For the NaCl eutectic I've seen two values widely used:
23.3wt% NaCl - James G. Speight: Lange's Handbook of Chemistry 2017)
5.1 M NaCl (Marion + Kargel 2008 - Cold Aqueous Planetary Geochemistry with FREZCHEM)
both of which are at about -21C.
Am I being silly - these values aren't the same (they're both at atmospheric pressure)?
Is my maths right? [23.3wt%=3.99M and 5.1M= 29.8wt%]