# How to experimentally determine the chemical potential of salt water? [closed]

I have NaCl dissolved in water. How do I find the chem potential?

• It depends of the amount of oxygen gas dissolved in the water. Sep 16, 2020 at 8:55
• @Maurice what do u mean? how do i measure this? Sep 16, 2020 at 9:51
• @Maurice i thought i could get chemical potential by dividing enthalpy by number of moles Sep 16, 2020 at 9:51
• Sorry. I was thinking of the redox potential, when I spoke of dissolved oxygen. Sep 16, 2020 at 12:24
• The chemical potential in what regard? Freezing point of solution, vapor pressure of water, conductivity, redox potential of $\ce{Cl-}$. and so on. Are you considering oxygen and carbon dioxide in the water? "Chemical potential" needs some context.
– MaxW
Sep 16, 2020 at 16:58

A simple aqueous salt solution contains water and salt. One does not generally discuss the "chemical potential of a solution" but rather that of the individual components. Chemical potential is defined by the following equation

$$\mu _i = \left(\frac{\partial G}{\partial n_i}\right)_{p,T}$$

where G is the Gibbs free energy and $$n_i$$ is the amount of substance i in the system (the solution).

One may want to discuss the Gibbs free energy of the solution, which can be related to the composition and chemical potential of individual components as follows:

$$G = \sum_i \mu_i n_i$$

The chemical potential requires a reference scale, so is usually reported as a difference relative to that of pure water, and in lieu of potential one can report the activity since there is a simple relation between the two ($$\mu_i \propto RT\ln(a_i)$$).

Water activity can be derived from the partial pressure of water vapor in equilibrium with a sample. The activity is equal to the ratio of the partial pressure for the sample to the partial pressure of pure water at the same temperature. There are a variety of methods that can be employed to determine the partial pressure of water, see for instance the Wikipedia.

• i wanna find the chemical potential of salt water Sep 17, 2020 at 4:07
• @RazeXIX I congratulate you on this ambitious goal, but it might help to understand a little better exactly what you are trying to accomplish. Measuring the chemical potential is not exactly like counting beans. Sep 17, 2020 at 7:29
• what other information do i provide oof Sep 18, 2020 at 18:10
• @RazeXIX What are you going to do with the "chemical potential" values? If you can explain this it would go some way to explaining what it is you actually want to know. My guess is you are looking for either the chemical potential of the water in a salt solution, OR the chemical potential of the ions. Sep 19, 2020 at 12:10
• I am looking at freezing point depression, dissolving salt should reduce the chemical potential of the solution Sep 20, 2020 at 9:48