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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: agg on January 26, 2014, 11:05:49 AM

Title: Activity coefficients.
Post by: agg on January 26, 2014, 11:05:49 AM
I need to know how can you experimentally measure mean activity coefficients in a saturated solution of Ca(OH)2.
Thank you.
Title: Re: Activity coefficients.
Post by: Radu on February 10, 2014, 12:43:39 PM
   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.
Title: Re: Activity coefficients.
Post by: Borek on February 10, 2014, 03:41:11 PM
Quote from: Radu on February 10, 2014, 12:43:39 PM
Me/Me(OH)n

Are you suggesting to put metallic calcium into water solution? Or am I missing something?
Title: Re: Activity coefficients.
Post by: Radu on February 10, 2014, 04:29:04 PM
  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.
Title: Re: Activity coefficients.
Post by: Borek on February 10, 2014, 07:07:57 PM
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?
Title: Re: Activity coefficients.
Post by: Radu on February 11, 2014, 12:40:56 AM
  Yes, you're right, we can use already calibrated pH electrodes.