Why -21.3 °C? The floor that salt water cannot go below

However much salt you add to water, the freezing point stops falling at a certain place. For sodium chloride that floor is about -21.2 °C, and the coldest eutectic plate sits just above it.

31 January 2026Updated 28 September 2026

Short answer

The freezing point of a sodium chloride solution falls to about -21.21 °C at 23.20 percent salt by weight and stops there; this is the eutectic point. Extra salt stays undissolved. GEBHARDT's coldest plate melts at -21 °C, so salt-water plates cannot hold a temperature below that.

Adding salt to water lowers its freezing point, but not without limit. For sodium chloride the limit is the eutectic point: about -21.21 °C at 23.20 percent NaCl by weight. More salt stays undissolved at the bottom and does not pull the freezing point lower. Sources give the value between -21.1 and -21.3 °C; the -21.3 °C in the title names the same floor.

A eutectic composition freezes and melts as a whole at one temperature, with no softening range in between. That is the behaviour cold transport looks for: while the plate melts, its temperature stays constant.

In GEBHARDT plates the fill is a non-toxic salt-water solution, and the melting points in the catalogue are -3, -14, -18 and -21 °C. The coldest plate therefore sits just above the ceiling of salt water. If your product must travel at -25 °C, the plates on this list cannot hold that temperature; write the target first, from the highest temperature the product may reach, and choose the plate after.