[agg]

I need to know how can you experimentally measure mean activity coefficients in a saturated solution of Ca(OH)₂.
Thank you.

[Radu]

   Humm, why not measuring them using an electrochemical cell , one electrode being Me/Me(OH)_n and the other the standard calomel or hidrogen electrode, where Me(OH)_n is an insoluble hydroxide.( e.g: Cd/Cd(OH)₂) . The measurable cell potential , which depends on pH, defined in activity terms, is recorded for different Ca(OH)₂ concentrations. I suppose if you have an accurate voltmeter, you can measure the activity coefficeints, though I don't know how precise they are.
       Besides, I don't know how easily you can put it into practice, so a general idea would be finding a cell whose potential is dependent on pH.

[Borek]

Me/Me(OH)_n

Are you suggesting to put metallic calcium into water solution? Or am I missing something?

[Radu]

  No, I am saying to insert an electrode covered in its hard soluble metal hydroxide ( a second degree electrode, shall it be Pd/Pd(OH)₂, OH^-_(aq) , or maybe Fe/Fe(OH)₃, OH^-, I don't know which one is best suitable, . ) in different solutions of known concentrations of Ca(OH)₂. We will notice that the calculated  hydroxide concentrations don't match the Nernst equation, we will correct it using activities, and from these data we find γ_+-. Experimentally, we can draw a function of the activity coefficient versus molality or molarity of solution.

[Borek]

I still don't understand what the metal electrode is for. If the solution doesn't contain the Me cation, electrode will respond mostly to OH^-, and we can measure pH using just a pH electrode. Why do you want to complicate your life using untested electrode of unknown response?

[Radu]

  Yes, you're right, we can use already calibrated pH electrodes.