Processing math: 100%

Saturday, May 26, 2012

Two versions of the Debye-Hückel limiting law

The Debye-Hückel limiting law is usually written for the mean activity coefficient\log(\gamma_\pm)=-|z_+z_-|A\sqrt{I} However, occasionally you will also see it written for the (unmeasurable) activity coefficient of each individual ion \log(\gamma_+)=-z_+^2A\sqrt{I} For the compound M_pX_q \gamma_\pm=(\gamma_+^p\gamma_-^q)^{1/(p+q)} But that implies \log(\gamma_\pm)=-\frac{(pz_+^2+qz_-^2)}{p+q}A\sqrt{I}I couldn't find the connection between these seemingly different definitions of \gamma_\pm anywhere on the web so here it comes.  It's actually rather simple in hindsight.  Charge neutrality dictates pz_++qz_-=0 and this means pz_+^2+qz_-z_+=0\\ pz_+z_-+qz_-^2=0\\pz_+^2+(p+q)z_-z_++qz_-^2=0 which is what we need.  For some reason z_-z_+ is usually written -|z_-z_+|.

One things that's usually not mentioned is that the activities based on the mean and ionic activity coefficients will be different, i.e. \gamma_\pm b_+ \ne \gamma_+ b_+.  But of course things like the equilibrium constant will of course.


Creative Commons License
This work is licensed under a Creative Commons Attribution 3.0 Unported License

No comments: