THE UNIVERSITY OF OKLAHOMA GRADUATE COLLEGE A METHOD FOR DETERMINING LIQUID-JUNCTION ELECTROMOTIVE FORCE BY CONTACT POTENTIAL MEASUREMENTS A DISSERTATION SUBMITTED TO THE GRADUATE FACULTY in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY BY MARVIN MARTIN MUELLER Norman, Oklahoma 1959 A METHOD FOR DETERMINING LIQUID-JUNCTION ELECTROMOTIVE FORCE BY CONTACT POTENTIAL MEASUREMENTS A^ROTOS BY ÿ "5 ^ 1 / ‘ m , . DISSERTATION CŒiMITTEE ACKNOWLEDGMENTS It is with pleasure that I declare my indebtedness to the late Professor William Schriever who initially suggested that we make an at­ tempt at measuring phase-boundary potential differences. His friendly interest and encouragement, specifically in the earlier portion of the research, were of considerable benefit. In particular, it should be mentioned that the idea of using a liquid-quadrant electrometer is due to Professor Schriever. To Professor J. Rud Nielsen goes my gratitude for his interest in the latter stages of the research and for his concern in seeing that the apparatus not fall into desuetude after this work is terminated. To the physics shop and, especially, Mr. Jim Hood go ny thanks and commendation for a job well and patiently done. Mr. Hood's care and concern over many months is manifest in the good mechanical performance of the final apparatus. Finally and most personally, 1 wish to record ray heartfelt grat­ itude for the myriad ways in which ny wife, Elvira, has been of help dur­ ing the years of this research. Without her assistance this work would not have been carried out. iii TABLE OF CONTENTS Page LIST OF TABLES .................................................. v U S T OF ILLUSTRATIONS ........................................... vi Chapter I . HISTORICAL INTRODUCTION..... 1 II. THE CONCEPT OF INTERPHASE POTENTIALS ..................... 9 III. THE LIQUID-JUNCTION POTENTIAL DIFFERENCE ................. 30 IV. THE VARIABLE CAPACITANCE METHOD OF MEASURING VOLTA POTENTIAL DIFFERENCE .................................... 35 V. A METHOD FOR DETERMININGLIQUID-JUNCTION POTENTIAL DIFFERENCES ............................................. 6L V I . THE EXPLORATORY EXPERIMENTAL WORK ........................ 73 VII. THE FINAL APPARATUS ...................................... 91 VIII. EXPERIMENTAL TECHNIQUES AND RESULTS ...................... 120 BIBLIOGRAPHY .................................................... 1^2 APPENDIX ........................................................ 133 IV LIST OF TABLES Table Page 1. Values of Ej in Millivolts Calculated by Four Methods as Described in the Text for Aqueous HGl Concentration Junctions at 25°C ..................................... U7 2. The Effect of Various Applied Field Strengths on the Apparent Contact Potential Difference of the Aluminum-Lucite-Air-Brass Condenser System ...................................... 1^7 LEST OF ILLUSTRATIONS Figure Page 1. The Variable Capacitance Instrument .................. 58 2. The Equivalent Circuit of the Variable Capacitance Instrument .................................................. 6l 3. The Principle of the Liquid-liquid Contact Potential Measurement ................................................. 66 U. An Over-all View of the Final Apparatus ................... 96 5. The Solution Troughs and the Probe Assembly .... 101 6. The Junction Chamber Apparatus ....................... 103 ?. The Underside Instrumentation ........................ 112 VI A METHOD FOR DETERMINING IIQUID-JUNCTION ELECTRCMOTIVE FORCE BY CONTACT POTENTIAL MEASUREMENTS CHAPTER I HISTORICAL INTRODUCTION The chronicle of attempts at determining individual phase- boundary potential differences is a long and tortuous comedy of errors without a satisfying ending. Even today, after more than a century of work on this question, no single potential difference is known to within reasonable- certainty, - In the pages of this chapter we shall outline the main courses of thought concerning this formidable problem. This discourse will, naturally, not be exhaustive and some of it can be found elsewhere^ with detailed references which are omitted here. Furthermore, we shall re­ serve the precise conceptual picture for the next chapter which will give a modem interpretation of the problem. Here we shall be content to use terms loosely in their historical context. The beginning of thought about phase-boundary potentials is co­ incident with the beginning of electrochemistry. About I8OO Volta dis­ covered his celebrated "pile” and immediately began to conjecture ^J, A. V, Butler, Electrical Phenomena at Interfaces (Methuen and Co„, London, 19^1), 2 concerning its seat of electrical energy which he tentatively put at the metal-metal contact. Volta was led to this postulate by his discovery of what has come