RETURNING MATERIALS: MSU Place in book drop to LIBRARIES remove this checkout from ._:—. your record. FINES wiII be charged if book is returned after the date stamped beIow. ELECTRON TRANSFER KINETICS AND THERMODYNAMIC ACTIVATION PARAMETERS FOR SEVERAL AOUO COMPLEXES AT MERCURY-AQUEOUS INTERFACE by Sayed Muneebullah Husaini A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Chemistry 1982 ABSTRACT ELECTRON TRANSFER KINETICS AND THERMODYNAMIC ACTIVATION PARAMETERS FOR SEVERAL AOUO COMPLEXES AT MERCURY-AQUEOUS INTERFACE by Sayed Muneebullah Husaini A disparity was observed by Anson and Parkinsonfbetween the electrocapillary data and kinetic results for electro- chemical reactions abbut the.double-1ayer structure in very dilute solutions of electrolytes at mercury-aqueous inter- face. This decrepancy was shown to emerge because of employing a potential independent transfer coefficient a = 0.5) to calcuate the double-layer potential ¢2 from kinetic results. The disagreement of results was resolved and exper- imentally investigated, for the oxidation reaction of Bug; in different dilutions of various electrolytes, employ- ing a potential dependent a. 3+/2+ The electrochemical redox reaction for V aq couple demonstrated a unique behavior among the transition metal redox couples by exhibiting a pH dependent reaction rate. The activation parameters for the electrodeposition 2+/O 2+/O 2+/0' .2+/O reaction for Mn aq , Feaq , Coaq and Nlaq couples were determined in aqueous solutions. An increase in the activation. entropy with increasing cathodic potential can be attributed to a partial desolvation of aqueous ligands at the mercury-aqueous interface. *J. Electroanal. Chem., 85, 317 (19771. In the name of God (Allah) who is most merciful most compassionate ii ACKNOWLEDGEMENTS Few words of thanks are like few drops of water returned to the ocean. All thanks to God who gave me the strength and courage to accomplish this work and who created a number of people who were helpful in my endeavor. I owe a lot to Dr. Michael Weaver, my preceptor, who tried his best to bear with me despite my weak- nesses. I feel a great respect for Dr. Fredrick Horne who tried to help me out in difficult times. My thanks are due to my parents, my brothers, sisters and wife, without their love and encouragement, I would not have been able to accomplish this task. iii TABLE OF CONTENTS PAGE List of Tables. vi List of Figures . vii Chapter I INTRODUCTION. II EXPERIMENTAL METHODS. 10 A. Materials 1. Solvent (water) . 10 2. Oxygen removal. 10 3. Electrolytes. 10 4. Reactant solutions. 11 B. Apparatus 1. Electrochemical Cell. 11 2. geference electrode . 3. Polarographic Instrument. 12 C. Electrochemical Techniques l. Egand_pulse experiments. 12 2. Cyclic v61tammetry. 13 III DOUBLE-LAXER STRUCTURE IN DILUTE ELECTROLYTES. 14 A. Double-Layer Structure and Electrode Kinetics . 14 B. Frumkin Double-Layer Correction. 18 C. Verification of Double-Layer Structure in Dilute Solutions. 21 Experimental Procedure and Analysis Of Data 0 O O O O O 0 24 Results and Discussion . 25 iv TABLE OF CONTENTS (cont'd.) PAGE IV THE 93 DEPENDENCE? OF ELECTROCHEMICAL RATE CONSTANTS OF TRANSITION METAL‘ AQUO COMPLEXES IN AQUEOUS SOLUTION. 76 A. pH in Heterogeneous kinetics . 76 B. Experimental procedure and analysis Of data I O C O O O O O O O O O O O O O 77 C. Results and Discussion . 78 V ACTIVATION PARAMETERS OF ELECTRODEPOSIf TION RMCTION O O O O C I C C O O O O O O 90 A. Activation Parameters. 90 B. Experimental Procedures and Analysis of Data . 96 C. Results and Discussion . 99 BIBLIOGRAPHY O O O O O O O O O O O O O O O O O O O O 113 LIST OF TABLES TABLE PAGE 4.1. Related values for the hydrolysis constant for some redox couples . 80 Var/2+ 4.2. Redox Reaction at Different [H I a 25°C 0 O O O C O O O O O O O O 86 5.1 Manganese: an+/0 (1 mM) reduction reaction (Activation Parameters). 101 5.2 Iron: Fe2+/o (1.0 mM) reduction reaction (Activation Parameters) . 102 Cobalt: Coz+/O (1.0 mM) reduction reaction (Activation Parameters) . 103 Nickel: Ni2+/0 (0.4 mM) reduction reaction (Activation Parameters) - - - 104 5.5 The variation of the electrochemical transfer coefficient with temperature at different ionic strengths for the electrodeposition reaction of some transition metal aquo complexes in water at mercury-aqueous interface . 109 5.6 Entropy of some transition metal aqueous ions according to Powell and Latimer . 