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
Type I Liquid Junctions For Type 1, potential, in V, given by: rT E (2t-1)In Cz where_.>c F Note RTF=0.02569V at 298K C checks When C2=CL EJ=0 When t-t, E=0(good electrolyte!) at_298K,E()005916
Type I Liquid Junctions For Type 1, potential, in V, given by: Note: RT/F = 0.02569 V at 298 K Checks: When c2=c1 , EJ = 0 When t+=t - , EJ = 0 (good electrolyte!) ( ) c c c c E t where zF RT J 1 2 1 2 = − 2 −1 ln _ + ( ) c c E t z at K VJ 1 2 2 1 log 0.05916 _ 298 , ( ) = − −+
Observations about Type I liquid Junction potentials Consider lft>t→E Ifc1>c2→E1 So this tells us if we want to minimize Ej we must Keep c's Use electrolytes with t t
Observations about Type I Liquid Junction Potentials Consider: If t+ > t - EJ _______ If c1 > c2 EJ _______ So, this tells us if we want to minimize EJ we must: Keep c’s ___________________ Use electrolytes with t+______t -
EXAMPLE(B&F 2.4e) 0.01M01M KCI KCI Ag/AgCl(s)/K, CI(I M)/ K+, Cl(0. 1 M)/AgCl(S)Ag cK Left(1), right (2)so C2=0. 1 and C1=1.0 =73.52/(73.52+7634)=0.49 E1=-0.05916(2*0.49)-1)og(0.1) =-0.05916(-0.02)(-1)=-0.001V Note: this represents one approach to measurement of junction potentials
EXAMPLE (B&F 2.4e) Ag/AgCl(s)/K+ ,Cl- (1 M)/ K+ , Cl- (0.1 M)/AgCl(s)/Ag Left (1), right (2) so c2 = 0.1 and c1 =1.0 t+ = 73.52/(73.52+76.34) = 0.49 EJ = - 0.05916 ((2 * 0.49)-1)log(0.1) = - 0.05916 (- 0.02) (-1) = - 0.001 V Note: this represents one approach to measurement of junction potentials 0.01 M KCl 0.1 M KCl K+ Cl-
EXAMPLE 2 0.01M0.1M HCI HC Same junction -HCl not KCl Ag/AgCl(S)/H+, CI(1 M) H H+, Cl(0. 1 M)/AgCI(S)Ag Left (1), right(2)So C2=0. 1 and C,=1.0 t=349.82/(34982+76.34)=0.82 E1=-0.05916((2*0.82)-1)og(0.1) =-005916(-0.64)(-1)=-0.038V Note: junction potential much larger significant perturbation on Ecell
Same junction -HCl not KCl: Ag/AgCl(s)/H+ ,Cl- (1 M)/ H+ , Cl- (0.1 M)/AgCl(s)/Ag Left (1), right (2) so c2 = 0.1 and c1 =1.0 t+ = 349.82/(349.82+76.34) = 0.82 EJ = - 0.05916 ((2 * 0.82)-1)log(0.1) = - 0.05916 (- 0.64) (-1) = - 0.038 V Note: junction potential much larger; significant perturbation on Ecell EXAMPLE 2 0.01 M HCl 0.1 M HCl H+ Cl-
Type 2 Liquid Junctions-the Lewis Sargent relation Type 2: solutions of two different electrolytes having common anion/cation and same concentration in contact with each other rT E + In\M, +if_common_cation △Mx
Type 2 Liquid Junctions - the Lewis Sargent Relation Type 2: solutions of two different electrolytes having common anion/cation and same concentration in contact with each other if common cation M M F RT X X EJ ln ; _ _ 1 2 = +
