# How do I calculate the humidity inside sealed containers containing mixing salts?

At $$\pu{25 ^\circ C}$$:

• saturated Magnesium chloride $$\ce{MgCl2}$$ keeps humidity 33% inside sealed containers.
• saturated Sodium chloride $$\ce{NaCl}$$ keeps humidity 75% inside sealed containers.

I want to keep humidity at 60%. Which ratio should I mix this two chemicals? How we can calculate humidity for mixing salts? Is there a formula or table list for this?

At $$\pu{25 ^\circ C}$$ temperature, $$\ce{NaCl}$$ sets 75% humidity and $$\ce{KCl}$$ sets 85% humidity. NaCl mixed 1:1 with $$\ce{KCl}$$ will give an RH% around 70–71 between $$20$$ and $$\pu{30 ^\circ C}$$.

What is the formula to calculate 70% with using 75% and 85%?

• There is no formula. It is pretty complicated. Commented Jun 15, 2020 at 22:29
• thanks for answer, i'll buy salts and do mixing experiments. Commented Jun 16, 2020 at 7:19

As Ivan Neretin pointed out in his comment, there is no simple formula on this matter. Howevr, if you have access to ammonium nitrate ($$\ce{NH4NO3}$$) or sodium nitrite ($$\ce{NaNO2}$$), saturated solution of both give 64% relative humidity (RH) at $$\pu{25 ^\circ C}$$ (Engineering Toolbox).
Interestingly, magnesium nitrate ($$\ce{Mg(NO3)2}$$) gives exactly 60% RH at $$\pu{0 ^\circ C}$$, but it was only 54% at $$\pu{25 ^\circ C}$$. A few salts and their measured RH values in different temperatures are given in the following Table for readers' convenience:
$$\bf{\text{Relative Humidity over the Saturation Salt Solution (%)}}\\ \begin{array}{c|ccc} \hline \text{Salt} & \pu{0 ^\circ C} & \pu{10 ^\circ C} & \pu{20 ^\circ C} & \pu{25 ^\circ C} & \pu{30 ^\circ C} & \pu{40 ^\circ C} & \pu{50 ^\circ C} & \pu{60 ^\circ C}\\ \hline \ce{NH4NO3} & - & 75 & 67 & 60 & - & - & - & - \\ \ce{(NH4)2SO4} & 82 & 82 & 81 & 81 & 81 & 80 & 79 & - \\ \ce{MgCl2} & 34 & 33 & 33 & 33 & 32 & 32 & 31 & 29 \\ \ce{Mg(NO3)2} &99 & 98 & 98 & 97 & 97 & 96 & 96 & - \\ \ce{KNO3} & 96 & 96 & 95 & 94 & 92 & 89 & 85 & - \\ \ce{KCl} & 89 & 87 & 85 & 84 & 84 & 82 & 81 & 80 \\ \ce{NaCl} & 76 & 76 & 75 & 75 & 75 & 75 & 74 & 75 \\ \ce{NaNO2} & - & - & 65 & 64 & 63 & 61 & - & - \\ \hline \end{array}$$