111 vi LIST OF FIGURES FIGURE PAGE 1.1. Double-layer structure at electrode- solution interphase. .'. 1.2. Potential-distance profile at the electrode-solution interphase according to Guoy-Chapman—Stern (GSC) model. 3.1. Rate-potential data from Anson and Parkinson 5) for the oxidation of 0.2 mg Eu + in aqueous solutions of various electrolytes: 1.55 mg HClO (circles); 0.8 mM HCl (triangles); 0.48 m! HBr (squares). Also Tafel lines at ¢ = 0 from Anson and Parkinson(§) (dashed line) and from Tyma and Weaver(7) (solid line). 29 3.2. Rate-potential data for the oxidation of 0.2 m! Eu2+ in aqueous solutions of various electrolytes: 1.55 mg HClO4 (circles); 0.8 mg HCl (triangles); 0.48 mM HBr (squares); Tafel lines at ¢ = 0 from Anson and Parkinson (5) (dashed line) and from Tyma and Weaver (7) (solid line) . 30 Rate-potential data for the oxidation 3.3. of 0.2 mg Eu2+ in aqueous solutions of HClO : 1.55 mM (circles); 4.0 mM (triangIes ; 7.75 mg (squares). AISO Tafel lines at ¢ = 0 from Anson and Parkinson (5) (dashed line) and from Tyma and Weaver (7) (solid line) . 31 3.4. Rate-potential data for the oxidation of 0.2 mM Eu2+ in aqueous solutions of HCl: 0.8 mM_(circles); 2.0 mg} (triangles); 4.0 mg (squares); 20.0 mM (stars). Also Tafel lines at ¢ = O—from.Anson and Parkinson (5) (daghed line) and from Tyma and Weaver (7) (solid line) . 32 vii LIST OF FIGURES (cont'd.) FIGURE PAGE 3.5. Rate-potential data for the oxidation of 0.2 mM Eu + in aqueous solutions of HBr: 0.48 mM (circles), 1.2 mM (triangles);2.4'mM (squares). AIso Tafel lines at O = 0 from Anson and Parkinson (5) (dgshed line) and from Tyma and Weaver (7) (solid line). 33 3.6. Double-layer potential (O2) vs. applied potential (E) curves from the data of Anson and Parkinsgn (5) for the oxida- tion of 0.2 mM Eu + in aqueous solutions of various electrolytes: 1.55 mM’HClO4 (Circles): 0.8 mg HCl (triangles); 0.48 mM HBr (squares). (O; values cal- culated using Tafel line at O = 0 constructed by Anson and Parkinson (5)). 35 3.7. Double-layer potential (O ) vs. applied potential (E) curves from the data of Anson and Parkinson (5).for the oxida- tion of 0.2 mM Eu2+ in aqueous solutions of various electrolytes: 1.55 mM’HClO4 (circles): 0.8 m! HCl (triangles); 0.48 mg HBr (squares). (O values cal- culated using Tafel line ag O = 0 constructed by Tyma and weave (7)) 37 Double-layer potential (O ) vs. applied potential (E5 curves for; he oxidation of 0.2 mM Eu + in aqueous solutions of various electrolytes: 1.55 mg HClO (circles): 0.8 mg HCl (triangles); 0.48 m! HBr (squares). (O values calculated using Tafel line at O = 0 constructed by Anson and Parkinsoa (5)) . 39 3.9 Double-layer potential (O ) vs. applied potential (E curves for Ehe oxidation of 0.2 mg Eu + in aqueous solutions of various electrolytes: 1.55 mM HClO (circles); 0.8 mg HCl (triangles): 0.48 mM_HBr (squares). (O values calculated using Tafel line at O = 0 constructed by Tyma and Weaver (7?. 41 viii LIST OF FIGURES (cont'd.) FIGURE PAGE 3.10. Double-layer potential (O ) vs. applied potential (E) guraes for the oxidation of 0.2 mM Eu + in aqueous solutions of HClO : 1.55 mM (circles): 4.0 mg (triangles ; 7.75 mg (squares). (O2 values calculated using Tafel line atwO constructed by Anson and Parkin- son(§))................ 43 3.11. Double-layer potential (O ) vs. applied potential (E) curaes for the oxidation of 0.2 mM Eu2+ in aqueous solutions of HClO : 1.55 mg (circles); 4.0 mM(trianglesf; 7.75 mg (squares). (O2 values calculated using Tafel line at O = 0 constructed by Tyma and Weavgr (7) . 45 3.12. Double-layer potential (O2) vs. applied potential (E) Surves for the oxidation of 0.2 mM Eu + in aqueous solutions of HCl: 0.8 mM (circles): 2.0 mg (triangles); 4.0 me(squares); 20.0 mg (stars). (3 values calcu- lated using Tafel 1 3e at $2 = 0 constructed by Anson and Parkinson (5)) . 47 3.13. Double-layer potential (O ) vs. applied (E) curves for the oxidation of 0.2 mM Eu2+ in aqueous solutions of HCl: 0.8 mM (circles); 2.0 mM (triangles); 4.0 mg (squares); 20.0 mg (stars). (O values calculated using Tafel ling at O = 0 con- structed.by Tyma and aeaver(7)) . 49 3.14. Double-layer potential (O ) vs. applied potential (E) curafis for the oxidation of 0.2 meEu + in aqueous solutions of HBr: 0.48 mM (circles); 1.2 mM_(triangles); 2.4 m! (squares). (O values calculated using Tafel line at O = 0 con- structed by Anson and Parkinson (5)) . 51 ix LIST OF FIGURES (cont'd.) FIGURE PAGE 3.15. Double-layer potential (O ) vs.