How do cold temperatures affect liquid deicer in storage?
In cold temperatures, we are concerned about physical changes to the liquid, and in particular, the liquid freezing or precipitating settleable solids.
When temperatures decrease, the motion of the molecules decrease as well. We don’t worry about chemical decomposition in cold temperatures. We are concerned about is physical changes to the liquid, and in particular, the liquid freezing or precipitating settleable solids.
One of the best tools that we have for understanding the low temperature storage characteristics of liquids is the freezing point curve. We provide many customers with freezing point curves for commercial liquid deicer products. Even though they are very commonly asked for, they are often frequently misunderstood. People will often try and use the freezing point curve to determine the lowest effective deicing temperature of a particular liquid or to determine what concentration it will melt ice best in. It turns out they do not work very well for this purpose, for a variety of reasons, as they are useful for understanding low temperature storage characteristics.
At 0% sodium chloride, of course we just have pure water, so we know water freezes at 32 degrees Fahrenheit. As we dissolve more and more salt in brine, the freezing point steadily drops. As I dissolve salt in my water, in fact if I dissolve anything in water, doesn’t matter what it is, essentially what we are doing is introducing foreign particles. We are introducing molecules that are different from the water molecules and they will get in the way of the water molecules and they will make it harder for the water molecules to come together and find each other so they can freeze together. That is a little bit of an oversimplified explanation, but it gives you the gist of what is going on. So, the more salt we can dissolve the farther we are going to drive down the freezing point. We run into a brick wall at about 23% sodium chloride. At that point the freezing point is not going to go any lower. This is what we call the eutectic. The eutectic is the lowest freezing point that you can get when I dissolve a given chemical in water.
Now why do we run into a brick wall? Well the reason is that there is a limit to how much salt I can dissolve in water. I can't keep dissolving salt infinitely. At some point that water is going to be holding all that it can hold and it is going to be saturated. That is what is happening at the eutectic. The water is holding all the salt it can and the only way I can get more salt to dissolve is if I raise the temperature.
One other bit of information that is useful in understanding how your liquid is going to behave at cold temperature is results of settleable solids tests. There is a settleable solid standard that was developed by the Pacific Northwest Snowfighters or the PNS. It is based on a fairly simple test. You take a one liter cone, you fill it up with a liter of the liquid, and you store it at a characteristic cold temperature that is set for that chemical. It is going to be lower for calcium chloride than it is for plain salt brine because calcium chloride will store at a much colder temperature than salt brine. You store it there for a week. That gives it enough time for any solids that are going to form to drop out of the brine and settle to the bottom of the cone and therefore we can measure the volume of the solids, and the typical PNS specification is that liquid deicers should contain a less than 1% settleable solids at that particular cold storage temperature, and so most commercial liquids you buy will probably be formulated to be stable to this extent, down to the “typical cold storage temperature”.