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I intend to do some experiments with building an ammonia-water gas adsorption refrigerator similar to a Crosley Icyball Wikipedia - Icyball. When recharging the icyball, it is important to not overheat the 'hot' side (containing water and ammonia) when driving off the ammonia. Too much heat will cause water to also evaporate from the hot side, and condense on the cold side along with the ammonia, compromising the efficiency of the device.

While experimenting with the device, I would like to be able to measure the concentration of water/ammonia within the two chambers of the device. The operating pressure would nominally be around 2,000 kPa, temperature range ~-10 to +50 degrees C. Other than water and ammonia, the only other gases present would be an inert/insoluble gas such as helium or hydrogen.

Would using a pH meter be a reasonable way of determining the concentration of water/ammonia in such a system? Or is there a more practical way of measuring water/ammonia concentration?

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    $\begingroup$ pH measurement will not be applicable for several reasons. I would suggest measuring physical parameters, like thermal conductivity, heat capacity, relative permitivity, refraction index etc. $\endgroup$
    – Poutnik
    Commented Dec 12, 2021 at 8:25

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pH is usually measured with a glass electrode. This electrode gives a voltage depending on its constitution. If dipped in dilute aqueous solutions, this voltage is proportional to $\ce{[H^+]}$. It can be used to determine the concentration $\ce{[H^+]}$. But if this electrode is not dipped into a dilute aqueous solution, it gives a voltage which is related to the concentrations in an unknown way. When the aqueous solution is concentrated, it is possible to introduce activity coefficients, which are able to be transformed into sensible concentration values. But if the electrode is dipped into a non-aqueous solution, the measured tension will have no relation with any concentration. It may even be unstable, and change without any reason from positive to negative values.

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  • $\begingroup$ Speculation: Perhaps the autoprotolysis of ammonia (formally $\ce{2 NH3 <=> NH^+_4 + NH^-_2}$) in addition to the one of water renders the situation even more complex. $\endgroup$
    – Buttonwood
    Commented Dec 12, 2021 at 13:10
  • $\begingroup$ @Buttonwood. You may be right. This autoprotolytic reaction should increase the conductivity of the ammonia solution. Is it the case ? I don't know. $\endgroup$
    – Maurice
    Commented Dec 15, 2021 at 17:08
  • $\begingroup$ The potential complication is inferred from the Birch reduction of arenes in ammonia (though e.g., Phil Baran's group worked on electrochemical alternatives easier to scale without this solvent) and speculation only. Possibly US American farmers know/know better how to check if liquid ammonia is dry, or/and those active in the business of industrial refrigeration with ammonia. $\endgroup$
    – Buttonwood
    Commented Dec 15, 2021 at 18:39

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