A second way to remove ice is to use heat--such as from the sun or an artificial source. Obviously, heat reverses the freezing process. However, due to cost and inefficiency, removal of snow and ice by a combination of mechanical means and heat is simply not practical or economical.
- Chemical. The third way to melt or prevent ice is by the use of a deicer chemical. Here, you use heat, but not in the same manner as from the sun. Water has a heat-of-fusion rating of 80 calories per gram (a calorie is the amount of heat required to raise the temperature of 1 gram of water by 1°C at 1 atmosphere pressure). A solution of water and chemical has a higher heat-of-fusion rating and thus requires a colder temperature to cause it to turn to ice.
Exactly how cold this temperature is depends on the chemical and its concentration in the solution, which affects hydrating. Hydrating occurs when molecules of the deicer chemical and water attach to each other. Molecules in deicer chemicals have both negative and postive electrical charges. Water molecules are electrically neutral, but are "polar." Polar means one end of the molecule has a positive charge and the other a negative charge of equal strength. The opposite electrical charges in the chemical attract and attach to the water molecules, thus causing hydration.
With the presence of the deicer chemical and more heat, colder temperatures are necessary for the water molecules to bond together. This higher heat of fusion is hwat prevents correctly mixed chemicals and water solutions from freezing at temperatures well below the normal freeze point of water. An example of this is the proper mixture of anti-freeze and water in the radiator of your vehicle.
Liquid chemicals applied to ice have a higher heat of fusion or colder freeze-point temperature than the ice. Indirectly, the chemical solution is "hotter" than the ice. The ice absorbs the "heat" from the solution, melts the hydrogen bonds and creates water at temperatures below 32°F. As this melting process continues, the chemical solution cools or dilutes until it reaches its freeze-point temperature at which point the solution freezes.
Terms you often hear when discussing deicer chemicals are "effective" and "eutectic" temperatures. Effective temperature is the lowest temperature at which the results you obtain justify the cost of the application. In other words, this is a budgetary decision. You must base your decision concerning at what temperature you use a particular chemical on an understanding of the costs of your operation and how deicer chemicals work.
Eutectic temperature is the freeze point of a solution, based on the concentration of chemical in the solution--not the volume. Simply put, the ratio of water to chemical determines the freeze point of the deicer, just as in the radiator of your vehicle. Phase curves typically show freeze points based on concentration. (See figure, page 8, which represents a fictitious product called Supermelt.) All phase-curve points represent the freeze point of the solution based on the ratio of chemical to water. For any product, the point of the "V" in its phase curve is he optimum concentration for the lowest freeze-point protection. As concentrations change, so does the freezing temperature of the solution.
All deicer products essentially work in the same manner, although they have different technical characteristics and phase curves. Their function is to lower the freeze-point temperature of water. This ability depends on the percentage of chemical in the solution. For example, if you fill a 10-quart container with 3 quarts of Supermelt and the rest with water, you have a 30-percent solution. On the phase curve at right, locate the 30-percent-solution point. Looking to the left of this point, you see a freeze-point-protection value of -50°F.
Dilution of solution
In the winter, Mother Nature supplies water in teh form of snow. By melting the snow with deicer and creating water, the concentration of the deicer solution continually changes--and so does the temperature at which it works. I call this concept "DOS" or "Dilution of Solution." Understanding DOW is essential in the understanding of how ice-control products work. Specifically, an ice-control product will work (melting ice and creating water) until the freeze-point temperature of the increasingly diluted solution equals the pavement temperature. At this point, the chemical no longer works. The material stops melting and you may experience refreeze. By looking at the phase curve, you can see this product (at 20°F) will not dilute past 2 percent before it freezes.
DOS also provides the foundation for determining how long a product application will last. Let us say, for example, that from your own experience, you know it will take 0.2 inches of water to dilute your application to the point of freezing. Knowing that 1 inch of snow typcially contains 0.1 inches of water, how long will an application last before it freezes? The answer: As long as it takes 2 inches of snow to accululate. Two inches of snow provides the 0.2 inches of water required to dilute the application to the point of freezing--regardless of how long that accumulation takes.
By looking at the phase curve, you see that four factors determine the total time required to dilute an application to freeze point:
- Surface temperature. This determines the concentration you must use, or when the dilution proces stops.
- Application rate. The colder it is, the more deicer chemical you need.
- Moisture received. At any given temperature, due to the dilution process, it takes more deicer to liquify 2 inches of snow than 1 inch of snow.
- Beginning concentration. The higher the concentration of deicer in a product, the more liquid it can make and the longer it will work.
Understanding these principles gives you the necessary resources to meet most challenges you face in anti-icing and deicing programs.
Dale Keep has worked for the Washington State department of transportation for 25 years. He is currently a maintenance-methods specialist and works statewide in winter operations.
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