I read with interest Jan Ellen Spiegel’s article about salting the roads, but was a little dismayed at some of the conclusions drawn by some of the individuals quoted. Specifically, Brian Burne of the Maine DOT, who “just laughs” at the effects of these chemicals and instead blames how the trucks and other vehicles are constructed. This is a ridiculous assumption.
The science behind using calcium or magnesium chloride instead of sodium chloride to melt snow and ice is really rather straight forward. It is based on something called colligative properties.
Ice will normally melt at zero degrees Celsius if the ice is pure water. However, if particles of a foreign substance are added, usually some soluble substance like a salt, then the ice undergoes what is called a freezing point depression, i.e.; the ice only begins to freeze at a lower temperature and will, therefore, melt at a lower temperature. The amount of freezing point depression is not dependent on the kind of particle in the ice but rather on the number of particles in the ice. That is what a colligative property is.
Since sodium chloride (…salt) is composed of just two particles – sodium and one chloride — it introduces just two particles per molecule to the ice. Both magnesium and calcium have two chlorine molecules attached to them and so you are introducing three particles to the ice for every molecule that is introduced.
It is really not quite as straight forward as that since the molecular weight of both calcium and magnesium is greater than that of sodium, but the reality turns out that, despite that difference, a pound of calcium chloride or magnesium chloride will increase the freezing point depression of water/ice to a greater degree than will a pound of sodium chloride.
The corrosive part of all three molecules is the chloride ion. There are two issues that are involved then.
First, for every pound of salt there are approximately twice as many chloride ions released with CaCl2 and MgCl2 as there are with NaCl (an adjustment must be made for the difference in molecular weight). The second issue is that when the compounds are released as a liquid, then the active chloride ion has already been released and any spray onto the trucks that are spraying the substance onto the road will already be corrosive.
That is not the case with a solid salt that is being dispersed. The salt has to be solubilized before it becomes corrosive.
The difference on the corrosiveness on cars should only be seen as a result of the first issue — that is, more chloride ion on the road per pound of salt since it is unlikely that the cars would receive any direct spray from a truck dispersing the liquid.
The “science” behind the corrosive effects of these chemicals on metal is quite clear. I would also suggest that the effects of releasing twice as many chloride ions into our environment certainly can’t be good for our fresh water surface and ground water supplies.
While I applaud the efforts of the State of Connecticut and the DOT to eliminate ice on our roads, I think we should seriously take a step back and weigh the long term effects on our vehicles and the environmental impact of doing so. While “old fashion” sand and salt may not be as effective in melting ice as the hybrid mixes, they clearly aren’t as destructive to our environment and vehicles.
The long term “costs” of each need to be intelligently measured.
Thank you for taking the time to research and write this article. It’s certainly an issue that needs to be addressed.
Jeff Coppage lives in Farmington.