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I am currently investigating the effect of adding Salt(NaCl) to water on its triple point. Let us assume the mass ratio of water to NaCl is $1:1$. I asked a similar question previously. But, I still haven't been able to find the temperature(s) of the triple point(s).

I understand that according to the Gibbs equation $f = C-P+2$ Therefore, in my case of Salt water, $f = 1$. Thus we have a range of temperatures/presures where the triple point exists.

But how can we find this range of temperature or pressures? IS there an experimental methods?

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But how can we find this range of temperature or pressures? IS there an experimental methods?

The range of temperatures is fairly easy. Mix saturated aqueous NaCl (liquid solution, concentration about 5 mol/L) with ice (pure water, solid state), and measure the temperature. It will be about equal to the temperature of a household freezer (no coincident, freezers were using salt/water mixtures in the old days), i.e. -20 $^\circ$C. If you keep measuring while the ice melts and the NaCl concentration drops, you will get the temperature ranges, which will have the normal freezing point of water at the high end.

Partial pressure of water measured at atmospheric pressure

Using the same setup, you could measure the humidity above the ice/solution slush. In the experiment, water exists in the solid, liquid, and gas phase, just like pure water at the triple point. The gas phase contains mostly nitrogen and oxygen, so you would have to measure the partial pressure of water rather than the total pressure (which is about 1 atm and depends on the weather). The partial pressure will be a bit lower than that of pure water at the same temperature (the activity of water is less than one in a solution, so the partial pressure will be lower relative to pure water, Roult's law).

Pressure of water measured with the aid of a vacuum pump

In a slightly more involved setup, you would place the slush into a vessel that you evacuate with a vacuum pump. This removes all gas components (nitrogen, oxygen) from the gas phase, and replaces them with evaporating water until the water boils and there is pure water in the gas phase. At that point, you can close off the system and measure the pressure. The result should be the same as the first experiment.

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