Understanding the NaCl Phase Diagram


The sodium chloride phase diagram is a visual representation of the behavior of NaCl solutions across a range of temperatures and salt concentrations. Winter maintenance practitioners are primarily concerned with the portion of the phase diagram where ice is formed. The NaCl phase diagram shows a familiar process: salt depressing the freezing point of ice. However, there has been some confusion regarding the portion of the diagram to the right of the eutectic point. In brine solutions with salt concentrations beyond the eutectic point, salt precipitate and ice begin to form in the solution as the temperature drops. Practitioners are unclear as to whether spraying a supersaturated brine can cause roads to become more slippery under certain circumstances.


The goals of this project were to provide winter maintenance practitioners with a better understanding of the sodium chloride phase diagram and of how brines with salt concentrations higher or lower than the eutectic point of 23.3% will behave on the roadway.


Project deliverables included the development of training materials (a fact sheet and a video) to help provide winter maintenance practitioners with a better understanding of the phase diagram for sodium chloride and how to apply it to yield the best results in roadway deicing. This knowledge will help winter maintenance agencies apply salt and salt brines effectively for the best performance on winter roadways.

Educational video describing the NaCl Phase Diagram


Time-lapse video. 4.5 hrs of salt precip / ice formation. 27% NaCl at 5 F.


Final project webinar was conducted on June 17, 2022.