to be called the Volta effect— the contact charging of un­ like metals. He found that whenever two dissimilar pieces of metal are put into contact and then separated, they bear opposite electric charges. Thus, it seemed to him that the observed electromotive force of an electrochemical cell should be due largely to the metal-metal contact. By 1870 quantitative methods of measuring the Volta effect had been developed. Kelvin made the opposite pairs of quadrants of his quad­ rant electrometer out of unlike metals and thus determined the contact potential difference between them. (A similar, though much more compli­ cated, method using liquid quadrants is described in Chapter VI.) F. W. Kohlrausch used a null method in which an adjustable potentiometer is connected across two parallel plates of unlike metals. At balance, no deflection of a quadrant electrometer occurs when the plates are touched and separated. The potentiometric potential difference is then equal and opposite to the contact potential difference. It was Kelvin, however, who, in later decades, refined this technique to a relatively high degree of precision. The metal-metal contact potential difference was investi­ gated for many metal pairs and was usually found to be of the order of a few volts and to be markedly dependent of the state of the surface. In particular, oxidation and adsorption of water vapor were found to have a most pronounced effect. In some cases even the sign of the potential difference could be changed by burnishing or heating in air. In summary: the various experimental methods are capable, if suitable precautions are taken, of unambiguously measuring the contact potential difference between 3 two surfaces— the difficulty arises only when an attempt is made to measure the contact potential difference between pure phases. Up till the last two decades, it was believed that a measurement of the contact potential difference between absolutely clean metals in vacuum would be ipso facto a measurement of the potential difference at the metal-metal contact. That this is not so (due to metal-vacuum po­ tential jumps) seems to be at present universally conceded by those few workers who are nowadays concerned with such matters. (We shall discuss this question more thoroughly in the next chapter.) Apart from this, needless to say, the problem of obtaining pure metal surfaces is exceed­ ingly difficult. Duplication of results among workers using the same as well as different techniques has been poor until very recently when mod­ ern ultra-high vacuum techniques were brought to bear on the problem.^ Even now most contact potential measurements between presumably pure met­ als are uncertain to within roughly eighty millivolts or more. For several succeeding decades the study of contact potential difference lay relatively dormant until awakened in the early part of this century by the thorough and refined investigations of thermionic and photoelectric electron emmission conducted largely by Richardson and Millikan. These new and totally different techniques rediscovered the phenomenon and its attendant dependence on surface conditions. It was also found that the contact potential difference between two metal sur­ faces is equal to the difference of their work functions. (This is only approximately true due to the fact that a photoelectric measurement is weighted toward that exposed crystal face which has the smallest woik ^J. C. Riviere, Proc. Roy. Soc. 70B, 676 (1957). u function while the Kelvin method of measuring contact potential difference measures the statistical average over all exposed crystal faces.) In 1916, using the new work function data, Langmuir showed that the contact potential difference between two metals is very roughly equal to the emf of an electrochemical cell composed of the same two metals dipping into normal solutions of their salts. This he took to be a strong indication of the essential correctness of Volta's original hypothesis. 1 2 Butler and Gurney and Fowler seized upon this idea and incorporated it in their kinetic classical and (later) quantum mechanical explanation of electrode potentials. Their treatment, which analyzes the electrode po­ tential into parts contributed by the metallic work function, sublimation energy of the metal atom, ionization energy of the metal ion, andhydra­ tion energy of the ion, has entirely supplanted the vague and inconsistent "solution pressure" hypothesis of Nemst.^ Strangely enough though, even the kinetic picture of electrode processes does not, as we shall see later, answer the old question regarding the seat of the emf. Thus far, we have dealt with only one side of the celebrated con­ troversy concerning the seat of emf in electrochemical cells which has persisted