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With a hydro aspirator with re-circulating water (see here), the maximum vacuum you can get is set by the water vapor pressure given the temperature.

Can you lower the vapor pressure measurably by adding salt to the water (I get −700 mmHg, at 25 °C, with regular tap water already)?

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  • $\begingroup$ Note that “$-700\ \mathrm{mmHg}$” is a gauge pressure, which is unfortunately still widespread in engineering but not useful for thermodynamic calculations since it depends on the barometric reference pressure. The equilibrium pressure of water at $T=25\ \mathrm{^\circ C}$ is $p=23.776\ \mathrm{mmHg}$. For example, at a reference pressure of $p_0=760\ \mathrm{mmHg}$, the corresponding gauge pressure would be $p_\mathrm g=-736.22\ \mathrm{mmHg}$. Also note that the use of the unit $\mathrm{mmHg}$ is deprecated. $\endgroup$ – user7951 Oct 13 '18 at 8:01
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According to this 1927 paper, "THE VAPOR PRESSURE OF THE SATURATED AQUEOUS SOLUTIONS OF CERTAIN SALTS" at 25 °C, 6 mol per kg salt solution gives a pressure of 18.5 mmHg instead of 24.5 mmHg for pure water. So yes there should be a measurable difference (unless salt destroys the diaphragm pump).

Edit: This recent paper "Water vapour pressure above saturated salt solutions at low temperatures", gives the same results

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