Electroanalytical Chemistry Lecture 3: Electrolyte-the Unassuming and Unappreciated Electrochemical Workhorse
Electroanalytical Chemistry Lecture 3: Electrolyte - the Unassuming and Unappreciated Electrochemical Workhorse
Junction potentials Whenever we change electrolyte or solvent there is a cost in that we create a double layer(electrode) which has a small but often measureable potential We call this potential a junction potential
Junction Potentials Whenever we change electrolyte or solvent there is a cost in that we create a double layer (electrode) which has a small but often measureable potential We call this potential a junction potential
General- Junction Potentials RTr E t1 F REgione n a 2 To solve this we must know 3 things How the concentration of each ion changes How the ion activity varies with concentration How the transport number varies with the concentration of the ions
General - Junction Potentials To solve this we must know 3 things: How the concentration of each ion changes How the ion activity varies with concentration How the transport number varies with the concentration of the ions i n i i i J d F RT z t E ln 2 Region1 1 = = −
The Henderson Equation We cannot solve analytically so we make 2 simplifying assumptions concentration of each ion varies linearly from region I to region 2(1) a=Ci i.e,Yi= 1 for each ion(2) Then we obtain the Henderson equation ∑2u(c(2)-c() RT E h<|2LC;(2) ∑zll:2.co)]∑ Zilu
The Henderson Equation We cannot solve analytically so we make 2 simplifying assumptions: concentration of each ion varies linearly from region 1 to region 2 (1) i = ci , i.e., i 1 for each ion (2) Then we obtain the Henderson equation: ( ) = = = = − − = n i i i i n i i i i n i i i i i n i i i i i i J z u c z u c z u c c c c z z u E F RT 1 1 1 1 (1) (2) ln (2) (1) (2) (1)
Classification of Junction Potentials 3 Categories of Junction Potentials Type 1: 2 solutions of same electrolyte with different concentrations in contact Type 2: 2 solutions of same concentrations of electrolytes in contact which share a common univalent(z-1)ion Type 3: 2 different solutions containing different electrolytes and/or different concentrations in contact
Classification of Junction Potentials 3 Categories of Junction Potentials: Type 1: 2 solutions of same electrolyte with different concentrations in contact Type 2: 2 solutions of same concentrations of electrolytes in contact which share a common univalent (z=1) ion Type 3: 2 different solutions containing different electrolytes and/or different concentrations in contact