# Deliquescence/efflorescence: How can I convert between salt concentration and RH?

I am looking at two phase diagrams, one has axes of $T$ and $\%\ce{MgClO4}$ in a $\ce{H2O-MgClO4}$ mix, and the other has axes of $T$ and relative humidity. The authors of the second phase diagram say "The stability diagram is modiﬁed from [the earlier phase diagram] by converting salt concentration to relative humidity using the Pitzer model and parameters." How does changing the salt concentration affect the relative humidity, and how can I reproduce this calculation?

(I am a geologist with minimal chemistry background).

Top figure from Chevrier et al., GRL, 2009; bottom figure from Gough et al., EPSL, 2011.