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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%?

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    $\begingroup$ There is no formula. It is pretty complicated. $\endgroup$ Jun 15 '20 at 22:29
  • $\begingroup$ thanks for answer, i'll buy salts and do mixing experiments. $\endgroup$ Jun 16 '20 at 7:19
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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:

$$ \bf{\text{Relative Humidity over the Saturation Salt Solution (%)}}\\ \begin{array}{c|ccc} \hline \text{Salt} & \pu{0 ^\circ C} & \pu{10 ^\circ C} & \pu{20 ^\circ C} & \pu{25 ^\circ C} & \pu{30 ^\circ C} & \pu{40 ^\circ C} & \pu{50 ^\circ C} & \pu{60 ^\circ C}\\ \hline \ce{NH4NO3} & - & 75 & 67 & 60 & - & - & - & - \\ \ce{(NH4)2SO4} & 82 & 82 & 81 & 81 & 81 & 80 & 79 & - \\ \ce{MgCl2} & 34 & 33 & 33 & 33 & 32 & 32 & 31 & 29 \\ \ce{Mg(NO3)2} &99 & 98 & 98 & 97 & 97 & 96 & 96 & - \\ \ce{KNO3} & 96 & 96 & 95 & 94 & 92 & 89 & 85 & - \\ \ce{KCl} & 89 & 87 & 85 & 84 & 84 & 82 & 81 & 80 \\ \ce{NaCl} & 76 & 76 & 75 & 75 & 75 & 75 & 74 & 75 \\ \ce{NaNO2} & - & - & 65 & 64 & 63 & 61 & - & - \\ \hline \end{array} $$

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